Aspen Insurance Holdings Limited (AHL-PC) 10-K 23 Feb 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Billions, except Share data, unless otherwise specified	Dec. 31, 2014	Feb. 13, 2015	Jun. 30, 2014
Document Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	AHL
Entity Registrant Name	ASPEN INSURANCE HOLDINGS LTD
Entity Central Index Key	1267395
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		62,238,945
Entity Public Float			$2.80

CONSOLIDATED_STATEMENTS_OF_OPE

CONSOLIDATED STATEMENTS OF OPERATIONS AND OTHER COMPREHENSIVE INCOME (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Revenues
Net earned premium	$2,405.30		$2,171.80	$2,083.50
Net investment income	190.3		186.4	204.9
Realized and unrealized investment gains	46.3	[1]	56.9	35.4
Other income	4.5		8.2	5.6
Total Revenues	2,646.40		2,423.30	2,329.40
Expenses
Losses and loss adjustment expenses	1,307.50		1,223.70	1,238.50
Amortization of deferred policy acquisition costs	451.2		422	381.2
General, administrative and corporate expenses	445.7		368.1	345.1
Interest on long-term debt	29.5		32.7	30.9
Change in fair value of derivatives	15.2		-1.3	28.4
Change in fair value of loan notes issued by variable interest entities	18.6		0	0
Realized and unrealized investment losses	14.7	[1]	20.5	8.6
Net realized and unrealized foreign exchange losses/(gains)	-5.6	[1]	13.2	-3.4
Other expenses	1.7		1.7	4.7
Total expenses	2,278.50		2,080.60	2,034
Income from operations before income tax	367.9		342.7	295.4
Income tax expense	-12.1		-13.4	-15
Net income	355.8		329.3	280.4
Proportion due to non-controlling interest	-0.8		0.5	0.2
Net income attributable to Aspen Insurance Holdings Limited ordinary shareholders	355		329.8	280.6
Available for sale investments:
Reclassification adjustment for net realized losses/(gains) on investments included in net income	-7.7		-24.1	2.6
Change in net unrealized gains/(losses) on available for sale securities held	45.4		-174.3	16.5
Amortization of loss on derivative contract	0		0.5	0.2
Net change from current period hedged transactions	-3.8		0	0
Change in foreign currency translation adjustment	-23.8		-24.1	-11.5
Other comprehensive income, gross of tax	10.1		-222	7.8
Tax thereon:
Reclassification adjustment for net realized losses on investments included in net income	0.2		0.7	-0.6
Change in net unrealized (gains)/losses on available for sale securities held	-3		13	-8.7
Change in foreign currency translation adjustment	7.9		0	0
Total tax on other comprehensive income	5.1		13.7	-9.3
Other comprehensive income	15.2		-208.3	-1.5
Total comprehensive income attributable to Aspen Holdings’ ordinary shareholders	$370.20		$121.50	$279.10
Weighted average number of ordinary share and share equivalents
Basic, in Shares	64,536,491		66,872,048	71,095,856	[2]
Diluted, in Shares	65,872,949		69,417,903	73,689,423	[2]
Basic earnings per ordinary share adjusted for preference share dividends	$4.92		$4.29	$3.51
Diluted earnings per ordinary share adjusted for preference share dividends	$4.82		$4.14	$3.39
[1]	Adjusted for a representation of foreign exchange in relation to investment securities from realized and unrealized exchange gains/(losses) to realized and unrealized investment gains/(losses).
[2]	The basic and diluted number of ordinary shares for the three months ended December 31, 2012 is the same, as the inclusion of dilutive securities in a loss-making period would be anti-dilutive.

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Dec. 31, 2014	Dec. 31, 2013
Investments:
Fixed income securities, available for sale at fair value (amortized cost — $5,462.9 and $5,449.9)	$5,630,000,000	$5,569,100,000
Fixed income securities, trading at fair value (amortized cost — $760.9 and $712.1)	771,000,000	716,200,000
Equity securities, available for sale at fair value (cost — $82.6 and $112.2)	109,900,000	149,500,000
Equity securities, trading at fair value (cost — $585.2 and $281.6)	616,000,000	310,900,000
Short-term investments, available for sale at fair value (amortized cost — $258.2 and $160.3)	258,300,000	160,300,000
Short-term investments, trading at fair value (amortized cost — $0.2 and $Nil)	200,000	0
Catastrophe bonds, trading at fair value (cost — $34.4 and $5.8)	34,800,000	5,800,000
Other investments, equity method	8,700,000	48,000,000
Total investments	7,428,900,000	6,959,800,000
Cash and cash equivalents (including cash within consolidated variable interest entities of $176.7 and $50.0)	1,178,500,000	1,293,600,000
Reinsurance recoverables:
Unpaid losses	350,000,000	332,700,000
Ceded unearned premiums	206,800,000	151,900,000
Receivables:
Underwriting premiums	1,011,700,000	999,000,000
Other	90,200,000	90,300,000
Funds withheld	46,900,000	46,500,000
Deferred policy acquisition costs	299,000,000	262,200,000
Derivatives at fair value	8,000,000	7,000,000
Receivable for securities sold	2,300,000	5,200,000
Office properties and equipment	62,200,000	60,100,000
Deferred taxation	0	1,600,000
Other assets	13,600,000	2,200,000
Intangible assets	18,200,000	18,400,000
Total assets	10,716,300,000	10,230,500,000
Insurance reserves
Losses and loss adjustment expenses	4,750,800,000	4,678,900,000
Unearned premiums	1,441,800,000	1,280,600,000
Total insurance reserves	6,192,600,000	5,959,500,000
Payables
Reinsurance premiums	92,000,000	88,200,000
Current taxation	18,300,000	15,700,000
Deferred taxation	3,100,000	0
Accrued expenses and other payables	356,900,000	265,600,000
Liabilities under derivative contracts	14,300,000	2,900,000
Total payables	484,600,000	372,400,000
Loan notes issued by variable interest entities, at fair value	70,700,000	50,000,000
Long-term debt	549,100,000	549,000,000
Total liabilities	7,297,000,000	6,930,900,000
Commitments and contingent liabilities (see Note 20)	0	0
SHAREHOLDERS’ EQUITY
62,017,368 shares of par value 0.15144558¢ each (December 31, 2013 — 65,546,976)	94,000	99,000
Preference shares:
Non-controlling interest	500,000	-300,000
Additional paid-in capital	1,134,300,000	1,297,400,000
Retained earnings	2,050,100,000	1,783,300,000
Accumulated other comprehensive income, net of taxes	234,300,000	219,100,000
Total shareholders’ equity	3,419,300,000	3,299,600,000
Total liabilities and shareholders’ equity	10,716,300,000	10,230,500,000
5.950% Preference Shares
Preference shares:
Preference shares, value	0	0
7.401% Preference Shares
Preference shares:
Preference shares, value	0	0
7.250% Preference Shares
Preference shares:
Preference shares, value	$0	$0

CONSOLIDATED_BALANCE_SHEETS_Co

CONSOLIDATED BALANCE SHEETS Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, except Share data, unless otherwise specified
Fixed income securities, available for sale at fair value (amortized cost — $5,462.9 and $5,449.9)	$5,462.90	$5,449.90
Fixed income securities, trading at fair value (amortized cost — $760.9 and $712.1)	760.9	712.1
Equity securities, available for sale at fair value (cost — $82.6 and $112.2)	82.6	112.2
Equity securities, trading at fair value (cost — $585.2 and $281.6)	585.2	281.6
Short-term investments, available for sale at fair value (amortized cost — $258.2 and $160.3)	258.2	160.3
Short-term investments, trading at fair value (amortized cost — $0.2 and $Nil)	0.2	0
Cash and cash equivalents (including cash within consolidated variable interest entities of $176.7 and $50.0)	1,178.50	1,293.60
Catastrophe bonds, trading at fair value (cost — $34.4 and $5.8)	34.4	5.8
Ordinary shares, par value	$0.15	$0.15
Ordinary shares, issued	62,017,368	65,546,976
Preference shares, par value	$0.15	$0.15
5.950% Preference Shares
Preference shares, issued	11,000,000	11,000,000
Preference shares, rate	5.95%	5.95%
Preference shares, par value	$0.15	$0.15
7.401% Preference Shares
Preference shares, issued	5,327,500	5,327,500
Preference shares, rate	7.40%	7.40%
Preference shares, par value	$0.15	$0.15
7.250% Preference Shares
Preference shares, issued	6,400,000	6,400,000
Preference shares, rate	7.25%	7.25%
Preference shares, par value	$0.15	$0.15
Consolidated Variable Interest Entities
Cash and cash equivalents (including cash within consolidated variable interest entities of $176.7 and $50.0)	$176.70	$50

CONSOLIDATED_STATEMENTS_OF_CHA

CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (USD $)	Total		Ordinary Shares	Preference Shares	Series A Preferred Stock	Non-controlling interest	Additional paid-in capital	Additional paid-in capital	Additional paid-in capital		Retained earnings		Retained earnings	Retained earnings		Cumulative foreign currency translation adjustments, net of taxes:	Deferred loss on derivatives, net of taxes:	Unrealized appreciation on available for sale investments, net of taxes:
In Millions, unless otherwise specified	USD ($)		USD ($)	USD ($)		USD ($)	USD ($)	Ordinary Shares	Preference Shares		USD ($)		Ordinary Shares	Preference Shares		USD ($)	USD ($)	USD ($)
								USD ($)	USD ($)				USD ($)	USD ($)
Shareholder's equity beginning balance at Dec. 31, 2011			$0.10	$0		$0.40	$1,385				$1,341.60					$124.20	($0.70)	$305.40
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change attributable to non-controlling interest for the year	-0.2					-0.2					0.2
Shares issued								22.1	154.5
Ordinary shares repurchased and cancelled	-59.9							-62.7
PIERS redeemed and cancelled									0
Change in Redemption Value	0	[1]							0	[2]				0	[2]
Share-based compensation							17.8
Net income	280.4										280.4
Dividends													-47	-31.1
Dividends due to non-controlling interest											-0.1
Change for the year, net of income tax																-11.5		9.8
Reclassification to interest on long-term debt																	0.2
Net change from current period hedged transactions																	0
Total accumulated other comprehensive income, net of taxes	427.4
Shareholder's equity ending balance at Dec. 31, 2012	3,488.40		0.1	0		0.2	1,516.70				1,544	[2]				112.7	-0.5	315.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change attributable to non-controlling interest for the year	-0.5					-0.5					0.5
Shares issued								21.2	270.6
Ordinary shares repurchased and cancelled								-309.6
PIERS redeemed and cancelled				-230					-230
Change in Redemption Value	-7.1	[1]							7.1	[2]				-7.1	[2]
Share-based compensation							21.4
Net income	329.3										329.3
Dividends													-47.8	-35.5
Dividends due to non-controlling interest											-0.1
Change for the year, net of income tax																-24.1		-184.7
Reclassification to interest on long-term debt																	0.5
Net change from current period hedged transactions																	0
Total accumulated other comprehensive income, net of taxes	219.1
Shareholder's equity ending balance at Dec. 31, 2013	3,299.60		0.1	0		-0.3	1,297.40				1,783.30	[2]				88.6	0	130.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change attributable to non-controlling interest for the year	0.8					0.8					-0.8
Shares issued								2.7	0
Ordinary shares repurchased and cancelled								-180.9
PIERS redeemed and cancelled				-230					0
Change in Redemption Value	0	[1]							0	[2]				0	[2]
Share-based compensation							15.1
Net income	355.8
Dividends													-50.3	-37.8
Dividends due to non-controlling interest											-0.1
Change for the year, net of income tax																-15.9		34.9
Reclassification to interest on long-term debt																	0
Net change from current period hedged transactions																	-3.8
Total accumulated other comprehensive income, net of taxes	234.3
Preference shares, rate					5.63%
Shareholder's equity ending balance at Dec. 31, 2014	3,419.30		0.1	0		0.5	1,134.30				2,050.10	[2]				72.7	-3.8	165.4
Shareholder's equity beginning balance at Sep. 30, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	67.2
Total accumulated other comprehensive income, net of taxes	234.3
Preference shares, rate					5.63%
Shareholder's equity ending balance at Dec. 31, 2014	$3,419.30		$0.10
[1]	The $7.1 million deduction from net income in 2013 is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million. For more information, please refer to Note 15, “Capital Structure” of these consolidated financial statements.
[2]	The $7.1 million reclassification from additional paid-in capital to retained earnings is the difference between the capital raised upon issuance of the 5.625% Perpetual Preferred Income Equity Replacement Securities (“PIERS”), net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million.

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net income	$355.80		$329.30	$280.40
Proportion due to non-controlling interest	-0.8		0.5	0.2
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	29.6		41.1	38.8
Share-based compensation	15.1		21.4	17.8
Realized and unrealized investment (gains)	-46.3		-56.9	-35.4
Realized and unrealized investment losses	14.7	[1]	20.5	8.6
Change in fair value of loan notes issued by variable interest entities	18.6		0	0
Other investments gains/(losses)	0		2.3	0
Net realized and unrealized investment foreign exchange losses	0.8		3.7	1.9
Loss on derivative contracts	0		0.5	0.2
Insurance reserves:
Losses and loss adjustment expenses	159.3		-82.9	211.9
Unearned premiums	152.6		158.5	198.3
Reinsurance recoverables:
Unpaid losses	-19.3		164.1	-70.4
Ceded unearned premiums	-51.8		-29.4	-34
Other receivables	-4.5		12.4	1.6
Deferred policy acquisition costs	-41.5		-39.1	-37.6
Reinsurance premiums payable	4.5		-32.8	-2.3
Funds withheld	-0.4		37.8	6.4
Premiums receivable	-28.5		52.1	-165.9
Deferred taxes	4.7		-19.5	-20.8
Income tax payable	-10.8		21.3	18.3
Accrued expenses and other payable	51.6		-9.6	52.4
Fair value of derivatives and settlement of liabilities under derivatives	10.4		-9.2	7.7
Long-term debt	0.1		0.2	0.1
Intangible assets	-0.2		-0.1	0
Other assets	-6.3		-19.8	18.2
Net cash generated by operating activities	607.4		566.4	496.4
Cash flows (used in) investing activities:
(Purchases) of fixed income securities — Available for sale	-2,005		-2,129.80	-1,529.60
(Purchases) of fixed income securities — Trading	-653.4		-763.4	-300.8
Proceeds from sales and maturities of fixed income securities — Available for sale	1,909.50		1,872.30	1,416.50
Proceeds from sales and maturities of fixed income securities — Trading	615.9		486	257.2
(Purchases) of equity securities — Available for sale	0		-2.5	-53.1
(Purchases) of equity securities — Trading	-361		-304.4	0
Net (purchases) of catastrophe bonds — Trading	-28.7		-5.8	0
Proceeds from sales of equity securities — Available for sale	40		82.2	46.9
Proceeds from sales of equity securities — Trading	62.2		24.1	0
Net (purchases)/sales of short-term investments — Available for sale	-110.3		260.6	-122.7
Net (purchases)/sales of short-term investments — Trading	-0.2		0	0
Net change in (payable)/receivable for securities sold	2.8		-0.9	1.1
Investment in Chaspark Maritime Holdings Ltd	0		0	-8.7
Purchase of equipment	-26.1		-16.3	-24
Net proceeds from other investments	39.3		0	0
Net cash (used in)/from investing activities	-515		-497.9	-317.2
Cash flows (used in)/from financing activities:
Proceeds from the issuance of ordinary shares, net of issuance costs	2.7		21.2	22.1
Proceeds from the issuance of preference shares, net of issuance costs	0		270.6	154.5
PIERS repurchased and cancelled	0		-230	0
Ordinary shares repurchased	-180.9		-309.6	-62.7
Proceeds from long-term debt issuances by Silverton	70		50	0
Dividends paid on ordinary shares	-50.3		-47.8	-47
Dividends paid on preference shares	-37.8		-35.5	-31.1
Dividends paid to non-controlling interest	-0.1		-0.1	-0.1
Proceeds from note issuances by Aspen Holdings	0		299.7	0
Long-term debt redeemed	0		-250	0
Make whole payment	0		-9.3	0
Net cash (used in)/from financing activities	-196.4		-240.8	35.7
Effect of exchange rate movements on cash and cash equivalents	-11.1		2.3	9.6
Increase in cash and cash equivalents	-115.1		-170	224.5
Cash and cash equivalents at beginning of period	1,293.60		1,463.60	1,239.10
Cash and cash equivalents at end of period	1,178.50		1,293.60	1,463.60
Supplemental disclosure of cash flow information:
Net cash paid/(received) during the period for income tax	1.8		-6.3	11
Cash paid during the period for interest	$29		$35	$30
[1]	Adjusted for a representation of foreign exchange in relation to investment securities from realized and unrealized exchange gains/(losses) to realized and unrealized investment gains/(losses).

History_and_Organization

History and Organization	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
History and Organization	History and Organization
	Aspen Insurance Holdings Limited (“Aspen Holdings”) was incorporated on May 23, 2002 and holds subsidiaries that provide insurance and reinsurance on a worldwide basis. Its principal operating subsidiaries are Aspen Insurance UK Limited (“Aspen U.K.”), Aspen Bermuda Limited (“Aspen Bermuda”), Aspen Specialty Insurance Company (“Aspen Specialty”), Aspen American Insurance Company (“AAIC”) and Aspen Underwriting Limited (corporate member of Lloyd’s Syndicate 4711, “AUL”) (collectively, the “Operating Subsidiaries”). We also established Aspen Capital Management Ltd and other related entities collectively (“ACM”) which are used to leverage our existing franchise and underwriting expertise to offer investors access to diversified products. In such regard, Silverton Re Ltd. (“Silverton”), a sidecar, was established in 2013 to attract third-party capital and to provide additional collateralized capacity to support Aspen Re’s global reinsurance business. References to the “Company,” “we,” “us” or “our” refer to Aspen Holdings or Aspen Holdings and its subsidiaries.

Basis_of_Preparation_and_Signi

Basis of Preparation and Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Basis of Preparation and Significant Accounting Policies	Basis of Presentation and Significant Accounting Policies
	The consolidated financial statements of Aspen Holdings are prepared in accordance with United States Generally Accepted Accounting Principles (“U.S. GAAP”) and are presented on a consolidated basis including the transactions of all operating subsidiaries. Transactions between Aspen Holdings and its subsidiaries are eliminated within the consolidated financial statements.
	(a)	Use of Estimates
	Assumptions and estimates made by management have a significant effect on the amounts reported within the consolidated financial statements. The most significant of these relate to the losses and loss adjustment expenses, reinsurance recoverables, gross written premiums and commissions which have not been reported to the Company such as those relating to proportional treaty reinsurance contracts, the fair value of derivatives and the fair value of other investments. All material assumptions and estimates are regularly reviewed and adjustments made as necessary, but actual results could turn out significantly different from those expected when the assumptions or estimates were made.
	(b)	Accounting for Insurance and Reinsurance Operations
	Premiums Earned. Premiums are recognized as revenues proportionately over the coverage period. Premiums earned are recorded in the statements of operations, net of the cost of purchased reinsurance. Premiums written which are not yet recognized as earned premium are recorded in the consolidated balance sheet as unearned premiums, gross of any ceded unearned premiums. Written and earned premiums, and the related costs, include estimates for premiums which have not yet been finally determined. These relate mainly to contractual provisions for the payment of adjustment or additional premiums, premiums payable under proportional treaties and delegated underwriting authorities, and reinstatement premiums.
	Adjustment and additional premiums are premiums charged which relate to experience during the policy term. The proportion of adjustable premiums included in the premium estimates varies between business lines with the largest adjustment premiums being in property and casualty reinsurance, marine, aviation and energy insurance and the smallest in property and casualty insurance.
	Premiums payable under proportional treaty contracts and delegated underwriting authorities are generally not reported to the Company until after the reinsurance coverage is in force. As a result, an estimate of these “pipeline” premiums is recorded. The Company estimates pipeline premiums based on projections of ultimate premium taking into account reported premiums and expected development patterns.
	Reinstatement premiums on assumed excess of loss reinsurance contracts are provided for based on experience under such contracts. Reinstatement premiums are the premiums charged for the restoration of the reinsurance limit of an excess of loss contract to its full amount after payment by the reinsurer of losses as a result of an occurrence and are recognized as revenue in full at the date of loss, triggering the payment of the reinstatement premiums. The payment of reinstatement premiums provides future insurance cover for the remainder of the initial policy term. An allowance for uncollectible premiums is established for possible non-payment of premium receivables, as deemed necessary.
	Outward reinsurance premiums, for when the Company purchases reinsurance or retrocessional coverage, are accounted for using the same accounting methodology as we use for inwards premiums. Premiums payable under reinsurance contracts that operate on a “losses occurring during” basis are accounted for in full over the period of coverage while those arising from “risks attaching during” policies are expensed over the earnings period of the premiums receivable from the reinsured business.
	Losses and Loss Adjustment Expenses. Losses represent the amount paid or expected to be paid to claimants in respect of events that have occurred on or before the balance sheet date. The costs of investigating, resolving and processing these claims are known as loss adjustment expenses (“LAE”). The statement of operations records these losses net of reinsurance, meaning that gross losses and loss adjustment expenses incurred are reduced by the amounts recovered or expected to be recovered under reinsurance contracts.
	Reinsurance. Written premiums, earned premiums, incurred claims, LAE and the amortization of deferred policy acquisition costs all reflect the net effect of assumed and ceded reinsurance transactions. Assumed reinsurance refers to the Company’s acceptance of certain insurance risks that other insurance companies have underwritten. Ceded reinsurance arises from contracts under which other insurance companies agree to share certain risks with the Company.
	Reinsurance accounting is followed when there is significant timing risk, significant underwriting risk and a reasonable possibility of significant loss.
	Reinsurance and retrocession does not isolate the ceding company from its obligations to policyholders. In the event that a reinsurer or retrocessionaire fails to meet its obligations, the ceding company’s obligations remain. The Company regularly evaluates the financial condition of its reinsurers and retrocessionaires and monitors the concentration of credit risk to minimize its exposure to financial loss from reinsurers’ and retrocessionaires’ insolvency. Where it is considered required, appropriate provision is made for balances deemed irrecoverable from reinsurers.
	Reserves. Insurance reserves are established for the total unpaid cost of claims and LAE in respect of events that have occurred by the balance sheet date, including the Company’s estimates of the total cost of claims incurred but not yet reported (“IBNR”). Claim reserves are reduced for estimated amounts of salvage and subrogation recoveries. Estimated amounts recoverable from reinsurers on unpaid losses and LAE are reflected as assets.
	For reported claims, reserves are established on a case-by-case basis within the parameters of coverage provided in the insurance policy or reinsurance agreement. For IBNR claims, reserves are estimated using a number of established actuarial methods to establish a range of estimates from which a management best estimate is selected. Both case and IBNR reserve estimates consider variables such as past loss experience, changes in legislative conditions, changes in judicial interpretation of legal liability, policy coverages and inflation.
	As many of the coverages underwritten involve claims that may not be ultimately settled for many years after they are incurred, subjective judgments as to the ultimate exposure to losses are an integral and necessary component of the loss reserving process. The Company regularly reviews its reserves, using a variety of statistical and actuarial techniques to analyze current claims costs, frequency and severity data, and prevailing economic, social and legal factors. Reserves established in prior periods are adjusted as claim experience develops and new information becomes available. Adjustments to previously estimated reserves are reflected in the financial results of the period in which the adjustments are made.
	The process of estimating required reserves does, by its very nature, involve considerable uncertainty. The level of uncertainty can be influenced by factors such as the existence of coverage with long duration payment patterns and changes in claims handling practices, as well as the factors noted above. Ultimate actual payments for claims and LAE could turn out to be significantly different from the Company’s estimates.
	Amortization of Deferred Policy Acquisition Costs. The costs directly related to writing an insurance policy are referred to as policy acquisition expenses and include commissions, premium taxes and profit commissions. With the exception of profit commissions, these expenses are incurred when a policy is issued, and only the costs directly related to the successful acquisition of new and renewal insurance and reinsurance contracts are deferred and amortized over the same period as the corresponding premiums are recorded as revenues. Profit commissions are estimated based on the related performance criteria evaluated at the balance sheet date, with subsequent changes to those estimates recognized when they occur.
	On a regular basis a recoverability analysis is performed of the deferred policy acquisition costs in relation to the expected recognition of revenues, including anticipated investment income, and adjustments, if any, are reflected as period costs. Should the analysis indicate that the acquisition costs are unrecoverable, further analyses are performed to determine if a reserve is required to provide for losses which may exceed the related unearned premium.
	General, Administrative and Corporate Expenses. These costs represent the expenses incurred in running the business and include, but are not limited to compensation costs for employees, rental costs, IT development and operating costs and professional and consultancy fees. General, policy and administrative costs directly attributable to the successful acquisition of business are deferred and amortized over the same period as the corresponding premiums are recorded as revenues. When reporting the results for its operating segments, the Company includes expenses which are directly attributable to the segment plus an allocation of central administrative costs. Corporate expenses are not allocated to the Company’s operating segments as they typically do not fluctuate with the levels of premium written and are related to the Company’s operations which include group executive costs, group finance costs, group legal and actuarial costs and certain strategic and non-recurring costs.
	(c)	Accounting for Investments, Cash and Cash Equivalents
	Fixed Income Securities. The fixed income securities portfolio comprises securities issued by governments and government agencies, corporate bonds, mortgage and other asset-backed securities and bank loans. Investments in fixed income securities are classified as available for sale or trading and are reported at estimated fair value in the consolidated balance sheet. Investment transactions are recorded on the trade date with balances pending settlement reflected in the consolidated balance sheet as a receivable for investments sold or a payable for investments purchased. Fair values are based on quoted market prices and other data provided by third-party pricing services and index providers.
	Equity Securities. The Company’s equity securities comprise U.S. and foreign equity securities. They are classified as either trading or available for sale and are carried on the consolidated balance sheet at estimated fair value. The fair values are based on quoted market prices in active markets from independent pricing sources.
	Short-term Investments. Short-term investments primarily comprise highly liquid debt securities with a maturity greater than three months but less than one year from the date of purchase and are held as part of the investment portfolio of the Company. Short-term investments are classified as either trading or available for sale and carried at estimated fair value.
	Gains and Losses. Realized gains or losses on the sale of investments are determined on the basis of the first in first out cost method and, for fixed income maturity available for sale securities, include adjustments to the cost basis of investments for declines in value that are considered to be other-than-temporary. Unrealized gains and losses represent the difference between the cost, or the cost as adjusted by amortization of any difference between its cost and its redemption value (“amortized cost”), of the security and its fair value at the reporting date and are included within other comprehensive income for securities classified as available for sale and in realized and unrealized investment gains or losses in the consolidated statement of operations for securities classified as trading.
	Other Investments. Other investments represent the Company’s investments that are recorded using the equity method of accounting. Adjustments to the fair value of these investments are made based on the net asset value of the investment.
	Catastrophe Bonds. Investments in catastrophe bonds are classified as trading and are carried on the consolidated balance sheet at estimated fair value.  The fair values are based on independent broker-dealer quotes.
	Cash and Cash Equivalents. Cash and cash equivalents are carried at fair value. Cash and cash equivalents comprise cash on hand, deposits held on call with banks and other short-term highly liquid investments due to mature within three months from the date of purchase and which are subject to insignificant risk of change in fair value.
	Other-than-temporary Impairment of Investments. A security is impaired when its fair value is below its cost or amortized cost. The Company reviews its investment portfolio each quarter on an individual security basis for potential other-than-temporary impairment (“OTTI”) based on criteria including issuer-specific circumstances, credit ratings actions and general macro-economic conditions.
	OTTI is deemed to occur when there is no objective evidence to support recovery in value of a security and a) the Company intends to sell the security or more likely than not will be required to sell the security before recovery of its cost or adjusted amortized cost basis or b) it is deemed probable that the Company will be unable to collect all amounts due according to the contractual terms of the individual security. In the first case, the entire unrealized loss position is taken as an OTTI charge to realized losses in earnings. In the second case, the unrealized loss is separated into the amount related to credit loss and the amount related to all other factors. The OTTI charge related to credit loss is recognized in realized losses in earnings and the amount related to all other factors is recognized in other comprehensive income. The cost basis of the investment is reduced accordingly and no adjustments to the cost basis are made for subsequent recoveries in value.
	Equity securities do not have a maturity date and therefore the Company’s review of these securities utilizes a higher degree of judgment. In its review, the Company considers its ability and intent to hold an impaired equity security for a reasonable period of time to allow for a full recovery. Where an equity security is considered to be other-than-temporarily impaired, the entire charge is recognized in realized losses in earnings. The cost basis of the investment is reduced accordingly and no adjustments to the cost basis are made for subsequent recoveries in value.
	Although the Company reviews each security on a case by case basis, it has also established parameters focusing on the extent and duration of impairment to help identify securities in an unrealized loss position which are other-than-temporarily impaired. For fixed income securities in the available for sale portfolio, the Company considers securities which have been in an unrealized loss position for 12 months or more which currently have a market value of more than 20% below cost should be other-than-temporarily impaired. For equities in the available for sale portfolio, the Company considers declines in value to a level of 20% or more below cost for 12 consecutive months to indicate the security should be other-than-temporarily impaired.
	Investment Income. Investment income includes amounts received and accrued in respect of periodic interest (“coupons”) payable to the Company by the issuer of fixed income securities, equity dividends and interest credited on cash and cash equivalents. It also includes amortization of premium and accretion of discount in respect of fixed income securities. Investment management and custody fees are charged against net investment income reported in the consolidated statement of operations.
	(d)	Accounting for Derivative Financial Instruments
	The Company enters into derivative instruments such as interest rate swaps and forward exchange contracts in order to manage certain market and credit risks. The Company records derivative instruments at fair value on the Company’s balance sheet as either assets or liabilities, depending on their rights and obligations. As at December 31, 2014, the Company held foreign exchange contracts for a notional amount of $135.8 million which qualify for hedge accounting, which are further described in Note 10, “Derivative Contracts” of these consolidated financial statements.
	The accounting for the gain or loss due to the changes in the fair value of these instruments is dependent on whether the derivative qualifies as a hedge. If the derivative does not qualify as a hedge, the gains or losses are reported in earnings when they occur. If the derivative does qualify as a hedge, the accounting treatment varies based on the type of risk being hedged.
	(e)	Intangible Assets
	Intangible assets are held in the consolidated balance sheet at cost less amortization and impairment. Amortization applies on a straight-line basis in respect of assets having a finite estimated useful economic life. The Company performs a qualitative assessment annually to determine whether it is more likely than not that an intangible asset considered to have an indefinite life, other than goodwill, is impaired.
	(f)	Office Properties and Equipment
	Office properties and equipment are carried at cost less accumulated depreciation. These assets are depreciated on a straight-line basis over the estimated useful lives of the assets. Computer equipment and software is depreciated between three and five years with depreciation for software commencing on the date the software is brought into use. Furniture and fittings are depreciated over four years and leasehold improvements are depreciated over the lesser of 15 years or the lease term.
	(g)	Foreign Currencies Translation
	The reporting currency of the Company is the U.S. Dollar. The functional currencies of the Company’s foreign operations and branches are the currencies in which the majority of their business is transacted.
	Transactions in currencies other than the functional currency are measured in the functional currency of that operation at the exchange rate prevailing at the date of the transaction. Monetary assets and liabilities denominated in non-functional currencies are remeasured at the exchange rate prevailing at the balance sheet date and any resulting foreign exchange gains or losses are reflected in the statement of operations.
	Monetary and non-monetary assets and liabilities of the Company’s functional currency operations are translated into U.S. Dollars at the exchange rate prevailing at the balance sheet date. Income and expenses of these operations are translated at the exchange rate prevailing at the date of the transaction. Unrealized gains or losses arising from the translation of functional currencies are recorded net of tax as a component of other comprehensive income.
	(h)	Earnings per Ordinary Share
	Basic earnings per ordinary share is determined by dividing net income available to ordinary shareholders by the weighted average number of ordinary shares outstanding during the period. Net income available to ordinary shareholders is calculated by deducting preference share dividends and net income/(loss) attributable to non-controlling interest from net income after tax for the period. Diluted earnings per share reflect the effect on earnings of the average number of ordinary shares outstanding associated with dilutive securities. The dilutive effect of potentially dilutive securities is calculated using the treasury stock method.
	(i)	Accounting for Income Tax
	Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. When the Company does not believe that, on the basis of available information, it is more likely than not that the deferred tax asset will be fully recovered, it recognizes a valuation allowance against its deferred tax assets to reduce assets to the recoverable amount. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date.
	(j)	Preference Shares
	The Company has in issue three classes of perpetual preference shares. The Company has no obligation to pay interest on these securities but they do carry entitlements to dividends payable at the discretion of the Board of Directors. In the event of non-payment of dividends for six consecutive periods, holders of preference shares have director appointment rights. They are therefore accounted for as equity instruments and included within total shareholders’ equity.
	(k)	Share-Based Employee Compensation
	The Company operates an employee incentive plan, a director plan and employee share purchase plans, the terms and conditions of which are described in Note 18. The Company applies a fair-value based measurement method including estimates for future forfeitures in the calculation of the compensation costs of stock options, performance shares, phantom shares and restricted share units.
	(l)	Long-term debt issued by Silverton
	The consolidated variable interest entity, Silverton, has issued debt instruments due to mature on September 16, 2016 and September 18, 2017, which are further described in Note 7, “Variable Interest Entities” of these consolidated financial statements. This debt is separately identified on the Company’s balance sheet and the Company has elected to record the debt at fair value. The fair value option was elected due to the potential variability over the ultimate settlement value of the debt instruments.
	(m)	New Accounting Policies
	New Accounting Policies Adopted in 2014
	In June 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-12, “Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period” to provide guidance on how to account for those share-based payments where performance targets could be achieved after the requisite service period. ASU 2014-12 can be applied prospectively to all awards granted or modified after the effective date or using a modified retrospective approach. The modified retrospective approach would apply to all awards outstanding on or after the beginning of the first annual period presented as of the adoption date. The effective date would be for annual and interim reporting periods beginning after December 15, 2015 with early adoption permitted. The Company adopted this ASU on January 1, 2015 and applied the guidance retrospectively to all of its outstanding awards. None of the Company’s outstanding share-based awards contain performance targets that are achievable after the requisite service period. Therefore, this ASU does not have an impact on its consolidated financial statements.
	In March 2014, the FASB issued ASU 2014-06, “Technical Corrections and Improvements Related to Glossary Terms” which includes technical corrections related to glossary links, glossary term deletions, and glossary term name changes. In addition, ASU 2014-06 includes more substantive, limited-scope improvements to reduce instances of the same term appearing multiple times in the Master Glossary with similar, but not entirely identical, definitions. The amendments apply to all reporting entities within the scope of the affected accounting guidance, do not have transition guidance and are effective upon issuance for both public entities and nonpublic entities. The Company adopted this ASU in the first quarter of 2014 and it does not have a material impact on its consolidated financial statements.
	In March 2013, the FASB issued ASU 2013-05, “Parent’s Accounting for the Cumulative Translation Adjustment Upon Derecognition of Certain Subsidiaries or Groups of Assets Within a Foreign Entity or of an Investment in a Foreign Entity” to standardize the release of the cumulative translation adjustment into net income when a parent either sells a part or all of its investment in its foreign entities or no longer holds a controlling financial interest in a subsidiary. ASU 2013-05 is applied prospectively and is effective for annual reporting periods beginning after December 15, 2013, and interim periods within those years. The Company adopted this ASU in the first quarter of 2014 and this standard does not have a material impact on its consolidated financial statements unless it should choose to sell its investments in its foreign entities or cease to hold a controlling financial interest.
	In February 2013, the FASB issued ASU 2013-04, “Obligations Resulting from Joint and Several Liability Arrangements for Which the Total Amount of the Obligation is Fixed at the Reporting Date” which provides guidance for the recognition, measurement, and disclosure of obligations resulting from joint and several liability arrangements for which the total amount of the obligation within the scope of this guidance is fixed at the reporting date. Examples of obligations include debt arrangements, other contractual obligations and settled litigation and judicial rulings. ASU 2013-04 is applied retrospectively and is effective for annual reporting periods beginning after December 15, 2013, and interim periods within those years. The Company adopted this ASU in the first quarter of 2014 and this standard does not have a material impact on its consolidated financial statements.
	In July 2013, the FASB issued ASU 2013-11, “Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists” which provides guidance on financial statement presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. ASU 2013-11 is effective for fiscal years beginning after December 15, 2013 or interim periods within those years. The Company adopted this ASU in the first quarter of 2014 and it does not have a material impact on its consolidated financial statements as its presentation of unrecognized tax benefits is already as required by ASU 2013-11.
	Accounting Pronouncements Not Yet Adopted
	In February 2015, the FASB issued ASU 2015-02, “Business Combinations (Topic 810)” which provides guidance for reporting entities that are required to evaluate whether they should consolidate certain legal entities. ASU 2015-02 is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2016. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
In November 2014, the FASB issued ASU 2014-16, “Derivatives and Hedging (Topic 815)” which provides guidance on reducing existing diversity under U.S. GAAP in the accounting for hybrid financial instruments issued in the form of a share. ASU 2014-16 is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2015. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
On August 27, 2014, the FASB issued ASU 2014-15, “Presentation of Financial Statements - Going Concern (Subtopic 204-40)” which provides U.S. GAAP guidance on management’s responsibility to evaluate whether there is substantial doubt about an entity’s ability to continue as a going concern or to provide related footnote disclosures. ASU 2014-15 is effective for fiscal years beginning after December 15, 2016 and interim periods beginning after December 15, 2016. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
On August 8, 2014, the FASB issued ASU 2014-14, “Receivables - Troubled debt restructuring by creditors (Subtopic 310-40)” which reduces diversity in practice by addressing the classification of certain foreclosed mortgage loans held by creditors that are either fully or partially guaranteed under government programs. ASU 2014-14 is effective for fiscal years beginning after December 15, 2014 and interim periods beginning after December 15, 2014. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
On August 5, 2014, the FASB issued ASU 2014-13, “Measuring the Financial Assets and the Financial Liabilities of a Consolidated Collateralized Financing Entity” which addresses the measurement difference in both the initial consolidation and the subsequent measurement of the financial assets and the financial liabilities of a collateralized financing entity. ASU 2014-13 is effective for fiscal years beginning after December 15, 2015 and interim periods beginning after December 15, 2015. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
On June 12, 2014, the FASB issued ASU 2014-11, “Repurchase-to-Maturity Transactions, Repurchase Financings, and Disclosures” which provides guidance on accounting and disclosures for repurchase agreements and similar transactions. ASU 2014-11 is effective for fiscal years beginning after December 15, 2014 and interim periods beginning after December 15, 2015. Early application for a public business entity is prohibited. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
On April 10, 2014, the FASB issued ASU 2014-08, “Presentation of Financial Statements (Topic 205) and Property, Plant, and Equipment (Topic 360)” which improves the definition of discontinued operations by limiting discontinued operations reporting to disposals of components of an entity that represent strategic shifts that have a major effect on an entity’s operations and financial results. The amendments in this ASU require expanded disclosures for discontinued operations. ASU 2014-08 is effective for fiscal years beginning after December 15, 2014 and interim periods therein. Early application is permitted, but only for disposals that have not been reported in financial statements previously issued or available for issuance. The Company does not expect this ASU to have a material impact on its consolidated financial statements as no disposals were made by the Company for the twelve months ended December 31, 2014.

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions
	The following summarizes the related party transactions of the Company.
	APJ Continuation Limited
	On January 22, 2010, the Company entered into a sale and purchase agreement to purchase APJ Continuation Limited and its subsidiaries (“APJ”) for an aggregate consideration of $4.8 million. The Company closed the transaction on March 22, 2010. The business writes a specialist account of Kidnap & Ransom (“K&R”) insurance which complements our existing political and financial risk business. The directors of Aspen Holdings assessed the fair value of the net tangible and financial assets acquired at $1.2 million. An amount of $3.6 million was the estimated goodwill on acquisition that is treated as an intangible asset. Mr. Villers, an executive officer of Aspen Holdings, held a 30% shareholding in APJ prior to its acquisition by Aspen Holdings.

Earnings_per_Ordinary_Share

Earnings per Ordinary Share	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Earnings per Ordinary Share	Earnings per Ordinary Share
	Basic earnings per ordinary share are calculated by dividing net income available to holders of Aspen Holdings’ ordinary shares by the weighted average number of ordinary shares outstanding. Net income available to ordinary shareholders is calculated by deducting preference share dividends and net income/(loss) attributable to non-controlling interest from net income/(loss) after tax for the period. Diluted earnings per ordinary share are based on the weighted average number of ordinary shares and dilutive potential ordinary shares outstanding during the period of calculation using the treasury stock method. The following table sets forth the computation of basic and diluted earnings per share for the twelve months ended December 31, 2014, 2013, and 2012, respectively:
			Twelve Months Ended December 31,
			2014			2013			2012
		($ in millions, except share and per share amounts)
	Net income/(loss)		$	355.8		$	329.3		$	280.4
	Preference share dividends		(37.8		)	(35.5		)	(31.1		)
	Change in redemption value (1)		—			(7.1		)	—
	Net profit attributable to non-controlling interest		(0.8		)	0.5			0.2
	Basic and diluted net income available to ordinary shareholders		317.2			287.2			249.5
	Ordinary shares:
	Basic weighted average ordinary shares		64,536,491			66,872,048			71,095,856
	Weighted average effect of dilutive securities		1,336,458			2,545,855			2,593,567
	Total diluted weighted average ordinary shares		65,872,949			69,417,903			73,689,423
	Earnings per ordinary share:
	Basic		$	4.92		$	4.29		$	3.51
	Diluted		$	4.82		$	4.14		$	3.39
	_______________
	(1)	The $7.1 million deduction from net income in 2013 is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million. For more information, please refer to Note 15, “Capital Structure” of these consolidated financial statements.
	Dividends. On February 5, 2015, the Company’s Board of Directors declared the following quarterly dividends:
			Dividend			Payable on:		Record Date:
	Ordinary shares		$	0.2		March 9, 2015		February 20, 2015
	7.401% Preference Shares		$	0.462563		April 1, 2015		March 15, 2015
	7.250% Preference Shares		$	0.4531		April 1, 2015		March 15, 2015
	5.95% Preference Shares		$	0.3719		April 1, 2015		March 15, 2015

Segment_Reporting

Segment Reporting	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Segment Reporting	Segment Reporting
	The Company has two reporting business segments: Insurance and Reinsurance. In addition to the way the Company manages its business, the Company has considered similarities in economic characteristics, products, customers, distribution, the regulatory environment of the Company’s operating segments and quantitative thresholds to determine the Company’s reportable segments. Segment profit or loss for each of the Company’s operating segments is measured by underwriting profit or loss. Underwriting profit is the excess of net earned premiums over the sum of losses and loss expenses, amortization of deferred policy acquisition costs and general and administrative expenses. Underwriting profit or loss provides a basis for management to evaluate the segment’s underwriting performance.
	Reinsurance Segment. The reinsurance segment consists of property catastrophe reinsurance, other property reinsurance (risk excess, pro rata, facultative), casualty reinsurance (U.S. treaty, international treaty and global facultative) and specialty reinsurance (credit and surety, agriculture, marine, aviation and other specialty lines). Our recently established Aspen Capital Markets forms part of our property catastrophe reinsurance line of business, as it currently focuses entirely on property catastrophe business through using alternative capital. For a more detailed description of this segment, see Part I, Item 1, “Business — Business Segments — Reinsurance” above.
	Insurance Segment. The insurance segment consists of property and casualty insurance, marine, aviation and energy insurance and financial and professional lines insurance. As previously stated, effective January 1, 2014, our property and casualty insurance lines of business were integrated into a combined property and casualty line. This includes the programs business, previously reported separately. For a more detailed description of this segment, see Part I, Item 1 “Business — Business Segments — Insurance” above.
	Non-underwriting Disclosures. The Company has provided additional disclosures for corporate and other (non-underwriting) income and expenses. Corporate and other income and expenses include net investment income, net realized and unrealized investment gains or losses, expenses associated with managing the group, certain strategic and non-recurring costs, changes in fair value of derivatives and changes in fair value of the loan notes issued by variable interest entities, interest expenses, net realized and unrealized foreign exchange gains or losses and income taxes, which are not allocated to the underwriting segments. Corporate expenses are not allocated to the Company’s operating segments as they typically do not fluctuate with the levels of premiums written and are not directly related to the Company’s segment operations. The Company does not allocate its assets by segment as it evaluates underwriting results of each segment separately from the results of the Company’s investment portfolio.
	The following tables provide a summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of the Company’s business segments for the twelve months ended December 31, 2014, 2013 and 2012:
			Twelve Months Ended December 31, 2014
			Reinsurance			Insurance			Total
			($ in millions)
	Underwriting Revenues
	Gross written premiums		$	1,172.80		$	1,729.90		$	2,902.70
	Net written premiums		1,124.00			1,391.20			2,515.20
	Gross earned premiums		1,137.60			1,599.00			2,736.60
	Net earned premiums		1,088.20			1,317.10			2,405.30
	Underwriting Expenses
	Losses and loss adjustment expenses		497.8			809.7			1,307.50
	Amortization of deferred policy acquisition costs		200			251.2			451.2
	General and administrative expenses		146.4			205.5			351.9
	Underwriting income		244			50.7			294.7
	Corporate expenses								(93.8		)
	Net investment income								190.3
	Realized and unrealized investment gains								46.3
	Realized and unrealized investment (losses)								(14.7		)
	Change in fair value of loan notes issued by variable interest entities								(18.6		)
	Change in fair value of derivatives								(15.2		)
	Interest expense on long term debt								(29.5		)
	Net realized and unrealized foreign exchange (losses)								5.6
	Other income								4.5
	Other expenses								(1.7		)
	Income before tax								367.9
	Income tax expense								(12.1		)
	Net income								$	355.8
	Net reserves for loss and loss adjustment expenses		$	2,493.30		$	1,907.50		$	4,400.80
	Ratios
	Loss ratio		45.7		%	61.5		%	54.4		%
	Policy acquisition expense ratio		18.4			19.1			18.8
	General and administrative expense ratio (1)		13.5			15.6			18.5
	Expense ratio		31.9			34.7			37.3
	Combined ratio		77.6		%	96.2		%	91.7		%
	_______________
	(1)	The general and administrative expense ratio in the total column includes corporate expenses.
			Twelve Months Ended December 31, 2013
			Reinsurance			Insurance			Total
			( $ in millions)
	Underwriting Revenues
	Gross written premiums		$	1,133.90		$	1,512.80		$	2,646.70
	Net written premiums		1,082.00			1,217.70			2,299.70
	Gross earned premiums		1,126.60			1,366.80			2,493.40
	Net earned premiums		1,073.00			1,098.80			2,171.80
	Underwriting Expenses
	Losses and loss adjustment expenses		481.7			742			1,223.70
	Amortization of deferred policy acquisition costs		207.2			214.8			422
	General and administrative expenses		131			185.9			316.9
	Underwriting income/(loss)		253.1			(43.9		)	209.2
	Corporate expenses								(51.2		)
	Net investment income								186.4
	Realized and unrealized investment gains								56.9
	Realized and unrealized investment (losses)								(20.5		)
	Change in fair value of derivatives								1.3
	Interest expense on long term debt								(32.7		)
	Net realized and unrealized foreign exchange (losses)								(13.2		)
	Other income								8.2
	Other expenses								(1.7		)
	Income before tax								342.7
Income tax expense								(13.4		)
Net income								$	329.3
Net reserves for loss and loss adjustment expenses		$	2,646.80		$	1,699.40		$	4,346.20
Ratios
Loss ratio		44.9		%	67.5		%	56.3		%
Policy acquisition expense ratio		19.3			19.5			19.4
General and administrative expense ratio (1)		12.2			16.9			16.9
Expense ratio		31.5			36.4			36.3
Combined ratio		76.4		%	103.9		%	92.6		%
________________
(1)	The general and administrative expense ratio in the total column includes corporate expenses.
		Twelve Months Ended December 31, 2012
		Reinsurance			Insurance			Total
		($ in millions)
Underwriting Revenues
Gross written premiums		$	1,227.90		$	1,355.40		$	2,583.30
Net written premiums		1,156.90			1,090.00			2,246.90
Gross earned premiums		1,208.00			1,177.00			2,385.00
Net earned premiums		1,132.40			951.1			2,083.50
Underwriting Expenses
Losses and loss adjustment expenses		635.3			603.2			1,238.50
Amortization of deferred policy acquisition costs		207.8			173.4			381.2
General and administrative expenses		123.9			168.2			292.1
Underwriting income		165.4			6.3			171.7
Corporate expenses								(53.0		)
Net investment income								204.9
Realized and unrealized investment gains								35.4
Realized and unrealized investment (losses)								(8.6		)
Change in fair value of derivatives								(28.4		)
Interest expense on long term debt								(30.9		)
Net realized and unrealized foreign exchange gains								3.4
Other income								5.6
Other expenses								(4.7		)
Income before tax								295.4
Income tax expense								(15.0		)
Net income								$	280.4
Net reserves for loss and loss adjustment expenses		$	2,811.30		$	1,469.40		$	4,280.70
Ratios
Loss ratio		56.1		%	63.4		%	59.4		%
Policy acquisition expense ratio		18.4			18.2			18.3
General and administrative expense ratio (1)		10.9			17.7			16.6
Expense ratio		29.3			35.9			34.9
Combined ratio		85.4		%	99.3		%	94.3		%
_______________
(1)	The general and administrative expense ratio in the total column includes corporate expenses.
Geographical Areas — The following summary presents the Company’s gross written premiums based on the location of the insured risk.
		For the Twelve Months Ended
		31-Dec-14			31-Dec-13			31-Dec-12
		($ in millions)
Australia/Asia		$	130.1		$	108.4		$	139.3
Caribbean		19.7			14.4			12.2
Europe		113.9			112.2			113
United Kingdom		209.3			166.4			168.6
United States & Canada(1)		1,357.30			1,179.60			1,106.90
Worldwide excluding United States (2)		116.2			145.7			151.7
Worldwide including United States(3)		851.8			827.4			810.8
Others		104.4			92.6			80.8
Total		$	2,902.70		$	2,646.70		$	2,583.30
______________
(1)	“United States and Canada” comprises individual policies that insure risks specifically in the United States and/or Canada, but not elsewhere.
(2)	“Worldwide excluding the United States” comprises individual policies that insure risks wherever they may be across the world but specifically excludes the United States.
(3)	“Worldwide including the United States” comprises individual policies that insure risks wherever they may be across the world but specifically includes the United States.

Investments

Investments	12 Months Ended
	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Investments	Investments
	Income Statement
	Investment Income. The following table summarizes investment income for the twelve months ended December 31, 2014, 2013 and 2012:
		For the Twelve Months Ended
		31-Dec-14			31-Dec-13			31-Dec-12
		($ in millions)
	Fixed income securities — Available for sale	$	151.1		$	155.6		$	181.3
	Fixed income securities — Trading	26.7			20.3			16.5
	Short-term investments — Available for sale	1.4			2.1			3.1
	Fixed term deposits (included in cash and cash equivalents)	3.3			5.3			6.5
	Equity securities — Available for sale	4.1			5.6			6.2
	Equity securities — Trading	13			7			—
	Catastrophe bonds — Trading	1.3			—			—
	Total	200.9			195.9			213.6
	Investment expenses	(10.6		)	(9.5		)	(8.7		)
	Net investment income	$	190.3		$	186.4		$	204.9
	The following table summarizes the net realized and unrealized investment gains and losses recorded in the statement of operations and the change in unrealized gains and losses on investments recorded in other comprehensive income:
		For the Twelve Months Ended
		31-Dec-14			31-Dec-13			31-Dec-12
		($ in millions)
	Available for sale:
	Fixed income securities — gross realized gains	$	10.3		$	18.2		$	7.6
	Fixed income securities — gross realized (losses)	(5.9		)	(7.4		)	(0.4		)
	Equity securities — gross realized gains	12.9			18			4.3
	Equity securities — gross realized (losses)	(0.8		)	(0.3		)	(4.9		)
	Total other-than-temporary impairments	(2.4		)	—			(3.0		)
	Trading:
	Fixed income securities — gross realized gains	7.3			9.5			9.8
	Fixed income securities — gross realized (losses)	(2.5		)	(2.9		)	(0.3		)
	Equity securities — gross realized gains	7.8			2.1			—
	Equity securities — gross realized (losses)	(3.1		)	(0.6		)	—
	Catastrophe bonds — trading	0.4			—			—
	Net change in gross unrealized gains	7.6			6.1			10.5
	Other investments:
	Gross realized and unrealized gains in Cartesian	—			3			3.2
	Other realized losses	—			(9.3		)	—
	Total net realized and unrealized investment gains recorded in the statement of operations	$	31.6		$	36.4		$	26.8
	Change in available for sale net unrealized gains/(losses):
	Fixed income securities	47.7			(209.6		)	2.7
	Equity securities	(10.0		)	11.2			16.4
	Total change in pre-tax available for sale unrealized gains/(losses)	37.7			(198.4		)	19.1
	Change in taxes	(2.8		)	13.7			(9.3		)
	Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income	$	34.9		$	(184.7	)	$	9.8
	Other-than-temporary Impairments. A security is potentially impaired when its fair value is below its cost or amortized cost. The Company reviews its available for sale fixed income and equity portfolios on an individual security basis for potential OTTI each quarter based on criteria including issuer-specific circumstances, credit ratings actions and general macro-economic conditions. The total OTTI charge for the twelve months ended December 31, 2014 was $2.4 million (2013 —$Nil). For a more detailed description of accounting policies for OTTI, please refer to Note 2 (c), “Basis of Preparation and Significant Accounting Policies — Accounting for Investments, Cash and Cash Equivalents” of these consolidated financial statements.
	Other realized losses. On December 16, 2013, the Company redeemed its $250.0 million 6.00% Senior Notes due to mature August 16, 2014. This early redemption resulted in a realized loss of $9.3 million, which is reflected in realized and unrealized investment losses recorded in the statement of operations.
	Balance Sheet
	Fixed Income Securities, Short-Term Investments and Equities — Available For Sale. The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income securities, short-term investments and equity securities as at December 31, 2014 and December 31, 2013:
		As at December 31, 2014
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	1,074.20		$	21.5		$	(1.3	)	$	1,094.40
	U.S. agency	190			7.5			(0.1		)	197.4
	Municipal	29.1			2.4			—			31.5
	Corporate	2,244.70			79.9			(5.2		)	2,319.40
	Non-U.S. government-backed corporate	76.8			1.2			—			78
	Foreign government	648.6			17.3			(0.2		)	665.7
	Asset-backed	141.3			2.4			(0.2		)	143.5
	Non-agency commercial mortgage-backed	41.5			3.3			—			44.8
	Agency mortgage-backed	1,016.70			40.8			(2.2		)	1,055.30
	Total fixed income securities — Available for sale	5,462.90			176.3			(9.2		)	5,630.00
	Total short-term investments — Available for sale	258.2			0.1			—			258.3
	Total equity securities — Available for sale	82.6			27.3			—			109.9
Total	$	5,803.70		$	203.7		$	(9.2	)	$	5,998.20
	As at December 31, 2013
	Cost or			Gross			Gross			Fair Market
	Amortized Cost			Unrealized			Unrealized			Value
				Gains			Losses
	($ in millions)
U.S. government	$	1,004.70		$	21.2		$	(5.5	)	$	1,020.40
U.S. agency	258.5			11.4			(0.8		)	269.1
Municipal	32.3			0.9			(0.4		)	32.8
Corporate	2,005.60			82.5			(18.7		)	2,069.40
Non-U.S. government-backed corporate	83.4			1.4			(0.2		)	84.6
Foreign government	772			11.2			(4.3		)	778.9
Asset-backed	119.8			2.8			(0.3		)	122.3
Non-agency commercial mortgage-backed	56.9			5.7			—			62.6
Agency mortgage-backed	1,116.70			30.6			(18.3		)	1,129.00
Total fixed income securities — Available for sale	5,449.90			167.7			(48.5		)	5,569.10
Total short-term investments — Available for sale	160.3			—			—			160.3
Total equity securities — Available for sale	112.2			37.8			(0.5		)	149.5
Total	$	5,722.40		$	205.5		$	(49.0	)	$	5,878.90
Fixed Income Securities, Short Term Investments, Equities and Catastrophe Bonds — Trading. The following tables present the cost or amortized cost, gross unrealized gains and losses, and estimated fair market value of trading investments in fixed income securities, short-term investments, equity securities and catastrophe bonds as at December 31, 2014 and December 31, 2013:
	As at December 31, 2014
	Cost or			Gross			Gross			Fair Market
	Amortized Cost			Unrealized			Unrealized			Value
				Gains			Losses
	($ in millions)
U.S. agency	$	0.2		$	—		$	—		$	0.2
Municipal	1.1			—			—			1.1
Corporate	520.9			11.7			(2.8		)	529.8
Foreign government	137.3			4.3			(1.5		)	140.1
Asset-backed	14.6			0.1			—			14.7
Bank loans	86.8			—			(1.7		)	85.1
Total fixed income securities — Trading	760.9			16.1			(6.0		)	771
Total short-term investments — Trading	0.2			—			—			0.2
Total equity securities — Trading	585.2			55.5			(24.7		)	616
Total catastrophe bonds — Trading	34.4			0.4			—			34.8
Total	$	1,380.70		$	72		$	(30.7	)	$	1,422.00
	As at December 31, 2013
	Cost or			Gross			Gross			Fair Market
	Amortized Cost			Unrealized			Unrealized			Value
				Gains			Losses
	($ in millions)
U.S. government	$	22.7		$	—		$	(0.7	)	$	22
U.S. agency	0.2			—			—			0.2
Municipal	1.1			—			—			1.1
Corporate	469.8			10.3			(5.3		)	474.8
Foreign government	136.5			1.2			(1.5		)	136.2
Asset-backed	12.7			0.1			—			12.8
Bank loans	69.1			0.3			(0.3		)	69.1
Total fixed income securities — Trading	712.1			11.9			(7.8		)	716.2
Total equity securities — Trading	281.6			34			(4.7		)	310.9
Total catastrophe bonds — Trading	5.8			—			—			5.8
Total	$	999.5		$	45.9		$	(12.5	)	$	1,032.90
The Company classifies these financial instruments as held for trading as this most closely reflects the facts and circumstances of the investments held.
In May 2014, the Company sold its BB High Yield Bonds portfolio for net proceeds of $25.1 million. As of December 31, 2014, the Company had invested $85.1 million in a U.S. Dollar BB Bank Loans trading portfolio and increased its investments in equities by $240.0 million in the twelve months ended December 31, 2014. In August 2013, the Company invested in a $200.0 million BBB Emerging Market Debt portfolio, which is reported above in corporate and foreign government securities.
Catastrophe bonds. The Company has invested in catastrophe bonds with a total value of $34.8 million, as of December 31, 2014.  The bonds receive quarterly interest payments based on variable interest rates with scheduled maturities ranging from 2016 to 2020.  The redemption value of the bonds will adjust based on the occurrence of a covered event, such as windstorms and earthquakes which occur in the geographic region of the United States, Canada, the North Atlantic, Japan and Australia.
Other Investments. The Company previously had an investment in Cartesian Iris Offshore Fund L.P (“Cartesian”), which provided capital to Iris Re, a Class 3 Bermuda reinsurer (“Iris Re”). The Company determined that Cartesian had the characteristics of a variable interest entity that are addressed by the guidance in ASC 810, Consolidation and was equity accounted rather than being consolidated by the Company. On June 29, 2013, the Company notified Cartesian Capital Group of its intention to withdraw the Company’s investment in Cartesian and to terminate the services provided to Iris Re. The termination took effect on January 1, 2014 and the Company received a final settlement of $39.3 million. In the twelve months ended December 31, 2014, the change in the value of the Company’s investment in Cartesian was $Nil (December 31, 2013 — gain of $3.0 million; December 31, 2012 — gain of $3.2 million). Changes in the value were recognized in realized and unrealized investment gains and losses in the consolidated statement of operations.
On October 2, 2012, the Company established a subsidiary, Aspen Recoveries Limited, to take ownership of a 58.5% shareholding in Chaspark Maritime Holdings Ltd., a Singaporean registered company (“Chaspark”), with the remaining shareholding owned by other insurers. The shareholding in Chaspark was received as a settlement for subrogation rights associated with a contract frustration claim settlement. The Company has determined that Chaspark has the characteristics of a variable interest entity as addressed by the guidance in ASC 810-10, Consolidation. However, having considered the provisions of ASC 810-10, the Company’s investment in Chaspark does not permit the Company to direct the activities which most significantly impact Chaspark’s economic performance and the Company is not acting as principal or agent for a related party group of investors. Under these circumstances, the Company is not required to consolidate Chaspark. The investment is therefore accounted for under the equity method and adjustments to the carrying value of this investment are made based on the Company’s share of capital including share of income and expenses, which is provided in the quarterly management accounts. The adjusted carrying value approximates fair value.
The table below shows the Company’s investments in Cartesian and Chaspark for the twelve months ended December 31, 2014 and 2013:
	Cartesian			Chaspark			Total
	($ in millions)
Opening undistributed value of investment as at January 1, 2014	$	39.3		$	8.7		$	48
Distribution for the twelve months to December 31, 2014	(39.3		)	—			(39.3		)
Closing value of investment as at December 31, 2014	$	—		$	8.7		$	8.7
Opening undistributed value of investment as at January 1, 2013	$	36.3		$	8.7		$	45
Unrealized gain for the twelve months to December 31, 2013	3			—			3
Closing value of investment as at December 31, 2013	$	39.3		$	8.7		$	48
Fixed Income Securities. The scheduled maturity distribution of the Company’s available for sale fixed income securities as at December 31, 2014 and December 31, 2013 is set forth below. Actual maturities may differ from contractual maturities because issuers of securities may have the right to call or prepay obligations with or without call or prepayment penalties.
	As at December 31, 2014
	Amortized			Fair Market			Average
	Cost or Cost			Value			S&P Ratings by
							Maturity
	($ in millions)
Due one year or less	$	590.2		$	594.7		AA
Due after one year through five years	2,552.00			2,620.80			AA-
Due after five years through ten years	1,023.50			1,059.90			A+
Due after ten years	97.7			111			A+
Subtotal	4,263.40			4,386.40
Non-agency commercial mortgage-backed	41.5			44.8			AA+
Agency mortgage-backed	1,016.70			1,055.30			AA+
Asset-backed	141.3			143.5			AAA
Total fixed income securities — Available for sale	$	5,462.90		$	5,630.00
	At December 31, 2013
	Amortized			Fair Market			Average
	Cost or Cost			Value			S&P Ratings by
							Maturity
	($ in millions)
Due one year or less	$	694.8		$	700		AA
Due after one year through five years	2,376.10			2,438.00			AA-
Due after five years through ten years	1,003.90			1,032.80			A+
Due after ten years	81.7			84.4			AA-
Subtotal	4,156.50			4,255.20
Non-agency commercial mortgage-backed	56.9			62.6			AA+
Agency mortgage-backed	1,116.70			1,129.00			AA+
Asset-backed	119.8			122.3			AAA
Total fixed income securities — Available for sale	$	5,449.90		$	5,569.10
Guaranteed Investments. As at December 31, 2014, the Company held $2.5 million (December 31, 2013 — $2.3 million) in investments which are guaranteed by mono-line insurers, excluding those with explicit government guarantees, and the Company’s holding was limited to two municipal securities, both rated Caa3 or higher (December 31, 2013 — two municipal securities, both rated BBB- or higher). The standalone rating (rating without guarantee) is determined as the senior unsecured debt rating of the issuer. Where the credit ratings were split between the two main rating agencies, Standard & Poor’s Financial Services LLC (“S&P”) and Moody’s Investors Service Inc. (“Moody’s”), the lowest rating was used. The Company’s exposure to other third-party guaranteed debt is primarily to investments backed by non-U.S. government guaranteed issuers.
Gross Unrealized Losses. The following tables summarize as at December 31, 2014 and December 31, 2013, by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position for the Company’s available for sale portfolio:
	December 31, 2014
	0-12 months						Over 12 months						Total
	Fair			Gross			Fair			Gross			Fair		Gross			Number of
	Market			Unrealized			Market			Unrealized			Market		Unrealized			Securities
	Value			Losses			Value			Losses			Value		Losses
	($ in millions)
U.S. government	$	166.1		$	(0.5	)	$	79.4		$	(0.8	)	$	245.5	$	(1.3	)	39
U.S. agency	25.1			—			4.9			(0.1		)	30		(0.1		)	7
Municipal	—			—			—			—			—		—			0
Corporate	459.4			(2.1		)	171.3			(3.1		)	630.7		(5.2		)	274
Non-U.S. government-backed corporate	0.7			—			—			—			0.7		—			1
Foreign government	30.4			—			44.2			(0.2		)	74.6		(0.2		)	16
Asset-backed	43.7			(0.1		)	11.7			(0.1		)	55.4		(0.2		)	43
Agency mortgage-backed	64.7			(0.3		)	111.7			(1.9		)	176.4		(2.2		)	48
Total fixed income securities — Available for sale	790.1			(3.0		)	423.2			(6.2		)	1,213.30		(9.2		)	428
Total short-term investments — Available for sale	4.6			—			—			—			4.6		—			3
Total	$	794.7		$	(3.0	)	$	423.2		$	(6.2	)	$	1,217.90	$	(9.2	)	431
	December 31, 2013
	0-12 months						Over 12 months						Total
	Fair			Gross			Fair			Gross			Fair		Gross			Number of
	Market			Unrealized			Market			Unrealized			Market		Unrealized			Securities
	Value			Losses			Value			Losses			Value		Losses
	($ in millions)
U.S. government	$	293.9		$	(5.5	)	$	—		$	—		$	293.9	$	(5.5	)	51
U.S. agency	72.1			(0.8		)	—			—			72.1		(0.8		)	18
Municipal	5.5			(0.2		)	1.3			(0.2		)	6.8		(0.4		)	7
Corporate	695.4			(16.8		)	23.4			(1.9		)	718.8		(18.7		)	372
Non-U.S. government-backed corporate	21.8			(0.2		)	4.9			—			26.7		(0.2		)	8
Foreign government	239.7			(4.1		)	8.5			(0.2		)	248.2		(4.3		)	44
Asset-backed	50.2			(0.3		)	—			—			50.2		(0.3		)	51
Agency mortgage-backed	491.8			(18.3		)	1.2			—			493		(18.3		)	123
Total fixed income securities — Available for sale	1,870.40			(46.2		)	39.3			(2.3		)	1,909.70		(48.5		)	674
Total short-term investments — Available for sale	7.7			—			—			—			7.7		—			6
Total equity securities — Available for sale	6			(0.4		)	2.3			(0.1		)	8.3		(0.5		)	7
Total	$	1,884.10		$	(46.6	)	$	41.6		$	(2.4	)	$	1,925.70	$	(49.0	)	687
Investment Purchases and Sales. The following table summarizes investment purchases, sales and maturities for the twelve months ended December 31, 2014, 2013 and 2012:
	For the Twelve Months Ended
	31-Dec-14			31-Dec-13			31-Dec-12
	($ in millions)
(Purchases) of fixed income securities — Available for sale	$	(2,005.0	)	$	(2,129.8	)	$	(1,529.6	)
(Purchases) of fixed income securities — Trading	(653.4		)	(763.4		)	(300.8		)
(Purchases) of equity securities — Available for sale	—			(2.5		)	(53.1		)
(Purchases) of equity securities — Trading	(361.0		)	(304.4		)	—
Proceeds from sales and maturities of fixed income securities — Available for sale	1,909.50			1,872.30			1,416.50
Proceeds from sales and maturities of fixed income securities — Trading	615.9			486			257.2
Proceeds from sales of equity securities — Available for sale	40			82.2			46.9
Proceeds from sales of equity securities — Trading	62.2			24.1			—
Net change in (payable)/receivable for securities (purchased)/sold	2.8			(0.9		)	1.1
Net (purchases)/sales of short-term investments — Available for sale	(110.3		)	258.2			(122.7		)
Net (purchases)/sales of short-term investments — Trading	(0.2		)	2.4			—
Investment in Chaspark Maritime Holdings Ltd	—			—			(8.7		)
Net (purchases) of catastrophe bonds — Trading	$	(28.7	)	$	(5.8	)	$	—
Net sales of other investments	$	39.3		$	—		$	—
Net (purchases) for the year	$	(488.9	)	$	(481.6	)	$	(293.2	)

Variable_Interest_Entities

Variable Interest Entities	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Variable Interest Entities	Variable Interest Entities
	As at December 31, 2014, the Company had two investments in two variable interest entities (“VIE”), Chaspark and Silverton.
	Chaspark. On October 2, 2012, the Company established a subsidiary, Aspen Recoveries Limited, to take ownership of a 58.5% shareholding in Chaspark, with the remaining shareholding owned by other insurers. The shareholding in Chaspark was received as a settlement for subrogation rights associated with a contract frustration claim settlement. The Company has determined that Chaspark has the characteristics of a VIE as addressed by the guidance in ASC 810, Consolidation. As discussed further in Note 6, “Investments” in these consolidated financial statements, the investment in Chaspark is accounted for under the equity method. In the twelve months ended December 31, 2014, there was no change in the value of the Company’s investment in Chaspark (December 31, 2013 —$Nil. The adjusted carrying value approximates fair value.
	Silverton. On September 10, 2013, the Company established Silverton, a Bermuda domiciled special purpose insurer, formed to provide additional collateralized capacity to support Aspen Re’s reinsurance business through retrocession agreements which will be collateralized and funded by Silverton through the issuance of one or more series of loan notes. Silverton is a non-rated insurer and the risks are fully collateralized by way of funds held in trust for the benefit of Aspen Bermuda.
	The proceeds of $65.0 million (of which $50.0 million was issued to third parties) from the issuance of Silverton’s Series 2014-1 Participating Notes on December 27, 2013 (“2014 Loan Notes”) were deposited into a collateral account to fund Silverton’s obligations under a retrocession property quota share agreement entered into with Aspen Bermuda effective January 1, 2014. The holders of the 2014 Loan Notes participate in any profit or loss generated by Silverton attributable to the operations of Silverton’s Series 2014-1 Segregated Account. Any existing value of the 2014 Loan Notes will be returned to the noteholders in installments after the expiration of the risk period of the retrocession agreement issued by Silverton for the related series with the final payment being contractually due on the September 16, 2016 maturity date. The fair value of the 2014 Loan Notes at December 31, 2014 was $89.2 million (of which $68.6 million is held by external investors). Using current loss estimates Silverton will distribute $88.2 million (of which $67.9 million is held by external investors) to its noteholders during 2015. The $67.9 million due to external investors has been classified as a current liability in the Company’s consolidated financial statements with the balance of $0.7 million classified as long term debt. The total aggregate unpaid balance of the 2014 Loan Notes held by third parties and those held by Aspen Holdings is $89.2 million. The Company’s maximum loss exposure to the 2014 Loan Notes is $20.6 million which is the fair value of its holdings as at December 31, 2014 due to mature on September 16, 2016.
	The proceeds of $85.0 million (of which $70.0 million was issued to third parties) from the issuance of Silverton’s Series 2015-1 Participating Notes on December 23, 2014 (“2015 Loan Notes”) were deposited into a collateral account to fund Silverton’s obligations under a retrocession property quota share agreement entered into with Aspen Bermuda effective January 1, 2015. The holders of the 2015 Loan Notes participate in any profit or loss generated by Silverton attributable to the operations of Silverton’s Series 2015-1 Segregated Account. Any existing value of the 2015 Loan Notes will be returned to the noteholders after the expiration of the risk period of the retrocession agreement issued by Silverton for the related series with the final payment being contractually due on the September 18, 2017 maturity date. The fair value of the 2015 Loan Notes at December 31, 2014 is $85.0 million (of which $70.0 million is held by external investors). The $70.0 million of the 2015 Loan Notes held by external investors is classified as long term debt in the Company’s consolidated financial statements. The total aggregate unpaid balance of the 2015 Loan Notes held by third parties and those held by Aspen Holdings is $85.0 million. The Company’s maximum loss exposure to the 2015 Loan Notes is its $15.0 million which is the fair value of its holdings as at December 31, 2014 due to mature on September 18, 2017.
	The Company has determined that Silverton has the characteristics of a VIE that are addressed by the guidance in ASC 810, Consolidation. The Company concluded that it is the primary beneficiary and has consolidated the subsidiary upon its formation as it owns 100% of the voting shares, 100% of the issued share capital and has a significant financial interest and the power to control Silverton. The Company has no other obligation to provide financial support to Silverton. Neither the creditors nor beneficial interest holders of Silverton have recourse to the Company’s general credit.
	In the event of either an extreme catastrophic property reinsurance event or severe credit related event there is a risk that Aspen Bermuda would be unable to recover losses from Silverton. These two risks are mitigated as follows:
	i.	Silverton has collateralized the aggregate limit provided to Aspen Bermuda by way of a trust in favor of Aspen Bermuda as the beneficiary;
	ii.	the trustee is a large, well-established regulated entity; and
	iii.	all funds within the trust account are bound by investment guidelines restricting investments to one of the institutional class money market funds run by large international investment managers.
	The tables below show the Company’s liabilities associated with the third-party investments in Silverton for the twelve months ended December 31, 2014 and 2013:
			Twelve Months Ended December 31, 2014
			($ in millions)
	Beginning balance as at January 1, 2014		$	(50.0	)
	Total change in fair value included in the statement of operations		(18.6		)
	Notes issued during the period		(70.0		)
	Balance as at December 31, 2014		$	(138.6	)
			Twelve Months Ended December 31, 2013
			($ in millions)
	Beginning balance as at January 1, 2013		$	—
	Notes issued during the period		(50.0		)
	Balance as at December 31, 2013		$	(50.0	)

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements
	The Company’s estimates of fair value for financial assets and liabilities are based on the framework established in the fair value accounting guidance included in ASC Topic 820, “Fair Value Measurements and Disclosures.” The framework prioritizes the inputs, which refer broadly to assumptions market participants would use in pricing an asset or liability, into three levels.
	The Company considers prices for actively traded securities to be derived based on quoted prices in an active market for identical assets, which are Level 1 inputs in the fair value hierarchy. The majority of these securities are valued using prices supplied by index providers.
	The Company considers prices for other securities that may not be as actively traded which are priced via pricing services, index providers, vendors and broker-dealers, or with reference to interest rates and yield curves, to be derived based on inputs that are observable for the asset, either directly or indirectly, which are Level 2 inputs in the fair value hierarchy. The majority of these securities are also valued using prices supplied by index providers.
	The Company considers securities, other financial instruments and derivative insurance contracts subject to fair value measurement whose valuation is derived by internal valuation models to be based largely on unobservable inputs, which are Level 3 inputs in the fair value hierarchy.
	The following tables present the level within the fair value hierarchy at which the Company’s financial assets and liabilities are measured on a recurring basis at December 31, 2014 and December 31, 2013:
			As at December 31, 2014
			Level 1				Level 2				Level 3				Total
			($ in millions)
	Available for sale financial assets, at fair value
	U.S. government		$	1,094.40			$	—			$	—			$	1,094.40
	U.S. agency		—				197.4				—				197.4
	Municipal		—				31.5				—				31.5
	Corporate		—				2,319.40				—				2,319.40
	Non-U.S. government-backed corporate		—				78				—				78
	Foreign government		456.5				209.2				—				665.7
	Asset-backed		—				143.5				—				143.5
	Non-agency commercial mortgage-backed		—				44.8				—				44.8
	Agency mortgage-backed		—				1,055.30				—				1,055.30
	Total fixed income securities available for sale, at fair value		1,550.90				4,079.10				—				5,630.00
	Short-term investments available for sale, at fair value		229.3				29				—				258.3
	Equity investments available for sale, at fair value		109.9				—				—				109.9
	Held for trading financial assets, at fair value
	U.S. agency		—				0.2				—				0.2
	Municipal		—				1.1				—				1.1
	Corporate		—				529.8				—				529.8
	Foreign government		36.1				104				—				140.1
	Asset-backed		—				14.7				—				14.7
	Bank loans		—				85.1				—				85.1
	Catastrophe bonds		—				34.8				—				34.8
	Total fixed income securities trading, at fair value		36.1				769.7				—				805.8
	Short-term investments trading, at fair value		0.1				0.1				—				0.2
	Equity investments trading, at fair value		616				—				—				616
	Other financial assets and liabilities, at fair value
	Derivatives at fair value — foreign exchange contracts		—				7.9				—				7.9
	Derivatives at fair value — interest rate swaps		—				0.1				—				0.1
	Liabilities under derivative contracts — foreign exchange contracts		—				(14.3		)		—				(14.3		)
	Loan notes issued by variable interest entities, at fair value		—				—				(70.7		)		(70.7		)
	Loan notes issued by variable interest entities, at fair value (classified as a current liability)		—				—				(67.9		)		(67.9		)
	Total		$	2,542.30			$	4,871.60			$	(138.6	)		$	7,275.30
	There were no maturities, settlements or transfers between Level 1, Level 2 and Level 3 during the twelve months ended December 31, 2014. There were no assets or liabilities that were classified as Level 3 as at December 31, 2014, except for the loan notes issued by the VIEs. There were no maturities, settlements, gains or transfers in or out of Level 3. For more information, see Note 7, “Variable Interest Entities” of these consolidated financial statements.
		At December 31, 2013
		Level 1				Level 2				Level 3				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	1,020.40			$	—			$	—			$	1,020.40
	U.S. agency	—				269.1				—				269.1
	Municipal	—				32.8				—				32.8
	Corporate	—				2,069.40				—				2,069.40
	Non-U.S. government-backed corporate	—				84.6				—				84.6
	Foreign government	596.2				182.7				—				778.9
	Asset-backed	—				122.3				—				122.3
	Non-agency commercial mortgage-backed	—				62.6				—				62.6
	Agency mortgage-backed	—				1,129.00				—				1,129.00
	Total fixed income securities available for sale, at fair value	1,616.60				3,952.50				—				5,569.10
	Short-term investments available for sale, at fair value	129.5				30.8				—				160.3
	Equity investments available for sale, at fair value	149.5				—				—				149.5
	Held for trading financial assets, at fair value
	U.S. government	22				—				—				22
	U.S. agency	—				0.2				—				0.2
	Municipal	—				1.1				—				1.1
	Corporate	—				474.8				—				474.8
	Foreign government	44.2				92				—				136.2
	Asset-backed	—				12.8				—				12.8
Bank loans	—				69.1				—				69.1
Total fixed income securities trading, at fair value	66.2				650				—				716.2
Equity investments trading, at fair value	310.9				—				—				310.9
Catastrophe bonds trading, at fair value	—				5.8				—				5.8
Other financial assets and liabilities, at fair value
Derivatives at fair value — foreign exchange contracts	—				5.9				—				5.9
Derivatives at fair value — interest rate swaps	—				1.1				—				1.1
Liabilities under derivative contracts — foreign exchange contracts	—				(2.9		)		—				(2.9		)
Loan notes issued by variable interest entities, at fair value	—				—				(50.0		)		(50.0		)
Total	$	2,272.70			$	4,643.20			$	(50.0	)		$	6,865.90
There were no maturities, settlements or transfers between Level 1, Level 2 and Level 3 during the twelve months ended December 31, 2013 and no assets or liabilities were classified as Level 3 as at December 31, 2013, except for the loan notes issued by the VIEs.
The following table presents a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Level 3 inputs for the twelve months ended December 31, 2014:
Reconciliation of Liabilities Using Level 3 Inputs			Twelve Months Ended December 31, 2014				Twelve Months Ended December 31, 2013
			($ in millions)
Balance at the beginning of the period			$	(50.0	)		$	—
Total change in fair value included in the statement of operations			(18.6		)		—
Notes issued during period			(70.0		)		(50.0		)
Balance at the end of the period			$	(138.6	)		$	(50.0	)
Valuation of Fixed Income Securities. The Company’s fixed income securities are classified as either available for sale or trading and are carried at fair value. At December 31, 2014 and December 31, 2013, the Company’s fixed income securities were valued by pricing services, index providers or broker-dealers using standard market conventions. The market conventions utilize market quotations, market transactions in comparable instruments and various relationships between instruments including, but not limited to, yield to maturity, dollar prices and spread prices in determining value.
Independent Pricing Services and Index Providers. The underlying methodology used to determine the fair value of securities in the Company’s available for sale and trading portfolios by the pricing services and index providers the Company uses is very similar. Pricing services will gather observable pricing inputs from multiple external sources, including buy and sell-side contacts and broker-dealers, in order to develop their internal prices. Index providers are those firms which provide prices for a range of securities within one or more asset classes, typically using their own in-house market makers (traders) as the primary pricing source for the indices, although ultimate valuations may also rely on other observable data inputs to derive a dollar price for all index-eligible securities. Index providers without in-house trading desks will function similarly to a pricing service in that they will gather their observable pricing inputs from multiple external sources. All prices for the Company’s securities attributed to index providers are for an individual security within the respective indices.
Pricing services and index providers provide pricing for less complex, liquid securities based on market quotations in active markets. Pricing services and index providers supply prices for a broad range of securities including those for actively traded securities, such as Treasury and other Government securities, in addition to those that trade less frequently or where valuation includes reference to credit spreads, pay down and pre-pay features and other observable inputs. These securities include Government Agency, Municipals, Corporate and Asset-Backed Securities.
For securities that may trade less frequently or do not trade on a listed exchange, these pricing services and index providers may use matrix pricing consisting of observable market inputs to estimate the fair value of a security. These observable market inputs include: reported trades, benchmark yields, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, reference data, and industry and economic factors. Additionally, pricing services and index providers may use a valuation model such as an option adjusted spread model commonly used for estimating fair values of mortgage-backed and asset-backed securities. Neither the Company, nor its index providers, derives dollar prices using an index as a pricing input for any individual security.
Broker-Dealers. The Company obtains quotes from broker-dealers who are active in the corresponding markets when prices are unavailable from independent pricing services or index providers. Generally, broker-dealers value securities through their trading desks based on observable market inputs. Their pricing methodologies include mapping securities based on trade data, bids or offers, observed spreads and performance of newly issued securities. They may also establish pricing through observing secondary trading of similar securities. Quotes from broker-dealers are non-binding.
The Company obtains prices for all of its fixed income investment securities via its third-party accounting service provider, and in the majority of cases receiving a number of quotes so as to obtain the most comprehensive information available to determine a security’s fair value. A single valuation is applied to each security based on the vendor hierarchy maintained by the Company’s third-party accounting service provider.
At December 31, 2014, the Company obtained an average of 2.0 quotes per fixed income investment, compared to 2.6 quotes at December 31, 2013. Pricing sources used in pricing fixed income investments at December 31, 2014 and December 31, 2013 were as follows:
		As at December 31, 2014			At December 31, 2013
Index providers		84	%		85	%
Pricing services		11			12
Broker-dealers		5			3
Total		100	%		100	%
Summary Pricing Information Table. A summary of securities priced using pricing information from index providers at December 31, 2014 and December 31, 2013 is provided below:
		As at December 31, 2014						At December 31, 2013
		Fair Market				% of Total		Fair Market				% of Total
		Value Determined				Fair Value by		Value Determined				Fair Value by
		using Prices from				Security Type		using Prices from				Security Type
		Index Providers						Index Providers
		($ in millions, except for percentages)
U.S. government		$	1,044.40			95%		$	998.5			96%
U.S. agency		186.9				95%		255.3				95%
Municipal		13.7				42%		14.5				43%
Corporate		2,731.10				96%		2,400.80				94%
Non-U.S. government-backed corporate		48.7				62%		55.9				66%
Foreign government		504.4				63%		605.8				66%
Asset-backed		140.5				89%		130.6				97%
Non-agency commercial mortgage-backed		44.8				100%		61				97%
Agency mortgage-backed		680.6				64%		830.6				74%
Total fixed income securities		$	5,395.10			84%		$	5,353.00			85%
Equities		$	725.9			100%		$	460.4			100%
Total fixed income securities and equity investments		$	6,121.00			86%		$	5,813.40			86%
The Company, in conjunction with its third-party accounting service provider, obtains an understanding of the methods, models and inputs used by the third-party pricing service and index providers to assess the ongoing appropriateness of vendors’ prices. The Company and its third-party accounting service provider also have controls in place to validate that amounts provided represent fair values. Processes to validate and review pricing include, but are not limited to:
•	quantitative analysis (e.g., comparing the quarterly return for each managed portfolio to its target benchmark, with significant differences identified and investigated);
•	comparison of market values obtained from pricing services, index providers and broker-dealers against alternative price sources for each security where further investigation is completed when significant differences exist for pricing of individual securities between pricing sources;
•	initial and ongoing evaluation of methodologies used by outside parties to calculate fair value; and
•	comparison of the fair value estimates to the Company’s knowledge of the current market.
Prices obtained from pricing services, index providers and broker-dealers are not adjusted by us; however, prices provided by a pricing service, index provider or broker-dealer in certain instances may be challenged based on market or information available from internal sources, including those available to the Company’s third-party investment accounting service provider. Subsequent to any challenge, revisions made by the pricing service, index provider or broker-dealer to the quotes are supplied to the Company’s investment accounting service provider.
Management reviews the vendor hierarchy maintained by the Company’s third-party accounting service provider in order to determine which price source provides the most appropriate fair value (i.e., a price obtained from a pricing service with more seniority in the hierarchy will be used over a less senior one in all cases). The hierarchy level assigned to each security in the Company’s available for sale and trading portfolios is based upon its assessment of the transparency and reliability of the inputs used in the valuation as of the measurement date. The hierarchy of index providers and pricing services is determined using various qualitative and quantitative points arising from reviews of the vendors conducted by the Company’s third-party accounting service provider. Vendor reviews include annual onsite due diligence meetings with index providers and pricing services vendors covering valuation methodology, operational walkthroughs and legal and compliance updates. Index providers are assigned the highest priority in the pricing hierarchy due primarily to availability and reliability of pricing information.
Fixed Income Securities. Fixed income securities are traded on the over-the-counter (“OTC”) market based on prices provided by one or more market makers in each security. Securities such as U.S. Government, U.S. Agency, Foreign Government and investment grade corporate bonds have multiple market makers in addition to readily observable market value indicators such as expected credit spread, except for Treasury securities, over the yield curve. The Company uses a variety of pricing sources to value fixed income securities including those securities that have pay down/prepay features such as mortgage-backed securities and asset-backed securities in order to ensure fair and accurate pricing. The fair value estimates for the investment grade securities in the Company’s portfolio do not use significant unobservable inputs or modeling techniques.
U.S. Government and Agency. U.S. government and agency securities consist primarily of bonds issued by the U.S. Treasury and corporate debt issued by agencies such as the Federal National Mortgage Association (“FNMA”), the Federal Home Loan Mortgage Corporation (“FHLMC”) and the Federal Home Loan Bank. As the fair values of U.S. Treasury securities are based on unadjusted market prices in active markets, they are classified within Level 1. The fair values of U.S. government agency securities are priced using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of U.S. government agency securities are classified within Level 2.
Municipals. The Company’s municipal portfolio comprises bonds issued by U.S. domiciled state and municipality entities. The fair value of these securities is determined using spreads obtained from broker-dealers, trade prices and the new issue market which are Level 2 inputs in the fair value hierarchy. Consequently, these securities are classified within Level 2.
Foreign Government. The issuers for securities in this category are non-U.S. governments and their agencies. The fair values of non-U.S. government bonds, primarily sourced from international indices, are based on unadjusted market prices in active markets and are therefore classified within Level 1. The fair values of the non-U.S. agency securities, again primarily sourced from international indices, are priced using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of non-U.S. agency securities are classified within Level 2. In addition, foreign government securities includes a portion of the BBB Emerging Market Debt portfolio which is also classified within Level 2.
Corporate. Corporate securities consist primarily of U.S. and foreign corporations covering a variety of industries and are for the most part priced by index providers and pricing vendors. Some issuers may participate in the FDIC program or other similar non-U.S. government programs which guarantee timely payment of principal and interest in the event of a default. The fair values of these securities are generally determined using the spread above the risk-free yield curve. Inputs used in the evaluation of these securities include credit data, interest rate data, market observations and sector news, broker-dealer quotes and trade volumes. In addition, corporate securities includes BB High Yield Bonds and a portion of the BBB Emerging Market Debt portfolio. The Company classifies all of these securities within Level 2.
Mortgage-backed Securities. Residential and commercial mortgage-backed securities consist of bonds issued by the Government National Mortgage Association, the FNMA and the FHLMC as well as private non-agency issuers. The fair values of these securities are determined through the use of a pricing model (including Option Adjusted Spread) which uses prepayment speeds and spreads to determine the appropriate average life of the mortgage-backed security. These spreads are generally obtained from broker-dealers, trade prices and the new issue market. As the significant inputs used to price mortgage-backed securities are observable market inputs, these securities are classified within Level 2.
Asset-backed Securities. The underlying collateral for the Company’s asset-backed securities consists mainly of student loans, automobile loans and credit card receivables. These securities are primarily priced by index providers and pricing vendors. Inputs to the valuation process include broker-dealer quotes and other available trade information, prepayment speeds, interest rate data and credit spreads. The Company classifies these securities within Level 2.
Bank Loans. These are variable rate, senior secured debt instruments issued by non-investment grade companies that are not publicly registered but are the most senior debt in a capital structure and are generally secured by company assets. Although these assets do not trade in as liquid a market as traditional fixed income instruments, they are valued in similar fashion to other fixed maturities, using similar inputs such as yield curves, interest rates and credit spreads. These securities are primarily priced by a third party pricing vendor. Bank loans are therefore classified within Level 2.
Short-term Investments. Short-term investments comprise highly liquid debt securities with a maturity greater than three months but less than one year from the date of purchase. Short-term investments are valued in a manner similar to the Company’s fixed maturity investments and are classified within Levels 1 and 2.
Equity Securities. Equity securities include U.S. and foreign common stocks and are classified either as trading or available for sale and carried at fair value. These securities are classified within Level 1 as their fair values are based on quoted market prices in active markets from independent pricing sources. At December 31, 2014, the Company obtained an average of 4.0 quotes per equity investment, compared to 4.9 quotes as at December 31, 2013. Pricing sources used in pricing equities at December 31, 2014 and December 31, 2013 were all provided by index providers.
Catastrophe Bonds. Catastrophe bonds held by the Company are variable rate fixed income instruments with redemption values adjusted based on the occurrence of a covered event, usually windstorms and earthquakes.  These bonds have been classified as trading and carried at fair value.  Bonds are priced using an average of multiple broker-dealer quotes and as such, are considered Level 2.
Foreign Exchange Contracts. The foreign exchange contracts which the Company uses to mitigate currency risk are characterized as OTC due to their customized nature and the fact that they do not trade on a major exchange. These instruments trade in a very deep liquid market, providing substantial price transparency and accordingly are classified as Level 2.
Interest Rate Swaps. The interest rate swaps which the Company uses to mitigate interest rate risk are also characterized as OTC and are valued by the counterparty using quantitative models with multiple market inputs. The market inputs, such as interest rates and yield curves, are observable and the valuation can be compared for reasonableness with third party pricing services. Consequently, these instruments are classified as Level 2.
Loan Notes Issued by Variable Interest Entities. Silverton, a licensed special purpose insurer, is consolidated into the Company’s group accounts as a VIE. In the fourth quarter of 2013, Silverton issued $65.0 million ($50.0 million third-party funded) loan notes with a maturity date of September 16, 2016. During the fourth quarter of 2014, Silverton issued an additional $85.0 million ($70.0 million third-party funded) loan notes with a maturity date of September 18, 2017. The Company has elected to account for the loan notes at fair value using the guidance as prescribed under ASC 825, Financial Instruments as the Company believes it represents the most meaningful measurement basis for these liabilities. The loan notes are recorded at fair value at each reporting period and, as they are not quoted on an active market and contain significant unobservable inputs, they have been classified as a Level 3 instrument in the fair value hierarchy. The loan notes are unique because their valuation is linked to the specific risks of the Company’s property catastrophe reinsurance contracts.
To determine the fair value of the loan notes, the Company runs an internal model which considers the seasonality of the risk assumed under the retrocessional agreements. The seasonality used in the model is determined by applying the percentage of property catastrophe losses planned by the Company’s actuaries to the estimated written premium to determine earned premium for each quarter. The inputs to the internal valuation model are based on Company specific data due to the lack of availability of observable market inputs. Reserves for losses is the most significant unobservable input. An increase in reserves for losses would normally result in a decrease in the fair value of the loan notes while a decrease in reserves would normally result in an increase in the fair value of the loan notes. The observable and unobservable inputs used to determine the fair value of the 2015-1 and 2014-1 loan notes as at December 31, 2014 and 2013 are presented in the tables below:
At December 31, 2014		Fair Value				Valuation Method		Observable (O) and		Low				High
		Level 3						Unobservable (U) inputs
	($ in millions)									($ in millions)
Loan notes held by third parties		$	138.6			Internal Valuation Model		Gross premiums written (O)		$	—			$	40
								Reserve for losses (U)		$	—			$	4.6
								Contract period (O)		N/A				365 days
								Initial value of issuance (O)		$	120			$	120
At December 31, 2013		Fair Value				Valuation Method		Observable (O) and		Low				High
		Level 3						Unobservable (U) inputs
	($ in millions)									($ in millions)
Loan notes held by third parties		$	50			Internal Valuation Model		Gross premiums written (O)		$	—			$	40
								Reserve for losses (U)		$	—			$	1.4
								Contract period (O)		N/A				365 days
								Initial value of issuance (O)		$	50			$	50
The observable and unobservable inputs represent the potential variation around the inputs used in the valuation model. The 2015-1 loan notes were not on risk as at December 31, 2014 and as no gross premiums were written at that date the minimum value of gross premiums written value was $Nil. The high premium value represents the actual premiums assumed by Silverton for the 2014-1 loan notes. Reserves for losses for the 2015-1 loan notes were $Nil as no contracts were written as at December 31, 2014, the high value is the estimate of losses assumed by the 2014-1 Loan notes. The contract period is defined in the Silverton loan agreements and the initial value represents the funds received from third parties.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Reinsurance	Reinsurance
	The Company purchases retrocession and reinsurance to limit and diversify the Company’s risk exposure and to increase its own insurance and reinsurance underwriting capacity. These agreements provide for recovery of a portion of losses and loss adjustment expenses from reinsurers. As is the case with most reinsurance contracts, the Company remains liable to the extent that reinsurers do not meet their obligations under these agreements, and therefore, in line with its risk management objectives, the Company evaluates the financial condition of its reinsurers and monitors concentrations of credit risk.
	Balances pertaining to reinsurance transactions are reported “gross” on the consolidated balance sheet, meaning that reinsurance recoverable on unpaid losses and ceded unearned premiums are not deducted from insurance reserves but are recorded as assets. For more information on reinsurance recoverables, please refer to Note 21, “Concentrations of Credit Risk — Reinsurance recoverables” of these consolidated financial statements.
	The effect of assumed and ceded reinsurance on premiums written, premiums earned and insurance losses and loss adjustment expenses is as follows:
		Twelve Months Ended December 31,
		2014			2013			2012
		($ in millions)
	Premiums written:
	Direct	$	1,729.90		$	1,512.80		$	1,355.40
	Assumed	1,172.80			1,133.90			1,227.90
	Ceded	(387.5		)	(347.0		)	(336.4		)
	Net premiums written	$	2,515.20		$	2,299.70		$	2,246.90
	Premiums earned:
	Direct	$	1,599.00		$	1,366.80		$	1,177.00
	Assumed	1,137.60			1,126.60			1,208.00
	Ceded	(331.3		)	(321.6		)	(301.5		)
	Net premiums earned	$	2,405.30		$	2,171.80		$	2,083.50
	Insurance losses and loss adjustment expenses:
	Direct	$	908.2		$	829.4		$	763
	Assumed	496.9			459.4			650.1
	Ceded	(97.6		)	(65.1		)	(174.6		)
	Net insurance losses and loss adjustment expenses	$	1,307.50		$	1,223.70		$	1,238.50

Derivative_Contracts

Derivative Contracts	12 Months Ended
	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Contracts	Derivative Contracts
	The following table summarizes information on the location and amounts of derivative fair values on the consolidated balance sheet as at December 31, 2014 and 2013:
				As at December 31, 2014							At December 31, 2013
	Derivatives Not Designated as Hedging Instruments		Balance Sheet Location	Notional			Fair				Notional		Fair
	Under ASC 815			Amount			Value				Amount		Value
				($ in millions)							($ in millions)
	Interest Rate Swaps		Derivatives at Fair Value	$	951.3		$	0.1		(1)	$	1,000.00	$	1.1		(1)
	Foreign Exchange Contracts		Derivatives at Fair Value	$	165.8		$	7.9			$	224.4	$	5.9
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	237.6		$	(10.5	)		$	57.5	$	(2.9	)
	(1)	Net of $22.3 million of cash collateral provided to counterparties, Goldman Sachs International ($451.3 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2013 — $34.3 million).
				As at December 31, 2014							At December 31, 2013
	Derivatives Designated as Hedging Instruments Under ASC 815		Balance Sheet Location	Notional			Fair				Notional		Fair
				Amount			Value				Amount		Value
				($ in millions)							($ in millions)
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	135.8		$	(3.8	)	(1)	$	—	$	—		(1)
	(1)	Net of $Nil cash collateral (December 31, 2013 — $Nil).
	The following tables provide the unrealized and realized gains/(losses) recorded in the statement of operations for the twelve months ended December 31, 2014 and 2013:
				Amount of Income/(Loss)
				Recognized in the Statement
				of  Operations and Other Comprehensive Income
				Twelve Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	31-Dec-14			31-Dec-13
	ASC 815		Statement of Operations and Other Comprehensive Income
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(7.7	)	$	(1.3	)
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	(7.2	)	$	2.6
				Amount of Income/(Loss)
				Recognized in the Statement
				of  Operations and Other Comprehensive Income
				Twelve Months Ended
	Derivatives Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	31-Dec-14			31-Dec-13
	ASC 815		Statement of Operations and Other Comprehensive Income
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(0.3	)	$	—
	Foreign Exchange Contracts		Net change from current period hedged transactions	$	(3.8	)	$	—
	Foreign Exchange Contracts. The Company uses foreign exchange contracts to manage foreign currency risk. A foreign exchange contract involves an obligation to purchase or sell a specified currency at a future date at a price set at the time of the contract. Foreign exchange contracts will not eliminate fluctuations in the value of the Company’s assets and liabilities denominated in foreign currencies but rather allow it to establish a rate of exchange for a future point in time.
	As at December 31, 2014, the Company held foreign exchange contracts that were not designated as hedging under ASC 815 with an aggregate value of $403.4 million (2013— $281.9 million). The foreign exchange contracts are recorded as derivatives at fair value in the balance sheet with changes recorded as a change in fair value of derivatives in the statement of operations. For the twelve months ended December 31, 2014, the impact of foreign exchange contracts on net income was a charge of $7.7 million (December 31, 2013 — charge of $1.3 million).
	As at December 31, 2014, the Company held foreign exchange contracts that were designated as hedging under ASC 815 with an aggregate value of $135.8 million (2013— $Nil). The foreign exchange contracts are recorded as derivatives at fair value in the balance sheet with the effective portion recorded in other comprehensive income and the ineffective portion recorded as a change in fair value of derivatives in the statement of operations. The contracts are considered to be effective and therefore, for the twelve months ended December 31, 2014, the movement in other comprehensive income representing the effective portion was a net unrealized loss of $3.8 million (December 31, 2013 — $Nil) and the impact of foreign exchange contracts on net income representing the expired contracts was a charge of $0.3 million (December 31, 2013 — charge of $Nil).
	Interest Rate Swaps. As at December 31, 2014, the Company held fixed for floating interest rate swaps with a total notional amount of $951.3 million (December 31, 2013 — $1.0 billion) that are due to mature between November 26, 2015 and November 9, 2020. The interest rate swaps are used in the ordinary course of the Company’s investment activities to partially mitigate the negative impact of rises in interest rates on the market value of the Company’s fixed income portfolio. For the twelve months ended December 31, 2014, there was a loss of $7.2 million (December 31, 2013 — gain of $2.6 million). During 2014, $48.7 million in notional amount of our interest rate swaps rolled off.
	As at December 31, 2014, cash collateral with a fair value of $22.3 million was held by the Company’s counterparties to support the current valuation of the interest rate swaps (December 31, 2013 — $34.3 million). As at December 31, 2014, no non-cash collateral was transferred to the Company by its counterparties (December 31, 2013 — $Nil). Transfers of cash collateral are recorded on the consolidated balance sheet within Derivatives at Fair Value, while transfers in respect of non-cash collateral are disclosed but not recorded. As at December 31, 2014, no amount was recorded in the consolidated balance sheet for the pledged assets.

Deferred_Policy_Acquisition_Co

Deferred Policy Acquisition Costs	12 Months Ended
	Dec. 31, 2014
Insurance [Abstract]
Deferred Policy Acquisition Costs	Deferred Policy Acquisition Costs
	The following table represents a reconciliation of beginning and ending deferred policy acquisition costs for the twelve months ended December 31, 2014 and 2013:
			Twelve Months Ended			Twelve Months Ended December 31, 2013
			December 31, 2014
			($ in millions)
	Balance at the beginning of the period		$	262.2		$	223
		Acquisition costs deferred	488			461.2
		Amortization of deferred policy acquisition costs	(451.2		)	(422.0		)
	Balance at the end of the period		$	299		$	262.2

Reserves_for_Losses_and_Adjust

Reserves for Losses and Adjustment Expenses	12 Months Ended
	Dec. 31, 2014
Insurance [Abstract]
Reserves for Losses and Adjustment Expenses	Reserves for Losses and Loss Adjustment Expenses
	The following table represents a reconciliation of beginning and ending consolidated loss and LAE reserves for the twelve months ended December 31, 2014, 2013 and 2012:
		As at December 31,
		2014			2013			2012
		($ in millions)
	Provision for losses and LAE at the start of the year	$	4,678.90		$	4,779.70		$	4,525.20
	Less reinsurance recoverable	(332.7		)	(499.0		)	(426.6		)
	Net loss and LAE at the start of the year	4,346.20			4,280.70			4,098.60
	Net loss and LAE expenses (disposed)	(24.2		)	(34.6		)	(9.0		)
	Provision for losses and LAE for claims incurred:
	Current year	1,411.60			1,331.40			1,375.90
	Prior years	(104.1		)	(107.7		)	(137.4		)
	Total incurred	1,307.50			1,223.70			1,238.50
	Losses and LAE payments for claims incurred:
	Current year	(112.1		)	(172.8		)	(244.3		)
	Prior years	(995.6		)	(912.3		)	(835.7		)
	Total paid	(1,107.7		)	(1,085.1		)	(1,080.0		)
	Foreign exchange (gains)/losses	(121.0		)	(38.5		)	32.6
	Net losses and LAE reserves at the end of the year	4,400.80			4,346.20			4,280.70
	Plus reinsurance recoverable on unpaid losses at the end of the year	350			332.7			499
	Provision for losses and LAE at the end of the year	$	4,750.80		$	4,678.90		$	4,779.70
	For the twelve months ended December 31, 2014, there was a reduction of $104.1 million in the Company’s estimate of the ultimate claims to be paid in respect of prior accident years compared to $107.7 million for the twelve months ended December 31, 2013. The Company disposed of $24.2 million of its reserves relating to commutations during the twelve months ended December 31, 2014 (December 31, 2013 — $34.6 million, December 31, 2012 — $9.0 million). For additional information on the reserve releases, please refer to Part II, Item 7, “Management’s Discussion and Analysis of Financial Condition and Results of Operations — Reserves for Losses and Loss Adjustment Expenses” above.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes
	Aspen Holdings and Aspen Bermuda are incorporated under the laws of Bermuda. Under current Bermudian law, they are not taxed on any Bermudian income or capital gains and they have received an undertaking from the Bermuda Minister of Finance that, in the event of any Bermudian income or capital gains taxes being imposed, they will be exempt from those taxes until March 31, 2035. The Company’s U.S. operating companies are subject to United States corporate tax at a rate of 34%. Under the current laws of England and Wales, Aspen U.K., AUL and Aspen Managing Agency Limited (“AMAL”) are taxed at the U.K. corporate tax rate which has reduced from 23% to 21% effective as at April 1, 2014. This rate reduction was enacted on July 17, 2013 and has been reflected in current year income tax disclosures. A further reduction of the U.K. corporate tax rate to 20% from April 1, 2015 was also enacted on July 17, 2013. The reduction in 2015 has been reflected in deferred taxation disclosures.
	Total income tax for the twelve months ended December 31, 2014, 2013 and 2012 is allocated as follows:
		Twelve Months Ended December 31,
		2014			2013			2012
		($ in millions)
	Income tax expense on income	$	12.1		$	13.4		$	15
	Income tax (benefit)/expense on other comprehensive income	(5.1		)	(13.7		)	9.3
	Income tax (benefit) charged directly to shareholders’ equity	(1.2		)	(1.5		)	(2.4		)
	Total income tax expense/(benefit)	$	5.8		$	(1.8	)	$	21.9
	Income/(loss) before tax and income tax expense/(benefit) attributable to that income/(loss) consists of:
		Twelve Months Ended December 31, 2014
		(Loss)/income			Current			Deferred			Total
		before tax			income tax			income tax			income tax
					expense			(benefit)			expense
		($ in millions)
	U.S.	$	(24.1	)	$	0.8		$	—		$	0.8
	Non-U.S.	392			22.6			(11.3		)	11.3
	Total	$	367.9		$	23.4		$	(11.3	)	$	12.1
		Twelve Months Ended December 31, 2013
		(Loss)/income			Current			Deferred			Total
		before tax			income tax			income tax			income tax
					expense			expense			expense
		($ in millions)
	U.S.	$	(12.2	)	$	3.8		$	—		$	3.8
	Non-U.S.	354.9			5.9			3.7			9.6
	Total	$	342.7		$	9.7		$	3.7		$	13.4
		Twelve Months Ended December 31, 2012
		(Loss)/income			Current			Deferred			Total
		before tax			income tax			income tax			income tax
					expense			(benefit)			expense
		($ in millions)
	U.S.	$	(58.3	)	$	4.4		$	—		$	4.4
	Non-U.S.	353.7			18.4			(7.8		)	10.6
	Total	$	295.4		$	22.8		$	(7.8	)	$	15
	The weighted average expected tax provision has been calculated using the pre-tax accounting income/loss in each jurisdiction multiplied by that jurisdiction’s applicable statutory tax rate. The expected tax rate in Bermuda, the Company’s country of domicile, is zero. Application of the weighted average tax rate for operations in other jurisdictions produces an expected tax (benefit)/expense as shown in the table below.
	The reconciliation between the provision for income taxes and the expected tax at the weighted average rate provision is provided below:
		Twelve Months Ended December 31,
		2014			2013			2012
	Income Tax Reconciliation	($ in millions)
	Expected tax (benefit)/expense at weighted average rate	$	(7.3	)	$	(7.5	)	$	(14.4	)
	Prior year adjustments (1)	(0.6		)	(4.2		)	(4.9		)
	Valuation provision on U.S. deferred tax assets	12.7			15.1			26.7
	Uncertain tax positions	5.3			8.5			9.6
	Non-utilizable foreign tax credits	—			2.6			—
	Disallowable expenses	1.8			1.6			1.2
	Other non-taxable items	—			(0.2		)	(2.4		)
	Impact of changes in statutory tax rates	0.2			(2.5		)	(0.8		)
	Total income tax expense	$	12.1		$	13.4		$	15
	________________
	(1) The submission dates for filing income tax returns for the Company’s U.S. and U.K. operating subsidiaries are after the submission date of the Company’s Annual Report on Form 10-K. The final tax liabilities may differ from the estimated tax provisions included in the Annual Report on Form 10-K and may result in prior year adjustments being reported.
	For 2014, the prior period adjustment of $0.6 million relates to the determination of results under U.K. GAAP, upon which the tax returns are based.  This can only be reasonably determined on an accurate basis after the Company’s Annual Report on Form 10-K has been filed.
	For 2013, the prior period adjustment of $4.2 million includes a $2.0 million credit in respect of a change of accounting policy related to deferred acquisition costs under U.K. GAAP and a $2.0 million credit relating to the final determination of the equalization reserves required under U.K. GAAP, which can only be reasonably calculated on an accurate basis once the Prudential Regulation Authority Return has been finalized. Finalization of this return takes place after the Company’s Annual Report on Form 10-K has been filed.
	For 2012, the $4.9 million prior year adjustment included a reclassification of a $3.9 million income tax charge to Other Comprehensive Income relating to unrealized investment gains (giving rise to a credit in the Income Statement), and a $1.0 million credit relating to an adjustment to the valuation of deductible employee stock awards.
	Uncertain tax positions.  Unrecognized tax benefits relate to prior period tax positions. As at December 31, 2013 they totaled $23.9 million, representing $10.0 million in relation to tax deductions for certain interest payments, $13.5 million relating to the adjustment to equity reserves and $0.4 million relating to tax deductions for certain expenses. During the year ended December 31, 2014, there was an increase in unrecognized benefits of $5.3 million in respect of tax deductions for interest payments, bringing the balance to $29.2 million.
	Unrecognized tax benefits may reduce the effective tax rate if recognized. It is possible that the entire balance of unrecognized tax benefits, totaling $29.2 million, could be eliminated following completion of tax examinations into these matters. During the twelve months ended December 31, 2014, the Company did not recognize or accrue any costs in respect of interest or penalties relating to uncertain tax positions (December 31, 2013 — $Nil).
		Twelve Months Ended December 31,
		2014			2013
		($ in millions)
	Unrecognized tax benefits balance at January 1	$	23.9		$	15.4
	Gross increases/(decreases) for tax positions of prior years	5.3			8.5
	Gross increases/(decreases) for tax positions of current year	—			—
	Unrecognized tax benefits balance at December 31	$	29.2		$	23.9
	The Company accrues interest and penalties, if applicable, as income tax expenses. The Company does not believe it will be subject to any penalties in any open tax years and has not accrued any such amounts during the twelve months ended December 31, 2014 (December 31, 2013 — $Nil).
Income tax returns that have been filed by the U.S. operating subsidiaries are subject to examination for 2010 and later tax years. The U.K. operating subsidiaries’ income tax returns are subject to examination for the 2011 and later tax years.

Deferred_Taxation

Deferred Taxation	12 Months Ended
	Dec. 31, 2014
Deferred Income Taxes and Other Assets [Abstract]
Deferred Taxation	Deferred Taxation
	The tax effects of temporary differences that give rise to deferred tax assets and deferred tax liabilities are presented in the following table:
		As at December 31,
		2014			2013
		($ in millions)
	Deferred tax assets:
	Share options	$	2.8		$	3.9
	Operating loss carry forwards	94.4			80.9
	Loss reserves	1.8			1.7
	Unrealized losses on investments	—			13.3
	Accrued expenses	11.2			8.4
	Foreign tax credits	13			7.5
	Unearned premiums	4.9			6.5
	Timing differences on fixed assets	8.7			5.6
	Other temporary differences	6.7			8.5
	Total gross deferred tax assets	143.5			136.3
	Less valuation allowance	(106.5		)	(93.8		)
	Net deferred tax assets	$	37		$	42.5
	Deferred tax liabilities:
	Equalization provision reserves	$	(32.1	)	$	(34.1	)
	Intangible assets (other)	(2.2		)	(1.8		)
	Unrealized (gains) on investments	(1.5		)	(0.2		)
	Deferred policy acquisition costs	(2.0		)	(4.2		)
	Other temporary differences	(2.3		)	(0.6		)
	Total gross deferred tax (liabilities)	(40.1		)	(40.9		)
	Net deferred tax (liability)/asset	$	(3.1	)	$	1.6
	Deferred tax liabilities and assets represent the tax effect of temporary differences between the value of assets and liabilities for financial statement purposes and such values as measured by U.K. and U.S. tax laws and regulations. Deferred tax assets and liabilities from the same tax jurisdiction have been netted off resulting in assets and liabilities being recorded under the deferred taxation captions on the consolidated balance sheet.
	In assessing the realizability of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences and operating losses become deductible. Management considers the scheduled reversal of deferred tax liabilities, projected future taxable income, and tax planning strategies in making this assessment. At December 31, 2014, the Company had net operating losses carried forward for U.S. Federal income tax purposes of $267.4 million (2013 — $230.6 million) which are available to offset future U.S. Federal taxable income, if any, with expiry periods between 2026 and 2034. For U.S. Federal income tax purposes, the Company also has capital loss carryforwards of $1.3 million, with expiry periods between 2015 and 2016, and charitable contribution carryforwards of $0.6 million, with expiry periods between 2015 and 2019. A full valuation provision on U.S. deferred tax assets (which includes these loss carryforwards) has been recognized at December 31, 2014 as management believes that it is more likely than not that a tax benefit will not be realized. A valuation allowance of $106.5 million has been established against U.S. deferred tax assets (2013 — $93.8 million). The increase in valuation allowance totals $12.7 million with $12.7 million recorded in the consolidated income statement and $Nil recorded in other comprehensive income.

Capital_Structure

Capital Structure	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Capital Structure	Capital Structure
	The following table provides a summary of the Company’s authorized and issued share capital at December 31, 2014 and 2013:
		As at December 31, 2014				At December 31, 2013
		Number		$ in		Number	$ in
				Thousands			Thousands
	Authorized share capital:
	Ordinary Shares 0.15144558¢ per share	969,629,030		1,469		969,629,030	1,469
	Non-Voting Shares 0.15144558¢ per share	6,787,880		10		6,787,880	10
	Preference Shares 0.15144558¢ per share	100,000,000		152		100,000,000	152
	Total authorized share capital			1,631			1,631
	Issued share capital:
	Issued ordinary shares of 0.15144558¢ per share	62,017,368		94		65,546,976	99
	Issued 7.401% Preference Shares of 0.15144558¢ each with a liquidation preference of $25 per share	5,327,500		8		5,327,500	8
	Issued 7.250% Preference Shares of 0.15144558¢ each with a liquidation preference of $25 per share	6,400,000		10		6,400,000	10
	Issued 5.95% Preference Shares of 0.15144558¢ each with a liquidation preference of $25 per share	11,000,000		17		11,000,000	17
	Total issued share capital			129			134
	Additional paid-in capital as at December 31, 2014 was $1,134.3 million (December 31, 2013 — $1,297.4 million). Additional paid-in capital includes the aggregate liquidation preferences of the Company’s preference shares of $568.2 million (December 31, 2013 — $568.2 million) less issue costs of $12.4 million (December 31, 2013 — $12.4 million).
	(a)    Ordinary Shares
	The following table summarizes transactions in the Company’s ordinary shares during the years ended December 31, 2014 and 2013:
		Number of Ordinary Shares
		2014		2013
	Ordinary shares in issue at the beginning of the year	65,546,976		70,753,723
	Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or 2008 share purchase plan	756,676		1,374,567
	Ordinary shares issued to non-employee directors	3,573		44,000
	Ordinary shares repurchased	(4,289,857	)	(8,461,174	)
	Ordinary shares issued in respect of the redemption of the PIERS	—		1,835,860
	Ordinary shares in issue at the end of the year	62,017,368		65,546,976
	Ordinary Share Repurchases in 2012. On February 2, 2012, our Board of Directors extended the authorization for the remaining amount of the $400.0 million ordinary share repurchase program originally authorized on February 9, 2010. As at December 31, 2011, $192.4 million remained available under the share repurchase plan. In 2012, we initiated an open market share repurchase program to repurchase ordinary shares in the open market and subsequently repurchased and cancelled a total of 2,064,643 ordinary shares for the year ended December 31, 2012 for a total consideration of $59.9 million.
	On October 24, 2012, the Company’s Board of Directors approved a new share repurchase authorization for up to $400.0 million of outstanding ordinary shares. The share repurchase authorization replaced the previous authorization and permits the Company to effect the repurchases from time to time through a combination of transactions, including open market repurchases, privately negotiated transactions and accelerated share repurchase transactions.
	In addition to the share repurchase program, the Company purchases shares offered from time to time by Appleby Services (Bermuda) Ltd. (the “Names’ Trustee”). On March 9, 2012, an agreement was entered into to repurchase 42,578 shares from the Names’ Trustee for a total consideration of $1.1 million and on March 23, 2012, an agreement was signed to repurchase 26,708 shares from the Names’ Trustee for a total consideration of $0.7 million. The shares under both transactions were repurchased on May 8, 2012 and subsequently cancelled.
	On August 10, 2012, an agreement was entered into to repurchase 34,151 ordinary shares from the Names’ Trustee for a total consideration of $1.0 million. The shares were purchased and subsequently cancelled on October 24, 2012.
	Ordinary Share Repurchases in 2013. On February 7, 2013, the Company’s Board of Directors replaced the then existing share repurchase authorization of $400.0 million with a new authorization of $500.0 million. The total share repurchase authorization, which was effective immediately through February 7, 2015, permits the Company to effect the repurchases from time to time through a combination of transactions, including open market repurchases, privately negotiated transactions and accelerated share repurchase transactions.
	On February 26, 2013, the Company entered into an Accelerated Share Repurchase Agreement (“ASR”) with Goldman Sachs & Co. (“Goldman”) to repurchase an aggregate of $150.0 million of the Company’s ordinary shares. Under the ASR, the Company initially acquired and cancelled 3,348,214 ordinary shares on March 1, 2013. On August 29, 2013, Goldman terminated the ASR and delivered to the Company an additional 705,062 ordinary shares. The total amount repurchased under the ASR was 4,053,276 ordinary shares at an average price of $37.01. Settlement was made entirely in the Company’s ordinary shares and was accounted for as an equity transaction under the guidelines specified under ASC 815 Derivatives and Hedging.
	On March 4, 2013, the Company entered into a share repurchase agreement with the Names’ Trustee for the purchase of 54,437 ordinary shares for a total purchase price of $2.0 million. This share repurchase closed on March 21, 2013.
	Ordinary Share Repurchases in 2014. Under open market repurchases, the Company acquired and cancelled a total of 4,289,857 ordinary shares for the twelve months ended December 31, 2014. The total consideration paid for the twelve months ended December 31, 2014 was $180.9 million with the average price for the twelve months ended December 31, 2014 being $42.16. The Company had $43.3 million remaining under its current share repurchase authorization as at December 31, 2014. On February 5, 2015, we announced a new share repurchase program of $500 million.
	(b)    Preference Shares
	Preference Shares Redemption. During 2005 and 2006, the Company issued 4.6 million 5.625% Perpetual Preferred Income Equity Replacement Securities (“PIERS”). The PIERS were convertible at the Company’s option if, at any time on or after January 1, 2009, the closing sale price of the Company’s ordinary shares equaled or exceeded 130% of the then prevailing conversion price for 20 trading days during any consecutive 30-trading day period, as well as the last day of such 30-day period.
	The PIERS were dilutive to the Company’s ordinary shares when the Company’s share price exceeded the prevailing conversion price and therefore, as the Company’s share price was generally above the 130% conversion price test, they were included in the Company’s fully diluted share count until the Company announced it would mandatorily redeem the PIERS.
	On April 25, 2013, the Company announced it would mandatorily redeem all of its PIERS outstanding based on the terms of the PIERS. Each holder of a PIERS unit received $50.00, equating to a total payment of $230.0 million in cash plus a number of the Company’s ordinary shares based on the conversion rate calculated in accordance with the average trading price of the Company’s ordinary shares over a 20-trading day settlement period following the Company’s issuance of the press release announcing the mandatory conversion. The conversion rate was 1.7121 shares of the Company’s ordinary shares per $50.00 liquidation preference of the PIERS equating to a total issuance of 1,835,860 ordinary shares. The Company settled the amount on May 30, 2013. In accordance with the terms of the PIERS, no further dividends were paid on the PIERS following the announcement of their mandatory redemption. As a result of the redemption, the difference of $7.1 million between the capital raised upon issuance of the PIERS, net of original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million was reclassified from additional paid-in capital to retained earnings.
	Preference Shares Issuance. On November 15, 2006, the Company issued 8,000,000 preference shares with a liquidation preference of $25 for an aggregate amount of $200.0 million. Each share will receive dividends on a non-cumulative basis only when declared by our Board of Directors initially at an annual rate of 7.401% (the “7.401% Preference Shares”) (NYSE: AHL-PRA). Starting on January 1, 2017, the dividend rate for the 7.401% Preference Shares will be paid at a floating annual rate, reset quarterly, equal to 3-month LIBOR plus 3.28%. The 7.401% Preference Shares have no stated maturity but are callable at the option of the Company on or after the 10th anniversary of the date of issuance. The Company raised proceeds of $196.3 million, net of total costs of $3.7 million, from this issuance.
	On March 31, 2009, the Company repurchased 2,672,500 of its 7.401% Preference Shares at a price of $12.50 per share. For earnings per share purposes, the repurchase resulted in a $31.5 million gain, net of a non-cash charge of $1.2 million reflecting the write off of the pro-rata portion of the original issuance costs of the 7.401% Preference Shares.
	On April 11, 2012, the Company issued 6,400,000 shares of 7.250% Perpetual Non-Cumulative Preference Shares (the “7.250% Preference Shares”) (NYSE: AHL-PRB). The 7.250% Preference Shares have a liquidation preference of $25 per share. Net proceeds were $154.5 million, comprising $160.0 million of total liquidation preference less $5.5 million of issue expenses.
	The 7.250% Preference Shares ranked equally with the PIERS and rank equally with the 7.401% Preference Shares and the 5.95% Preference Shares, discussed below, and have no fixed maturity date. The Company may redeem all or a portion of the Preference Shares at a redemption price of $25 per share on or after July 1, 2017.
	In the event of liquidation of the Company, the holders of outstanding preference shares would have preference over the ordinary shareholders and would receive a distribution equal to the liquidation preference per share, subject to availability of funds. In connection with the issuance of the 7.401% Preference Shares, the Company entered into a Replacement Capital Covenant, initially for the benefit of persons that hold the Company’s Senior Notes, that the Company will not redeem or repurchase the 7.401% Preference Shares on or before November 15, 2046, unless, during the six months prior to the date of that redemption or repurchase, the Company receives a specified amount of proceeds from the sale of ordinary shares.
	On August 17, 2012, the Company designated the 6.00% Senior Notes due December 15, 2020, as the covered debt in accordance with the terms of the Replacement Capital Covenant.
	On May 2, 2013, the Company issued 11.0 million shares of 5.95% of Fixed-to-Floating Perpetual Non-Cumulative Preference Shares (the “5.95% Preference Shares”). Each preference shareholder will receive dividends on a non-cumulative basis only when declared by the Board of Directors initially at an annual fixed rate of 5.95% until July 1, 2023 at which time a floating rate, reset quarterly, of 3-month LIBOR plus 4.06% will commence per annum. The 5.95% Preference Shares have a liquidation preference of $25.00 per share and net proceeds were $270.6 million (comprising $275.0 million of total liquidation preference less $4.4 million of issue expenses).
	The Company used $230.0 million of the net proceeds from this offering for settling the cash portion of the mandatory conversion of the PIERS.
	The 5.95% Preference Shares rank equally with preference shares previously issued by the Company and have no fixed maturity date. The Company may redeem all or a portion of the 5.95% Preference Shares at a redemption price of $25.00 per share on or after July 1, 2023. The Company has listed the 5.95% Preference Shares on the New York Stock Exchange under the symbol “AHLPRC.”
	Rights Agreement. On April 17, 2014, the Board of Directors of the Company resolved to issue one preferred share purchase right (a “Right”) for each outstanding ordinary share, and adopted a shareholder rights plan, as set forth in the Rights Agreement dated as of April 17, 2014. Each Right will allow its holder to purchase from the Company one one-thousandth of a share of Series A Junior Participating Preference Shares for $160, once the Rights become exercisable. The Rights will not be exercisable until 10 business days after the public announcement that a person or group has acquired the beneficial ownership of 10% or more of the outstanding ordinary shares of the Company (or 15% in the case of passive institutional investors). The Rights may be redeemed at any time at the discretion of the Board of Directors of the Company. As of December 31, 2014, no Rights have been exercisable or exercised. The rights agreement expires on April 17, 2015.

Statutory_Requirements_and_Div

Statutory Requirements and Dividends Restrictions	12 Months Ended
	Dec. 31, 2014
Text Block [Abstract]
Statutory Requirements and Dividends Restrictions	Statutory Requirements and Dividends Restrictions
	As a holding company, Aspen Holdings relies on dividends and other distributions from its Operating Subsidiaries to provide cash flow to meet ongoing cash requirements, including any future debt service payments and other expenses, and to pay dividends, if any, to our preference and ordinary shareholders. Aspen Holdings must comply with the provisions of the Bermuda Companies Act 1981, as amended, (the “Companies Act”) regulating the payment of dividends and distributions. As of December 31, 2014, there were no restrictions under Bermudian law or the law of any other jurisdiction on the payment of dividends from retained earnings by Aspen Holdings.  The ability of the Company’s Operating Subsidiaries to pay the Company dividends or other distributions is subject to the laws and regulations applicable to each jurisdiction, as well as the Operating Subsidiaries’ need to maintain capital requirements adequate to maintain their insurance and reinsurance operations and their financial strength ratings issued by independent rating agencies.
	The company law of England and Wales prohibits Aspen U.K. or AUL from declaring a dividend to its shareholders unless it has “profits available for distribution.” The determination of whether a company has profits available for distribution is based on its accumulated realized profits and other distributable reserves less its accumulated realized losses. While the U.K. insurance regulatory laws impose no statutory restrictions on a general insurer’s ability to declare a dividend, the PRA’s rules require each insurance company within its jurisdiction to maintain its solvency margin at all times. On October 21, 2013, and in line with common market practice for regulated institutions, the PRA requested that it be afforded with the opportunity to provide a “non-objection” prior to all future dividend payments made by Aspen U.K.  As at December 31, 2014 Aspen U.K. had an accumulated balance of retained losses of approximately $137.5 million and AUL had an accumulated balance of retained income of approximately £6.1 million. In addition, Aspen U.K. held a capital contribution reserve of $470.0 million which under certain circumstances would also be distributable.
	Aspen Bermuda must comply with the provisions of the Companies Act regulating the payment of dividends and distributions. There were no significant restrictions under company law on the ability of Aspen Bermuda to pay dividends funded from its accumulated balances of retained income as at December 31, 2014.  Aspen Bermuda may not in any financial year pay dividends which would exceed 25% of its total statutory capital and surplus, as shown on its statutory balance sheet in relation to the previous financial year, unless it files with the BMA a solvency affidavit at least seven days in advance. As at December 31, 2014, 25% of Aspen Bermuda’s statutory capital and surplus amounted to $513.1 million. Further, Aspen Bermuda must obtain the prior approval of the BMA before reducing by 15% or more its total statutory capital as set out in its previous year’s financial statements.
	Under both North Dakota and Texas law, insurance companies may only pay dividends out of earned surplus as distinguished from contributed surplus. As such, Aspen Specialty and AAIC could not pay a dividend as of December 31, 2014.
	Actual and required statutory capital and surplus for the principal operating subsidiaries of the Company, excluding its Lloyd’s syndicate, as at December 31, 2014 and December 31, 2013 were:
		As at December 31, 2014
		U.S.		Bermuda		U.K.
		($ in millions)
	Required statutory capital and surplus	$	56.9	$	1,097.60	$	202.2
	Statutory capital and surplus	$	394.1	$	2,052.30	$	989.8
		As at December 31, 2013
		U.S.		Bermuda		U.K.
		($ in millions)
	Required statutory capital and surplus	$	40.9	$	1,068.20	$	218.8
	Statutory capital and surplus	$	375.3	$	1,907.40	$	1,005.10
	AUL as the sole corporate member of our Lloyd’s Syndicate is required to maintain Funds at Lloyd’s of $395.1 million. As at December 31, 2014, AUL had total funds at Lloyd’s of $411.2 million of which $377.3 million was provided by Aspen Bermuda.
	The Bermuda Monetary Authority is the group supervisor of the Company. The laws and regulations of Bermuda require that the Company maintain a minimum amount of group statutory capital and surplus based on the enhanced capital requirement using the group standardized risk-based capital model of the Bermuda Monetary Authority. As of December 31, 2014, the Company’s enhanced capital requirement is 60% of the amount calculated. The Company is also subject to an early-warning level based on 120% of the enhanced capital requirement which may trigger additional reporting requirements or other enhanced oversight. As of December 31, 2014, the amount of group statutory capital and surplus maintained by the Company satisfied these regulatory requirements.

Retirement_Plans

Retirement Plans	12 Months Ended
	Dec. 31, 2014
Compensation and Retirement Disclosure [Abstract]
Retirement Plans	Retirement Plans
	The Company operates defined contribution retirement plans for the majority of its employees at varying rates of their salaries, up to a maximum of 20.0%. Total contributions by the Company to the retirement plans were $13.7 million in the twelve months ended December 31, 2014, $11.7 million in the twelve months ended December 31, 2013 and $11.1 million in the twelve months ended December 31, 2012.

ShareBased_Payments

Share-Based Payments	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share Based Payments	Share-Based Payments
	The Company issued options and other equity incentives under three arrangements: investor options, the employee equity incentive plan and the non-employee equity incentive plan. When options are exercised or other equity awards vest, new shares are issued as the Company does not currently hold treasury shares.
	(a)	Investor Options
	The investor options were issued on June 21, 2002 in connection with the transfer to Aspen Holdings of part of the operations of Wellington Underwriting plc (“Wellington”), the Company’s predecessor company. The Company conferred the option to subscribe for up to 6,787,880 ordinary shares of Aspen Holdings to Wellington and members of Syndicate 2020 who were not corporate members of the Lloyd’s syndicate managed by Wellington (the “Wellington Names”). All of the options issued to Wellington were exercised on March 28, 2007 resulting in the issuance of 426,083 ordinary shares by the Company.
	The options issued to the Wellington Names were held for their benefit by the Names’ Trustee. The subscription price payable under the options was initially £10 and increased by 5% per annum, less any dividends paid. Option holders were not entitled to participate in any dividends prior to exercise and would not rank as a creditor in the event of liquidation. All options were exercised prior to the expiry date of June 21, 2012.
	The table below shows the number of Names’ options exercised and the number of shares issued since our initial public offering:
			Options Granted			Options Exercised			Ordinary Shares Issued
	2002		3,006,760			—			—
	2003		—			440,144			152,583
	2004		—			856,218			135,321
	2005		—			303,321			56,982
	2006		—			34,155			3,757
	2007		—			66,759			7,381
	2008		—			20,641			3,369
	2009		—			9,342			3,056
	2010		—			149,895			49,538
	2011		—			761,037			255,504
	2012		—			365,248			116,510
	Total as at December 31, 2013 and 2014		3,006,760			3,006,760			784,001
	(b)	Employee Equity Incentives
	Employee options and other awards are granted under the Aspen 2003 Share Incentive Plan prior to April 24, 2013 and thereafter, the new 2013 Share Incentive Plan. The total number of ordinary shares that may be issued under the 2013 Share Incentive Plan is 2,845,683 shares, which includes 595,683 shares available to grant under the 2003 Share Incentive Plan as of February 25, 2013. The number of ordinary shares that may be issued under the 2013 Share Incentive Plan is adjusted per the number of awards that may be forfeited under the 2003 Share Incentive Plan.
	Options. Stock options were granted with an exercise price equivalent to the fair value of the share on the grant date. The weighted average value at grant date was determined using the Black-Scholes option pricing model. Stock options typically vest over a three-year period with a ten-year contract period (except for options granted in 2007 which have a seven-year exercise period) with vesting dependent on time and performance conditions established at the time of grant. In the case of Mr. O’Kane, the Compensation Committee on April 22, 2014, approved the extension of the expiration of the 2007 options by one year to May 4, 2015. No options were granted during the twelve months ended December 31, 2014 (2013 — Nil) and 84,018 options were exercised and shares issued in the twelve months ended December 31, 2014 (2013 — 904,242). No charges or tax charges against income were made in respect of employee options for the twelve months ended December 31, 2014 (2013 — $Nil; 2012 — $Nil).
	The following table summarizes information about employee options outstanding to purchase ordinary shares at December 31, 2014.
			As at December 31, 2014
			Options			Options			Options			Outstanding			Exercise		Weighted Average	Remaining
			Granted			Forfeited			Exercised			and			Price		Fair Value at	Contractual
												Exercisable					Grant Date	Time
	Option Holder
	2003 Option grants		3,884,030			712,906			3,171,124			—			$16.20		$5.31	expired
	2004 Option grants		500,113			276,082			224,031			—			$24.44		$5.74	expired
	2006 Option grants February 16		1,072,490			450,567			483,347			138,576			$23.65		$6.99	1 year, 2 months
	2007 Option grants May 4 (1)		607,635			152,276			376,241			79,118			$27.28		$6.14	expired
	________________
	(1)	In the case of Mr. O’Kane, the expiration date for the 2007 options were extended for one year to May 4, 2015.
	With respect to the 2003 options, 65% of the options were subject to time-based vesting with 20% vesting upon grant and 20% vesting on each December 31 of the calendar years 2003, 2004, 2005 and 2006. The remaining 35% of the initial grant options were subject to performance-based vesting and in any event cliff vested on December 31, 2009. The 2003 options expired on August 20, 2013.
	The 2004 options vested over a three-year period with vesting subject to the achievement of Company performance targets. The options lapse if the criteria are not met. As at December 31, 2004, not all performance targets were met and 242,643 options for non-performance were cancelled.
	The 2006 options vested at the end of a three-year period with vesting subject to the achievement of one-year and three-year performance targets. The options lapse if the criteria were not met. A total of 695,643 of 2006 options vested.
	The 2007 option grants are not subject to performance conditions and 476,250 options vested at the end of the three-year period from the date of grant on May 4, 2010. The options are exercisable for a period of seven years from the date of grant.
	The intrinsic value of options exercised in the twelve months ended December 31, 2014 was $1.5 million (2013 — $17.2 million; 2012 — $18.7 million).
	The following table shows the per share weighted average fair value and the related underlying assumptions using a modified Black-Scholes option pricing model by date of grant:
			Grant Date
			October 22, 2007				May 4,				August 4,				February 16,			December 23,			August 20,
							2007				2006				2006			2004			2003(1)
	Per share weighted average fair value		$	5.76			$	6.14			$	4.41			$	6.99		$	5.74		$	5.31
	Risk free interest rate		4.09		%		4.55		%		5.06		%		4.66		%	3.57		%	4.7		%
	Dividend yield		2.1		%		2.2		%		2.6		%		2.7		%	0.5		%	0.6		%
	Expected life		5 years				5 years				5 years				5 years			5 years			7 years
	Share price volatility		20.28		%		23.76		%		19.33		%		35.12		%	19.68		%	—
	Foreign currency volatility		—				—				—				—			—			9.4		%
	________________
	(1)	The 2003 options had a price volatility of zero. The minimum value method was utilized because the Company was unlisted on the date that the options were issued. Foreign currency volatility of 9.4% was applied as the exercise price was initially in British Pounds and the share price of the Company is in U.S. Dollars.
	The above table does not show the per share weighted average fair value and the related underlying assumptions for the 2005 options as the performance targets were not met and all options were forfeited.
	Restricted Share Units. Restricted share units (“RSUs”) granted to employees vest over a two or three-year period, based on continued service. Some of the grants vest at year-end, while some other grants vest on the anniversary of the date of grant or when the Compensation Committee of the Board of Directors agrees to deliver them. Holders of restricted share units will be paid one ordinary share for each unit that vests as soon as practicable following the vesting date. Holders of restricted share units generally will not be entitled to any rights of a holder of ordinary shares, including the right to vote, unless and until their units vest and ordinary shares are issued but they are entitled to receive dividend equivalents. Dividend equivalents will be denominated in cash and paid in cash if and when the underlying RSUs vest.
	The following table summarizes information about RSUs as at December 31, 2014:
				As at December 31, 2014
				Restricted Share Units
	RSU Holder			Amount			Amount			Amount			Amount
				Granted			Vested			Forfeited			Outstanding
	2004 - 2011 Grants			965,911			897,080			68,831			—
	2012 Grants			350,899			208,744			47,533			94,622
	2013 Grants			307,441			100,012			27,511			179,918
	2014 Grants			259,640			—			15,068			244,572
	Total			1,883,891			1,205,836			158,943			519,112
	The fair value of the RSUs is based on the closing price on the date of the grant. The fair value is expensed through the consolidated income statement evenly over the vesting period. Compensation cost in respect of RSUs charged against income was $9.3 million for the twelve months ended December 31, 2014 (2013 — $7.6 million ; 2012 — $7.6 million) with a fair value adjustment for the twelve months ended December 31, 2014 of $3.1 million (2013 — $0.4 million; 2012 — $0.3 million). The total tax credit recognized by the Company in relation to RSUs in the twelve months ended December 31, 2014 was $2.2 million (2013 — $1.9 million; 2012 — $1.6 million).
Performance Shares. Performance share awards are not entitled to dividends before they vest. Performance shares that vest will only be issued following the assessment of the final performance target in the three-year period, and subject to the participant’s continued employment. The following table summarizes information about performance shares as at December 31, 2014:
		As at December 31, 2014
		Performance Share Awards
		Amount			Amount			Amount			Amount Outstanding
		Granted			Vested			Forfeited
2004 - 2011 Grants (1)		4,194,638			2,220,442			1,974,196			—
2012 Grants		344,131			241,367			102,764			—
2013 Grants (2)		250,066			127,153			43,728			79,185
2014 Grants (2)		315,389			122,056			4,100			189,233
Total		5,104,224			2,711,018			2,124,788			268,418
______________________
(1)	The amounts vested and forfeited in respect of the 2004 - 2011 performance share awards have been updated to reflect employees leaving after the financial reporting date but before the final vesting date.
(2)	These balances could increase depending on future performance.
On February 3 and 4, 2011, the Compensation Committee approved the grant of 853,223 performance shares with a grant date of February 9, 2011. Additional grants of 31,669 and 5,902 performance shares were made on March 21, 2011 and May 2, 2011, respectively. The performance shares will be subject to a 3-year vesting period with a separate annual ROE test for each year. One-third of the grant will be eligible for vesting each year based on a formula, and will only be issuable at the end of the 3-year period.
If the ROE achieved in 2011 was:
•	less than 6%, then the portion of the performance shares subject to the vesting conditions in such year will be forfeited (i.e., 33.33% of the initial grant);
•	between 6% and 11%, then the percentage of the performance shares eligible for vesting in such year will be between 10% and 100% on a straight-line basis; and
•	between 11% and 21%, then the percentage of the performance shares eligible for vesting in such year will be between 100% and 200% on a straight-line basis.
At its meeting held on February 1, 2012, the Compensation Committee approved the vesting conditions for the portion of the 2011 performance shares subject to 2012 performance testing. If the ROE achieved in 2012 is less than 5%, then the portion of the performance shares subject to the vesting conditions in such year will be forfeited (i.e. 33.33% of the initial grant). If the ROE achieved in 2012 is between 5% and 10%, then the percentage of the performance shares eligible for vesting will be between 10% and 100% on a straight-line basis. If the ROE achieved in 2012 is between 10% and 20%, then the percentage of the performance shares eligible for vesting will be between 100% and 200% on a straight-line basis.
At its meeting held on February 6, 2013, the Compensation Committee approved the vesting conditions for the portion of the 2011 performance share awards subject to 2013 performance testing.
If the ROE achieved in 2013 was:
•	less than 5%, then the portion of the performance shares subject to the vesting conditions in such year will be forfeited (i.e., 33.33% of the initial grant);
•	between 5% and 10%, then the percentage of the performance shares eligible for vesting in such year will be between 10% and 100% on a straight-line basis; and
•	between 10% and 20%, then the percentage of the performance shares eligible for vesting in such year will be between 100% and 200% on a straight-line basis.
2011 Performance Shares
Year				Split			ROE			Banked
2011				33.3	%		(5.3	)%		—	%
2012				33.3	%		10	%		33.3	%
2013				33.3	%		11.7	%		39	%
Total				100	%					72.3	%
Based on the achievement of a negative ROE of (5.3)% in 2011, one-third of the 2011 performance share award based on 2011 performance test was forfeited. Based on the achievement of a 2012 ROE of 10.0%, 100.0% of one-third of the 2011 performance share award based on the 2012 performance test was eligible for vesting, resulting in 169,981 performance shares being banked. Based on the achievement of a 2013 ROE of 11.7%, 117.0% of one-third of the 2011 performance share award was eligible for vesting, resulting in 303,247 performance shares being banked. A total of 473,228 performance shares were issued in 2014 in respect to the 2011 performance share award.
On February 2, 2012, the Compensation Committee approved the grant of 334,125 performance shares with a grant date of February 8, 2012. An additional grant of 10,006 performance shares was made on November 1, 2012. The performance shares are subject to a three-year vesting period with a separate annual diluted book value per share (“BVPS”) growth test for each year, adjusted to add back ordinary dividends to shareholders’ equity at the end of the relevant year. One-third of the grant will be eligible for vesting each year based on a formula, and will only be issuable at the end of the three-year period.
If the diluted BVPS growth achieved in 2012 was:
•	less than 5%, then the portion of the performance shares subject to the vesting conditions in such year will be forfeited (i.e., 33.33% of the initial grant);
•	between 5% and 10%, then the percentage of the performance shares eligible for vesting in such year will be between 10% and 100% on a straight-line basis; and
•	between 10% and 20%, then the percentage of the performance shares eligible for vesting in such year will be between 100% and 200% on a straight-line basis.
The 2013 and 2014 performance tests applicable to the 2012 performance share awards are described below under the 2013 performance share awards and the 2014 performance share awards, respectively.
2012 Performance Shares
Year				Split			Increase in BVPS			Banked
2012				33.3	%		8.1	%		21.9	%
2013				33.3	%		6.2	%		10.5	%
2014				33.3	%		13.3	%		43	%
Total				100	%					75.4	%
Based on the achievement of a BVPS growth in 2012 of 8.1%, 65.8% of one-third of the 2012 performance share awards were eligible for vesting, resulting in 62,930 performance shares being banked. Based on the achievement of a BVPS growth of 6.2% in 2013, as refined by the Compensation Committee as discussed further below, 31.6% of one-third of the 2012 performance award was eligible for vesting, resulting in 33,012 performance shares being banked. Based on the achievement of a BVPS growth in 2014 of 13.3%, as described further below, 129.0% of one-third of the 2012 performance share award is eligible for vesting, resulting in 145,425 performance share being banked. All banked 2012 performance share awards will be issuable upon filing of the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2014.
On February 6, 2013, the Compensation Committee approved the grant of 250,066 performance shares with a grant date of February 11, 2013. The performance shares are subject to a three-year vesting period with a separate BVPS growth test for each year, adjusted to add back ordinary dividends to shareholders’ equity at the end of the relevant year. One-third of the grant will be eligible for vesting each year based on a formula and will only be issuable at the end of the three-year period.
If the diluted BVPS growth achieved in 2013 was:
•	less than 5%, then the portion of the performance shares subject to the vesting conditions in such year will be forfeited (i.e., 33.33% of the initial grant);
•	between 5% and 10%, then the percentage of the performance shares eligible for vesting in such year will be between 10% and 100% on a straight-line basis; or
•	between 10% and 20%, then the percentage of the performance shares eligible for vesting in such year will be between 100% and 200% on a straight-line basis.
On February 5, 2014, the Compensation Committee approved the testing conditions of the performance share awards that were subject to the Company’s 2013 annual growth in BVPS test. For purposes of the annual growth in the diluted BVPS test for 2013, diluted BVPS was initially defined as the diluted BVPS, after adding back dividends, as described above. The approval by the Compensation Committee revises within the original terms the definition of diluted BVPS for purposes of the annual growth in diluted BVPS test for 2013 to reflect (i) the impact of all of the Company’s PIERS retired during the second quarter of 2013 and (ii) the variance between the Company’s assumptions of the price at which it would execute its share repurchase program in 2013 against the price at which it actually repurchased its ordinary shares. As a result of the 28.8% increase in the Company’s share price in 2013, the Company purchased a smaller quantity of ordinary shares than anticipated which adversely impacted the Company’s BVPS.
The Compensation Committee approved the testing conditions to ensure that the Company’s officers would not be penalized as a result of the increase in the Company’s ordinary share price, which benefited the Company and its shareholders, or as result of the impact on the Company’s diluted BVPS as a result of the retirement of the PIERS. Each of these factors were regarded by the Compensation Committee as sufficiently unusual or outside the control of the Company’s management and therefore justified revising (within the original terms) the BVPS test applicable to the 2013 tested performance share awards. As a result, after consideration of all factors involved, including the importance of retaining key talent, the Compensation Committee believed it was appropriate to make the above-described awards. The awards resulted in a vesting of 31.6% of one-third of each of the 2012 and 2013 performance share awards that are subject to the 2013 BVPS test.
The 2014 performance test applicable to the 2013 performance share awards is described below under the 2014 performance share awards.
2013 Performance Shares
Year				Split			Increase in BVPS			Banked
2013				33.3	%		6.2	%		10.5	%
2014				33.3	%		13.3	%		43	%
2015				33.3	%		NA			NA
Total				100	%					53.5	%
Based on the achievement of a BVPS growth of 6.2% in 2013, as refined by the Compensation Committee as discussed above, 31.6% of one-third of the 2012 performance award was eligible for vesting, resulting in 25,001 performance shares being banked. Based on the achievement of a BVPS growth in 2014 of 13.3%, as described below, 129.0% of one-third of the 2013 performance share award is eligible for vesting upon the filing of this report, in 102,152 performance shares being banked.
During the twelve months ended December 31, 2014, the Company granted 315,389 performance shares.
The performance shares are subject to a three-year vesting period with a separate BVPS growth test for each year, adjusted to add back ordinary dividends and movements in AOCI to shareholders’ equity at the end of the relevant year. One-third of the grant will be eligible for vesting each year based on a formula, and will only be issuable at the end of the three-year period.
If the diluted BVPS growth achieved in 2014 is:
• less than 5.2%, then the portion of the performance shares subject to the vesting conditions in such year will be forfeited (i.e., 33.3% of the initial grant);
• between 5.2% and 10.4% then the percentage of the performance shares eligible for vesting in such year will be between 10% and 100% on a straight-line basis; or
• between 10.4% and 20.8%, then the percentage of the performance shares eligible for vesting in such year will be between 100% and 200% on a straight-line basis.
In calculating BVPS for 2014, the entire movement in AOCI will be excluded. Interest rate movements and credit spread movements in AOCI can be fairly significant and adversely impact growth in BVPS which management does not have any control over. The Compensation Committee also agreed that it will review the impact of any capital management actions undertaken during 2014, including share repurchases and special dividends, and consider whether any further adjustments to growth in BVPS should be may be made in the context of such actions. The Compensation Committee also further agreed to
exclude from the calculation of BVPS for 2014 the costs payable to third-party service providers resulting from the Company’s response to the proposals received from Endurance Specialty Holdings Ltd. (“Endurance”). The Compensation Committee believes that it would not be appropriate for employees’ performance-related compensation to be impacted by these costs.
The Compensation Committee will determine the vesting conditions for the 2015 and 2016 portions of the grant in such years taking into consideration the market conditions and the Company’s business plans at the commencement of the years concerned. Notwithstanding the vesting criteria for each given year, if in any given year, the shares eligible for vesting are greater than 100% for the portion of such year’s grant and the average diluted BVPS growth over such year and the preceding year is less than the average of the minimum vesting thresholds for such year and the preceding year (which in the case of the 2013 portion of the grant, the average BVPS is less than 5%, then only 100% (and no more) of the shares that are eligible for vesting in such year shall vest. Notwithstanding the foregoing, if in the judgment of the Compensation Committee the main reason for the BVPS metric in the earlier year falling below the minimum threshold (or below 5% in the case of 2013) is due to the impact of rising interest rates and bond yields, then the Compensation Committee may, in its discretion, disapply this limitation on 100% vesting.
2014 Performance Shares
Year				Split			Increase in BVPS			Banked
2014				33.3	%		13.3	%		43	%
2015				33.3	%		NA			NA
2016				33.3	%		NA			NA
Total				100	%					43	%
Based on the achievement of a BVPS growth in 2014 of 13.3% as described above, 129.0% of one-third of the 2014 performance share award is eligible for vesting, upon the filing of this report, resulting in 122,056 performance shares being banked.
The fair value of performance share awards is based on the value of the closing share price on the date of the grant less a deduction for expected dividends which would not accrue during the vesting period. Compensation costs charged against income in the twelve months ended December 31, 2014 in respect of performance shares was a charge of $8.9 million (2013 — $8.1 million; 2012 — $11.1 million). The total tax credit recognized by the Company in relation to performance share awards in the twelve months ended December 31, 2014 was $2.4 million (2013 — $2.0 million; 2012 — $3.0 million).
A summary of performance share activity under Aspen’s 2003 and 2013 Share Incentive Plans for the twelve months ended December 31, 2014 is presented below:
		Twelve Months Ended December 31, 2014
		Number of			Weighted Average
		Shares			Grant Date
					Fair Value
Outstanding performance share awards, beginning of period		597,479			$27.15
Granted		315,389			$38.46
Earned		(373,935	)		$30.12
Forfeited		(270,515	)		$27.61
Outstanding performance share awards, end of period		268,418			$25.35
Phantom Shares.    On February 2, 2012, the Compensation Committee approved the grant of 278,143 phantom shares with a grant date of February 8, 2012 (2011 — Nil). The phantom shares are subject to a three-year vesting period with a separate annual diluted BVPS growth test for each year, in accordance with the test described above for the 2012 performance shares, with the difference being that any vested amount would be paid in cash in lieu of shares. As shares are not issued, these instruments have no dilutive effect.
2012 Phantom Shares
Year				Split			Increase in BVPS			Banked
2012				33.3	%		8.1	%		21.9	%
2013				33.3	%		6.2	%		10.5	%
2014				33.3	%		13.3	%		43	%
Total				100	%					75.4	%
The total number of 2012 phantom shares that have been banked based on the 2012 performance test was 61,006. The total number of 2012 phantom shares that have been banked based on the 2013 performance test was 9,258. The total number of 2012 phantom shares that that will be banked, upon the filing of this report, based on the 2014 performance test will be 88,658. Cash equal to the vested amount based on the closing share price on the date of filing of this report will be paid to employees upon the filing of this report.
On February 6, 2013, the Compensation Committee approved the grant of 152,541 phantom shares with a grant date of February 11, 2013. Additional grants of 6,521 and 542 phantom shares were made on April 8, 2013 and June 11, 2013, respectively. The phantom shares are subject to a three-year vesting period with a separate annual diluted BVPS growth test for each year, in accordance with the test described above for the 2013 performance shares, with the difference being that any vested amount would be paid in cash in lieu of shares. As shares are not issued, these instruments have no dilutive effect.
2013 Phantom Shares
Year				Split			Increase in BVPS			Banked
2013				33.3	%		6.2	%		10.5	%
2014				33.3	%		13.3	%		43	%
2015				33.3	%		NA			NA
Total				100	%					53.5	%
The total number of 2013 phantom shares that that have been banked based on the 2013 performance test was 16,812. The total number of 2013 phantom shares that will be banked, upon the filing of this report, based on the 2014 performance test will be 64,357.
On April 22, 2014, the Compensation Committee approved the grant of 154,512 phantom shares with a grant date of April 25, 2014. The phantom shares are subject to a three-year vesting period with a separate annual diluted BVPS growth test for each year, in accordance with the test described above for the 2014 performance shares, with the difference being that any vested amount would be paid in cash in lieu of shares. As shares are not issued, these instruments have no diluted effect.
2014 Phantom Shares
Year				Split			Increase in BVPS			Banked
2014				33.3	%		13.3	%		43	%
2015				33.3	%		NA			NA
2016				33.3	%		NA			NA
Total				100	%					43	%
The total number of 2014 phantom shares that will that will be banked, upon the filing of this report, based on the 2014 performance test will be 59,796.
The fair value of the phantom shares is based on the closing share price on the date of the grant, less estimated dividends payable over the vesting period. The fair value is expensed through the consolidated income statement evenly over the vesting period, but as the payment to beneficiaries will ultimately be in cash rather than shares, an adjustment is required each quarter to revalue the accumulated liability to the balance sheet date fair value.
Compensation costs charged against income in the twelve months ended December 31, 2014 in respect of phantom shares was $6.1 million (2013 — $1.5 million; 2012 — $1.3 million) with a fair value adjustment for the twelve months ended December 31, 2014 of $2.9 million (2013 — $1.4 million; 2012 — $0.1 million). The total tax credit recognized by the Company in relation to phantom share awards in the twelve months ended December 31, 2014 was $2.2 million (2013 — $0.4 million; 2012 — $0.3 million).
Employee Share Purchase Plans. On April 30, 2008, the shareholders of the Company approved the Employee Share Purchase Plan , the 2008 Sharesave Scheme, as amended, and the International Employee Share Purchase Plan (collectively, the “ESPP”), which are implemented by a series of consecutive offering periods as determined by the Board of Directors. In respect of the ESPP, employees can save up to $500 per month over a two-year period, at the end of which they will be eligible to purchase Company shares at a discounted price, subject to a further one year holding period. In respect of the 2008 Sharesave Scheme, employees can save up to £250 per month over a three-year period, at the end of which they will be eligible to purchase Company shares at a discounted price. The amount employees can save increased to £500 per month effective April 6, 2014. The purchase price will be eighty-five percent 85% of the fair market value of a share on the offering date which may be adjusted upon changes in capitalization of the Company. Under the ESPP, 11,821 ordinary shares were issued during the twelve months ended December 31, 2014 (2013 —  38,915 shares; 2012 —75,066). Compensation costs charged against income in the twelve months ended December 31, 2014 in respect of the ESPP was $0.3 million (2013 — $1.3 million; 2012 — $0.2 million). The total tax credit recognized by the Company in relation to the ESPP in the twelve months ended December 31, 2014 was $0.1 million (2013 — $0.1 million; 2012 — $Nil).
The fair value of the employee options granted was estimated on the date of grant using a modified Black-Scholes option pricing model under the following assumptions:
Grant Date		Per share		Risk free			Dividend yield			Expected life		Share price
		weighted		interest rate								volatility
		average
		fair value
		($)		(%)			(%)			(in years)		(%)
November 4, 2008		$3.18		0.48	%		2.7	%		3		68	%
December 4, 2008		2.87		(0.41	)		3.16			2		102
November 23, 2009		3.76		0.01			2.28			3		22
December 21, 2009		3.82		0.04			2.34			2		18
December 22, 2010		4.24		0.13			2.07			3		14
December 22, 2010		4.46		0.13			2.07			2		14
December 13, 2011		4.2		0.05			2.8			3		26.2
December 13, 2011		3.85		0.05			2.75			2		26.2
March 20, 2013		7.79		0.38			1.88			3		2.8
March 20, 2013		5.75		0.25			1.88			2		3.2
26-Sep-14		6.61		1.06			1.87			3		6.2
26-Sep-14		6.43		0.58			1.87			2		4
(c)    Non-employee equity incentives
Non-employee director options are granted under the Aspen 2006 Stock Option Plan for Non-Employee Directors (the “Director Stock Option Plan”).
Options.    The following table summarizes information about non-employee director options outstanding to purchase ordinary shares at December 31, 2014.
Option Holder		Options Outstanding			Options Exercisable			Exercise		Fair Value at		Remaining
								Price		Grant Date		Contractual  Time
Non-employee directors - 2006 Option grants (May 25)		4,435			4,435			$21.96		$4.24		1 year, 5 months
Non-employee directors - 2007 Option grants (July 30)		2,012			2,012			$24.76		$4.97		2 years, 7 months
The options granted in 2006 and 2007 vested at the end of a three-year period from the date of grant subject to continued service as a director. Vested options are exercisable for a period of ten years from the date of grant. No options were granted during the twelve months ended December 31, 2014 (2013 — Nil) and no options were exercised and shares issued in the twelve months ended December 31, 2014 (2013 — 2,012). No charges or tax charges against income were made in respect of non-employee directors options for the twelve months ended December 31, 2014 (2013 — $Nil; 2012 — $Nil).
The fair value of the non-employee director options granted were estimated on the date of grant using a modified Black-Scholes option pricing model under the following assumptions:
		Grant Date
		July 30, 2007			May 25, 2006
Per share weighted average fair value		$4.97			$4.24
Risk-free interest rate		4.64	%		4.85	%
Dividend yield		2.4	%		2.7	%
Expected life		5 years			5 years
Share price volatility		19.55	%		20.05	%
Restricted Share Units.    The following table summarizes information about restricted share units issued to non-employee directors as at December 31, 2014.
		As at December 31, 2014
		Restricted Share Units
		Amount			Amount			Amount			Amount
		Granted			Vested			Forfeited			Outstanding
Non-Employee Directors — 2012 and prior		138,504			131,866			6,638			—
Non-Employee Directors — 2013		29,092			26,727			2,365			—
Non-Employee Directors — 2014		27,180			22,640			—			4,540
Chairman — 2012 and prior		75,799			75,799			—			—
Chairman — 2013		14,188			14,188			—			—
Chairman — 2014		13,590			11,324			—			2,266
Total		298,353			282,544			9,003			6,806
One-twelfth of the RSUs vest on each one month anniversary of the date of grant, with 100% of the restricted share units becoming vested and issued on the first anniversary of the grant date, or on the date of departure of a director (for the amount vested through such date). A portion of the shares that is eligible to vest following the final vesting date in the calendar year of the date of grant is delivered as soon as practicable thereafter and the remaining shares under the restricted share units are delivered on the first anniversary of the grant date. If a director leaves the Board for any reason other than “cause” (as defined in the award agreement), then the director would receive the shares under the restricted share units that had vested through the date the director leaves the Board. RSUs entitle the holder to receive one ordinary share unit for each unit that vests. Holders of RSUs are not entitled to any of the rights of a holder of ordinary shares, including the right to vote, unless and until their units vest and ordinary shares are issued but they are entitled to receive dividend equivalents with respect to their units. Dividend equivalents will be denominated in cash and paid in cash if and when the underlying units vest.
In respect of the RSUs granted to the Chairman up to December 31, 2009, one-third of the grants vests on the anniversary date of grant over a three-year period. For grants from January 1, 2010, onwards, one-twelfth of the RSUs vest on each one month anniversary of the date of grant, with 100% of the restricted share units becoming vested and issued on the first anniversary of the grant date, or on the date of departure.
The fair value of the RSUs is based on the closing price on the date of the grant. Compensation cost charged against income was $0.4 million for the twelve months ended December 31, 2014 (2013 — $1.3 million; 2012 — $1.4 million).
The total tax charge recognized by the Company in relation to non-employee RSUs in the twelve months ended December 31, 2014 was $Nil (2013 — $Nil; 2012 — $Nil).
(d)	Summary of investor options, employee and non-employee share options and restricted share units.
A summary of option activity and restricted share unit activity discussed above is presented in the tables below:
			Twelve Months Ended December 31, 2014
Option activity			Number of Options			Weighted Average
						Exercise  Price
Outstanding options, beginning of period			302,460			$25.02
Exercised and issued			(74,520	)		25.26
Forfeited or expired			(3,799	)		27.28
Outstanding and exercisable options, end of period			224,141			$24.91
			Twelve Months Ended December 31, 2013
Option activity			Number of Options			Weighted Average
						Exercise  Price
Outstanding options, beginning of period			1,208,787			$23.07
Exercised and issued			(906,254	)		22.45
Forfeited or expired			(73	)		24.21
Outstanding and exercisable options, end of period			302,460			$25.02
			Twelve Months Ended December 31, 2014
Restricted share unit activity			Number of			Weighted Average
			Shares			Grant Date
						Fair Value
Outstanding restricted stock, beginning of period			544,751			$32.13
Granted			300,410			38.6
Vested			(291,468	)		32.12
Forfeited			(27,775	)		36.29
Outstanding restricted stock, end of period			525,918			$35.83
			Twelve Months Ended December 31, 2013
Restricted share unit activity			Number of			Weighted Average
			Shares			Grant Date
						Fair Value
Outstanding restricted stock, beginning of period			488,013			$27.81
Granted			350,721			35.61
Vested			(253,700	)		23.66
Forfeited			(40,283	)		31.45
Outstanding restricted stock, end of period			544,751			$32.13

Intangible_Assets

Intangible Assets	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets	Intangible Assets
	The following table provides a summary of the Company’s intangible assets for the twelve months ended December 31, 2014 and 2013:
		Twelve Months Ended December 31, 2014										Twelve Months Ended December 31, 2013
		Trade		Insurance		Other			Total			Trade		Insurance		Other			Total
		Mark		Licenses								Mark		Licenses
		($ in millions)										($ in millions)
	Intangible Assets
	Beginning of the period	$	1.6	$	16.6	$	0.2		$	18.4		$	1.6	$	16.6	$	0.8		$	19
	Amortization	—		—		(0.2		)	(0.2		)	—		—		(0.6		)	(0.6		)
	End of the period	$	1.6	$	16.6	$	—		$	18.2		$	1.6	$	16.6	$	0.2		$	18.4
	License to use the “Aspen” Trademark. On April 5, 2005, the Company entered into an agreement with Aspen (Actuaries and Pension Consultants) Plc to acquire the right to use the Aspen trademark in the United Kingdom. The consideration paid was approximately $1.6 million. As at December 31, 2014, the value of the license to use the Aspen trademark was $1.6 million (December 31, 2013 — $1.6 million). The Company performed its annual qualitative assessment and determined that it was not more likely than not that the Aspen trademark was impaired as at December 31, 2014.
	Insurance Licenses. The total value of the licenses as at December 31, 2014 was $16.6 million (December 31, 2013 — $16.6 million). This includes $10.0 million of acquired licenses held by AAIC, $4.5 million of acquired licenses held by Aspen Specialty and $2.1 million of acquired licenses held by Aspen U.K. The insurance licenses are considered to have an indefinite life and are not being amortized. The Company performed its annual qualitative assessment and determined that it was not more likely than not that the insurance licenses were impaired as at December 31, 2014.
	Other. In 2010, the Company purchased APJ for an aggregate consideration of $4.8 million. The directors of Aspen Holdings assessed the fair value of the net tangible and financial assets acquired at $1.2 million. The $3.6 million intangible asset represented the Company’s assessment of the value of renewal rights and distribution channels ($2.2 million) and the lock-in period for employees associated with the business ($1.4 million). The asset was amortized over a five-year period and the value as at December 31, 2014 was $Nil (December 31, 2013 — $0.2 million).

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingent Liabilities
	(a)	Restricted assets
	The Company’s subsidiaries are obliged by the terms of its contractual obligations to U.S. policyholders and by obligations to certain regulatory authorities to facilitate issue of letters of credit or maintain certain balances in trust funds for the benefit of policyholders.
	The following table details the forms and value of Company’s restricted assets as at December 31, 2014 and 2013:
			As at December 31, 2014			At December 31, 2013
			($ in millions, except percentages)
	Regulatory trusts and deposits:
	Affiliated transactions		$	1,086.90		$	685.8
	Third party		2,183.40			2,236.40
	Letters of credit / guarantees(1)		778.7			830.4
	Total restricted assets		$	4,049.00		$	3,752.60
	Total as percent of cash and invested assets		47		%	45.5		%
	_______________
	-1	As of December 31, 2014, the Company had pledged funds of $764.4 million and £9.2 million (December 31, 2013 — $803.7 million and £16.1 million) as collateral for the secured letters of credit.
	Our current arrangements with our bankers for the issue of letters of credit require us to provide collateral in the form of cash and investments for the full amount of all secured and undrawn letters of credit that are outstanding. We monitor the proportion of our otherwise liquid assets that are committed to trust funds or to the collateralization of letters of credit. As at December 31, 2014 and 2013, these funds amounted to approximately 47.0% of the $8.6 billion and approximately 45.5% of the $8.3 billion of cash and investments held by the Company, respectively. We do not consider that this unduly restricts our liquidity at this time. Refer to Note 23, “Credit Facility and Long-term Debt” of these consolidated financial statements for further discussion of our credit facilities and long-term debt arrangements.
	Funds at Lloyd’s. AUL operates at Lloyd’s as the corporate member for Syndicate 4711. Lloyd’s determines Syndicate 4711’s required regulatory capital principally through the syndicate’s annual business plan. Such capital, called Funds at Lloyd’s, comprising of cash and investments at December 31, 2014 in the amount of $411.9 million (2013 — $346.4 million).
	The amounts provided as Funds at Lloyd’s will be drawn upon and become a liability of the Company in the event of Syndicate 4711 declaring a loss at a level that cannot be funded from other resources, or if Syndicate 4711 requires funds to cover a short term liquidity gap. The amount which the Company provides as Funds at Lloyd’s is not available for distribution to the Company for the payment of dividends. AMAL is also required by Lloyd’s to maintain a minimum level of capital which as at December 31, 2014 was £0.4 million (December 31, 2013 — £0.4 million). This is not available for distribution by the Company for the payment of dividends.
	U.S. Reinsurance Trust Fund.    For its U.S. reinsurance activities, Aspen U.K. has established and must retain a multi-beneficiary U.S. trust fund for the benefit of its U.S. cedants so that they are able to take financial statement credit without the need to post cedant-specific security. The minimum trust fund amount is $20.0 million plus an amount equal to 100% of Aspen U.K.’s U.S. reinsurance liabilities, which were $1,071.4 million at December 31, 2014 and $1,035.9 million at December 31, 2013. At December 31, 2014, the total value of assets (including applicable letter of credit facilities) held in the trust was $1,323.6 million (2013 — $1,352.2 million).
	Aspen Bermuda has also established and must retain a multi-beneficiary U.S. trust fund for the benefit of its U.S. cedants so that they are able to take financial statement credit without the need to post cedant-specific security. The minimum trust fund amount is $20.0 million plus an amount equal to 100% of Aspen Bermuda’s liabilities to its U.S. cedants which was $694.8 million and $581.3 million as at December 31, 2014 and 2013, respectively. At December 31, 2014, the total assets held in the U.S. trust fund and other Aspen Bermuda trusts were $1,020.1 million (2013 —$918.6 million).
	U.S. Surplus Lines Trust Fund.    Aspen U.K. has also established a U.S. surplus lines trust fund with a U.S. bank to secure liabilities under U.S. surplus lines policies. The balance held in the trust at December 31, 2014 was $170.9 million (2013 — $146.6 million).
	U.S. Credit and Surety Lines Trust Fund.    Aspen U.K. has also established a U.S. credit and surety lines trust fund with a U.S. bank to secure liabilities under U.S. credit and surety lines policies. The balance held in the trust at December 31, 2014 was Nil (2013 —Nil).
	U.S.  Regulatory Deposits.    As at December 31, 2014, Aspen Specialty had a total of $6.2 million (2013 — $6.2 million) on deposit with six U.S. states in order to satisfy state regulations for writing business in those states. AAIC had a further $7.2 million (2013 — $7.2 million) on deposit with twelve U.S. states.
	Canadian Trust Fund.    Aspen U.K. has established a Canadian trust fund with a Canadian bank to secure a Canadian insurance license. As at December 31, 2014, the balance held in trust was CAD$314.8 million (2013 — CAD$354.4 million).
	Australian Trust Fund.    Aspen U.K. has established an Australian trust fund with an Australian bank to secure policyholder liabilities and as a condition for maintaining an Australian insurance license. As at December 31, 2014, the balance held in trust was AUD$114.4 million (2013 — AUD$182.1 million).
	Swiss Trust Fund.    Aspen U.K. has established a Swiss trust fund with a Swiss bank to secure policyholder liabilities and as a condition for maintaining a Swiss insurance license. As at December 31, 2014, the balance held in trust was CHF12.3 million (2013— CHF16.6 million).
	Singapore Fund.    Aspen U.K. has established a segregated Singaporean bank account to secure policyholder liabilities and as a condition for maintaining a Singaporean insurance license and meet local solvency requirements. As at December 31, 2014, the balance in the account was SGD$72.6 million (2013 — SGD$67.5 million).
	Interest Rate Swaps. As at December 31, 2014, cash collateral with a fair value of $22.3 million was held by the Company’s counterparties to support the current valuation of the interest rate swaps (December 31, 2013 — $34.3 million). For more information, please refer to Note 10, “Derivative Contracts” of these consolidated financial statements.
	(b)Operating leases
	Amounts outstanding under operating leases net of subleases as of December 31, 2014 and 2013 were:
	As at December 31, 2014		2015			2016			2017		2018		2019		Later		Total
															Years
			($ in millions)
	Operating Lease Obligations		$	13.4		$	9.3		$	8.5	$	7.3	$	6.4	$	1.5	$	46.4
	As at December 31, 2013		2014			2015			2016		2017		2018		Later		Total
															Years
			($ in millions)
	Operating Lease Obligations		$	11.9		$	12.1		$	8.3	$	7.5	$	6.3	$	6.5	$	52.6
	Total rental and premises expenses for 2014 was $16.7 million (2013 — $19.9 million). For all leases, all rent incentives, including reduced-rent and rent-free periods, are spread on a straight-line basis over the term of the lease. We believe that our office space is sufficient for us to conduct our operations for the foreseeable future in these locations.
	The total depreciation for fixed assets was $8.2 million for the twelve months ended December 31, 2014 (2013 — $14.4 million). Accumulated depreciation as at December 31, 2014 was $97.6 million (2013 — $89.4 million).
	(c)	Variable interest entities
	As at December 31, 2014, the Company had two investments in variable interest entities, Chaspark Maritime Holdings Ltd and Silverton Re Ltd.
	Chaspark Maritime Holdings Ltd. See Note 6, “Investments” of these consolidated financial statements for further information regarding the Company’s investment in Chaspark Maritime Holdings Ltd.
	Silverton Re Ltd. See Note 7, “Variable Interest Entities” of these consolidated financial statements for further information regarding the Company’s investment in Silverton Re Ltd.
	(d)	Contingent liabilities
	In common with the rest of the insurance and reinsurance industry, the Company is also subject to litigation and arbitration in the ordinary course of business. The Company’s Operating Subsidiaries are regularly engaged in the investigation, conduct and defense of disputes, or potential disputes, resulting from questions of insurance or reinsurance coverage or claims activities. Pursuant to insurance and reinsurance arrangements, many of these disputes are resolved by arbitration or other forms of alternative dispute resolution. Such legal proceedings are considered in connection with estimating the Company’s Insurance Reserves — Loss and Loss Adjustment Expenses, as provided on the Company’s consolidated balance sheet.
	In some jurisdictions, noticeably the U.S., a failure to deal with such disputes or potential disputes in an appropriate manner could result in an award of “bad faith” punitive damages against the Company’s Operating Subsidiaries. In accordance with ASC 450-20-50-4b, for (a) reasonably possible losses for which no accrual is made because any of the conditions for accrual in ASC 450-20-25-2 are not met and (b) reasonably possible losses in excess of the amounts accrued pursuant to ASC 450-20-30-1, the Company will provide an estimate of the possible loss or range of possible loss or state that such an estimate cannot be made.
	As of December 31, 2014, based on available information, it was the opinion of the Company’s management that the probability of the ultimate resolution of pending or threatened litigation or arbitrations having a material effect on the Company’s financial condition, results of operations or liquidity would be remote.

Concentration_of_Credit_Risk

Concentration of Credit Risk	12 Months Ended
	Dec. 31, 2014
Risks and Uncertainties [Abstract]
Concentrations of Credit Risk	Concentrations of Credit Risk
	The Company is potentially exposed to concentrations of credit risk in respect of amounts recoverable from reinsurers, investments and cash and cash equivalents, and insurance and reinsurance balances owed by the brokers with whom the Company transacts business.
	The Company’s Reinsurance Credit Committee defines credit risk tolerances in line with the risk appetite set by our Board and they, together with the group’s risk management function, monitor exposures to individual counterparties. Any exceptions are reported to senior management and our Board’s Risk Committee.
	Reinsurance recoverables
	The total amount recoverable by the Company from reinsurers at December 31, 2014 is $350.0 million (2013 — $332.7 million). Of the balance at December 31, 2014, 27.3% of the Company’s reinsurance recoverables are with Lloyd’s of London Syndicates rated A by A.M. Best and A+ by S&P, 18.6% are with Munich Re which is rated A+ by A.M. Best and AA- by S&P and 8.4% are with Arch Re which is rated A+ by A.M. Best and A+ by S&P. These are the Company’s largest exposures to individual reinsurers. The Company has made no provision for doubtful debts from any of its reinsurers as at December 31, 2014.
	Underwriting premium receivables
	The total underwriting premium receivable by the Company at December 31, 2014 was $1,011.7 million (2013 — $999.0 million). Of the balance receivable at December 31, 2014, $2.3 million has been due for settlement for more than one year. The Company assesses the recoverability of premium receivables through a review of policies and the concentration of receivables by broker. A bad debt provision was included of $4.3 million as at December 31, 2014 (2013 — $Nil) for underwriting premiums unlikely to be collected.
	Investments and cash and cash equivalents
	The Company’s investment policies include specific provisions that limit the allowable holdings of a single issue and issuer. At December 31, 2014, there were no investments in any single issuer, other than the U.S. government, U.S. government agencies, U.S. government sponsored enterprises, Canadian government and the U.K. government in excess of 2% of the aggregate investment portfolio.
	Balances owed by brokers
	The Company underwrites a significant amount of its business through brokers and a credit risk exists should any of these brokers be unable to fulfill their contractual obligations in respect of insurance or reinsurance balances due to the Company. The following table shows the largest brokers that the Company transacted business with in the three years ended December 31, 2014 and the proportion of gross written premiums from each of those brokers.
			Twelve Months Ended December 31,
			2014			2013			2012
	Broker		(in percentages)
	Aon Corporation		17.8		%	16.8		%	18.5		%
	Marsh & McLennan Companies, Inc.		15.1			15			15.8
	Willis Group Holdings, Ltd.		13.7			14.4			15.1
	Others(1)		53.4			53.8			50.6
	Total		100		%	100		%	100		%
	Gross written premiums ($ millions)		$	2,902.70		$	2,646.70		$	2,583.30
	________________
	-1	No other individual broker accounted for more than 10% of total gross written premiums.

Reclassifications_from_Accumul

Reclassifications from Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Reclassifications from Accumulated Other Comprehensive Income	Reclassifications from Accumulated Other Comprehensive Income
	The following table sets out the components of the Company’s AOCI that are reclassified into the audited condensed consolidated statement of operations for the twelve months ended months ended December 31, 2014 and 2013:
		Amount Reclassified from AOCI
	Details about the AOCI Components	Twelve Months Ended December 31, 2014			Twelve Months Ended December 31, 2013			Affected Line Item in the
								Consolidated Statement of Operations
		($ in millions)
	Available for sale securities:
	Realized gain on sale of securities	$	13.9		$	24.6		Realized and unrealized investment gains
	Realized (losses) on sale of securities	(6.2		)	(0.5		)	Realized and unrealized investment losses
		7.7			24.1			Income from operations before tax
	Tax on realized gains and (losses) of securities	(0.2		)	(0.7		)	Income tax expense
		$	7.5		$	23.4		Net income
	Foreign currency translation adjustments:
	Foreign currency translation adjustments, before tax	$	—		$	(1.9	)	Net realized and unrealized foreign exchange gains/(losses)
	Tax on foreign currency translation adjustments	—			0.4			Income tax expense
		$	—		$	(1.5	)	Net income
	Amortization of derivatives:
	Amortization of long-term debt associated expenses, before tax	$	—		$	(0.5	)	Interest expense
		$	—		$	(0.5	)	Net income
	Total reclassifications from AOCI to the statement of operations, net of tax	$	7.5		$	21.4		Net income

Credit_Facility_and_Longterm_D

Credit Facility and Long-term Debt	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Credit Facility and Long-term Debt	Credit Facility and Long-term Debt
	Credit Facility. On July 30, 2010, the Company and certain of our direct and indirect subsidiaries (collectively, the “Borrowers”) entered into a three-year revolving credit facility with a syndicate of commercial banks under which it may, subject to the terms of the credit agreements, borrow up to $280.0 million or issue letters of credit with an aggregate value of up to $280.0 million. The facility could have been used by any of the Borrowers (as defined in the agreement) to provide funding for the operating subsidiaries of the Company, to finance the working capital needs of the Company and its subsidiaries and for general corporate purposes of the Company and its subsidiaries. The revolving credit facility further provided for the issuance of collateralized letters of credit. Initial availability under the facility was $280.0 million, and the Company had the option (subject to obtaining commitments from acceptable lenders) to increase the facility by up to $75.0 million. The expiry date of this facility was July 30, 2013.
	On June 12, 2013, the Borrowers entered into an amended and restated credit agreement (the “credit agreement”) with various lenders and Barclays Bank PLC, as administrative agent, which amends and restates the credit agreement dated as of July 30, 2010 among the Company, certain of its subsidiaries, various lenders and Barclays Bank PLC, as administrative agent. The credit facility is used to finance the Company’s working capital needs and those of its subsidiaries, for letters of credit in connection with its insurance and reinsurance businesses and for other general corporate purposes. Initial availability under the credit facility is $200.0 million with the option (subject to obtaining commitments from acceptable lenders) to increase the facility by up to $100.0 million. The facility will expire on June 12, 2017. As of December 31, 2014, no borrowings were outstanding under the credit facility.
	The fees and interest rates on the loans and the fees on the letters of credit payable by the Borrowers under the Credit Agreement are based upon the credit ratings for the Company’s long-term unsecured senior debt by S&P and Moody’s. In addition, the fees for a letter of credit vary based upon whether the applicable Borrower has provided collateral (in the form of cash or qualifying debt securities) to secure its reimbursement obligations with respect to such letter of credit.
	Under the credit facility, the Company must not permit (a) consolidated tangible net worth to be less than approximately $2,428.6 million plus 50% of consolidated net income and 50% of aggregate net cash proceeds from the issuance by the Company of its capital stock, in each case after January 1, 2013, (b) the ratio of its total consolidated debt to the sum of such debt plus our consolidated tangible net worth to exceed 35% or (c) any material insurance subsidiary to have a financial strength rating of less than “B++” from A.M. Best. In addition, the credit facility contains other customary affirmative and negative covenants as well as certain customary events of default, including with respect to a change in control. The various affirmative and negative covenants, include, among others, covenants that, subject to various exceptions, restrict the ability of the Company and its subsidiaries to: incur indebtedness; create or permit liens on assets; engage in mergers or consolidations; dispose of assets; pay dividends or other distributions; purchase or redeem the Company’s equity securities or those of its subsidiaries and make other restricted payments; make certain investments; agree with others to limit the ability of the Company’s subsidiaries to pay dividends or other restricted payments or to make loans or transfer assets to the Company or another of its subsidiaries. In addition, the credit facility has customary events of default, including (subject to certain materiality thresholds and grace periods) payment default, failure to comply with covenants, material inaccuracy of representation or warranty, bankruptcy or insolvency proceedings, change of control and cross-default to other debt agreements.
	On December 12, 2014, Aspen Holdings and the Borrowers entered into a first amendment to update and restate the credit agreement with various lenders and Barclays, which amends the Credit Agreement. Aspen Holdings has recently established, and may establish additional, special purpose entities that have issued or will issue insurance-linked securities to third-party investors (each, an “ILS Entity” and collectively, the “ILS Entities”). Accordingly, the Credit Agreement was amended, among other things, to (i) exclude ILS Entities from the definition of “Subsidiary”, (ii) permit the Borrowers to invest in ILS Entities and (iii) permit the Borrowers to engage in transactions with an ILS Entity.
	Other Credit Facilities. On February 28, 2011, Aspen U.K. and Aspen Bermuda entered into an amendment to the $200.0 million secured letter of credit facility agreement with Barclays Bank PLC dated as of October 6, 2009. The amendment extends the maturity date of the credit facility to December 31, 2013. On February 1, 2013, Aspen U.K. and Aspen Bermuda entered into a further amendment to the secured letter of credit facility to extend the maturity date of the credit facility to January 31, 2015. On August 21, 2013, the commitments were reduced to $100.0 million. All letters of credit issued under the facility are used to support reinsurance obligations of the parties to the agreement and their respective subsidiaries. As at December 31, 2014, $5.0 million collateralized letters of credit were outstanding under this facility (December 31, 2013 — $18.9 million). We did not extend the maturity date of the Barclays Bank PLC secured letter of credit facility and, as a result, it expired on January 31, 2015 and no new letters of credit can be issued under this facility.
	On April 29, 2009, Aspen Bermuda replaced its existing letter of credit facility with Citibank Europe plc dated October 29, 2008 in a maximum aggregate amount of up to $450.0 million with a new letter of credit facility in a maximum aggregate amount of up to $550.0 million. On August 12, 2011, the maximum aggregate amount was increased to $1,050.0 million. On July 30, 2012, Aspen Bermuda and Citibank Europe plc replaced the existing letter of credit facility dated August 12, 2011 in a maximum aggregate amount of up to $1,050.0 million with a new letter of credit facility in a maximum aggregate amount of up to $950.0 million (the “LOC Facility”) comprised of two maturity tranches (Tranche I with a limit of $650.0 million and Tranche II with a limit of $300.0 million) which expired on its own terms on June 30, 2014.
	On June 30, 2014, Aspen Bermuda and Citibank Europe plc replaced the LOC Facility with a new letter of credit facility in a maximum aggregate amount of up to $575.0 million (the “New LOC Facility”). Under the New LOC Facility, which will expire on June 30, 2016, Aspen Bermuda will pay to Citibank Europe plc (a) a letter of credit fee based on the available amounts of each letter of credit and (b) a commitment fee, which varies based upon usage, on the unutilized portion of the New LOC Facility. Aspen Bermuda will also pay interest on the amount drawn by any beneficiary under a credit provided under the New LOC Facility at a rate per annum of LIBOR plus 1% (plus reserve asset costs, if any) from the date of drawing until the date of reimbursement by Aspen Bermuda. The New LOC Facility is used to secure obligations of Aspen Bermuda to its policyholders. In addition to the New LOC Facility, we also use regulatory trusts to secure our obligations to policyholders. As at December 31, 2014, we had $463.6 million of outstanding collateralized letters of credit under this facility compared to $516.8 million at December 31, 2013.
	The terms of a Pledge Agreement between Aspen Bermuda and Citibank Europe plc (pursuant to an Assignment Agreement dated October 11, 2006) dated January 17, 2006, as amended, were also amended on June 30, 2014 to change the types of securities or other assets that are acceptable as collateral under the New LOC Facility. All other agreements relating to Aspen Bermuda’s LOC Facility, which now apply to the New LOC Facility with Citibank Europe plc, as previously filed with the United States Securities and Exchange Commission, remain in full force and effect.
	On December 18, 2014, Aspen Bermuda and Citi Europe entered into an amended and restated pledge agreement (“pledge agreement”) to, among other things, (i) change the types of securities or other assets that qualify as collateral pledged under the pledge agreement, (ii) provide Aspen Bermuda the right to give certain directions or entitlement orders to The Bank of New York Mellon (“BNY Mellon”), as securities intermediary, relating to the collateral without the consent of Citi Europe provided certain conditions are satisfied, (iii) provide Citi Europe, subject to the provisions set forth in the amended and restated account control agreement, dated December 18, 2014 (the “control agreement”), among Aspen Bermuda, Citi Europe and BNY Mellon, with the right and power to exercise exclusive control over the accounts set forth in the control agreement and (iv) provide a schedule of currency margins such that if the collateral is denominated in a currency other than the credit currency the collateral shall be reduced by a specified percentage.
	Long-term Debt. On August 16, 2004, the Company closed its offering of $250.0 million 6.00% coupon Senior Notes due August 15, 2014 (the “2014 Senior Notes”). The net proceeds from the 2014 Senior Notes offering, before offering expenses, were $249.3 million. On December 16, 2013, the Company redeemed the 2014 Senior Notes. The redemption resulted in a realized loss, or make-whole payment, of $9.3 million which is reflected in net realized and unrealized investment gains and losses of the statement of operations and other comprehensive income.
	On December 15, 2010, the Company closed its offering of $250.0 million 6.00% coupon Senior Notes due December 15, 2020. The net proceeds from this offering, before offering expenses, were $247.5 million.
	On November 13, 2013, the Company closed its offering of $300.0 million 4.65% Senior Notes due November 15, 2023 (the “2023 Senior Notes”). The net proceeds from the 2023 Senior Notes offering, before offering expenses, were $299.7 million and a portion of the proceeds was used to redeem the outstanding 2014 Senior Notes. Subject to applicable law, the 2023 Senior Notes will be the senior unsecured obligations of Aspen Holdings and will rank equally in right of payment with all of our other senior unsecured indebtedness from time to time outstanding.
	Subject to certain exceptions, so long as any of the Senior Notes remains outstanding, we have agreed that neither we nor any of our subsidiaries will (i) create a lien on any shares of capital stock of any designated subsidiary (currently Aspen U.K. and Aspen Bermuda, as defined in the Indenture), or (ii) issue, sell, assign, transfer or otherwise dispose of any shares of capital stock of any designated subsidiary. Certain events will constitute an event of default under the Indenture, including default in payment at maturity of any of our other indebtedness in excess of $50.0 million.
	On December 27, 2013, Silverton issued $65.0 million of loan notes (of which $50.0 million was issued to third parties), which will provide quota share support for Aspen Re’s global property catastrophe excess of loss reinsurance business. The operations of Silverton commenced on January 1, 2014. The Company’s maximum loss exposure to Silverton in relation to the 2013 notes issuance is its $20.6 million note holdings as at December 31, 2014 due to mature on September 16, 2016.
	On December 23, 2014, Silverton issued $85.0 million of participating notes (of which $70.0 million was issued to third parties), which will provide quota share support for Aspen Re’s global property catastrophe excess of loss reinsurance business. The operations of Silverton commenced on January 1, 2014. The Company’s maximum loss exposure to Silverton in relation to the 2014 notes issuance is its $15.0 million note holdings as at December 31, 2014 due to mature on September 18, 2017.
	The following table summarizes our contractual obligations under the long-term debts as of December 31, 2014.
		Payments Due By Period
	Contractual Basis	Less than		1-3 years		3-5 years		More than 5 years		Total
		1 year
		($ in millions)
	Long-term Debt Obligations	$	—	$	—	$	—	$	550	$	550
	The Senior Notes obligation disclosed above does not include the $29.0 million annual interest payable associated with the Senior Notes or the loan notes issued by Silverton. For more information on Silverton, please refer to Note 7, “Variable Interest Entities” of these consolidated financial statements.

Unaudited_Quarterly_Financial_

Unaudited Quarterly Financial Data	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Unaudited Quarterly Financial Data	Unaudited Quarterly Financial Data
	The following is a summary of the quarterly financial data for the twelve months ended December 31, 2014, 2013 and 2012.
		2014
		Quarter Ended			Quarter Ended			Quarter Ended September 30			Quarter Ended December 31			Year Ended December 31
		March 31			30-Jun
	Revenues	($ in millions)
	Net earned premium	$	566.5		$	616.2		$	610.4		$	612.2		$	2,405.30
	Net investment income	49.5			46.1			48			46.7			190.3
	Realized and unrealized investment gains/(losses)(1)	17.9			34.6			1.1			(7.3		)	46.3
	Other income	0.6			3.2			1			(0.3		)	4.5
	Total revenues	634.5			700.1			660.5			651.3			2,646.40
	Expenses
	Losses and loss adjustment expenses	288.1			337.1			342.7			339.6			1,307.50
	Amortization of deferred policy acquisition costs	112			108.9			115.5			114.8			451.2
	General, administrative and corporate expenses	95.6			108.8			119.8			121.5			445.7
	Interest on long-term debt	7.4			7.3			7.4			7.4			29.5
	Change in fair value of derivatives	(1.1		)	4.6			5.1			6.6			15.2
	Change in fair value of loan notes issued by variable interest entities	3.4			2.6			8.5			4.1			18.6
	Realized and unrealized investment losses/(gains)(1)	4.3			3.3			21.2			(14.1		)	14.7
	Net realized and unrealized foreign exchange (gains)/losses(1)	(0.1		)	(10.7		)	1.3			3.9			(5.6		)
	Other expenses	0.7			1.2			0.3			(0.5		)	1.7
	Total expenses	510.3			563.1			621.8			583.3			2,278.50
	Income from operations before income tax	124.2			137			38.7			68			367.9
	Income tax (expense)	(3.8		)	(6.2		)	(1.3		)	(0.8		)	(12.1		)
	Net income	$	120.4		$	130.8		$	37.4		$	67.2		$	355.8
	Per Share Data
	Weighted average number of ordinary share and share equivalents
	Basic	65,289,351			65,447,128			65,116,463			62,206,260			64,536,491
	Diluted	66,565,890			66,700,368			66,513,009			63,605,298			65,872,949
	Basic earnings per ordinary share adjusted for preference share dividends	$	1.7		$	1.85		$	0.43		$	0.92		$	4.92
	Diluted earnings per ordinary share adjusted for preference share dividends	$	1.66		$	1.82		$	0.42		$	0.9		$	4.82
	____________________
	(1) Adjusted for a representation of foreign exchange in relation to investment securities from realized and unrealized exchange gains/(losses) to realized and unrealized investment gains/(losses).
		2013
		Quarter Ended			Quarter Ended			Quarter Ended September 30			Quarter Ended December 31			Year Ended December 31
		31-Mar			30-Jun
	Revenues	($ in millions)
	Net earned premium	$	510.9		$	544		$	544.3		$	572.6		$	2,171.80
	Net investment income	48.3			45.9			45			47.2			186.4
	Realized and unrealized investment gains	16.3			14.4			23.6			2.6			56.9
	Other income	1.1			0.9			1.6			4.6			8.2
	Total revenues	576.6			605.2			614.5			627			2,423.30
	Expenses
	Losses and loss adjustment expenses	268.7			333.4			290.2			331.4			1,223.70
	Amortization of deferred policy acquisition costs	104.6			107.2			110.5			99.7			422
	General, administrative and corporate expenses	86.6			87.7			98.9			94.9			368.1
	Interest on long-term debt	7.7			7.8			7.7			9.5			32.7
	Change in fair value of derivatives	4.2			2.9			(6.6		)	(1.8		)	(1.3		)
	Realized and unrealized investment losses/(gains)	1.1			21			5.9			(7.5		)	20.5
	Net realized and unrealized foreign exchange losses/(gains)	5.4			4.1			(2.4		)	6.1			13.2
	Other expenses	0.6			—			—			1.1			1.7
	Total expenses	478.9			564.1			504.2			533.4			2,080.60
	Income from operations before income tax	97.7			41.1			110.3			93.6			342.7
	Income tax (expense)	(5.9		)	(1.0		)	(2.9		)	(3.6		)	(13.4		)
	Net income	$	91.8		$	40.1		$	107.4		$	90		$	329.3
	Per Share Data
	Weighted average number of ordinary share and share equivalents
	Basic	68,854,286			66,191,426			66,716,202			65,593,669			66,872,048
	Diluted	72,452,705			69,291,324			68,561,515			67,051,993			69,417,903
	Basic earnings per ordinary share adjusted for preference share dividends	$	1.21		$	0.38		$	1.47		$	1.23		$	4.29
	Diluted earnings per ordinary share adjusted for preference share dividends	$	1.15		$	0.36		$	1.43		$	1.21		$	4.14
		2012
		Quarter Ended			Quarter Ended			Quarter Ended September 30			Quarter Ended December 31			Year Ended December 31
		31-Mar			30-Jun
Revenues	($ in millions)
Net earned premium	$	495.4		$	513.4		$	516.2		$	558.5		$	2,083.50
Net investment income	52.4			52.8			48.6			51.1			204.9
Realized and unrealized investment gains	10.7			5.4			13.2			6.1			35.4
Other income	0.7			3.5			4.8			(3.4		)	5.6
Total revenues	559.2			575.1			582.8			612.3			2,329.40
Expenses
Losses and loss adjustment expenses	284			262.1			255			437.4			1,238.50
Amortization of deferred policy acquisition costs	96.1			102			103.1			80			381.2
General, administrative and corporate expenses	84.8			83.5			90.7			86.1			345.1
Interest on long-term debt	7.7			7.7			7.8			7.7			30.9
Change in fair value of derivatives	7.5			11.6			4.9			4.4			28.4
Realized and unrealized investment losses	1.7			4.1			2.4			0.4			8.6
Net realized and unrealized foreign exchange losses/(gains)	(7.7		)	12.7			(4.5		)	(3.9		)	(3.4		)
Other expenses	1			0.6			0.3			2.8			4.7
Total expenses	475.1			484.3			459.7			614.9			2,034.00
Income/(loss) from operations before income tax	84.1			90.8			123.1			(2.6		)	295.4
Income tax (expense)/credit	(5.4		)	(6.2		)	(8.0		)	4.6			(15.0		)
Net income	$	78.7		$	84.6		$	115.1		$	2		$	280.4
Per Share Data
Weighted average number of ordinary share and share equivalents
Basic (1)	70,943,997			71,303,855			71,129,102			71,007,079			71,095,856
Diluted (1)	73,832,734			73,845,903			73,397,796			71,007,079			73,689,423
Basic earnings/(loss) per ordinary share adjusted for preference share dividends	$	1.03		$	1.07		$	1.5		$	(0.09	)	$	3.51
Diluted earnings/(loss) per ordinary share adjusted for preference share dividends	$	0.99		$	1.03		$	1.45		$	(0.09	)	$	3.39
____________________
(1)    The basic and diluted number of ordinary shares for the three months ended December 31, 2012 is the same, as the inclusion of dilutive securities in a loss-making period would be anti-dilutive.

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events
	On February 5, 2015, the Company and its Board agreed a new share repurchase authorization program of $500.0 million The total share repurchase authorization, which was effective immediately through February 6, 2017, permits the Company to effect the repurchases from time to time through a combination of transactions, including open market repurchases, privately negotiated transactions and accelerated share repurchase transactions.

Schedule_I_Investments

Schedule I - Investments	12 Months Ended
	Dec. 31, 2014
Summary of Investments, Other than Investments in Related Parties [Abstract]
Schedule I - Investments	SCHEDULE I - INVESTMENTS
	For the Twelve Months Ended December 31, 2014, 2013 and 2012
	The Company’s investments comprise investments in subsidiaries.

Schedule_II_Condensed_Financia

Schedule II - Condensed Financial Information of Registrant	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Schedule II - Condensed Financial Information of Registrant	SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT
	BALANCE SHEETS
	As at December 31, 2014 and 2013
		As at December 31, 2014			As at December 31, 2013
		($ in millions, except per share amounts)
	ASSETS
	Cash and cash equivalents	$	86.8		$	94.2
	Investments in subsidiaries	3,368.50			3,153.70
	Other investments	8.7			48
	Eurobond issued by subsidiary	573.8			571.9
	Long-term debt issued by subsidiaries	35.6			15
	Intercompany funds due from affiliates	43.6			—
	Other assets	—			0.6
	Total assets	$	4,117.00		$	3,883.40
	LIABILITIES
	Accrued expenses and other payables	52.5			14.6
	Intercompany funds due to affiliates	96.1			20.2
	Long-term debt	549.1			549
	Total liabilities	$	697.7		$	583.8
	SHAREHOLDERS’ EQUITY
	Ordinary Shares:
	62,017,368 shares of par value 0.15144558¢ each	$	0.1		$	0.1
	(December 31, 2013 — 65,546,976)
	Preference Shares:
	11,000,000 5.950% shares of par value 0.15144558¢ each	—			—
	(December 31, 2013  — 11,000,000)
	5,327,500 7.401% shares of par value 0.15144558¢ each	—			—
	(December 31, 2013 — 5,327,500)
	6,400,000 7.250% shares of par value 0.15144558¢ each	—			—
	(December 31, 2013 — 6,400,000)
	Additional paid in capital	1,134.30			1,297.40
	Retained earnings	2,050.10			1,783.30
	Non-controlling interest	0.5			(0.3		)
	Accumulated other comprehensive income, net of taxes:
	Unrealized gains on investments	165.4			130.5
	Loss on derivatives	(3.8		)	—
	Gains on foreign currency translation	72.7			88.6
	Total accumulated other comprehensive income	234.3			219.1
	Total shareholders’ equity	3,419.30			3,299.60
	Total liabilities and shareholders’ equity	$	4,117.00		$	3,883.40
	ASPEN INSURANCE HOLDINGS LIMITED
	SCHEDULE II  - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - Continued
	STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME
	For the Twelve Months Ended December 31, 2014, 2013 and 2012
		Twelve Months Ended December 31, 2014			Twelve Months Ended December 31, 2013			Twelve Months Ended December 31, 2012
		($ in millions)
	Operating Activities:
	Equity in net earnings/(loss) of subsidiaries and other investments	$	146.9		$	40.6		$	160.6
	Dividend income	258.5			301.8			100
	Interest income on Eurobond	29.5			44.6			56.5
	Realized investment gains	5.6			(6.3		)	3.2
	Other income	1.9			1.9			3.4
	Total Revenues	442.4			382.6			323.7
	Expenses:
	Operating and administrative expenses	(57.1		)	(20.6		)	(12.4		)
	Interest expense	(29.5		)	(32.7		)	(30.9		)
	Income from operations before income tax	355.8			329.3			280.4
	Income tax	—			—			—
	Net income	355.8			329.3			280.4
	Add: Loss attributable to non-controlling interest	(0.8		)	0.5			0.2
	Net income attributable to Aspen Insurance Holdings Limited ordinary shareholders	355			329.8			280.6
	Other comprehensive income/(loss), net of taxes:
	Change in unrealized losses on investments	34.9			(184.7		)	9.8
	Loss on derivatives reclassified to interest expense	—			0.5			0.2
Net change from current period hedged transactions	(3.8		)
Change in unrealized gains on foreign currency translation	(15.9		)	(24.1		)	(11.5		)
Other comprehensive income	15.2			(208.3		)	(1.5		)
Comprehensive income	$	370.2		$	121.5		$	279.1
ASPEN INSURANCE HOLDINGS LIMITED
SCHEDULE II  - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - Continued
STATEMENTS OF CASH FLOWS
For the Twelve Months Ended December 31, 2014, 2013 and 2012
	Twelve Months Ended December 31, 2014			Twelve Months Ended December 31, 2013			Twelve Months Ended December 31, 2012
	($ in millions)
Cash Flows Provided By Operating Activities:
Net income (excluding equity in net earnings of subsidiaries)	$	209.8		$	288.8		$	119.8
Adjustments:
Share-based compensation expenses	15.1			21.4			17.8
Realized and unrealized losses/(gains)	(5.6		)	6.3			(3.2		)
Loss on derivative reclassified to interest expense	—			0.5			0.2
Change in other receivables	1.1			—			—
Change in other assets	0.6			(2.8		)	1.1
Change in accrued expenses and other payables	37.8			(5.5		)	4
Change in intercompany activities	32.3			104.3			(58.5		)
Net cash generated by operating activities	291.1			413			81.2
Cash Flows Used in Investing Activities:
Investment in subsidiaries	(56.6		)	(605.4		)	—
Investment in long-term debt issued by subsidiary	(15.0		)	(15.0		)	—
Net proceeds from other investments	39.3			—			—
Net cash (used in) investing activities	(32.3		)	(620.4		)	—
Cash Flows Used in Financing Activities:
Proceeds from issuance of ordinary shares, net of issuance costs	2.7			21.2			22.1
Proceeds from issuance of preference shares, net of issuance costs	—			270.6			154.5
PIERS redeemed and cancelled	—			(230.0		)	—
Ordinary share repurchase	(180.9		)	(309.6		)	(62.7		)
Make-whole payment	—			(9.3		)	—
Proceeds from long term debt	—			299.7			—
Debt redemption	—			(250.0		)	—
Ordinary and preference share dividends paid	(88.1		)	(83.3		)	(78.1		)
Proceeds from maturity of Eurobond	—			400			50
Eurobond purchased from subsidiary	—			—			(100.0		)
Net cash (used in)/generated by financing activities	(266.3		)	109.3			(14.2		)
Increase in cash and cash equivalents	(7.4		)	(98.1		)	67
Cash and cash equivalents — beginning of period	94.2			192.3			125.3
Cash and cash equivalents — end of period	$	86.8		$	94.2		$	192.3

Schedule_III_Supplementary_Ins

Schedule III - Supplementary Insurance Information	12 Months Ended
	Dec. 31, 2014
Supplementary Insurance Information [Abstract]
Schedule III - Supplementary Insurance Information	SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION
	For the Twelve Months Ended December 31, 2014, 2013 and 2012
	Supplementary Information
	($ in millions)
	Year Ended December 31, 2014	Deferred		Net		Net		Net		Net		Losses and		Policy		Net		General
		Policy		Reserves		Reserves		Premiums		Investment		LAE		Acquisition		Premium		and
		Acquisition		for Losses		for		Earned		Income		Expenses		Expenses		Written		Administrative
		Costs		and LAE		Unearned												Expenses
						Premiums
	Reinsurance	$	156.4	$	2,493.30	$	680.1	$	1,088.20			$	497.8	$	200	$	1,124.00	$	146.4
	Insurance	142.6		1,907.50		554.9		1,317.10				809.7		251.2		1,391.20		205.5
	Total	$	299	$	4,400.80	$	1,235.00	$	2,405.30	$	190.3	$	1,307.50	$	451.2	$	2,515.20	$	351.9
	Year to date December 31, 2013	Deferred		Net		Net		Net		Net		Losses and		Policy		Net		General
		Policy		Reserves		Reserves		Premiums		Investment		LAE		Acquisition		Premium		and
		Acquisition		for Losses		for		Earned		Income		Expenses		Expenses		Written		Administrative
		Costs		and LAE		Unearned												Expenses
						Premiums
	Reinsurance	$	131.9	$	2,646.80	$	529.9	$	1,073.00			$	481.7	$	207.2	$	1,082.00	$	131
	Insurance	130.3		1,699.40		598.8		1,098.80				742		214.8		1,217.70		185.9
	Total	$	262.2	$	4,346.20	$	1,128.70	$	2,171.80	$	186.4	$	1,223.70	$	422	$	2,299.70	$	316.9
	Year to date December 31, 2012	Deferred		Net		Net		Net		Net		Losses and		Policy		Net		General
		Policy		Reserves		Reserves		Premiums		Investment		LAE		Acquisition		Premium		and
		Acquisition		for Losses		for		Earned		Income		Expenses		Expenses		Written		Administrative
		Costs		and LAE		Unearned												Expenses
						Premiums
	Reinsurance	$	109.4	$	2,811.30	$	469.1	$	1,132.40			$	635.3	$	207.8	$	1,156.90	$	123.9
	Insurance	113.6		1,469.40		529.1		951.1				603.2		173.4		1,090.00		168.2
	Total	$	223	$	4,280.70	$	998.2	$	2,083.50	$	204.9	$	1,238.50	$	381.2	$	2,246.90	$	292.1

Schedule_IV_Reinsurance

Schedule IV - Reinsurance	12 Months Ended
	Dec. 31, 2014
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
Schedule IV - Reinsurance	SCHEDULE IV - REINSURANCE
	For the Twelve Months Ended December 31, 2014, 2013 and 2012
	Premiums Written
		Direct		Assumed			Ceded			Net Amount
		($ in millions)
	2014	$	1,729.90	$	1,172.80		$	(387.5	)	$	2,515.20
	2013	$	1,512.80	$	1,133.90		$	(347.0	)	$	2,299.70
	2012	$	1,355.40	$	1,227.90		$	(336.4	)	$	2,246.90
	Premiums Earned
		Gross Amount		Ceded to Other			Assumed From			Net Amount		Percentage of
				Companies			Other					Amount
							Companies					Assumed
												to Net
		($ in millions. except for percentages)
	2014	$	1,599.00	$	(331.3	)	$	1,137.60		$	2,405.30	47.3	%
	2013	$	1,366.80	$	(321.6	)	$	1,126.60		$	2,171.80	51.9	%
	2012	$	1,177.00	$	(301.5	)	$	1,208.00		$	2,083.50	58	%

Schedule_V_Valuation_and_Quali

Schedule V - Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2014
Valuation and Qualifying Accounts [Abstract]
Schedule V - Valuation and Qualifying Accounts	SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS
	For the Twelve Months Ended December 31, 2014, 2013 and 2012
		Balance at		Charged to			Charged to		Deductions		Balance at
		Beginning  of		Costs and			Other				End of  Year
		Year		Expenses			Accounts
		($ in millions)
	2014
	Premiums receivable from underwriting activities	$	1.1	$	3.2		$	—	$	—	$	4.3
	Reinsurance	$	—	$	—		$	—	$	—	$	—
	2013
	Premiums receivable from underwriting activities	$	0.1	$	1		$	—	$	—	$	1.1
	Reinsurance	$	0.2	$	(0.2	)	$	—	$	—	$	—
	2012
	Premiums receivable from underwriting activities	$	—	$	0.1		$	—	$	—	$	0.1
	Reinsurance	$	0.2	$	—		$	—	$	—	$	0.2

Basis_of_Preparation_and_Signi1

Basis of Preparation and Significant Accounting Policies Basis of Preparation and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates
	Assumptions and estimates made by management have a significant effect on the amounts reported within the consolidated financial statements. The most significant of these relate to the losses and loss adjustment expenses, reinsurance recoverables, gross written premiums and commissions which have not been reported to the Company such as those relating to proportional treaty reinsurance contracts, the fair value of derivatives and the fair value of other investments. All material assumptions and estimates are regularly reviewed and adjustments made as necessary, but actual results could turn out significantly different from those expected when the assumptions or estimates were made.
Accounting for Insurance and Reinsurance Operations	Accounting for Insurance and Reinsurance Operations
	Premiums Earned. Premiums are recognized as revenues proportionately over the coverage period. Premiums earned are recorded in the statements of operations, net of the cost of purchased reinsurance. Premiums written which are not yet recognized as earned premium are recorded in the consolidated balance sheet as unearned premiums, gross of any ceded unearned premiums. Written and earned premiums, and the related costs, include estimates for premiums which have not yet been finally determined. These relate mainly to contractual provisions for the payment of adjustment or additional premiums, premiums payable under proportional treaties and delegated underwriting authorities, and reinstatement premiums.
	Adjustment and additional premiums are premiums charged which relate to experience during the policy term. The proportion of adjustable premiums included in the premium estimates varies between business lines with the largest adjustment premiums being in property and casualty reinsurance, marine, aviation and energy insurance and the smallest in property and casualty insurance.
	Premiums payable under proportional treaty contracts and delegated underwriting authorities are generally not reported to the Company until after the reinsurance coverage is in force. As a result, an estimate of these “pipeline” premiums is recorded. The Company estimates pipeline premiums based on projections of ultimate premium taking into account reported premiums and expected development patterns.
	Reinstatement premiums on assumed excess of loss reinsurance contracts are provided for based on experience under such contracts. Reinstatement premiums are the premiums charged for the restoration of the reinsurance limit of an excess of loss contract to its full amount after payment by the reinsurer of losses as a result of an occurrence and are recognized as revenue in full at the date of loss, triggering the payment of the reinstatement premiums. The payment of reinstatement premiums provides future insurance cover for the remainder of the initial policy term. An allowance for uncollectible premiums is established for possible non-payment of premium receivables, as deemed necessary.
	Outward reinsurance premiums, for when the Company purchases reinsurance or retrocessional coverage, are accounted for using the same accounting methodology as we use for inwards premiums. Premiums payable under reinsurance contracts that operate on a “losses occurring during” basis are accounted for in full over the period of coverage while those arising from “risks attaching during” policies are expensed over the earnings period of the premiums receivable from the reinsured business.
	Losses and Loss Adjustment Expenses. Losses represent the amount paid or expected to be paid to claimants in respect of events that have occurred on or before the balance sheet date. The costs of investigating, resolving and processing these claims are known as loss adjustment expenses (“LAE”). The statement of operations records these losses net of reinsurance, meaning that gross losses and loss adjustment expenses incurred are reduced by the amounts recovered or expected to be recovered under reinsurance contracts.
	Reinsurance. Written premiums, earned premiums, incurred claims, LAE and the amortization of deferred policy acquisition costs all reflect the net effect of assumed and ceded reinsurance transactions. Assumed reinsurance refers to the Company’s acceptance of certain insurance risks that other insurance companies have underwritten. Ceded reinsurance arises from contracts under which other insurance companies agree to share certain risks with the Company.
	Reinsurance accounting is followed when there is significant timing risk, significant underwriting risk and a reasonable possibility of significant loss.
	Reinsurance and retrocession does not isolate the ceding company from its obligations to policyholders. In the event that a reinsurer or retrocessionaire fails to meet its obligations, the ceding company’s obligations remain. The Company regularly evaluates the financial condition of its reinsurers and retrocessionaires and monitors the concentration of credit risk to minimize its exposure to financial loss from reinsurers’ and retrocessionaires’ insolvency. Where it is considered required, appropriate provision is made for balances deemed irrecoverable from reinsurers.
	Reserves. Insurance reserves are established for the total unpaid cost of claims and LAE in respect of events that have occurred by the balance sheet date, including the Company’s estimates of the total cost of claims incurred but not yet reported (“IBNR”). Claim reserves are reduced for estimated amounts of salvage and subrogation recoveries. Estimated amounts recoverable from reinsurers on unpaid losses and LAE are reflected as assets.
	For reported claims, reserves are established on a case-by-case basis within the parameters of coverage provided in the insurance policy or reinsurance agreement. For IBNR claims, reserves are estimated using a number of established actuarial methods to establish a range of estimates from which a management best estimate is selected. Both case and IBNR reserve estimates consider variables such as past loss experience, changes in legislative conditions, changes in judicial interpretation of legal liability, policy coverages and inflation.
	As many of the coverages underwritten involve claims that may not be ultimately settled for many years after they are incurred, subjective judgments as to the ultimate exposure to losses are an integral and necessary component of the loss reserving process. The Company regularly reviews its reserves, using a variety of statistical and actuarial techniques to analyze current claims costs, frequency and severity data, and prevailing economic, social and legal factors. Reserves established in prior periods are adjusted as claim experience develops and new information becomes available. Adjustments to previously estimated reserves are reflected in the financial results of the period in which the adjustments are made.
	The process of estimating required reserves does, by its very nature, involve considerable uncertainty. The level of uncertainty can be influenced by factors such as the existence of coverage with long duration payment patterns and changes in claims handling practices, as well as the factors noted above. Ultimate actual payments for claims and LAE could turn out to be significantly different from the Company’s estimates.
	Amortization of Deferred Policy Acquisition Costs. The costs directly related to writing an insurance policy are referred to as policy acquisition expenses and include commissions, premium taxes and profit commissions. With the exception of profit commissions, these expenses are incurred when a policy is issued, and only the costs directly related to the successful acquisition of new and renewal insurance and reinsurance contracts are deferred and amortized over the same period as the corresponding premiums are recorded as revenues. Profit commissions are estimated based on the related performance criteria evaluated at the balance sheet date, with subsequent changes to those estimates recognized when they occur.
	On a regular basis a recoverability analysis is performed of the deferred policy acquisition costs in relation to the expected recognition of revenues, including anticipated investment income, and adjustments, if any, are reflected as period costs. Should the analysis indicate that the acquisition costs are unrecoverable, further analyses are performed to determine if a reserve is required to provide for losses which may exceed the related unearned premium.
	General, Administrative and Corporate Expenses. These costs represent the expenses incurred in running the business and include, but are not limited to compensation costs for employees, rental costs, IT development and operating costs and professional and consultancy fees. General, policy and administrative costs directly attributable to the successful acquisition of business are deferred and amortized over the same period as the corresponding premiums are recorded as revenues. When reporting the results for its operating segments, the Company includes expenses which are directly attributable to the segment plus an allocation of central administrative costs. Corporate expenses are not allocated to the Company’s operating segments as they typically do not fluctuate with the levels of premium written and are related to the Company’s operations which include group executive costs, group finance costs, group legal and actuarial costs and certain strategic and non-recurring costs.
Accounting for Investments, Cash and Cash Equivalents	Accounting for Investments, Cash and Cash Equivalents
	Fixed Income Securities. The fixed income securities portfolio comprises securities issued by governments and government agencies, corporate bonds, mortgage and other asset-backed securities and bank loans. Investments in fixed income securities are classified as available for sale or trading and are reported at estimated fair value in the consolidated balance sheet. Investment transactions are recorded on the trade date with balances pending settlement reflected in the consolidated balance sheet as a receivable for investments sold or a payable for investments purchased. Fair values are based on quoted market prices and other data provided by third-party pricing services and index providers.
	Equity Securities. The Company’s equity securities comprise U.S. and foreign equity securities. They are classified as either trading or available for sale and are carried on the consolidated balance sheet at estimated fair value. The fair values are based on quoted market prices in active markets from independent pricing sources.
	Short-term Investments. Short-term investments primarily comprise highly liquid debt securities with a maturity greater than three months but less than one year from the date of purchase and are held as part of the investment portfolio of the Company. Short-term investments are classified as either trading or available for sale and carried at estimated fair value.
	Gains and Losses. Realized gains or losses on the sale of investments are determined on the basis of the first in first out cost method and, for fixed income maturity available for sale securities, include adjustments to the cost basis of investments for declines in value that are considered to be other-than-temporary. Unrealized gains and losses represent the difference between the cost, or the cost as adjusted by amortization of any difference between its cost and its redemption value (“amortized cost”), of the security and its fair value at the reporting date and are included within other comprehensive income for securities classified as available for sale and in realized and unrealized investment gains or losses in the consolidated statement of operations for securities classified as trading.
	Other Investments. Other investments represent the Company’s investments that are recorded using the equity method of accounting. Adjustments to the fair value of these investments are made based on the net asset value of the investment.
	Catastrophe Bonds. Investments in catastrophe bonds are classified as trading and are carried on the consolidated balance sheet at estimated fair value.  The fair values are based on independent broker-dealer quotes.
	Cash and Cash Equivalents. Cash and cash equivalents are carried at fair value. Cash and cash equivalents comprise cash on hand, deposits held on call with banks and other short-term highly liquid investments due to mature within three months from the date of purchase and which are subject to insignificant risk of change in fair value.
	Other-than-temporary Impairment of Investments. A security is impaired when its fair value is below its cost or amortized cost. The Company reviews its investment portfolio each quarter on an individual security basis for potential other-than-temporary impairment (“OTTI”) based on criteria including issuer-specific circumstances, credit ratings actions and general macro-economic conditions.
	OTTI is deemed to occur when there is no objective evidence to support recovery in value of a security and a) the Company intends to sell the security or more likely than not will be required to sell the security before recovery of its cost or adjusted amortized cost basis or b) it is deemed probable that the Company will be unable to collect all amounts due according to the contractual terms of the individual security. In the first case, the entire unrealized loss position is taken as an OTTI charge to realized losses in earnings. In the second case, the unrealized loss is separated into the amount related to credit loss and the amount related to all other factors. The OTTI charge related to credit loss is recognized in realized losses in earnings and the amount related to all other factors is recognized in other comprehensive income. The cost basis of the investment is reduced accordingly and no adjustments to the cost basis are made for subsequent recoveries in value.
	Equity securities do not have a maturity date and therefore the Company’s review of these securities utilizes a higher degree of judgment. In its review, the Company considers its ability and intent to hold an impaired equity security for a reasonable period of time to allow for a full recovery. Where an equity security is considered to be other-than-temporarily impaired, the entire charge is recognized in realized losses in earnings. The cost basis of the investment is reduced accordingly and no adjustments to the cost basis are made for subsequent recoveries in value.
	Although the Company reviews each security on a case by case basis, it has also established parameters focusing on the extent and duration of impairment to help identify securities in an unrealized loss position which are other-than-temporarily impaired. For fixed income securities in the available for sale portfolio, the Company considers securities which have been in an unrealized loss position for 12 months or more which currently have a market value of more than 20% below cost should be other-than-temporarily impaired. For equities in the available for sale portfolio, the Company considers declines in value to a level of 20% or more below cost for 12 consecutive months to indicate the security should be other-than-temporarily impaired.
	Investment Income. Investment income includes amounts received and accrued in respect of periodic interest (“coupons”) payable to the Company by the issuer of fixed income securities, equity dividends and interest credited on cash and cash equivalents. It also includes amortization of premium and accretion of discount in respect of fixed income securities. Investment management and custody fees are charged against net investment income reported in the consolidated statement of operations.
Accounting for Derivative Financial Instruments	Accounting for Derivative Financial Instruments
	The Company enters into derivative instruments such as interest rate swaps and forward exchange contracts in order to manage certain market and credit risks. The Company records derivative instruments at fair value on the Company’s balance sheet as either assets or liabilities, depending on their rights and obligations. As at December 31, 2014, the Company held foreign exchange contracts for a notional amount of $135.8 million which qualify for hedge accounting, which are further described in Note 10, “Derivative Contracts” of these consolidated financial statements.
	The accounting for the gain or loss due to the changes in the fair value of these instruments is dependent on whether the derivative qualifies as a hedge. If the derivative does not qualify as a hedge, the gains or losses are reported in earnings when they occur. If the derivative does qualify as a hedge, the accounting treatment varies based on the type of risk being hedged.
Intangible Assets	Intangible Assets
	Intangible assets are held in the consolidated balance sheet at cost less amortization and impairment. Amortization applies on a straight-line basis in respect of assets having a finite estimated useful economic life. The Company performs a qualitative assessment annually to determine whether it is more likely than not that an intangible asset considered to have an indefinite life, other than goodwill, is impaired.
Office Properties and Equipment	Office Properties and Equipment
	Office properties and equipment are carried at cost less accumulated depreciation. These assets are depreciated on a straight-line basis over the estimated useful lives of the assets. Computer equipment and software is depreciated between three and five years with depreciation for software commencing on the date the software is brought into use. Furniture and fittings are depreciated over four years and leasehold improvements are depreciated over the lesser of 15 years or the lease term.
Foreign Currencies Translation	Foreign Currencies Translation
	The reporting currency of the Company is the U.S. Dollar. The functional currencies of the Company’s foreign operations and branches are the currencies in which the majority of their business is transacted.
	Transactions in currencies other than the functional currency are measured in the functional currency of that operation at the exchange rate prevailing at the date of the transaction. Monetary assets and liabilities denominated in non-functional currencies are remeasured at the exchange rate prevailing at the balance sheet date and any resulting foreign exchange gains or losses are reflected in the statement of operations.
	Monetary and non-monetary assets and liabilities of the Company’s functional currency operations are translated into U.S. Dollars at the exchange rate prevailing at the balance sheet date. Income and expenses of these operations are translated at the exchange rate prevailing at the date of the transaction. Unrealized gains or losses arising from the translation of functional currencies are recorded net of tax as a component of other comprehensive income.
Earnings per Ordinary Share	Earnings per Ordinary Share
	Basic earnings per ordinary share is determined by dividing net income available to ordinary shareholders by the weighted average number of ordinary shares outstanding during the period. Net income available to ordinary shareholders is calculated by deducting preference share dividends and net income/(loss) attributable to non-controlling interest from net income after tax for the period. Diluted earnings per share reflect the effect on earnings of the average number of ordinary shares outstanding associated with dilutive securities. The dilutive effect of potentially dilutive securities is calculated using the treasury stock method.
Accounting for Income Tax	Accounting for Income Tax
	Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. When the Company does not believe that, on the basis of available information, it is more likely than not that the deferred tax asset will be fully recovered, it recognizes a valuation allowance against its deferred tax assets to reduce assets to the recoverable amount. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date.
Preference Shares	Preference Shares
	The Company has in issue three classes of perpetual preference shares. The Company has no obligation to pay interest on these securities but they do carry entitlements to dividends payable at the discretion of the Board of Directors. In the event of non-payment of dividends for six consecutive periods, holders of preference shares have director appointment rights. They are therefore accounted for as equity instruments and included within total shareholders’ equity.
Share-Based Employee Compensation	Share-Based Employee Compensation
	The Company operates an employee incentive plan, a director plan and employee share purchase plans, the terms and conditions of which are described in Note 18. The Company applies a fair-value based measurement method including estimates for future forfeitures in the calculation of the compensation costs of stock options, performance shares, phantom shares and restricted share units.
Long-term debt issued by Silverton	Long-term debt issued by Silverton
	The consolidated variable interest entity, Silverton, has issued debt instruments due to mature on September 16, 2016 and September 18, 2017, which are further described in Note 7, “Variable Interest Entities” of these consolidated financial statements. This debt is separately identified on the Company’s balance sheet and the Company has elected to record the debt at fair value. The fair value option was elected due to the potential variability over the ultimate settlement value of the debt instruments.
New Accounting Policies	New Accounting Policies
	New Accounting Policies Adopted in 2014
	In June 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-12, “Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period” to provide guidance on how to account for those share-based payments where performance targets could be achieved after the requisite service period. ASU 2014-12 can be applied prospectively to all awards granted or modified after the effective date or using a modified retrospective approach. The modified retrospective approach would apply to all awards outstanding on or after the beginning of the first annual period presented as of the adoption date. The effective date would be for annual and interim reporting periods beginning after December 15, 2015 with early adoption permitted. The Company adopted this ASU on January 1, 2015 and applied the guidance retrospectively to all of its outstanding awards. None of the Company’s outstanding share-based awards contain performance targets that are achievable after the requisite service period. Therefore, this ASU does not have an impact on its consolidated financial statements.
	In March 2014, the FASB issued ASU 2014-06, “Technical Corrections and Improvements Related to Glossary Terms” which includes technical corrections related to glossary links, glossary term deletions, and glossary term name changes. In addition, ASU 2014-06 includes more substantive, limited-scope improvements to reduce instances of the same term appearing multiple times in the Master Glossary with similar, but not entirely identical, definitions. The amendments apply to all reporting entities within the scope of the affected accounting guidance, do not have transition guidance and are effective upon issuance for both public entities and nonpublic entities. The Company adopted this ASU in the first quarter of 2014 and it does not have a material impact on its consolidated financial statements.
	In March 2013, the FASB issued ASU 2013-05, “Parent’s Accounting for the Cumulative Translation Adjustment Upon Derecognition of Certain Subsidiaries or Groups of Assets Within a Foreign Entity or of an Investment in a Foreign Entity” to standardize the release of the cumulative translation adjustment into net income when a parent either sells a part or all of its investment in its foreign entities or no longer holds a controlling financial interest in a subsidiary. ASU 2013-05 is applied prospectively and is effective for annual reporting periods beginning after December 15, 2013, and interim periods within those years. The Company adopted this ASU in the first quarter of 2014 and this standard does not have a material impact on its consolidated financial statements unless it should choose to sell its investments in its foreign entities or cease to hold a controlling financial interest.
	In February 2013, the FASB issued ASU 2013-04, “Obligations Resulting from Joint and Several Liability Arrangements for Which the Total Amount of the Obligation is Fixed at the Reporting Date” which provides guidance for the recognition, measurement, and disclosure of obligations resulting from joint and several liability arrangements for which the total amount of the obligation within the scope of this guidance is fixed at the reporting date. Examples of obligations include debt arrangements, other contractual obligations and settled litigation and judicial rulings. ASU 2013-04 is applied retrospectively and is effective for annual reporting periods beginning after December 15, 2013, and interim periods within those years. The Company adopted this ASU in the first quarter of 2014 and this standard does not have a material impact on its consolidated financial statements.
	In July 2013, the FASB issued ASU 2013-11, “Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists” which provides guidance on financial statement presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. ASU 2013-11 is effective for fiscal years beginning after December 15, 2013 or interim periods within those years. The Company adopted this ASU in the first quarter of 2014 and it does not have a material impact on its consolidated financial statements as its presentation of unrecognized tax benefits is already as required by ASU 2013-11.
Accounting Pronouncements Not Yet Adopted	Accounting Pronouncements Not Yet Adopted
	In February 2015, the FASB issued ASU 2015-02, “Business Combinations (Topic 810)” which provides guidance for reporting entities that are required to evaluate whether they should consolidate certain legal entities. ASU 2015-02 is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2016. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	In November 2014, the FASB issued ASU 2014-16, “Derivatives and Hedging (Topic 815)” which provides guidance on reducing existing diversity under U.S. GAAP in the accounting for hybrid financial instruments issued in the form of a share. ASU 2014-16 is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2015. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	On August 27, 2014, the FASB issued ASU 2014-15, “Presentation of Financial Statements - Going Concern (Subtopic 204-40)” which provides U.S. GAAP guidance on management’s responsibility to evaluate whether there is substantial doubt about an entity’s ability to continue as a going concern or to provide related footnote disclosures. ASU 2014-15 is effective for fiscal years beginning after December 15, 2016 and interim periods beginning after December 15, 2016. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	On August 8, 2014, the FASB issued ASU 2014-14, “Receivables - Troubled debt restructuring by creditors (Subtopic 310-40)” which reduces diversity in practice by addressing the classification of certain foreclosed mortgage loans held by creditors that are either fully or partially guaranteed under government programs. ASU 2014-14 is effective for fiscal years beginning after December 15, 2014 and interim periods beginning after December 15, 2014. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	On August 5, 2014, the FASB issued ASU 2014-13, “Measuring the Financial Assets and the Financial Liabilities of a Consolidated Collateralized Financing Entity” which addresses the measurement difference in both the initial consolidation and the subsequent measurement of the financial assets and the financial liabilities of a collateralized financing entity. ASU 2014-13 is effective for fiscal years beginning after December 15, 2015 and interim periods beginning after December 15, 2015. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	On June 12, 2014, the FASB issued ASU 2014-11, “Repurchase-to-Maturity Transactions, Repurchase Financings, and Disclosures” which provides guidance on accounting and disclosures for repurchase agreements and similar transactions. ASU 2014-11 is effective for fiscal years beginning after December 15, 2014 and interim periods beginning after December 15, 2015. Early application for a public business entity is prohibited. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	On April 10, 2014, the FASB issued ASU 2014-08, “Presentation of Financial Statements (Topic 205) and Property, Plant, and Equipment (Topic 360)” which improves the definition of discontinued operations by limiting discontinued operations reporting to disposals of components of an entity that represent strategic shifts that have a major effect on an entity’s operations and financial results. The amendments in this ASU require expanded disclosures for discontinued operations. ASU 2014-08 is effective for fiscal years beginning after December 15, 2014 and interim periods therein. Early application is permitted, but only for disposals that have not been reported in financial statements previously issued or available for issuance. The Company does not expect this ASU to have a material impact on its consolidated financial statements as no disposals were made by the Company for the twelve months ended December 31, 2014.

Earnings_per_Ordinary_Share_Ta

Earnings per Ordinary Share (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Computation of Basic and Diluted Earnings per Share	The following table sets forth the computation of basic and diluted earnings per share for the twelve months ended December 31, 2014, 2013, and 2012, respectively:
			Twelve Months Ended December 31,
			2014			2013			2012
		($ in millions, except share and per share amounts)
	Net income/(loss)		$	355.8		$	329.3		$	280.4
	Preference share dividends		(37.8		)	(35.5		)	(31.1		)
	Change in redemption value (1)		—			(7.1		)	—
	Net profit attributable to non-controlling interest		(0.8		)	0.5			0.2
	Basic and diluted net income available to ordinary shareholders		317.2			287.2			249.5
	Ordinary shares:
	Basic weighted average ordinary shares		64,536,491			66,872,048			71,095,856
	Weighted average effect of dilutive securities		1,336,458			2,545,855			2,593,567
	Total diluted weighted average ordinary shares		65,872,949			69,417,903			73,689,423
	Earnings per ordinary share:
	Basic		$	4.92		$	4.29		$	3.51
	Diluted		$	4.82		$	4.14		$	3.39
	_______________
	(1)	The $7.1 million deduction from net income in 2013 is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million. For more information, please refer to Note 15, “Capital Structure” of these consolidated financial statements.
Summary of Declared Dividends	Dividends. On February 5, 2015, the Company’s Board of Directors declared the following quarterly dividends:
			Dividend			Payable on:		Record Date:
	Ordinary shares		$	0.2		March 9, 2015		February 20, 2015
	7.401% Preference Shares		$	0.462563		April 1, 2015		March 15, 2015
	7.250% Preference Shares		$	0.4531		April 1, 2015		March 15, 2015
	5.95% Preference Shares		$	0.3719		April 1, 2015		March 15, 2015

Segment_Reporting_Tables

Segment Reporting (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of Company's business segments	The following tables provide a summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of the Company’s business segments for the twelve months ended December 31, 2014, 2013 and 2012:
			Twelve Months Ended December 31, 2014
			Reinsurance			Insurance			Total
			($ in millions)
	Underwriting Revenues
	Gross written premiums		$	1,172.80		$	1,729.90		$	2,902.70
	Net written premiums		1,124.00			1,391.20			2,515.20
	Gross earned premiums		1,137.60			1,599.00			2,736.60
	Net earned premiums		1,088.20			1,317.10			2,405.30
	Underwriting Expenses
	Losses and loss adjustment expenses		497.8			809.7			1,307.50
	Amortization of deferred policy acquisition costs		200			251.2			451.2
	General and administrative expenses		146.4			205.5			351.9
	Underwriting income		244			50.7			294.7
	Corporate expenses								(93.8		)
	Net investment income								190.3
	Realized and unrealized investment gains								46.3
	Realized and unrealized investment (losses)								(14.7		)
	Change in fair value of loan notes issued by variable interest entities								(18.6		)
	Change in fair value of derivatives								(15.2		)
	Interest expense on long term debt								(29.5		)
	Net realized and unrealized foreign exchange (losses)								5.6
	Other income								4.5
	Other expenses								(1.7		)
	Income before tax								367.9
	Income tax expense								(12.1		)
	Net income								$	355.8
	Net reserves for loss and loss adjustment expenses		$	2,493.30		$	1,907.50		$	4,400.80
	Ratios
	Loss ratio		45.7		%	61.5		%	54.4		%
	Policy acquisition expense ratio		18.4			19.1			18.8
	General and administrative expense ratio (1)		13.5			15.6			18.5
	Expense ratio		31.9			34.7			37.3
	Combined ratio		77.6		%	96.2		%	91.7		%
	_______________
	(1)	The general and administrative expense ratio in the total column includes corporate expenses.
			Twelve Months Ended December 31, 2013
			Reinsurance			Insurance			Total
			( $ in millions)
	Underwriting Revenues
	Gross written premiums		$	1,133.90		$	1,512.80		$	2,646.70
	Net written premiums		1,082.00			1,217.70			2,299.70
	Gross earned premiums		1,126.60			1,366.80			2,493.40
	Net earned premiums		1,073.00			1,098.80			2,171.80
	Underwriting Expenses
	Losses and loss adjustment expenses		481.7			742			1,223.70
	Amortization of deferred policy acquisition costs		207.2			214.8			422
	General and administrative expenses		131			185.9			316.9
	Underwriting income/(loss)		253.1			(43.9		)	209.2
	Corporate expenses								(51.2		)
	Net investment income								186.4
	Realized and unrealized investment gains								56.9
	Realized and unrealized investment (losses)								(20.5		)
	Change in fair value of derivatives								1.3
	Interest expense on long term debt								(32.7		)
	Net realized and unrealized foreign exchange (losses)								(13.2		)
	Other income								8.2
	Other expenses								(1.7		)
	Income before tax								342.7
	Income tax expense								(13.4		)
	Net income								$	329.3
	Net reserves for loss and loss adjustment expenses		$	2,646.80		$	1,699.40		$	4,346.20
	Ratios
	Loss ratio		44.9		%	67.5		%	56.3		%
Policy acquisition expense ratio		19.3			19.5			19.4
General and administrative expense ratio (1)		12.2			16.9			16.9
Expense ratio		31.5			36.4			36.3
Combined ratio		76.4		%	103.9		%	92.6		%
________________
(1)	The general and administrative expense ratio in the total column includes corporate expenses.
		Twelve Months Ended December 31, 2012
		Reinsurance			Insurance			Total
		($ in millions)
Underwriting Revenues
Gross written premiums		$	1,227.90		$	1,355.40		$	2,583.30
Net written premiums		1,156.90			1,090.00			2,246.90
Gross earned premiums		1,208.00			1,177.00			2,385.00
Net earned premiums		1,132.40			951.1			2,083.50
Underwriting Expenses
Losses and loss adjustment expenses		635.3			603.2			1,238.50
Amortization of deferred policy acquisition costs		207.8			173.4			381.2
General and administrative expenses		123.9			168.2			292.1
Underwriting income		165.4			6.3			171.7
Corporate expenses								(53.0		)
Net investment income								204.9
Realized and unrealized investment gains								35.4
Realized and unrealized investment (losses)								(8.6		)
Change in fair value of derivatives								(28.4		)
Interest expense on long term debt								(30.9		)
Net realized and unrealized foreign exchange gains								3.4
Other income								5.6
Other expenses								(4.7		)
Income before tax								295.4
Income tax expense								(15.0		)
Net income								$	280.4
Net reserves for loss and loss adjustment expenses		$	2,811.30		$	1,469.40		$	4,280.70
Ratios
Loss ratio		56.1		%	63.4		%	59.4		%
Policy acquisition expense ratio		18.4			18.2			18.3
General and administrative expense ratio (1)		10.9			17.7			16.6
Expense ratio		29.3			35.9			34.9
Combined ratio		85.4		%	99.3		%	94.3		%
_______________
(1)	The general and administrative expense ratio in the total column includes corporate expenses.
Schedule of the Company's gross written premiums based on the location of the insured risk	Geographical Areas — The following summary presents the Company’s gross written premiums based on the location of the insured risk.
			For the Twelve Months Ended
			31-Dec-14			31-Dec-13			31-Dec-12
			($ in millions)
	Australia/Asia		$	130.1		$	108.4		$	139.3
	Caribbean		19.7			14.4			12.2
	Europe		113.9			112.2			113
	United Kingdom		209.3			166.4			168.6
	United States & Canada(1)		1,357.30			1,179.60			1,106.90
	Worldwide excluding United States (2)		116.2			145.7			151.7
	Worldwide including United States(3)		851.8			827.4			810.8
	Others		104.4			92.6			80.8
	Total		$	2,902.70		$	2,646.70		$	2,583.30
	______________
	(1)	“United States and Canada” comprises individual policies that insure risks specifically in the United States and/or Canada, but not elsewhere.
	(2)	“Worldwide excluding the United States” comprises individual policies that insure risks wherever they may be across the world but specifically excludes the United States.
	(3)	“Worldwide including the United States” comprises individual policies that insure risks wherever they may be across the world but specifically includes the United States.

Investments_Tables

Investments (Tables)	12 Months Ended
	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Summary of Investment Income	Investment Income. The following table summarizes investment income for the twelve months ended December 31, 2014, 2013 and 2012:
		For the Twelve Months Ended
		31-Dec-14			31-Dec-13			31-Dec-12
		($ in millions)
	Fixed income securities — Available for sale	$	151.1		$	155.6		$	181.3
	Fixed income securities — Trading	26.7			20.3			16.5
	Short-term investments — Available for sale	1.4			2.1			3.1
	Fixed term deposits (included in cash and cash equivalents)	3.3			5.3			6.5
	Equity securities — Available for sale	4.1			5.6			6.2
	Equity securities — Trading	13			7			—
	Catastrophe bonds — Trading	1.3			—			—
	Total	200.9			195.9			213.6
	Investment expenses	(10.6		)	(9.5		)	(8.7		)
	Net investment income	$	190.3		$	186.4		$	204.9
Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments	The following table summarizes the net realized and unrealized investment gains and losses recorded in the statement of operations and the change in unrealized gains and losses on investments recorded in other comprehensive income:
		For the Twelve Months Ended
		31-Dec-14			31-Dec-13			31-Dec-12
		($ in millions)
	Available for sale:
	Fixed income securities — gross realized gains	$	10.3		$	18.2		$	7.6
	Fixed income securities — gross realized (losses)	(5.9		)	(7.4		)	(0.4		)
	Equity securities — gross realized gains	12.9			18			4.3
	Equity securities — gross realized (losses)	(0.8		)	(0.3		)	(4.9		)
	Total other-than-temporary impairments	(2.4		)	—			(3.0		)
	Trading:
	Fixed income securities — gross realized gains	7.3			9.5			9.8
	Fixed income securities — gross realized (losses)	(2.5		)	(2.9		)	(0.3		)
	Equity securities — gross realized gains	7.8			2.1			—
	Equity securities — gross realized (losses)	(3.1		)	(0.6		)	—
	Catastrophe bonds — trading	0.4			—			—
	Net change in gross unrealized gains	7.6			6.1			10.5
	Other investments:
	Gross realized and unrealized gains in Cartesian	—			3			3.2
	Other realized losses	—			(9.3		)	—
	Total net realized and unrealized investment gains recorded in the statement of operations	$	31.6		$	36.4		$	26.8
	Change in available for sale net unrealized gains/(losses):
	Fixed income securities	47.7			(209.6		)	2.7
	Equity securities	(10.0		)	11.2			16.4
	Total change in pre-tax available for sale unrealized gains/(losses)	37.7			(198.4		)	19.1
	Change in taxes	(2.8		)	13.7			(9.3		)
	Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income	$	34.9		$	(184.7	)	$	9.8
Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities, Short-Term Investments and Equities	The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income securities, short-term investments and equity securities as at December 31, 2014 and December 31, 2013:
		As at December 31, 2014
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	1,074.20		$	21.5		$	(1.3	)	$	1,094.40
	U.S. agency	190			7.5			(0.1		)	197.4
	Municipal	29.1			2.4			—			31.5
	Corporate	2,244.70			79.9			(5.2		)	2,319.40
	Non-U.S. government-backed corporate	76.8			1.2			—			78
	Foreign government	648.6			17.3			(0.2		)	665.7
	Asset-backed	141.3			2.4			(0.2		)	143.5
	Non-agency commercial mortgage-backed	41.5			3.3			—			44.8
	Agency mortgage-backed	1,016.70			40.8			(2.2		)	1,055.30
	Total fixed income securities — Available for sale	5,462.90			176.3			(9.2		)	5,630.00
	Total short-term investments — Available for sale	258.2			0.1			—			258.3
	Total equity securities — Available for sale	82.6			27.3			—			109.9
	Total	$	5,803.70		$	203.7		$	(9.2	)	$	5,998.20
		As at December 31, 2013
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	1,004.70		$	21.2		$	(5.5	)	$	1,020.40
	U.S. agency	258.5			11.4			(0.8		)	269.1
	Municipal	32.3			0.9			(0.4		)	32.8
	Corporate	2,005.60			82.5			(18.7		)	2,069.40
	Non-U.S. government-backed corporate	83.4			1.4			(0.2		)	84.6
	Foreign government	772			11.2			(4.3		)	778.9
	Asset-backed	119.8			2.8			(0.3		)	122.3
	Non-agency commercial mortgage-backed	56.9			5.7			—			62.6
	Agency mortgage-backed	1,116.70			30.6			(18.3		)	1,129.00
	Total fixed income securities — Available for sale	5,449.90			167.7			(48.5		)	5,569.10
	Total short-term investments — Available for sale	160.3			—			—			160.3
	Total equity securities — Available for sale	112.2			37.8			(0.5		)	149.5
	Total	$	5,722.40		$	205.5		$	(49.0	)	$	5,878.90
Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities	The following tables present the cost or amortized cost, gross unrealized gains and losses, and estimated fair market value of trading investments in fixed income securities, short-term investments, equity securities and catastrophe bonds as at December 31, 2014 and December 31, 2013:
		As at December 31, 2014
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. agency	$	0.2		$	—		$	—		$	0.2
	Municipal	1.1			—			—			1.1
	Corporate	520.9			11.7			(2.8		)	529.8
	Foreign government	137.3			4.3			(1.5		)	140.1
	Asset-backed	14.6			0.1			—			14.7
	Bank loans	86.8			—			(1.7		)	85.1
	Total fixed income securities — Trading	760.9			16.1			(6.0		)	771
	Total short-term investments — Trading	0.2			—			—			0.2
	Total equity securities — Trading	585.2			55.5			(24.7		)	616
	Total catastrophe bonds — Trading	34.4			0.4			—			34.8
	Total	$	1,380.70		$	72		$	(30.7	)	$	1,422.00
		As at December 31, 2013
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	22.7		$	—		$	(0.7	)	$	22
	U.S. agency	0.2			—			—			0.2
	Municipal	1.1			—			—			1.1
	Corporate	469.8			10.3			(5.3		)	474.8
	Foreign government	136.5			1.2			(1.5		)	136.2
	Asset-backed	12.7			0.1			—			12.8
	Bank loans	69.1			0.3			(0.3		)	69.1
	Total fixed income securities — Trading	712.1			11.9			(7.8		)	716.2
	Total equity securities — Trading	281.6			34			(4.7		)	310.9
	Total catastrophe bonds — Trading	5.8			—			—			5.8
	Total	$	999.5		$	45.9		$	(12.5	)	$	1,032.90
Other Investments	The table below shows the Company’s investments in Cartesian and Chaspark for the twelve months ended December 31, 2014 and 2013:
		Cartesian			Chaspark			Total
		($ in millions)
	Opening undistributed value of investment as at January 1, 2014	$	39.3		$	8.7		$	48
	Distribution for the twelve months to December 31, 2014	(39.3		)	—			(39.3		)
	Closing value of investment as at December 31, 2014	$	—		$	8.7		$	8.7
	Opening undistributed value of investment as at January 1, 2013	$	36.3		$	8.7		$	45
	Unrealized gain for the twelve months to December 31, 2013	3			—			3
	Closing value of investment as at December 31, 2013	$	39.3		$	8.7		$	48
Summary of Fixed Maturities	The scheduled maturity distribution of the Company’s available for sale fixed income securities as at December 31, 2014 and December 31, 2013 is set forth below. Actual maturities may differ from contractual maturities because issuers of securities may have the right to call or prepay obligations with or without call or prepayment penalties.
		As at December 31, 2014
		Amortized			Fair Market			Average
		Cost or Cost			Value			S&P Ratings by
								Maturity
		($ in millions)
	Due one year or less	$	590.2		$	594.7		AA
	Due after one year through five years	2,552.00			2,620.80			AA-
	Due after five years through ten years	1,023.50			1,059.90			A+
	Due after ten years	97.7			111			A+
	Subtotal	4,263.40			4,386.40
	Non-agency commercial mortgage-backed	41.5			44.8			AA+
	Agency mortgage-backed	1,016.70			1,055.30			AA+
	Asset-backed	141.3			143.5			AAA
	Total fixed income securities — Available for sale	$	5,462.90		$	5,630.00
		At December 31, 2013
		Amortized			Fair Market			Average
		Cost or Cost			Value			S&P Ratings by
								Maturity
		($ in millions)
	Due one year or less	$	694.8		$	700		AA
	Due after one year through five years	2,376.10			2,438.00			AA-
	Due after five years through ten years	1,003.90			1,032.80			A+
	Due after ten years	81.7			84.4			AA-
	Subtotal	4,156.50			4,255.20
	Non-agency commercial mortgage-backed	56.9			62.6			AA+
	Agency mortgage-backed	1,116.70			1,129.00			AA+
	Asset-backed	119.8			122.3			AAA
	Total fixed income securities — Available for sale	$	5,449.90		$	5,569.10
Aggregate Fair Value and Gross Unrealized Loss by Type of Security	The following tables summarize as at December 31, 2014 and December 31, 2013, by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position for the Company’s available for sale portfolio:
		December 31, 2014
		0-12 months						Over 12 months						Total
		Fair			Gross			Fair			Gross			Fair		Gross			Number of
		Market			Unrealized			Market			Unrealized			Market		Unrealized			Securities
		Value			Losses			Value			Losses			Value		Losses
		($ in millions)
	U.S. government	$	166.1		$	(0.5	)	$	79.4		$	(0.8	)	$	245.5	$	(1.3	)	39
	U.S. agency	25.1			—			4.9			(0.1		)	30		(0.1		)	7
	Municipal	—			—			—			—			—		—			0
	Corporate	459.4			(2.1		)	171.3			(3.1		)	630.7		(5.2		)	274
	Non-U.S. government-backed corporate	0.7			—			—			—			0.7		—			1
	Foreign government	30.4			—			44.2			(0.2		)	74.6		(0.2		)	16
	Asset-backed	43.7			(0.1		)	11.7			(0.1		)	55.4		(0.2		)	43
	Agency mortgage-backed	64.7			(0.3		)	111.7			(1.9		)	176.4		(2.2		)	48
	Total fixed income securities — Available for sale	790.1			(3.0		)	423.2			(6.2		)	1,213.30		(9.2		)	428
	Total short-term investments — Available for sale	4.6			—			—			—			4.6		—			3
	Total	$	794.7		$	(3.0	)	$	423.2		$	(6.2	)	$	1,217.90	$	(9.2	)	431
		December 31, 2013
		0-12 months						Over 12 months						Total
		Fair			Gross			Fair			Gross			Fair		Gross			Number of
		Market			Unrealized			Market			Unrealized			Market		Unrealized			Securities
		Value			Losses			Value			Losses			Value		Losses
		($ in millions)
	U.S. government	$	293.9		$	(5.5	)	$	—		$	—		$	293.9	$	(5.5	)	51
	U.S. agency	72.1			(0.8		)	—			—			72.1		(0.8		)	18
	Municipal	5.5			(0.2		)	1.3			(0.2		)	6.8		(0.4		)	7
	Corporate	695.4			(16.8		)	23.4			(1.9		)	718.8		(18.7		)	372
	Non-U.S. government-backed corporate	21.8			(0.2		)	4.9			—			26.7		(0.2		)	8
	Foreign government	239.7			(4.1		)	8.5			(0.2		)	248.2		(4.3		)	44
	Asset-backed	50.2			(0.3		)	—			—			50.2		(0.3		)	51
	Agency mortgage-backed	491.8			(18.3		)	1.2			—			493		(18.3		)	123
	Total fixed income securities — Available for sale	1,870.40			(46.2		)	39.3			(2.3		)	1,909.70		(48.5		)	674
	Total short-term investments — Available for sale	7.7			—			—			—			7.7		—			6
	Total equity securities — Available for sale	6			(0.4		)	2.3			(0.1		)	8.3		(0.5		)	7
	Total	$	1,884.10		$	(46.6	)	$	41.6		$	(2.4	)	$	1,925.70	$	(49.0	)	687
Analysis of Investment Purchases/Sales and Maturities	The following table summarizes investment purchases, sales and maturities for the twelve months ended December 31, 2014, 2013 and 2012:
		For the Twelve Months Ended
		31-Dec-14			31-Dec-13			31-Dec-12
		($ in millions)
	(Purchases) of fixed income securities — Available for sale	$	(2,005.0	)	$	(2,129.8	)	$	(1,529.6	)
	(Purchases) of fixed income securities — Trading	(653.4		)	(763.4		)	(300.8		)
	(Purchases) of equity securities — Available for sale	—			(2.5		)	(53.1		)
	(Purchases) of equity securities — Trading	(361.0		)	(304.4		)	—
	Proceeds from sales and maturities of fixed income securities — Available for sale	1,909.50			1,872.30			1,416.50
	Proceeds from sales and maturities of fixed income securities — Trading	615.9			486			257.2
	Proceeds from sales of equity securities — Available for sale	40			82.2			46.9
	Proceeds from sales of equity securities — Trading	62.2			24.1			—
	Net change in (payable)/receivable for securities (purchased)/sold	2.8			(0.9		)	1.1
	Net (purchases)/sales of short-term investments — Available for sale	(110.3		)	258.2			(122.7		)
	Net (purchases)/sales of short-term investments — Trading	(0.2		)	2.4			—
	Investment in Chaspark Maritime Holdings Ltd	—			—			(8.7		)
	Net (purchases) of catastrophe bonds — Trading	$	(28.7	)	$	(5.8	)	$	—
	Net sales of other investments	$	39.3		$	—		$	—
	Net (purchases) for the year	$	(488.9	)	$	(481.6	)	$	(293.2	)

Variable_Interest_Entities_Tab

Variable Interest Entities (Tables)	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Schedule of Variable Interest Entities	The tables below show the Company’s liabilities associated with the third-party investments in Silverton for the twelve months ended December 31, 2014 and 2013:
		Twelve Months Ended December 31, 2014
		($ in millions)
	Beginning balance as at January 1, 2014	$	(50.0	)
	Total change in fair value included in the statement of operations	(18.6		)
	Notes issued during the period	(70.0		)
	Balance as at December 31, 2014	$	(138.6	)
		Twelve Months Ended December 31, 2013
		($ in millions)
	Beginning balance as at January 1, 2013	$	—
	Notes issued during the period	(50.0		)
	Balance as at December 31, 2013	$	(50.0	)

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Financial Assets Measured on Recurring Basis	The following tables present the level within the fair value hierarchy at which the Company’s financial assets and liabilities are measured on a recurring basis at December 31, 2014 and December 31, 2013:
			As at December 31, 2014
			Level 1				Level 2				Level 3				Total
			($ in millions)
	Available for sale financial assets, at fair value
	U.S. government		$	1,094.40			$	—			$	—			$	1,094.40
	U.S. agency		—				197.4				—				197.4
	Municipal		—				31.5				—				31.5
	Corporate		—				2,319.40				—				2,319.40
	Non-U.S. government-backed corporate		—				78				—				78
	Foreign government		456.5				209.2				—				665.7
	Asset-backed		—				143.5				—				143.5
	Non-agency commercial mortgage-backed		—				44.8				—				44.8
	Agency mortgage-backed		—				1,055.30				—				1,055.30
	Total fixed income securities available for sale, at fair value		1,550.90				4,079.10				—				5,630.00
	Short-term investments available for sale, at fair value		229.3				29				—				258.3
	Equity investments available for sale, at fair value		109.9				—				—				109.9
	Held for trading financial assets, at fair value
	U.S. agency		—				0.2				—				0.2
	Municipal		—				1.1				—				1.1
	Corporate		—				529.8				—				529.8
	Foreign government		36.1				104				—				140.1
	Asset-backed		—				14.7				—				14.7
	Bank loans		—				85.1				—				85.1
	Catastrophe bonds		—				34.8				—				34.8
	Total fixed income securities trading, at fair value		36.1				769.7				—				805.8
	Short-term investments trading, at fair value		0.1				0.1				—				0.2
	Equity investments trading, at fair value		616				—				—				616
	Other financial assets and liabilities, at fair value
	Derivatives at fair value — foreign exchange contracts		—				7.9				—				7.9
	Derivatives at fair value — interest rate swaps		—				0.1				—				0.1
	Liabilities under derivative contracts — foreign exchange contracts		—				(14.3		)		—				(14.3		)
	Loan notes issued by variable interest entities, at fair value		—				—				(70.7		)		(70.7		)
	Loan notes issued by variable interest entities, at fair value (classified as a current liability)		—				—				(67.9		)		(67.9		)
	Total		$	2,542.30			$	4,871.60			$	(138.6	)		$	7,275.30
	There were no maturities, settlements or transfers between Level 1, Level 2 and Level 3 during the twelve months ended December 31, 2014. There were no assets or liabilities that were classified as Level 3 as at December 31, 2014, except for the loan notes issued by the VIEs. There were no maturities, settlements, gains or transfers in or out of Level 3. For more information, see Note 7, “Variable Interest Entities” of these consolidated financial statements.
		At December 31, 2013
		Level 1				Level 2				Level 3				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	1,020.40			$	—			$	—			$	1,020.40
	U.S. agency	—				269.1				—				269.1
	Municipal	—				32.8				—				32.8
	Corporate	—				2,069.40				—				2,069.40
	Non-U.S. government-backed corporate	—				84.6				—				84.6
	Foreign government	596.2				182.7				—				778.9
	Asset-backed	—				122.3				—				122.3
	Non-agency commercial mortgage-backed	—				62.6				—				62.6
	Agency mortgage-backed	—				1,129.00				—				1,129.00
	Total fixed income securities available for sale, at fair value	1,616.60				3,952.50				—				5,569.10
	Short-term investments available for sale, at fair value	129.5				30.8				—				160.3
	Equity investments available for sale, at fair value	149.5				—				—				149.5
	Held for trading financial assets, at fair value
	U.S. government	22				—				—				22
	U.S. agency	—				0.2				—				0.2
	Municipal	—				1.1				—				1.1
	Corporate	—				474.8				—				474.8
	Foreign government	44.2				92				—				136.2
	Asset-backed	—				12.8				—				12.8
	Bank loans	—				69.1				—				69.1
	Total fixed income securities trading, at fair value	66.2				650				—				716.2
	Equity investments trading, at fair value	310.9				—				—				310.9
	Catastrophe bonds trading, at fair value	—				5.8				—				5.8
	Other financial assets and liabilities, at fair value
Derivatives at fair value — foreign exchange contracts	—				5.9				—				5.9
Derivatives at fair value — interest rate swaps	—				1.1				—				1.1
Liabilities under derivative contracts — foreign exchange contracts	—				(2.9		)		—				(2.9		)
Loan notes issued by variable interest entities, at fair value	—				—				(50.0		)		(50.0		)
Total	$	2,272.70			$	4,643.20			$	(50.0	)		$	6,865.90
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation	The following table presents a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Level 3 inputs for the twelve months ended December 31, 2014:
	Reconciliation of Liabilities Using Level 3 Inputs			Twelve Months Ended December 31, 2014				Twelve Months Ended December 31, 2013
				($ in millions)
	Balance at the beginning of the period			$	(50.0	)		$	—
	Total change in fair value included in the statement of operations			(18.6		)		—
	Notes issued during period			(70.0		)		(50.0		)
	Balance at the end of the period			$	(138.6	)		$	(50.0	)
Pricing Sources Used in Pricing Fixed Income Investments	Pricing sources used in pricing fixed income investments at December 31, 2014 and December 31, 2013 were as follows:
			As at December 31, 2014			At December 31, 2013
	Index providers		84	%		85	%
	Pricing services		11			12
	Broker-dealers		5			3
	Total		100	%		100	%
Summary of Securities Priced Using Pricing Information from Index Providers	A summary of securities priced using pricing information from index providers at December 31, 2014 and December 31, 2013 is provided below:
			As at December 31, 2014						At December 31, 2013
			Fair Market				% of Total		Fair Market				% of Total
			Value Determined				Fair Value by		Value Determined				Fair Value by
			using Prices from				Security Type		using Prices from				Security Type
			Index Providers						Index Providers
			($ in millions, except for percentages)
	U.S. government		$	1,044.40			95%		$	998.5			96%
	U.S. agency		186.9				95%		255.3				95%
	Municipal		13.7				42%		14.5				43%
	Corporate		2,731.10				96%		2,400.80				94%
	Non-U.S. government-backed corporate		48.7				62%		55.9				66%
	Foreign government		504.4				63%		605.8				66%
	Asset-backed		140.5				89%		130.6				97%
	Non-agency commercial mortgage-backed		44.8				100%		61				97%
	Agency mortgage-backed		680.6				64%		830.6				74%
	Total fixed income securities		$	5,395.10			84%		$	5,353.00			85%
	Equities		$	725.9			100%		$	460.4			100%
	Total fixed income securities and equity investments		$	6,121.00			86%		$	5,813.40			86%
Fair Value Inputs, Assets, Quantitative Information	The observable and unobservable inputs used to determine the fair value of the 2015-1 and 2014-1 loan notes as at December 31, 2014 and 2013 are presented in the tables below:
	At December 31, 2014		Fair Value				Valuation Method		Observable (O) and		Low				High
			Level 3						Unobservable (U) inputs
		($ in millions)									($ in millions)
	Loan notes held by third parties		$	138.6			Internal Valuation Model		Gross premiums written (O)		$	—			$	40
									Reserve for losses (U)		$	—			$	4.6
									Contract period (O)		N/A				365 days
									Initial value of issuance (O)		$	120			$	120
	At December 31, 2013		Fair Value				Valuation Method		Observable (O) and		Low				High
			Level 3						Unobservable (U) inputs
		($ in millions)									($ in millions)
	Loan notes held by third parties		$	50			Internal Valuation Model		Gross premiums written (O)		$	—			$	40
									Reserve for losses (U)		$	—			$	1.4
									Contract period (O)		N/A				365 days
									Initial value of issuance (O)		$	50			$	50
	The observable and unobservable inputs represent the potential variation around the inputs used in the valuation model. The 2015-1 loan notes were not on risk as at December 31, 2014 and as no gross premiums were written at that date the minimum value of gross premiums written value was $Nil. The high premium value represents the actual premiums assumed by Silverton for the 2014-1 loan notes. Reserves for losses for the 2015-1 loan notes were $Nil as no contracts were written as at December 31, 2014, the high value is the estimate of losses assumed by the 2014-1 Loan notes. The contract period is defined in the Silverton loan agreements and the initial value represents the funds received from third parties.

Reinsurance_Tables

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Summary of Assumed and Ceded Reinsurance on Premiums Written, Premiums Earned and Insurance Losses and Loss Adjustment Expenses	The effect of assumed and ceded reinsurance on premiums written, premiums earned and insurance losses and loss adjustment expenses is as follows:
		Twelve Months Ended December 31,
		2014			2013			2012
		($ in millions)
	Premiums written:
	Direct	$	1,729.90		$	1,512.80		$	1,355.40
	Assumed	1,172.80			1,133.90			1,227.90
	Ceded	(387.5		)	(347.0		)	(336.4		)
	Net premiums written	$	2,515.20		$	2,299.70		$	2,246.90
	Premiums earned:
	Direct	$	1,599.00		$	1,366.80		$	1,177.00
	Assumed	1,137.60			1,126.60			1,208.00
	Ceded	(331.3		)	(321.6		)	(301.5		)
	Net premiums earned	$	2,405.30		$	2,171.80		$	2,083.50
	Insurance losses and loss adjustment expenses:
	Direct	$	908.2		$	829.4		$	763
	Assumed	496.9			459.4			650.1
	Ceded	(97.6		)	(65.1		)	(174.6		)
	Net insurance losses and loss adjustment expenses	$	1,307.50		$	1,223.70		$	1,238.50

Derivative_Contracts_Tables

Derivative Contracts (Tables)	12 Months Ended
	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Fair Value of Derivative Instruments	The following table summarizes information on the location and amounts of derivative fair values on the consolidated balance sheet as at December 31, 2014 and 2013:
				As at December 31, 2014							At December 31, 2013
	Derivatives Not Designated as Hedging Instruments		Balance Sheet Location	Notional			Fair				Notional		Fair
	Under ASC 815			Amount			Value				Amount		Value
				($ in millions)							($ in millions)
	Interest Rate Swaps		Derivatives at Fair Value	$	951.3		$	0.1		(1)	$	1,000.00	$	1.1		(1)
	Foreign Exchange Contracts		Derivatives at Fair Value	$	165.8		$	7.9			$	224.4	$	5.9
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	237.6		$	(10.5	)		$	57.5	$	(2.9	)
	(1)	Net of $22.3 million of cash collateral provided to counterparties, Goldman Sachs International ($451.3 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2013 — $34.3 million).
				As at December 31, 2014							At December 31, 2013
	Derivatives Designated as Hedging Instruments Under ASC 815		Balance Sheet Location	Notional			Fair				Notional		Fair
				Amount			Value				Amount		Value
				($ in millions)							($ in millions)
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	135.8		$	(3.8	)	(1)	$	—	$	—		(1)
	(1)	Net of $Nil cash collateral (December 31, 2013 — $Nil).
Gain/(Loss) Recognized in Income on Derivative	The following tables provide the unrealized and realized gains/(losses) recorded in the statement of operations for the twelve months ended December 31, 2014 and 2013:
				Amount of Income/(Loss)
				Recognized in the Statement
				of  Operations and Other Comprehensive Income
				Twelve Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	31-Dec-14			31-Dec-13
	ASC 815		Statement of Operations and Other Comprehensive Income
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(7.7	)	$	(1.3	)
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	(7.2	)	$	2.6
				Amount of Income/(Loss)
				Recognized in the Statement
				of  Operations and Other Comprehensive Income
				Twelve Months Ended
	Derivatives Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	31-Dec-14			31-Dec-13
	ASC 815		Statement of Operations and Other Comprehensive Income
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(0.3	)	$	—
	Foreign Exchange Contracts		Net change from current period hedged transactions	$	(3.8	)	$	—

Deferred_Policy_Acquisition_Co1

Deferred Policy Acquisition Costs (Tables)	12 Months Ended
	Dec. 31, 2014
Insurance [Abstract]
Reconciliation of beginning and ending deferred policy acquisition costs	The following table represents a reconciliation of beginning and ending deferred policy acquisition costs for the twelve months ended December 31, 2014 and 2013:
			Twelve Months Ended			Twelve Months Ended December 31, 2013
			December 31, 2014
			($ in millions)
	Balance at the beginning of the period		$	262.2		$	223
		Acquisition costs deferred	488			461.2
		Amortization of deferred policy acquisition costs	(451.2		)	(422.0		)
	Balance at the end of the period		$	299		$	262.2

Reserves_for_Losses_and_Adjust1

Reserves for Losses and Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2014
Insurance [Abstract]
Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses (LAE) Reserves	The following table represents a reconciliation of beginning and ending consolidated loss and LAE reserves for the twelve months ended December 31, 2014, 2013 and 2012:
		As at December 31,
		2014			2013			2012
		($ in millions)
	Provision for losses and LAE at the start of the year	$	4,678.90		$	4,779.70		$	4,525.20
	Less reinsurance recoverable	(332.7		)	(499.0		)	(426.6		)
	Net loss and LAE at the start of the year	4,346.20			4,280.70			4,098.60
	Net loss and LAE expenses (disposed)	(24.2		)	(34.6		)	(9.0		)
	Provision for losses and LAE for claims incurred:
	Current year	1,411.60			1,331.40			1,375.90
	Prior years	(104.1		)	(107.7		)	(137.4		)
	Total incurred	1,307.50			1,223.70			1,238.50
	Losses and LAE payments for claims incurred:
	Current year	(112.1		)	(172.8		)	(244.3		)
	Prior years	(995.6		)	(912.3		)	(835.7		)
	Total paid	(1,107.7		)	(1,085.1		)	(1,080.0		)
	Foreign exchange (gains)/losses	(121.0		)	(38.5		)	32.6
	Net losses and LAE reserves at the end of the year	4,400.80			4,346.20			4,280.70
	Plus reinsurance recoverable on unpaid losses at the end of the year	350			332.7			499
	Provision for losses and LAE at the end of the year	$	4,750.80		$	4,678.90		$	4,779.70

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Summary of Total Income Tax	Total income tax for the twelve months ended December 31, 2014, 2013 and 2012 is allocated as follows:
		Twelve Months Ended December 31,
		2014			2013			2012
		($ in millions)
	Income tax expense on income	$	12.1		$	13.4		$	15
	Income tax (benefit)/expense on other comprehensive income	(5.1		)	(13.7		)	9.3
	Income tax (benefit) charged directly to shareholders’ equity	(1.2		)	(1.5		)	(2.4		)
	Total income tax expense/(benefit)	$	5.8		$	(1.8	)	$	21.9
Schedule of Income Tax by Taxing Authority	Income/(loss) before tax and income tax expense/(benefit) attributable to that income/(loss) consists of:
		Twelve Months Ended December 31, 2014
		(Loss)/income			Current			Deferred			Total
		before tax			income tax			income tax			income tax
					expense			(benefit)			expense
		($ in millions)
	U.S.	$	(24.1	)	$	0.8		$	—		$	0.8
	Non-U.S.	392			22.6			(11.3		)	11.3
	Total	$	367.9		$	23.4		$	(11.3	)	$	12.1
		Twelve Months Ended December 31, 2013
		(Loss)/income			Current			Deferred			Total
		before tax			income tax			income tax			income tax
					expense			expense			expense
		($ in millions)
	U.S.	$	(12.2	)	$	3.8		$	—		$	3.8
	Non-U.S.	354.9			5.9			3.7			9.6
	Total	$	342.7		$	9.7		$	3.7		$	13.4
		Twelve Months Ended December 31, 2012
		(Loss)/income			Current			Deferred			Total
		before tax			income tax			income tax			income tax
					expense			(benefit)			expense
		($ in millions)
	U.S.	$	(58.3	)	$	4.4		$	—		$	4.4
	Non-U.S.	353.7			18.4			(7.8		)	10.6
	Total	$	295.4		$	22.8		$	(7.8	)	$	15
Income Tax Reconciliation	The weighted average expected tax provision has been calculated using the pre-tax accounting income/loss in each jurisdiction multiplied by that jurisdiction’s applicable statutory tax rate. The expected tax rate in Bermuda, the Company’s country of domicile, is zero. Application of the weighted average tax rate for operations in other jurisdictions produces an expected tax (benefit)/expense as shown in the table below.
	The reconciliation between the provision for income taxes and the expected tax at the weighted average rate provision is provided below:
		Twelve Months Ended December 31,
		2014			2013			2012
	Income Tax Reconciliation	($ in millions)
	Expected tax (benefit)/expense at weighted average rate	$	(7.3	)	$	(7.5	)	$	(14.4	)
	Prior year adjustments (1)	(0.6		)	(4.2		)	(4.9		)
	Valuation provision on U.S. deferred tax assets	12.7			15.1			26.7
	Uncertain tax positions	5.3			8.5			9.6
	Non-utilizable foreign tax credits	—			2.6			—
	Disallowable expenses	1.8			1.6			1.2
	Other non-taxable items	—			(0.2		)	(2.4		)
	Impact of changes in statutory tax rates	0.2			(2.5		)	(0.8		)
	Total income tax expense	$	12.1		$	13.4		$	15
	________________
	(1) The submission dates for filing income tax returns for the Company’s U.S. and U.K. operating subsidiaries are after the submission date of the Company’s Annual Report on Form 10-K. The final tax liabilities may differ from the estimated tax provisions included in the Annual Report on Form 10-K and may result in prior year adjustments being reported.
Schedule of Unrecognized Tax Benefits	During the twelve months ended December 31, 2014, the Company did not recognize or accrue any costs in respect of interest or penalties relating to uncertain tax positions (December 31, 2013 — $Nil).
		Twelve Months Ended December 31,
		2014			2013
		($ in millions)
	Unrecognized tax benefits balance at January 1	$	23.9		$	15.4
	Gross increases/(decreases) for tax positions of prior years	5.3			8.5
	Gross increases/(decreases) for tax positions of current year	—			—
	Unrecognized tax benefits balance at December 31	$	29.2		$	23.9

Deferred_Taxation_Tables

Deferred Taxation (Tables)	12 Months Ended
	Dec. 31, 2014
Deferred Income Taxes and Other Assets [Abstract]
Tax Effects of Deferred Tax Assets and Deferred Tax Liabilities	The tax effects of temporary differences that give rise to deferred tax assets and deferred tax liabilities are presented in the following table:
		As at December 31,
		2014			2013
		($ in millions)
	Deferred tax assets:
	Share options	$	2.8		$	3.9
	Operating loss carry forwards	94.4			80.9
	Loss reserves	1.8			1.7
	Unrealized losses on investments	—			13.3
	Accrued expenses	11.2			8.4
	Foreign tax credits	13			7.5
	Unearned premiums	4.9			6.5
	Timing differences on fixed assets	8.7			5.6
	Other temporary differences	6.7			8.5
	Total gross deferred tax assets	143.5			136.3
	Less valuation allowance	(106.5		)	(93.8		)
	Net deferred tax assets	$	37		$	42.5
	Deferred tax liabilities:
	Equalization provision reserves	$	(32.1	)	$	(34.1	)
	Intangible assets (other)	(2.2		)	(1.8		)
	Unrealized (gains) on investments	(1.5		)	(0.2		)
	Deferred policy acquisition costs	(2.0		)	(4.2		)
	Other temporary differences	(2.3		)	(0.6		)
	Total gross deferred tax (liabilities)	(40.1		)	(40.9		)
	Net deferred tax (liability)/asset	$	(3.1	)	$	1.6

Capital_Structure_Tables

Capital Structure (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Summary of Authorized and Issued Share Capital	The following table provides a summary of the Company’s authorized and issued share capital at December 31, 2014 and 2013:
		As at December 31, 2014				At December 31, 2013
		Number		$ in		Number	$ in
				Thousands			Thousands
	Authorized share capital:
	Ordinary Shares 0.15144558¢ per share	969,629,030		1,469		969,629,030	1,469
	Non-Voting Shares 0.15144558¢ per share	6,787,880		10		6,787,880	10
	Preference Shares 0.15144558¢ per share	100,000,000		152		100,000,000	152
	Total authorized share capital			1,631			1,631
	Issued share capital:
	Issued ordinary shares of 0.15144558¢ per share	62,017,368		94		65,546,976	99
	Issued 7.401% Preference Shares of 0.15144558¢ each with a liquidation preference of $25 per share	5,327,500		8		5,327,500	8
	Issued 7.250% Preference Shares of 0.15144558¢ each with a liquidation preference of $25 per share	6,400,000		10		6,400,000	10
	Issued 5.95% Preference Shares of 0.15144558¢ each with a liquidation preference of $25 per share	11,000,000		17		11,000,000	17
	Total issued share capital			129			134
Summary of Ordinary Shares	The following table summarizes transactions in the Company’s ordinary shares during the years ended December 31, 2014 and 2013:
		Number of Ordinary Shares
		2014		2013
	Ordinary shares in issue at the beginning of the year	65,546,976		70,753,723
	Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or 2008 share purchase plan	756,676		1,374,567
	Ordinary shares issued to non-employee directors	3,573		44,000
	Ordinary shares repurchased	(4,289,857	)	(8,461,174	)
	Ordinary shares issued in respect of the redemption of the PIERS	—		1,835,860
	Ordinary shares in issue at the end of the year	62,017,368		65,546,976

Statutory_Requirements_and_Div1

Statutory Requirements and Dividends Restrictions (Tables)	12 Months Ended
	Dec. 31, 2014
Text Block [Abstract]
Summary of Statutory Requirements and Dividends Restrictions	Actual and required statutory capital and surplus for the principal operating subsidiaries of the Company, excluding its Lloyd’s syndicate, as at December 31, 2014 and December 31, 2013 were:
		As at December 31, 2014
		U.S.		Bermuda		U.K.
		($ in millions)
	Required statutory capital and surplus	$	56.9	$	1,097.60	$	202.2
	Statutory capital and surplus	$	394.1	$	2,052.30	$	989.8
		As at December 31, 2013
		U.S.		Bermuda		U.K.
		($ in millions)
	Required statutory capital and surplus	$	40.9	$	1,068.20	$	218.8
	Statutory capital and surplus	$	375.3	$	1,907.40	$	1,005.10

ShareBased_Payments_Tables

Share-Based Payments (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Number of Names Options Exercised and Number of Shares Issued	The table below shows the number of Names’ options exercised and the number of shares issued since our initial public offering:
			Options Granted			Options Exercised			Ordinary Shares Issued
	2002		3,006,760			—			—
	2003		—			440,144			152,583
	2004		—			856,218			135,321
	2005		—			303,321			56,982
	2006		—			34,155			3,757
	2007		—			66,759			7,381
	2008		—			20,641			3,369
	2009		—			9,342			3,056
	2010		—			149,895			49,538
	2011		—			761,037			255,504
	2012		—			365,248			116,510
	Total as at December 31, 2013 and 2014		3,006,760			3,006,760			784,001
Information About Employee Options Outstanding to Purchase Ordinary Shares	The following table summarizes information about employee options outstanding to purchase ordinary shares at December 31, 2014.
			As at December 31, 2014
			Options			Options			Options			Outstanding			Exercise		Weighted Average	Remaining
			Granted			Forfeited			Exercised			and			Price		Fair Value at	Contractual
												Exercisable					Grant Date	Time
	Option Holder
	2003 Option grants		3,884,030			712,906			3,171,124			—			$16.20		$5.31	expired
	2004 Option grants		500,113			276,082			224,031			—			$24.44		$5.74	expired
	2006 Option grants February 16		1,072,490			450,567			483,347			138,576			$23.65		$6.99	1 year, 2 months
	2007 Option grants May 4 (1)		607,635			152,276			376,241			79,118			$27.28		$6.14	expired
	________________
	(1)	In the case of Mr. O’Kane, the expiration date for the 2007 options were extended for one year to May 4, 2015.
Per share Weighted Average Fair Value and Related Underlying Assumptions Using Modified Black-Scholes Option Pricing Model by Date of Grant	The following table shows the per share weighted average fair value and the related underlying assumptions using a modified Black-Scholes option pricing model by date of grant:
			Grant Date
			October 22, 2007				May 4,				August 4,				February 16,			December 23,			August 20,
							2007				2006				2006			2004			2003(1)
	Per share weighted average fair value		$	5.76			$	6.14			$	4.41			$	6.99		$	5.74		$	5.31
	Risk free interest rate		4.09		%		4.55		%		5.06		%		4.66		%	3.57		%	4.7		%
	Dividend yield		2.1		%		2.2		%		2.6		%		2.7		%	0.5		%	0.6		%
	Expected life		5 years				5 years				5 years				5 years			5 years			7 years
	Share price volatility		20.28		%		23.76		%		19.33		%		35.12		%	19.68		%	—
	Foreign currency volatility		—				—				—				—			—			9.4		%
	________________
	(1)	The 2003 options had a price volatility of zero. The minimum value method was utilized because the Company was unlisted on the date that the options were issued. Foreign currency volatility of 9.4% was applied as the exercise price was initially in British Pounds and the share price of the Company is in U.S. Dollars.
Summary of Restricted Share Units	The following table summarizes information about RSUs as at December 31, 2014:
				As at December 31, 2014
				Restricted Share Units
	RSU Holder			Amount			Amount			Amount			Amount
				Granted			Vested			Forfeited			Outstanding
	2004 - 2011 Grants			965,911			897,080			68,831			—
	2012 Grants			350,899			208,744			47,533			94,622
	2013 Grants			307,441			100,012			27,511			179,918
	2014 Grants			259,640			—			15,068			244,572
	Total			1,883,891			1,205,836			158,943			519,112
Summary of Performance Shares by Year of Grants	The following table summarizes information about performance shares as at December 31, 2014:
			As at December 31, 2014
			Performance Share Awards
			Amount			Amount			Amount			Amount Outstanding
			Granted			Vested			Forfeited
	2004 - 2011 Grants (1)		4,194,638			2,220,442			1,974,196			—
	2012 Grants		344,131			241,367			102,764			—
	2013 Grants (2)		250,066			127,153			43,728			79,185
	2014 Grants (2)		315,389			122,056			4,100			189,233
	Total		5,104,224			2,711,018			2,124,788			268,418
	______________________
	(1)	The amounts vested and forfeited in respect of the 2004 - 2011 performance share awards have been updated to reflect employees leaving after the financial reporting date but before the final vesting date.
	(2)	These balances could increase depending on future performance.
Summary of Performance Shares Which are Vested, Roe Achieved and Vested
	2014 Performance Shares
	Year				Split			Increase in BVPS			Banked
	2014				33.3	%		13.3	%		43	%
	2015				33.3	%		NA			NA
	2016				33.3	%		NA			NA
	Total				100	%					43	%
	2011 Performance Shares
	Year				Split			ROE			Banked
	2011				33.3	%		(5.3	)%		—	%
	2012				33.3	%		10	%		33.3	%
	2013				33.3	%		11.7	%		39	%
	Total				100	%					72.3	%
	2012 Performance Shares
	Year				Split			Increase in BVPS			Banked
	2012				33.3	%		8.1	%		21.9	%
	2013				33.3	%		6.2	%		10.5	%
	2014				33.3	%		13.3	%		43	%
	Total				100	%					75.4	%
	2013 Performance Shares
	Year				Split			Increase in BVPS			Banked
	2013				33.3	%		6.2	%		10.5	%
	2014				33.3	%		13.3	%		43	%
	2015				33.3	%		NA			NA
	Total				100	%					53.5	%
Summary of Performance Share Activity Under Aspen's 2003 Share Incentive Plan	A summary of performance share activity under Aspen’s 2003 and 2013 Share Incentive Plans for the twelve months ended December 31, 2014 is presented below:
			Twelve Months Ended December 31, 2014
			Number of			Weighted Average
			Shares			Grant Date
						Fair Value
	Outstanding performance share awards, beginning of period		597,479			$27.15
	Granted		315,389			$38.46
	Earned		(373,935	)		$30.12
	Forfeited		(270,515	)		$27.61
	Outstanding performance share awards, end of period		268,418			$25.35
Summary of Phantom Shares Which Committee Approved
	2013 Phantom Shares
	Year				Split			Increase in BVPS			Banked
	2013				33.3	%		6.2	%		10.5	%
	2014				33.3	%		13.3	%		43	%
	2015				33.3	%		NA			NA
	Total				100	%					53.5	%
	2012 Phantom Shares
	Year				Split			Increase in BVPS			Banked
	2012				33.3	%		8.1	%		21.9	%
	2013				33.3	%		6.2	%		10.5	%
	2014				33.3	%		13.3	%		43	%
	Total				100	%					75.4	%
	2014 Phantom Shares
	Year				Split			Increase in BVPS			Banked
	2014				33.3	%		13.3	%		43	%
	2015				33.3	%		NA			NA
	2016				33.3	%		NA			NA
	Total				100	%					43	%
Amounts for Employee Options Granted were Estimated on Date of Grant Using Modified Black-Scholes Option Pricing Model	The fair value of the employee options granted was estimated on the date of grant using a modified Black-Scholes option pricing model under the following assumptions:
	Grant Date		Per share		Risk free			Dividend yield			Expected life		Share price
			weighted		interest rate								volatility
			average
			fair value
			($)		(%)			(%)			(in years)		(%)
	November 4, 2008		$3.18		0.48	%		2.7	%		3		68	%
	December 4, 2008		2.87		(0.41	)		3.16			2		102
	November 23, 2009		3.76		0.01			2.28			3		22
	December 21, 2009		3.82		0.04			2.34			2		18
	December 22, 2010		4.24		0.13			2.07			3		14
	December 22, 2010		4.46		0.13			2.07			2		14
	December 13, 2011		4.2		0.05			2.8			3		26.2
	December 13, 2011		3.85		0.05			2.75			2		26.2
	March 20, 2013		7.79		0.38			1.88			3		2.8
	March 20, 2013		5.75		0.25			1.88			2		3.2
	26-Sep-14		6.61		1.06			1.87			3		6.2
	26-Sep-14		6.43		0.58			1.87			2		4
Summary of Information About Non-employee Director Options Outstanding to Purchase Ordinary Shares	Options.    The following table summarizes information about non-employee director options outstanding to purchase ordinary shares at December 31, 2014.
	Option Holder		Options Outstanding			Options Exercisable			Exercise		Fair Value at		Remaining
									Price		Grant Date		Contractual  Time
	Non-employee directors - 2006 Option grants (May 25)		4,435			4,435			$21.96		$4.24		1 year, 5 months
	Non-employee directors - 2007 Option grants (July 30)		2,012			2,012			$24.76		$4.97		2 years, 7 months
Non-employee Director Options Granted	The fair value of the non-employee director options granted were estimated on the date of grant using a modified Black-Scholes option pricing model under the following assumptions:
			Grant Date
			July 30, 2007			May 25, 2006
	Per share weighted average fair value		$4.97			$4.24
	Risk-free interest rate		4.64	%		4.85	%
	Dividend yield		2.4	%		2.7	%
	Expected life		5 years			5 years
	Share price volatility		19.55	%		20.05	%
Restricted Share Units Issued to Non-Employee Directors	Restricted Share Units.    The following table summarizes information about restricted share units issued to non-employee directors as at December 31, 2014.
			As at December 31, 2014
			Restricted Share Units
			Amount			Amount			Amount			Amount
			Granted			Vested			Forfeited			Outstanding
	Non-Employee Directors — 2012 and prior		138,504			131,866			6,638			—
	Non-Employee Directors — 2013		29,092			26,727			2,365			—
	Non-Employee Directors — 2014		27,180			22,640			—			4,540
	Chairman — 2012 and prior		75,799			75,799			—			—
	Chairman — 2013		14,188			14,188			—			—
	Chairman — 2014		13,590			11,324			—			2,266
	Total		298,353			282,544			9,003			6,806
Summary of Option Activity and Restricted Share Unit Activity	A summary of option activity and restricted share unit activity discussed above is presented in the tables below:
				Twelve Months Ended December 31, 2014
	Option activity			Number of Options			Weighted Average
							Exercise  Price
	Outstanding options, beginning of period			302,460			$25.02
	Exercised and issued			(74,520	)		25.26
	Forfeited or expired			(3,799	)		27.28
	Outstanding and exercisable options, end of period			224,141			$24.91
				Twelve Months Ended December 31, 2013
	Option activity			Number of Options			Weighted Average
							Exercise  Price
	Outstanding options, beginning of period			1,208,787			$23.07
	Exercised and issued			(906,254	)		22.45
	Forfeited or expired			(73	)		24.21
	Outstanding and exercisable options, end of period			302,460			$25.02
				Twelve Months Ended December 31, 2014
	Restricted share unit activity			Number of			Weighted Average
				Shares			Grant Date
							Fair Value
	Outstanding restricted stock, beginning of period			544,751			$32.13
	Granted			300,410			38.6
	Vested			(291,468	)		32.12
	Forfeited			(27,775	)		36.29
	Outstanding restricted stock, end of period			525,918			$35.83
				Twelve Months Ended December 31, 2013
	Restricted share unit activity			Number of			Weighted Average
				Shares			Grant Date
							Fair Value
	Outstanding restricted stock, beginning of period			488,013			$27.81
	Granted			350,721			35.61
	Vested			(253,700	)		23.66
	Forfeited			(40,283	)		31.45
	Outstanding restricted stock, end of period			544,751			$32.13

Intangible_Assets_Tables

Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2014
Goodwill and Intangible Assets Disclosure [Abstract]
Summary of Changes in Intangible Assets	The following table provides a summary of the Company’s intangible assets for the twelve months ended December 31, 2014 and 2013:
		Twelve Months Ended December 31, 2014										Twelve Months Ended December 31, 2013
		Trade		Insurance		Other			Total			Trade		Insurance		Other			Total
		Mark		Licenses								Mark		Licenses
		($ in millions)										($ in millions)
	Intangible Assets
	Beginning of the period	$	1.6	$	16.6	$	0.2		$	18.4		$	1.6	$	16.6	$	0.8		$	19
	Amortization	—		—		(0.2		)	(0.2		)	—		—		(0.6		)	(0.6		)
	End of the period	$	1.6	$	16.6	$	—		$	18.2		$	1.6	$	16.6	$	0.2		$	18.4

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Schedule of Company's Restricted Assets	The following table details the forms and value of Company’s restricted assets as at December 31, 2014 and 2013:
			As at December 31, 2014			At December 31, 2013
			($ in millions, except percentages)
	Regulatory trusts and deposits:
	Affiliated transactions		$	1,086.90		$	685.8
	Third party		2,183.40			2,236.40
	Letters of credit / guarantees(1)		778.7			830.4
	Total restricted assets		$	4,049.00		$	3,752.60
	Total as percent of cash and invested assets		47		%	45.5		%
	_______________
	-1	As of December 31, 2014, the Company had pledged funds of $764.4 million and £9.2 million (December 31, 2013 — $803.7 million and £16.1 million) as collateral for the secured letters of credit.
Amounts Outstanding under Operating Leases	Amounts outstanding under operating leases net of subleases as of December 31, 2014 and 2013 were:
	As at December 31, 2014		2015			2016			2017		2018		2019		Later		Total
															Years
			($ in millions)
	Operating Lease Obligations		$	13.4		$	9.3		$	8.5	$	7.3	$	6.4	$	1.5	$	46.4
	As at December 31, 2013		2014			2015			2016		2017		2018		Later		Total
															Years
			($ in millions)
	Operating Lease Obligations		$	11.9		$	12.1		$	8.3	$	7.5	$	6.3	$	6.5	$	52.6

Concentration_of_Credit_Risk_T

Concentration of Credit Risk (Tables)	12 Months Ended
	Dec. 31, 2014
Risks and Uncertainties [Abstract]
Schedule Of Gross Written Premium From Major Brokers	The following table shows the largest brokers that the Company transacted business with in the three years ended December 31, 2014 and the proportion of gross written premiums from each of those brokers.
			Twelve Months Ended December 31,
			2014			2013			2012
	Broker		(in percentages)
	Aon Corporation		17.8		%	16.8		%	18.5		%
	Marsh & McLennan Companies, Inc.		15.1			15			15.8
	Willis Group Holdings, Ltd.		13.7			14.4			15.1
	Others(1)		53.4			53.8			50.6
	Total		100		%	100		%	100		%
	Gross written premiums ($ millions)		$	2,902.70		$	2,646.70		$	2,583.30
	________________
	-1	No other individual broker accounted for more than 10% of total gross written premiums.

Reclassifications_from_Accumul1

Reclassifications from Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income Reclassification	The following table sets out the components of the Company’s AOCI that are reclassified into the audited condensed consolidated statement of operations for the twelve months ended months ended December 31, 2014 and 2013:
		Amount Reclassified from AOCI
	Details about the AOCI Components	Twelve Months Ended December 31, 2014			Twelve Months Ended December 31, 2013			Affected Line Item in the
								Consolidated Statement of Operations
		($ in millions)
	Available for sale securities:
	Realized gain on sale of securities	$	13.9		$	24.6		Realized and unrealized investment gains
	Realized (losses) on sale of securities	(6.2		)	(0.5		)	Realized and unrealized investment losses
		7.7			24.1			Income from operations before tax
	Tax on realized gains and (losses) of securities	(0.2		)	(0.7		)	Income tax expense
		$	7.5		$	23.4		Net income
	Foreign currency translation adjustments:
	Foreign currency translation adjustments, before tax	$	—		$	(1.9	)	Net realized and unrealized foreign exchange gains/(losses)
	Tax on foreign currency translation adjustments	—			0.4			Income tax expense
		$	—		$	(1.5	)	Net income
	Amortization of derivatives:
	Amortization of long-term debt associated expenses, before tax	$	—		$	(0.5	)	Interest expense
		$	—		$	(0.5	)	Net income
	Total reclassifications from AOCI to the statement of operations, net of tax	$	7.5		$	21.4		Net income

Credit_Facility_and_Longterm_D1

Credit Facility and Long-term Debt (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Summary of Contractual Obligations Under Long-term Debts	The following table summarizes our contractual obligations under the long-term debts as of December 31, 2014.
		Payments Due By Period
	Contractual Basis	Less than		1-3 years		3-5 years		More than 5 years		Total
		1 year
		($ in millions)
	Long-term Debt Obligations	$	—	$	—	$	—	$	550	$	550

Unaudited_Quarterly_Financial_1

Unaudited Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Summary of Quarterly Financial Data	The following is a summary of the quarterly financial data for the twelve months ended December 31, 2014, 2013 and 2012.
		2014
		Quarter Ended			Quarter Ended			Quarter Ended September 30			Quarter Ended December 31			Year Ended December 31
		March 31			30-Jun
	Revenues	($ in millions)
	Net earned premium	$	566.5		$	616.2		$	610.4		$	612.2		$	2,405.30
	Net investment income	49.5			46.1			48			46.7			190.3
	Realized and unrealized investment gains/(losses)(1)	17.9			34.6			1.1			(7.3		)	46.3
	Other income	0.6			3.2			1			(0.3		)	4.5
	Total revenues	634.5			700.1			660.5			651.3			2,646.40
	Expenses
	Losses and loss adjustment expenses	288.1			337.1			342.7			339.6			1,307.50
	Amortization of deferred policy acquisition costs	112			108.9			115.5			114.8			451.2
	General, administrative and corporate expenses	95.6			108.8			119.8			121.5			445.7
	Interest on long-term debt	7.4			7.3			7.4			7.4			29.5
	Change in fair value of derivatives	(1.1		)	4.6			5.1			6.6			15.2
	Change in fair value of loan notes issued by variable interest entities	3.4			2.6			8.5			4.1			18.6
	Realized and unrealized investment losses/(gains)(1)	4.3			3.3			21.2			(14.1		)	14.7
	Net realized and unrealized foreign exchange (gains)/losses(1)	(0.1		)	(10.7		)	1.3			3.9			(5.6		)
	Other expenses	0.7			1.2			0.3			(0.5		)	1.7
	Total expenses	510.3			563.1			621.8			583.3			2,278.50
	Income from operations before income tax	124.2			137			38.7			68			367.9
	Income tax (expense)	(3.8		)	(6.2		)	(1.3		)	(0.8		)	(12.1		)
	Net income	$	120.4		$	130.8		$	37.4		$	67.2		$	355.8
	Per Share Data
	Weighted average number of ordinary share and share equivalents
	Basic	65,289,351			65,447,128			65,116,463			62,206,260			64,536,491
	Diluted	66,565,890			66,700,368			66,513,009			63,605,298			65,872,949
	Basic earnings per ordinary share adjusted for preference share dividends	$	1.7		$	1.85		$	0.43		$	0.92		$	4.92
	Diluted earnings per ordinary share adjusted for preference share dividends	$	1.66		$	1.82		$	0.42		$	0.9		$	4.82
	____________________
	(1) Adjusted for a representation of foreign exchange in relation to investment securities from realized and unrealized exchange gains/(losses) to realized and unrealized investment gains/(losses).
		2013
		Quarter Ended			Quarter Ended			Quarter Ended September 30			Quarter Ended December 31			Year Ended December 31
		31-Mar			30-Jun
	Revenues	($ in millions)
	Net earned premium	$	510.9		$	544		$	544.3		$	572.6		$	2,171.80
	Net investment income	48.3			45.9			45			47.2			186.4
	Realized and unrealized investment gains	16.3			14.4			23.6			2.6			56.9
	Other income	1.1			0.9			1.6			4.6			8.2
	Total revenues	576.6			605.2			614.5			627			2,423.30
	Expenses
	Losses and loss adjustment expenses	268.7			333.4			290.2			331.4			1,223.70
	Amortization of deferred policy acquisition costs	104.6			107.2			110.5			99.7			422
	General, administrative and corporate expenses	86.6			87.7			98.9			94.9			368.1
	Interest on long-term debt	7.7			7.8			7.7			9.5			32.7
	Change in fair value of derivatives	4.2			2.9			(6.6		)	(1.8		)	(1.3		)
	Realized and unrealized investment losses/(gains)	1.1			21			5.9			(7.5		)	20.5
	Net realized and unrealized foreign exchange losses/(gains)	5.4			4.1			(2.4		)	6.1			13.2
	Other expenses	0.6			—			—			1.1			1.7
	Total expenses	478.9			564.1			504.2			533.4			2,080.60
	Income from operations before income tax	97.7			41.1			110.3			93.6			342.7
	Income tax (expense)	(5.9		)	(1.0		)	(2.9		)	(3.6		)	(13.4		)
	Net income	$	91.8		$	40.1		$	107.4		$	90		$	329.3
	Per Share Data
	Weighted average number of ordinary share and share equivalents
	Basic	68,854,286			66,191,426			66,716,202			65,593,669			66,872,048
	Diluted	72,452,705			69,291,324			68,561,515			67,051,993			69,417,903
	Basic earnings per ordinary share adjusted for preference share dividends	$	1.21		$	0.38		$	1.47		$	1.23		$	4.29
	Diluted earnings per ordinary share adjusted for preference share dividends	$	1.15		$	0.36		$	1.43		$	1.21		$	4.14
		2012
		Quarter Ended			Quarter Ended			Quarter Ended September 30			Quarter Ended December 31			Year Ended December 31
		31-Mar			30-Jun
	Revenues	($ in millions)
Net earned premium	$	495.4		$	513.4		$	516.2		$	558.5		$	2,083.50
Net investment income	52.4			52.8			48.6			51.1			204.9
Realized and unrealized investment gains	10.7			5.4			13.2			6.1			35.4
Other income	0.7			3.5			4.8			(3.4		)	5.6
Total revenues	559.2			575.1			582.8			612.3			2,329.40
Expenses
Losses and loss adjustment expenses	284			262.1			255			437.4			1,238.50
Amortization of deferred policy acquisition costs	96.1			102			103.1			80			381.2
General, administrative and corporate expenses	84.8			83.5			90.7			86.1			345.1
Interest on long-term debt	7.7			7.7			7.8			7.7			30.9
Change in fair value of derivatives	7.5			11.6			4.9			4.4			28.4
Realized and unrealized investment losses	1.7			4.1			2.4			0.4			8.6
Net realized and unrealized foreign exchange losses/(gains)	(7.7		)	12.7			(4.5		)	(3.9		)	(3.4		)
Other expenses	1			0.6			0.3			2.8			4.7
Total expenses	475.1			484.3			459.7			614.9			2,034.00
Income/(loss) from operations before income tax	84.1			90.8			123.1			(2.6		)	295.4
Income tax (expense)/credit	(5.4		)	(6.2		)	(8.0		)	4.6			(15.0		)
Net income	$	78.7		$	84.6		$	115.1		$	2		$	280.4
Per Share Data
Weighted average number of ordinary share and share equivalents
Basic (1)	70,943,997			71,303,855			71,129,102			71,007,079			71,095,856
Diluted (1)	73,832,734			73,845,903			73,397,796			71,007,079			73,689,423
Basic earnings/(loss) per ordinary share adjusted for preference share dividends	$	1.03		$	1.07		$	1.5		$	(0.09	)	$	3.51
Diluted earnings/(loss) per ordinary share adjusted for preference share dividends	$	0.99		$	1.03		$	1.45		$	(0.09	)	$	3.39
____________________
(1)    The basic and diluted number of ordinary shares for the three months ended December 31, 2012 is the same, as the inclusion of dilutive securities in a loss-making period would be anti-dilutive.

Related_Party_Transactions_Add

Related Party Transactions - Additional Information (Narrative) (Details) (USD $)	0 Months Ended	12 Months Ended	0 Months Ended
In Millions, unless otherwise specified	Apr. 05, 2005	Dec. 31, 2010	Jan. 22, 2010
Related Party Transaction [Line Items]
Aggregate consideration for sale and purchase agreement	$1.60	$4.80
Fair value of tangible and financial assets, net		1.2
APJ Continuation Limited
Related Party Transaction [Line Items]
Aggregate consideration for sale and purchase agreement			4.8
Fair value of tangible and financial assets, net			1.2
Estimated goodwill on acquisition			$3.60
Percentage of shares formerly held by executive officer			30.00%

Basis_of_Preparation_and_Signi2

Basis of Preparation and Significant Accounting Policies - Additional Information (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
	period
Property, Plant and Equipment [Line Items]
Non-Payment Of Dividends, Number Of Periods	6
Computer Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Computer Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	4 years
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	15 years
Foreign Exchange Contracts
Property, Plant and Equipment [Line Items]
Derivative, Notional Amount	135,800,000
Fixed maturities
Property, Plant and Equipment [Line Items]
Percentage of Value Decline in Securities to be considered Other than Temporary Impairment	20.00%
Equity securities
Property, Plant and Equipment [Line Items]
Percentage of Value Decline in Securities to be considered Other than Temporary Impairment	20.00%

Earnings_Per_Ordinary_Share_Co

Earnings Per Ordinary Share - Computation of Basic and Diluted Earnings per Share (Details) (USD $)	3 Months Ended																12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Net income/(loss)	$67.20	$37.40	$130.80	$120.40	$90	$107.40	$40.10	$91.80	$2		$115.10		$84.60		$78.70		$355.80		$329.30		$280.40
Change in Redemption Value																	0	[1]	-7.1	[1]	0	[1]
Net change attributable to non-controlling interest for the year																	0.8		-0.5		-0.2
Basic and diluted net income available to ordinary shareholders																	317.2		287.2		249.5
Ordinary shares:
Basic weighted average ordinary shares	62,206,260	65,116,463	65,447,128	65,289,351	65,593,669	66,716,202	66,191,426	68,854,286	71,007,079	[2]	71,129,102	[2]	71,303,855	[2]	70,943,997	[2]	64,536,491		66,872,048		71,095,856	[2]
Weighted average effect of dilutive securities																	1,336,458		2,545,855		2,593,567
Total diluted weighted average ordinary shares	63,605,298	66,513,009	66,700,368	66,565,890	67,051,993	68,561,515	69,291,324	72,452,705	71,007,079	[2]	73,397,796	[2]	73,845,903	[2]	73,832,734	[2]	65,872,949		69,417,903		73,689,423	[2]
Earnings per ordinary share:
Basic, dollars per share	$0.92	$0.43	$1.85	$1.70	$1.23	$1.47	$0.38	$1.21	($0.09)		$1.50		$1.07		$1.03		$4.92		$4.29		$3.51
Diluted, dollars per share	$0.90	$0.42	$1.82	$1.66	$1.21	$1.43	$0.36	$1.15	($0.09)		$1.45		$1.03		$0.99		$4.82		$4.14		$3.39
Retained earnings
Net income/(loss)																			329.3		280.4
Net change attributable to non-controlling interest for the year																	-0.8		0.5		0.2
Retained earnings | Preferred Stock
Preference share dividends																	-37.8		-35.5		-31.1
Change in Redemption Value																	$0	[3]	($7.10)	[3]	$0	[3]
[1]	The $7.1 million deduction from net income in 2013 is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million. For more information, please refer to Note 15, “Capital Structure” of these consolidated financial statements.
[2]	The basic and diluted number of ordinary shares for the three months ended December 31, 2012 is the same, as the inclusion of dilutive securities in a loss-making period would be anti-dilutive.
[3]	The $7.1 million reclassification from additional paid-in capital to retained earnings is the difference between the capital raised upon issuance of the 5.625% Perpetual Preferred Income Equity Replacement Securities (“PIERS”), net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million.

Earnings_Per_Ordinary_Share_Su

Earnings Per Ordinary Share - Summary of Declared Dividends (Details) (USD $)	0 Months Ended		12 Months Ended	0 Months Ended
	Mar. 31, 2009	Nov. 15, 2006	Dec. 31, 2014	Feb. 05, 2015
7.401% Preference Shares
Dividends Payable [Line Items]
Preference shares, rate	7.40%	7.40%	7.40%
7.250% Preference Shares
Dividends Payable [Line Items]
Preference shares, rate			7.25%
5.950% Preference Shares
Dividends Payable [Line Items]
Preference shares, rate			5.95%
Subsequent Event | Ordinary shares
Dividends Payable [Line Items]
Dividend				$0.20
Payable on				9-Mar-15
Record Date				20-Feb-15
Subsequent Event | 7.401% Preference Shares
Dividends Payable [Line Items]
Dividend				$0.46
Payable on				1-Apr-15
Record Date				15-Mar-15
Preference shares, rate				7.40%
Subsequent Event | 7.250% Preference Shares
Dividends Payable [Line Items]
Dividend				$0.45
Payable on				1-Apr-15
Record Date				15-Mar-15
Preference shares, rate				7.25%
Subsequent Event | 5.950% Preference Shares
Dividends Payable [Line Items]
Dividend				$0.37
Payable on				1-Apr-15
Record Date				15-Mar-15
Preference shares, rate				5.95%

Earnings_Per_Ordinary_Share_Ad

Earnings Per Ordinary Share - Additional Information (Narrative) (Details) (USD $)	12 Months Ended						0 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Apr. 25, 2013
Earnings Per Share [Line Items]
PIERS redemption	$0	[1]	$7.10	[1]	$0	[1]
Preference Shares
Earnings Per Share [Line Items]
PIERS redeemed and cancelled	230		230				230
Reclassification from Additional Paid in Capital to Retained Earnings
Earnings Per Share [Line Items]
PIERS redemption	$7.10		$7.10
[1]	The $7.1 million deduction from net income in 2013 is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million. For more information, please refer to Note 15, “Capital Structure” of these consolidated financial statements.

Segment_Reporting_Summary_of_G

Segment Reporting - Summary of Gross and Net Written and Earned Premiums, Underwriting Results, Ratios and Reserves for Each of Company's Business Segments (Details) (USD $)	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Underwriting Revenues
Gross written premiums																	$2,902,700,000		$2,646,700,000		$2,583,300,000
Net written premiums																	2,515,200,000		2,299,700,000		2,246,900,000
Gross earned premiums																	2,736,600,000		2,493,400,000		2,385,000,000
Net earned premium	612,200,000		610,400,000		616,200,000		566,500,000		572,600,000	544,300,000	544,000,000	510,900,000	558,500,000	516,200,000	513,400,000	495,400,000	2,405,300,000		2,171,800,000		2,083,500,000
Underwriting Expenses
Losses and loss adjustment expenses	339,600,000		342,700,000		337,100,000		288,100,000		331,400,000	290,200,000	333,400,000	268,700,000	437,400,000	255,000,000	262,100,000	284,000,000	1,307,500,000		1,223,700,000		1,238,500,000
Amortization of deferred policy acquisition costs	114,800,000		115,500,000		108,900,000		112,000,000		99,700,000	110,500,000	107,200,000	104,600,000	80,000,000	103,100,000	102,000,000	96,100,000	451,200,000		422,000,000		381,200,000
General and administrative expenses																	351,900,000		316,900,000		292,100,000
Underwriting income																	294,700,000		209,200,000		171,700,000
Corporate expenses																	-93,800,000		-51,200,000		-53,000,000
Net investment income	46,700,000		48,000,000		46,100,000		49,500,000		47,200,000	45,000,000	45,900,000	48,300,000	51,100,000	48,600,000	52,800,000	52,400,000	190,300,000		186,400,000		204,900,000
Realized and unrealized investment gains	-7,300,000	[1]	1,100,000	[1]	34,600,000	[1]	17,900,000	[1]	2,600,000	23,600,000	14,400,000	16,300,000	6,100,000	13,200,000	5,400,000	10,700,000	46,300,000	[1]	56,900,000		35,400,000
Realized and unrealized investment (losses)	14,100,000	[1]	-21,200,000	[1]	-3,300,000	[1]	-4,300,000	[1]	7,500,000	-5,900,000	-21,000,000	-1,100,000	-400,000	-2,400,000	-4,100,000	-1,700,000	-14,700,000	[1]	-20,500,000		-8,600,000
Change in fair value of loan notes issued by variable interest entities	-4,100,000		-8,500,000		-2,600,000		-3,400,000										-18,600,000		0		0
Change in fair value of derivatives	-6,600,000		-5,100,000		-4,600,000		1,100,000		1,800,000	6,600,000	-2,900,000	-4,200,000	-4,400,000	-4,900,000	-11,600,000	-7,500,000	-15,200,000		1,300,000		-28,400,000
Interest expense on long term debt	-7,400,000		-7,400,000		-7,300,000		-7,400,000		-9,500,000	-7,700,000	-7,800,000	-7,700,000	-7,700,000	-7,800,000	-7,700,000	-7,700,000	-29,500,000		-32,700,000		-30,900,000
Net realized and unrealized foreign exchange (losses)	-3,900,000	[1]	-1,300,000	[1]	10,700,000	[1]	100,000	[1]	-6,100,000	2,400,000	-4,100,000	-5,400,000	3,900,000	4,500,000	-12,700,000	7,700,000	5,600,000	[1]	-13,200,000		3,400,000
Other income	-300,000		1,000,000		3,200,000		600,000		4,600,000	1,600,000	900,000	1,100,000	-3,400,000	4,800,000	3,500,000	700,000	4,500,000		8,200,000		5,600,000
Other expenses																	-1,700,000		-1,700,000		-4,700,000
Income from operations before income tax	68,000,000		38,700,000		137,000,000		124,200,000		93,600,000	110,300,000	41,100,000	97,700,000	-2,600,000	123,100,000	90,800,000	84,100,000	367,900,000		342,700,000		295,400,000
Income tax expense	-800,000		-1,300,000		-6,200,000		-3,800,000		-3,600,000	-2,900,000	-1,000,000	-5,900,000	4,600,000	-8,000,000	-6,200,000	-5,400,000	-12,100,000		-13,400,000		-15,000,000
Net income/(loss)	67,200,000		37,400,000		130,800,000		120,400,000		90,000,000	107,400,000	40,100,000	91,800,000	2,000,000	115,100,000	84,600,000	78,700,000	355,800,000		329,300,000		280,400,000
Losses and loss adjustment expenses	4,750,800,000								4,678,900,000				4,779,700,000				4,750,800,000		4,678,900,000		4,779,700,000		4,525,200,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net	4,400,800,000								4,346,200,000				4,280,700,000				4,400,800,000		4,346,200,000		4,280,700,000		4,098,600,000
Ratios
Loss ratio																	54.40%		56.30%		59.40%
Policy acquisition expense ratio																	18.80%		19.40%		18.30%
General and administrative expense ratio																	18.50%	[2]	16.90%	[2]	16.60%	[2]
Expense ratio																	37.30%		36.30%		34.90%
Combined ratio																	91.70%		92.60%		94.30%
Reinsurance
Underwriting Revenues
Net written premiums																	1,124,000,000		1,082,000,000		1,156,900,000
Net earned premium																	1,088,200,000		1,073,000,000		1,132,400,000
Underwriting Expenses
Losses and loss adjustment expenses	2,493,300,000								2,646,800,000				2,811,300,000				2,493,300,000		2,646,800,000		2,811,300,000
Ratios
Loss ratio																	45.70%		44.90%		56.10%
Policy acquisition expense ratio																	18.40%		19.30%		18.40%
General and administrative expense ratio																	13.50%	[2]	12.20%	[2]	10.90%	[2]
Expense ratio																	31.90%		31.50%		29.30%
Combined ratio																	77.60%		76.40%		85.40%
Insurance
Underwriting Revenues
Net written premiums																	1,391,200,000		1,217,700,000		1,090,000,000
Net earned premium																	1,317,100,000		1,098,800,000		951,100,000
Underwriting Expenses
Losses and loss adjustment expenses	1,907,500,000								1,699,400,000				1,469,400,000				1,907,500,000		1,699,400,000		1,469,400,000
Ratios
Loss ratio																	61.50%		67.50%		63.40%
Policy acquisition expense ratio																	19.10%		19.50%		18.20%
General and administrative expense ratio																	15.60%	[2]	16.90%	[2]	17.70%	[2]
Expense ratio																	34.70%		36.40%		35.90%
Combined ratio																	96.20%		103.90%		99.30%
Operating Segments [Member] | Reinsurance
Underwriting Revenues
Gross written premiums																	1,172,800,000		1,133,900,000		1,227,900,000
Net written premiums																	1,124,000,000		1,082,000,000		1,156,900,000
Gross earned premiums																	1,137,600,000		1,126,600,000		1,208,000,000
Net earned premium																	1,088,200,000		1,073,000,000		1,132,400,000
Underwriting Expenses
Losses and loss adjustment expenses																	497,800,000		481,700,000		635,300,000
Amortization of deferred policy acquisition costs																	200,000,000		207,200,000		207,800,000
General and administrative expenses																	146,400,000		131,000,000		123,900,000
Underwriting income																	244,000,000		253,100,000		165,400,000
Operating Segments [Member] | Insurance
Underwriting Revenues
Gross written premiums																	1,729,900,000		1,512,800,000		1,355,400,000
Net written premiums																	1,391,200,000		1,217,700,000		1,090,000,000
Gross earned premiums																	1,599,000,000		1,366,800,000		1,177,000,000
Net earned premium																	1,317,100,000		1,098,800,000		951,100,000
Underwriting Expenses
Losses and loss adjustment expenses																	809,700,000		742,000,000		603,200,000
Amortization of deferred policy acquisition costs																	251,200,000		214,800,000		173,400,000
General and administrative expenses																	205,500,000		185,900,000		168,200,000
Underwriting income																	$50,700,000		($43,900,000)		$6,300,000
[1]	Adjusted for a representation of foreign exchange in relation to investment securities from realized and unrealized exchange gains/(losses) to realized and unrealized investment gains/(losses).
[2]	The general and administrative expense ratio in the total column includes corporate expenses.

Segment_Reporting_Summary_of_t

Segment Reporting - Summary of the Schedule of the Company's gross written premiums based on the location of the insured risk (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross written premiums	$2,902,700,000		$2,646,700,000		$2,583,300,000
Australia/Asia
Segment Reporting Information [Line Items]
Gross written premiums	130,100,000		108,400,000		139,300,000
Caribbean
Segment Reporting Information [Line Items]
Gross written premiums	19,700,000		14,400,000		12,200,000
Europe
Segment Reporting Information [Line Items]
Gross written premiums	113,900,000		112,200,000		113,000,000
United Kingdom
Segment Reporting Information [Line Items]
Gross written premiums	209,300,000		166,400,000		168,600,000
United States & Canada
Segment Reporting Information [Line Items]
Gross written premiums	1,357,300,000	[1]	1,179,600,000	[1]	1,106,900,000	[1]
Worldwide excluding United States
Segment Reporting Information [Line Items]
Gross written premiums	116,200,000	[2]	145,700,000	[2]	151,700,000	[2]
Worldwide including United States
Segment Reporting Information [Line Items]
Gross written premiums	851,800,000	[3]	827,400,000	[3]	810,800,000	[3]
Others
Segment Reporting Information [Line Items]
Gross written premiums	$104,400,000		$92,600,000		$80,800,000
[1]	“United States and Canada” comprises individual policies that insure risks specifically in the United States and/or Canada, but not elsewhere.
[2]	“Worldwide excluding the United States” comprises individual policies that insure risks wherever they may be across the world but specifically excludes the United States.
[3]	“Worldwide including the United States” comprises individual policies that insure risks wherever they may be across the world but specifically includes the United States.

Segment_Reporting_Additional_I

Segment Reporting - Additional Information (Narrative) (Details)	12 Months Ended
	Dec. 31, 2014
	segement
Segment Reporting [Abstract]
Number of business segments	2

Investments_Summary_of_Investm

Investments - Summary of Investment Income (Details) (USD $)	3 Months Ended												12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total													$200.90	$195.90	$213.60
Investment expenses													-10.6	-9.5	-8.7
Net investment income	46.7	48	46.1	49.5	47.2	45	45.9	48.3	51.1	48.6	52.8	52.4	190.3	186.4	204.9
Fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total													151.1	155.6	181.3
Fixed income securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total													26.7	20.3	16.5
Short-term investments — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total													1.4	2.1	3.1
Fixed term deposits (included in cash and cash equivalents)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total													3.3	5.3	6.5
Equity securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total													4.1	5.6	6.2
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total													13	7	0
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total													$1.30	$0	$0

Investments_Net_Realized_and_U

Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (Details) (USD $)	0 Months Ended	12 Months Ended
	Dec. 16, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total other-than-temporary impairments		($2,400,000)	$0	($3,000,000)
Catastrophe bonds — trading		400,000	0	0
Net change in gross unrealized gains		7,600,000	6,100,000	10,500,000
Gross realized and unrealized gains in Cartesian			3,000,000
Other realized losses	-9,300,000	0	-9,300,000	0
Total net realized and unrealized investment gains recorded in the statement of operations		31,600,000	36,400,000	26,800,000
Change in available for sale net unrealized gains/(losses):
Fixed income securities		47,700,000	-209,600,000	2,700,000
Equity securities		-10,000,000	11,200,000	16,400,000
Total change in pre-tax available for sale unrealized gains/(losses)		37,700,000	-198,400,000	19,100,000
Change in taxes		-2,800,000	13,700,000	-9,300,000
Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income		34,900,000	-184,700,000	9,800,000
Cartesian
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross realized and unrealized gains in Cartesian		0	3,000,000	3,200,000
Available for sale - Fixed income maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities — gross realized gains		10,300,000	18,200,000	7,600,000
Fixed income securities — gross realized (losses)		-5,900,000	-7,400,000	-400,000
Available for sale - Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities — gross realized gains		12,900,000	18,000,000	4,300,000
Equity securities — gross realized (losses)		-800,000	-300,000	-4,900,000
Trading - Fixed income maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities — gross realized gains		7,300,000	9,500,000	9,800,000
Fixed income securities — gross realized (losses)		-2,500,000	-2,900,000	-300,000
Trading - Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities — gross realized gains		7,800,000	2,100,000	0
Equity securities — gross realized (losses)		($3,100,000)	($600,000)	$0

Investments_Cost_Gross_Unreali

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities, Short-Term Investments and Equities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	$5,803.70	$5,722.40
Gross Unrealized Gains	203.7	205.5
Gross Unrealized Losses	-9.2	-49
Fair Market Value	5,998.20	5,878.90
Total fixed income securities — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	5,462.90	5,449.90
Gross Unrealized Gains	176.3	167.7
Gross Unrealized Losses	-9.2	-48.5
Fair Market Value	5,630	5,569.10
Total short-term investments — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	258.2	160.3
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	0	0
Fair Market Value	258.3	160.3
Total equity securities — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	82.6	112.2
Gross Unrealized Gains	27.3	37.8
Gross Unrealized Losses	0	-0.5
Fair Market Value	109.9	149.5
U.S. government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,074.20	1,004.70
Gross Unrealized Gains	21.5	21.2
Gross Unrealized Losses	-1.3	-5.5
Fair Market Value	1,094.40	1,020.40
U.S. agency
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	190	258.5
Gross Unrealized Gains	7.5	11.4
Gross Unrealized Losses	-0.1	-0.8
Fair Market Value	197.4	269.1
Municipal
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	29.1	32.3
Gross Unrealized Gains	2.4	0.9
Gross Unrealized Losses	0	-0.4
Fair Market Value	31.5	32.8
Corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	2,244.70	2,005.60
Gross Unrealized Gains	79.9	82.5
Gross Unrealized Losses	-5.2	-18.7
Fair Market Value	2,319.40	2,069.40
Non-U.S. government-backed corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	76.8	83.4
Gross Unrealized Gains	1.2	1.4
Gross Unrealized Losses	0	-0.2
Fair Market Value	78	84.6
Foreign government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	648.6	772
Gross Unrealized Gains	17.3	11.2
Gross Unrealized Losses	-0.2	-4.3
Fair Market Value	665.7	778.9
Asset-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	141.3	119.8
Gross Unrealized Gains	2.4	2.8
Gross Unrealized Losses	-0.2	-0.3
Fair Market Value	143.5	122.3
Non-agency commercial mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	41.5	56.9
Gross Unrealized Gains	3.3	5.7
Gross Unrealized Losses	0	0
Fair Market Value	44.8	62.6
Agency mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,016.70	1,116.70
Gross Unrealized Gains	40.8	30.6
Gross Unrealized Losses	-2.2	-18.3
Fair Market Value	$1,055.30	$1,129

Investments_Cost_Gross_Unreali1

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	$760.90	$712.10
Fair Market Value	771	716.2
Total fixed income securities — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	760.9	712.1
Gross Unrealized Gains	16.1	11.9
Gross Unrealized Losses	-6	-7.8
Fair Market Value	771	716.2
Total short-term investments — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	0.2
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Market Value	0.2
Total equity securities — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	585.2	281.6
Gross Unrealized Gains	55.5	34
Gross Unrealized Losses	-24.7	-4.7
Fair Market Value	616	310.9
Total catastrophe bonds — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	34.4	5.8
Gross Unrealized Gains	0.4	0
Gross Unrealized Losses	0	0
Fair Market Value	34.8	5.8
Total
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,380.70	999.5
Gross Unrealized Gains	72	45.9
Gross Unrealized Losses	-30.7	-12.5
Fair Market Value	1,422	1,032.90
U.S. government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost		22.7
Gross Unrealized Gains		0
Gross Unrealized Losses		-0.7
Fair Market Value		22
U.S. agency
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	0.2	0.2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	0.2	0.2
Municipal
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1.1	1.1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	1.1	1.1
Corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	520.9	469.8
Gross Unrealized Gains	11.7	10.3
Gross Unrealized Losses	-2.8	-5.3
Fair Market Value	529.8	474.8
Foreign government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	137.3	136.5
Gross Unrealized Gains	4.3	1.2
Gross Unrealized Losses	-1.5	-1.5
Fair Market Value	140.1	136.2
Asset-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	14.6	12.7
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0	0
Fair Market Value	14.7	12.8
Bank loans
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	86.8	69.1
Gross Unrealized Gains	0	0.3
Gross Unrealized Losses	-1.7	-0.3
Fair Market Value	$85.10	$69.10

Investments_Other_Investments_

Investments - Other Investments (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance	$48,000,000	$45,000,000
Proceeds from Equity Method Investment, Dividends or Distributions	-39,300,000
Gain		3,000,000
Closing value of investment, ending balance	8,700,000	48,000,000
Cartesian
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance	39,300,000	36,300,000
Proceeds from Equity Method Investment, Dividends or Distributions	-39,300,000
Gain	0	3,000,000	3,200,000
Closing value of investment, ending balance	0	39,300,000	36,300,000
Chaspark
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance	8,700,000	8,700,000
Proceeds from Equity Method Investment, Dividends or Distributions	0
Gain	0	0
Closing value of investment, ending balance	$8,700,000	$8,700,000

Investments_Summary_of_Fixed_M

Investments - Summary of Fixed Maturities (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	$5,462.90	$5,449.90
Cost or Amortized Cost	5,803.70	5,722.40
Available for sale investments in fixed income maturities, Fair Market Value	5,630	5,569.10
Other Fixed Income Securities
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Due one year or less, Cost or Amortized Cost	590.2	694.8
Due after one year through five years, Cost or Amortized Cost	2,552	2,376.10
Due after five years through ten years, Cost or Amortized Cost	1,023.50	1,003.90
Due after ten years, Cost or Amortized Cost	97.7	81.7
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	4,263.40	4,156.50
Due one year or less, Fair Market Value	594.7	700
Due after one year through five years, Fair Market Value	2,620.80	2,438
Due after five years through ten years, Fair Market Value	1,059.90	1,032.80
Due after ten years, Fair Market Value	111	84.4
Available for sale investments in fixed income maturities, Fair Market Value	4,386.40	4,255.20
Due one year or less, Average Ratings by Maturity	AA	AA
Due after one year through five years, Average Ratings by Maturity	AA-	AA-
Due after five years through ten years, Average Ratings by Maturity	A+	A+
Due after ten years, Average Ratings by Maturity	A+	AA-
Non-agency commercial mortgage-backed
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Cost or Amortized Cost	41.5	56.9
Available for sale investments in fixed income maturities, Fair Market Value	44.8	62.6
Available for sale investments Average Ratings by Maturity	AA+	AA+
Agency mortgage-backed
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Cost or Amortized Cost	1,016.70	1,116.70
Available for sale investments in fixed income maturities, Fair Market Value	1,055.30	1,129
Available for sale investments Average Ratings by Maturity	AA+	AA+
Asset-backed
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Cost or Amortized Cost	141.3	119.8
Available for sale investments in fixed income maturities, Fair Market Value	143.5	122.3
Available for sale investments Average Ratings by Maturity	AAA	AAA
Total fixed income securities — Available for sale
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Cost or Amortized Cost	5,462.90	5,449.90
Available for sale investments in fixed income maturities, Fair Market Value	$5,630	$5,569.10

Investments_Aggregate_Fair_Val

Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified	security	security
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$794.70	$1,884.10
0-12 months, Gross Unrealized Losses	-3	-46.6
Over 12 months, Fair Market Value	423.2	41.6
Over 12 months, Gross Unrealized Losses	-6.2	-2.4
Total, Fair Market Value	1,217.90	1,925.70
Total, Gross Unrealized Losses	-9.2	-49
Total, Number of Securities	431	687
Fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	790.1	1,870.40
0-12 months, Gross Unrealized Losses	-3	-46.2
Over 12 months, Fair Market Value	423.2	39.3
Over 12 months, Gross Unrealized Losses	-6.2	-2.3
Total, Fair Market Value	1,213.30	1,909.70
Total, Gross Unrealized Losses	-9.2	-48.5
Total, Number of Securities	428	674
Short-term investments — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	4.6	7.7
0-12 months, Gross Unrealized Losses	0	0
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Losses	0	0
Total, Fair Market Value	4.6	7.7
Total, Gross Unrealized Losses	0	0
Total, Number of Securities	3	6
Total equity securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value		6
0-12 months, Gross Unrealized Losses		-0.4
Over 12 months, Fair Market Value		2.3
Over 12 months, Gross Unrealized Losses		-0.1
Total, Fair Market Value		8.3
Total, Gross Unrealized Losses		-0.5
Total, Number of Securities		7
U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	166.1	293.9
0-12 months, Gross Unrealized Losses	-0.5	-5.5
Over 12 months, Fair Market Value	79.4	0
Over 12 months, Gross Unrealized Losses	-0.8	0
Total, Fair Market Value	245.5	293.9
Total, Gross Unrealized Losses	-1.3	-5.5
Total, Number of Securities	39	51
U.S. agency
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	25.1	72.1
0-12 months, Gross Unrealized Losses	0	-0.8
Over 12 months, Fair Market Value	4.9	0
Over 12 months, Gross Unrealized Losses	-0.1	0
Total, Fair Market Value	30	72.1
Total, Gross Unrealized Losses	-0.1	-0.8
Total, Number of Securities	7	18
Municipal
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	0	5.5
0-12 months, Gross Unrealized Losses	0	-0.2
Over 12 months, Fair Market Value	0	1.3
Over 12 months, Gross Unrealized Losses	0	-0.2
Total, Fair Market Value	0	6.8
Total, Gross Unrealized Losses	0	-0.4
Total, Number of Securities	0	7
Corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	459.4	695.4
0-12 months, Gross Unrealized Losses	-2.1	-16.8
Over 12 months, Fair Market Value	171.3	23.4
Over 12 months, Gross Unrealized Losses	-3.1	-1.9
Total, Fair Market Value	630.7	718.8
Total, Gross Unrealized Losses	-5.2	-18.7
Total, Number of Securities	274	372
Non-U.S. government-backed corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	0.7	21.8
0-12 months, Gross Unrealized Losses	0	-0.2
Over 12 months, Fair Market Value	0	4.9
Over 12 months, Gross Unrealized Losses	0	0
Total, Fair Market Value	0.7	26.7
Total, Gross Unrealized Losses	0	-0.2
Total, Number of Securities	1	8
Foreign government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	30.4	239.7
0-12 months, Gross Unrealized Losses	0	-4.1
Over 12 months, Fair Market Value	44.2	8.5
Over 12 months, Gross Unrealized Losses	-0.2	-0.2
Total, Fair Market Value	74.6	248.2
Total, Gross Unrealized Losses	-0.2	-4.3
Total, Number of Securities	16	44
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	43.7	50.2
0-12 months, Gross Unrealized Losses	-0.1	-0.3
Over 12 months, Fair Market Value	11.7	0
Over 12 months, Gross Unrealized Losses	-0.1	0
Total, Fair Market Value	55.4	50.2
Total, Gross Unrealized Losses	-0.2	-0.3
Total, Number of Securities	43	51
Agency mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	64.7	491.8
0-12 months, Gross Unrealized Losses	-0.3	-18.3
Over 12 months, Fair Market Value	111.7	1.2
Over 12 months, Gross Unrealized Losses	-1.9	0
Total, Fair Market Value	176.4	493
Total, Gross Unrealized Losses	($2.20)	($18.30)
Total, Number of Securities	48	123

Investments_Analysis_of_Invest

Investments - Analysis of Investment Purchases/Sales and Maturities (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
(Purchases) of fixed income securities — Available for sale	($2,005)	($2,129.80)	($1,529.60)
(Purchases) of fixed income securities — Trading	-653.4	-763.4	-300.8
(Purchases) of equity securities — Available for sale	0	-2.5	-53.1
(Purchases) of equity securities — Trading	-361	-304.4	0
Proceeds from sales and maturities of fixed income securities — Available for sale	1,909.50	1,872.30	1,416.50
Proceeds from sales and maturities of fixed income securities — Trading	615.9	486	257.2
Proceeds from sales of equity securities — Available for sale	40	82.2	46.9
Proceeds from sales of equity securities — Trading	62.2	24.1	0
Net change in (payable)/receivable for securities (purchased)/sold	2.8	-0.9	1.1
Net (purchases)/sales of short-term investments — Available for sale	-110.3	260.6	-122.7
Purchases Of Catastrophe Bonds	28.7	5.8	0
Net proceeds from other investments	39.3	0	0
Net (purchases)/sales for the period	-488.9	-481.6	-293.2
Net (purchases)/sales of short-term investments — Available for sale
Gain (Loss) on Investments [Line Items]
Net (purchases)/sales of short-term investments — Available for sale		258.2	-122.7
Net (purchases)/sales of short-term investments — Trading
Gain (Loss) on Investments [Line Items]
Net (purchases)/sales of short-term investments — Available for sale	-0.2	2.4	0
Investment in Chaspark Maritime Holdings Ltd
Gain (Loss) on Investments [Line Items]
Net (purchases) of catastrophe bonds — Trading		$0	($8.70)

Investments_Additional_Informa

Investments - Additional Information (Narrative) (Details) (USD $)	0 Months Ended	12 Months Ended
	Dec. 16, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Aug. 31, 2013	31-May-14
		municipal_security	municipal_security
Investment Securities [Line Items]
Other-than-temporary impairment charge		$2,400,000	$0	$3,000,000
Other realized losses	-9,300,000	0	-9,300,000	0
Investment funded		2,500,000	2,300,000
Fair Market Value		771,000,000	716,200,000
Catastrophe bonds, trading at fair value (cost — $34.4 and $5.8)		34,800,000	5,800,000
Gross realized and unrealized gains in Cartesian			3,000,000
Number of municipal holdings		2	2
Cartesian
Investment Securities [Line Items]
Proceeds from Sale of Equity Method Investments		39,300,000
Gross realized and unrealized gains in Cartesian		0	3,000,000	3,200,000
Bank loans
Investment Securities [Line Items]
Fair Market Value		85,100,000	69,100,000
Equity securities
Investment Securities [Line Items]
Payments to Acquire Investments		240,000,000
Corporate and Foreign Government Debt Securities
Investment Securities [Line Items]
Fair Market Value					200,000,000
BB High Yield Bonds
Investment Securities [Line Items]
Investment funded						25,100,000
Senior Notes
Investment Securities [Line Items]
Senior note, aggregate principal amount	$250,000,000
Interest rate of senior notes	6.00%

Variable_Interest_Entities_Sch

Variable Interest Entities - Schedule of Variable Interest Entities (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Silverton
Business Acquisition [Line Items]
Balance at the beginning of the period		$0
Notes issued during the period		-50
Silverton Two
Business Acquisition [Line Items]
Total change in fair value included in the statement of operations	-18.6
Notes issued during the period	-70
Fair Value, Measurements, Recurring | Level 3
Business Acquisition [Line Items]
Balance at the beginning of the period	-50	0
Total change in fair value included in the statement of operations	-18.6	0
Balance at the end of the period	-138.6	-50
Fair Value, Measurements, Recurring | Level 3 | Silverton
Business Acquisition [Line Items]
Balance at the end of the period		($50)

Variable_Interest_Entities_Add

Variable Interest Entities - Additional Information (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Oct. 02, 2012	Dec. 27, 2013
Variable Interest Entity [Line Items]
Number Of Investments In Variable Interest Entities		2
Gross realized and unrealized gains in Cartesian	$3,000,000
Total aggregate unpaid balance	549,000,000	549,100,000
Chaspark
Variable Interest Entity [Line Items]
Percentage of ownership acquired			58.50%
Gross realized and unrealized gains in Cartesian	0	0
Silverton
Variable Interest Entity [Line Items]
Loans issued				65,000,000
Total aggregate unpaid balance		700,000
Percentage of interests acquired		100.00%
Silverton | Loan Notes 2014
Variable Interest Entity [Line Items]
Debt Instrument, Fair Value Disclosure		89,200,000
Silverton | Silverton Loan Notes
Variable Interest Entity [Line Items]
Repayments of Debt		88,200,000
Silverton | Third party
Variable Interest Entity [Line Items]
Loans issued				50,000,000
Silverton | Aspen Holdings and Third Parties | Silverton Loan Notes
Variable Interest Entity [Line Items]
Total aggregate unpaid balance		89,200,000
Silverton | Aspen Holdings | Silverton Loan Notes
Variable Interest Entity [Line Items]
Maximum loss exposure, amount		20,600,000
Silverton Two | Loan Notes 2015
Variable Interest Entity [Line Items]
Debt Instrument, Fair Value Disclosure		85,000,000
Silverton Two | Third party | Loan Notes 2015
Variable Interest Entity [Line Items]
Debt Instrument, Fair Value Disclosure		70,000,000
Silverton Two | Aspen Holdings | Silverton Loan Notes
Variable Interest Entity [Line Items]
Maximum loss exposure, amount		15,000,000
Silverton Two | Aspen Holdings | Loan Notes 2015
Variable Interest Entity [Line Items]
Debt Instrument, Fair Value Disclosure		85,000,000
Silverton Two
Variable Interest Entity [Line Items]
Loans issued		85,000,000
Loan notes issued by third parties		70,000,000
Investor | Silverton | Loan Notes 2014
Variable Interest Entity [Line Items]
Debt Instrument, Fair Value Disclosure		68,600,000
Repayments of Debt		$67,900,000

Fair_Value_Measurements_Financ

Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds, trading at fair value (cost — $5.8 and $5.8)	$34,400,000	$5,800,000
Derivatives at fair value	8,000,000	7,000,000
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued by variable interest entities, at fair value	-138,600,000	-50,000,000
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds, trading at fair value (cost — $5.8 and $5.8)		0
Loan notes issued by variable interest entities, at fair value	0	0
Loan notes issued by variable interest entities, at fair value (classified as a current liability)	0
Total	2,542,300,000	2,272,700,000
Recurring | Level 1 | Short-term investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	100,000
Recurring | Level 1 | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	616,000,000	310,900,000
Recurring | Level 1 | Derivatives at fair value | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	0	0
Recurring | Level 1 | Derivatives at fair value | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	0	0
Recurring | Level 1 | Liabilities under derivative contracts — foreign exchange contracts | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	0	0
Recurring | Level 1 | Available for sale financial assets, at fair value | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,094,400,000	1,020,400,000
Recurring | Level 1 | Available for sale financial assets, at fair value | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Available for sale financial assets, at fair value | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Available for sale financial assets, at fair value | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Available for sale financial assets, at fair value | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Available for sale financial assets, at fair value | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	456,500,000	596,200,000
Recurring | Level 1 | Available for sale financial assets, at fair value | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Available for sale financial assets, at fair value | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Available for sale financial assets, at fair value | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,550,900,000	1,616,600,000
Recurring | Level 1 | Short-term investments — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	229,300,000	129,500,000
Recurring | Level 1 | Total equity securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	109,900,000	149,500,000
Recurring | Level 1 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	36,100,000	66,200,000
Catastrophe bonds, trading at fair value (cost — $5.8 and $5.8)	0
Recurring | Level 1 | Held for trading financial assets, at fair value | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		22,000,000
Recurring | Level 1 | Held for trading financial assets, at fair value | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Held for trading financial assets, at fair value | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Held for trading financial assets, at fair value | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Held for trading financial assets, at fair value | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	36,100,000	44,200,000
Recurring | Level 1 | Held for trading financial assets, at fair value | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Held for trading financial assets, at fair value | Bank loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds, trading at fair value (cost — $5.8 and $5.8)		5,800,000
Loan notes issued by variable interest entities, at fair value	0	0
Loan notes issued by variable interest entities, at fair value (classified as a current liability)	0
Total	4,871,600,000	4,643,200,000
Recurring | Level 2 | Short-term investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	100,000
Recurring | Level 2 | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 2 | Derivatives at fair value | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	7,900,000	5,900,000
Recurring | Level 2 | Derivatives at fair value | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	100,000	1,100,000
Recurring | Level 2 | Liabilities under derivative contracts — foreign exchange contracts | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	-14,300,000	-2,900,000
Recurring | Level 2 | Available for sale financial assets, at fair value | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 2 | Available for sale financial assets, at fair value | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	197,400,000	269,100,000
Recurring | Level 2 | Available for sale financial assets, at fair value | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	31,500,000	32,800,000
Recurring | Level 2 | Available for sale financial assets, at fair value | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,319,400,000	2,069,400,000
Recurring | Level 2 | Available for sale financial assets, at fair value | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	78,000,000	84,600,000
Recurring | Level 2 | Available for sale financial assets, at fair value | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	209,200,000	182,700,000
Recurring | Level 2 | Available for sale financial assets, at fair value | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	143,500,000	122,300,000
Recurring | Level 2 | Available for sale financial assets, at fair value | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	44,800,000	62,600,000
Recurring | Level 2 | Available for sale financial assets, at fair value | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,055,300,000	1,129,000,000
Recurring | Level 2 | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	4,079,100,000	3,952,500,000
Recurring | Level 2 | Short-term investments — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	29,000,000	30,800,000
Recurring | Level 2 | Total equity securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 2 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	769,700,000	650,000,000
Catastrophe bonds, trading at fair value (cost — $5.8 and $5.8)	34,800,000
Recurring | Level 2 | Held for trading financial assets, at fair value | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Recurring | Level 2 | Held for trading financial assets, at fair value | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	200,000	200,000
Recurring | Level 2 | Held for trading financial assets, at fair value | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,100,000	1,100,000
Recurring | Level 2 | Held for trading financial assets, at fair value | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	529,800,000	474,800,000
Recurring | Level 2 | Held for trading financial assets, at fair value | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	104,000,000	92,000,000
Recurring | Level 2 | Held for trading financial assets, at fair value | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	14,700,000	12,800,000
Recurring | Level 2 | Held for trading financial assets, at fair value | Bank loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	85,100,000	69,100,000
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds, trading at fair value (cost — $5.8 and $5.8)		0
Loan notes issued by variable interest entities, at fair value	-70,700,000	-50,000,000
Loan notes issued by variable interest entities, at fair value (classified as a current liability)	-67,900,000
Total	-138,600,000	-50,000,000
Recurring | Level 3 | Short-term investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0
Recurring | Level 3 | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Derivatives at fair value | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	0	0
Recurring | Level 3 | Derivatives at fair value | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	0	0
Recurring | Level 3 | Liabilities under derivative contracts — foreign exchange contracts | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	0	0
Recurring | Level 3 | Available for sale financial assets, at fair value | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Available for sale financial assets, at fair value | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Available for sale financial assets, at fair value | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Available for sale financial assets, at fair value | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Available for sale financial assets, at fair value | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Available for sale financial assets, at fair value | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Available for sale financial assets, at fair value | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Available for sale financial assets, at fair value | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Available for sale financial assets, at fair value | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Short-term investments — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Total equity securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Catastrophe bonds, trading at fair value (cost — $5.8 and $5.8)	0
Recurring | Level 3 | Held for trading financial assets, at fair value | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Recurring | Level 3 | Held for trading financial assets, at fair value | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Held for trading financial assets, at fair value | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Held for trading financial assets, at fair value | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Held for trading financial assets, at fair value | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Held for trading financial assets, at fair value | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Held for trading financial assets, at fair value | Bank loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Estimate of Fair Value Measurement [Member] | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds, trading at fair value (cost — $5.8 and $5.8)		5,800,000
Loan notes issued by variable interest entities, at fair value	-70,700,000	-50,000,000
Loan notes issued by variable interest entities, at fair value (classified as a current liability)	-67,900,000
Total	7,275,300,000	6,865,900,000
Estimate of Fair Value Measurement [Member] | Recurring | Short-term investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	200,000
Estimate of Fair Value Measurement [Member] | Recurring | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	616,000,000	310,900,000
Estimate of Fair Value Measurement [Member] | Recurring | Derivatives at fair value | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	7,900,000	5,900,000
Estimate of Fair Value Measurement [Member] | Recurring | Derivatives at fair value | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	100,000	1,100,000
Estimate of Fair Value Measurement [Member] | Recurring | Liabilities under derivative contracts — foreign exchange contracts | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	-14,300,000	-2,900,000
Estimate of Fair Value Measurement [Member] | Recurring | Available for sale financial assets, at fair value | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,094,400,000	1,020,400,000
Estimate of Fair Value Measurement [Member] | Recurring | Available for sale financial assets, at fair value | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	197,400,000	269,100,000
Estimate of Fair Value Measurement [Member] | Recurring | Available for sale financial assets, at fair value | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	31,500,000	32,800,000
Estimate of Fair Value Measurement [Member] | Recurring | Available for sale financial assets, at fair value | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,319,400,000	2,069,400,000
Estimate of Fair Value Measurement [Member] | Recurring | Available for sale financial assets, at fair value | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	78,000,000	84,600,000
Estimate of Fair Value Measurement [Member] | Recurring | Available for sale financial assets, at fair value | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	665,700,000	778,900,000
Estimate of Fair Value Measurement [Member] | Recurring | Available for sale financial assets, at fair value | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	143,500,000	122,300,000
Estimate of Fair Value Measurement [Member] | Recurring | Available for sale financial assets, at fair value | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	44,800,000	62,600,000
Estimate of Fair Value Measurement [Member] | Recurring | Available for sale financial assets, at fair value | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,055,300,000	1,129,000,000
Estimate of Fair Value Measurement [Member] | Recurring | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5,630,000,000	5,569,100,000
Estimate of Fair Value Measurement [Member] | Recurring | Short-term investments — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	258,300,000	160,300,000
Estimate of Fair Value Measurement [Member] | Recurring | Total equity securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	109,900,000	149,500,000
Estimate of Fair Value Measurement [Member] | Recurring | Held for trading financial assets, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	805,800,000	716,200,000
Catastrophe bonds, trading at fair value (cost — $5.8 and $5.8)	34,800,000
Estimate of Fair Value Measurement [Member] | Recurring | Held for trading financial assets, at fair value | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		22,000,000
Estimate of Fair Value Measurement [Member] | Recurring | Held for trading financial assets, at fair value | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	200,000	200,000
Estimate of Fair Value Measurement [Member] | Recurring | Held for trading financial assets, at fair value | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,100,000	1,100,000
Estimate of Fair Value Measurement [Member] | Recurring | Held for trading financial assets, at fair value | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	529,800,000	474,800,000
Estimate of Fair Value Measurement [Member] | Recurring | Held for trading financial assets, at fair value | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	140,100,000	136,200,000
Estimate of Fair Value Measurement [Member] | Recurring | Held for trading financial assets, at fair value | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	14,700,000	12,800,000
Estimate of Fair Value Measurement [Member] | Recurring | Held for trading financial assets, at fair value | Bank loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	$85,100,000	$69,100,000

Fair_Value_Measurements_Reconc

Fair Value Measurements - Reconciliation of Level 3 Inputs (Details) (Fair Value, Measurements, Recurring, Level 3, USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance at the beginning of the period	($50)	$0
Total change in fair value included in the statement of operations	-18.6	0
Notes issued during the period	-70	-50
Balance at the end of the period	($138.60)	($50)

Fair_Value_Measurements_Pricin

Fair Value Measurements - Pricing Sources Used in Pricing Fixed Income Investments (Details)	Dec. 31, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]
Index providers	84.00%	85.00%
Pricing services	11.00%	12.00%
Broker-dealers	5.00%	3.00%
Total	100.00%	100.00%

Fair_Value_Measurements_Summar

Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$6,121	$5,813.40
% of Total Fair Value by Security Type	86.00%	86.00%
Fixed Income Maturities
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	5,395.10	5,353
% of Total Fair Value by Security Type	84.00%	85.00%
Fixed Income Maturities | U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	1,044.40	998.5
% of Total Fair Value by Security Type	95.00%	96.00%
Fixed Income Maturities | U.S. agency
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	186.9	255.3
% of Total Fair Value by Security Type	95.00%	95.00%
Fixed Income Maturities | Municipal
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	13.7	14.5
% of Total Fair Value by Security Type	42.00%	43.00%
Fixed Income Maturities | Corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	2,731.10	2,400.80
% of Total Fair Value by Security Type	96.00%	94.00%
Fixed Income Maturities | Non-U.S. government-backed corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	48.7	55.9
% of Total Fair Value by Security Type	62.00%	66.00%
Fixed Income Maturities | Foreign government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	504.4	605.8
% of Total Fair Value by Security Type	63.00%	66.00%
Fixed Income Maturities | Asset-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	140.5	130.6
% of Total Fair Value by Security Type	89.00%	97.00%
Fixed Income Maturities | Non-agency commercial mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	44.8	61
% of Total Fair Value by Security Type	100.00%	97.00%
Fixed Income Maturities | Agency mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	680.6	830.6
% of Total Fair Value by Security Type	64.00%	74.00%
Total equity securities — Available for sale
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$725.90	$460.40
% of Total Fair Value by Security Type	100.00%	100.00%

Fair_Value_Measurements_Additi

Fair Value Measurements- Additional Information (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 27, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	$2,902,700,000	$2,646,700,000	$2,583,300,000
Quotes per fixed income investment	2	2.6
Quotes per equity investment	4	4.9
Silverton
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans issued		65,000,000		65,000,000
Loan notes issued by third parties		50,000,000		50,000,000
Silverton Two
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans issued	85,000,000
Loan notes issued by third parties	70,000,000
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued by third parties	138,600,000	50,000,000
Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	0	0
Loan notes issued by third parties	120,000,000	50,000,000
Reserve for Losses	0	0
Minimum | Level 3 | Silverton
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	0
Reserve for Losses	$0

Fair_Value_Measurement_Inputs_

Fair Value Measurement - Inputs Used (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	$2,902,700,000	$2,646,700,000	$2,583,300,000
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes held by third parties	138,600,000	50,000,000
Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes held by third parties	120,000,000	50,000,000
Gross written premiums	0	0
Reserve for Losses	0	0
Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes held by third parties	120,000,000	50,000,000
Gross written premiums	40,000,000	40,000,000
Reserve for Losses	$4,600,000	$1,400,000
Contract Period	365 days	365 days

Reinsurance_Summary_of_Assumed

Reinsurance - Summary of Assumed and Ceded Reinsurance on Premiums Written, Premiums Earned and Insurance Losses and Loss Adjustment Expenses (Details) (USD $)	3 Months Ended												12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Premiums written:
Direct													$1,729.90	$1,512.80	$1,355.40
Assumed													1,172.80	1,133.90	1,227.90
Ceded													-387.5	-347	-336.4
Net premiums written													2,515.20	2,299.70	2,246.90
Premiums earned:
Direct													1,599	1,366.80	1,177
Assumed													1,137.60	1,126.60	1,208
Ceded													-331.3	-321.6	-301.5
Net premiums earned	612.2	610.4	616.2	566.5	572.6	544.3	544	510.9	558.5	516.2	513.4	495.4	2,405.30	2,171.80	2,083.50
Insurance losses and loss adjustment expenses:
Direct													908.2	829.4	763
Assumed													496.9	459.4	650.1
Ceded													-97.6	-65.1	-174.6
Net insurance losses and loss adjustment expenses	$339.60	$342.70	$337.10	$288.10	$331.40	$290.20	$333.40	$268.70	$437.40	$255	$262.10	$284	$1,307.50	$1,223.70	$1,238.50

Derivative_Contracts_Fair_Valu

Derivative Contracts - Fair Value of Derivative Instruments (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount			$1,000,000,000
Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	135,800,000
Not Designated as Hedging Instrument | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	403,400,000		281,900,000
Not Designated as Hedging Instrument | Derivatives at Fair Value | Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Notional Amount	951,300,000		1,000,000,000
Fair Value			1,100,000	[1]
Not Designated as Hedging Instrument | Derivatives at Fair Value | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Notional Amount	165,800,000		224,400,000
Fair Value			5,900,000
Not Designated as Hedging Instrument | Liabilities under Derivative Contracts | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Notional Amount	237,600,000		57,500,000
Fair Value			-2,900,000
Designated as Hedging Instrument | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	135,800,000		0
Derivative Financial Instruments, Assets | Level 2 | Fair Value, Measurements, Recurring | Not Designated as Hedging Instrument | Derivatives at Fair Value | Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Fair Value	100,000	[1]
Derivative Financial Instruments, Assets | Level 2 | Fair Value, Measurements, Recurring | Not Designated as Hedging Instrument | Derivatives at Fair Value | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Fair Value	7,900,000
Liabilities under derivative contracts — foreign exchange contracts | Level 2 | Fair Value, Measurements, Recurring | Not Designated as Hedging Instrument | Liabilities under Derivative Contracts | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Fair Value	($10,500,000)
[1]	Net of $22.3 million of cash collateral provided to counterparties, Goldman Sachs International ($451.3 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2013 — $34.3 million).

Derivative_Contracts_GainLoss_

Derivative Contracts - Gain/(Loss) Recognized in Income on Derivative (Details) (Change in Fair Value of Derivatives, USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Fair Value Hedges Recognized in Earnings	($0.30)	$0
Gain (charge) to net income from derivative instruments	-7.7	-1.3
Gain (Loss) on Foreign Currency Fair Value Hedge Derivatives	-3.8	0
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Gain (charge) to net income from derivative instruments	($7.20)	$2.60

Derivative_Contracts_Additiona

Derivative Contracts - Additional Information (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange	$135,800,000
Foreign Exchange Contracts | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange	403,400,000		281,900,000
Foreign Exchange Contracts | Not Designated as Hedging Instrument | Derivative At Fair Value
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value			5,900,000
Derivative Liability, Notional Amount	165,800,000		224,400,000
Foreign Exchange Contracts | Not Designated as Hedging Instrument | Derivative Financial Instruments, Assets | Level 2 | Fair Value, Measurements, Recurring | Derivative At Fair Value
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value	7,900,000
Foreign Exchange Contracts | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Collateral provided to counterparties as security for the Company's net liability position	0		0
Purchase of US and foreign exchange	135,800,000		0
Foreign Exchange Contracts | Change in Fair Value of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (charge) to net income from derivative instruments	-7,700,000		-1,300,000
Gain (Loss) on Foreign Currency Fair Value Hedge Derivatives	-3,800,000		0
Gain (Loss) on Fair Value Hedges Recognized in Earnings	-300,000		0
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Collateral provided to counterparties as security for the Company's net liability position	22,300,000		34,300,000
Purchase of US and foreign exchange			1,000,000,000
Pledged assets	0		0
Pledged assets recorded in balance sheet	0
Interest Rate Swaps | Contract Termination
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange	48,700,000
Interest Rate Swaps | Not Designated as Hedging Instrument | Derivative At Fair Value
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value			1,100,000	[1]
Derivative Liability, Notional Amount	951,300,000		1,000,000,000
Interest Rate Swaps | Not Designated as Hedging Instrument | Derivative Financial Instruments, Assets | Level 2 | Fair Value, Measurements, Recurring | Derivative At Fair Value
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value	100,000	[1]
Interest Rate Swaps | Change in Fair Value of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (charge) to net income from derivative instruments	-7,200,000		2,600,000
Goldman Sachs International
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange	451,300,000
Credit Agricole CIB
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange	$500,000,000
[1]	Net of $22.3 million of cash collateral provided to counterparties, Goldman Sachs International ($451.3 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2013 — $34.3 million).

Deferred_Policy_Acquisition_Co2

Deferred Policy Acquisition Costs - Reconciliation of beginning and ending deferred policy acquisition costs (Details) (USD $)	3 Months Ended												12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance at the beginning of the period				$262.20				$223					$262.20	$223
Acquisition costs deferred													488	461.2
Amortization of deferred policy acquisition costs	-114.8	-115.5	-108.9	-112	-99.7	-110.5	-107.2	-104.6	-80	-103.1	-102	-96.1	-451.2	-422	-381.2
Balance at the end of the period	$299				$262.20				$223				$299	$262.20	$223

Reserves_for_Losses_and_Adjust2

Reserves for Losses and Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses (LAE) Reserves (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Insurance [Abstract]
Provision for losses and LAE, beginning balance	$4,678.90	$4,779.70	$4,525.20
Less reinsurance recoverable		-332.7	-499	-426.6
Net losses and LAE reserves, beginning balance	4,346.20	4,280.70	4,098.60
Net loss and LAE expenses (disposed)	-24.2	-34.6	-9
Provision for losses and LAE for claims incurred:
Current year	1,411.60	1,331.40	1,375.90
Prior years	-104.1	-107.7	-137.4
Total incurred	1,307.50	1,223.70	1,238.50
Losses and LAE payments for claims incurred:
Current year	-112.1	-172.8	-244.3
Prior years	-995.6	-912.3	-835.7
Total paid	-1,107.70	-1,085.10	-1,080
Foreign exchange (gains)/losses	-121	-38.5	32.6
Net losses and LAE reserves, ending balance	4,400.80	4,346.20	4,280.70
Plus reinsurance recoverable on unpaid losses at the end of the year	350	332.7	499
Provision for losses and LAE, ending balance	$4,750.80	$4,678.90	$4,779.70

Reserves_for_Losses_and_Adjust3

Reserves for Losses and Adjustment Expenses - Additional Information (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Insurance [Abstract]
Prior years	$104.10	$107.70	$137.40
Liability For Unpaid Claims And Claims Adjustment Expense Amount Disposed Of	$24.20	$34.60	$9

Income_Taxes_Summary_of_Total_

Income Taxes - Summary of Total Income Tax (Details) (USD $)	3 Months Ended												12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Income tax expense on income	$0.80	$1.30	$6.20	$3.80	$3.60	$2.90	$1	$5.90	($4.60)	$8	$6.20	$5.40	$12.10	$13.40	$15
Income tax (benefit)/expense on other comprehensive income													5.1	13.7	-9.3
Income tax (benefit) charged directly to shareholders’ equity													1.2	1.5	2.4
Total income tax expense/(benefit)													$5.80	($1.80)	$21.90

Income_Taxes_IncomeLoss_Before

Income Taxes - Income/(Loss) Before Tax and Income Tax Expense/(Benefit) Attributable to that Income (Details) (USD $)	3 Months Ended												12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
(Loss)/income before tax, U.S.													($24.10)	($12.20)	($58.30)
(Loss)/income before tax, Non-U.S.													392	354.9	353.7
Income from operations before income tax	68	38.7	137	124.2	93.6	110.3	41.1	97.7	-2.6	123.1	90.8	84.1	367.9	342.7	295.4
Current income tax (benefit)/expense, U.S.													0.8	3.8	4.4
Current income tax (benefit)/expense, Non-U.S.													22.6	5.9	18.4
Current income tax (benefit)/expense, Total													23.4	9.7	22.8
Deferred income Tax (benefit), U.S.													0	0	0
Deferred income Tax (benefit), Non-U.S.													-11.3	3.7	-7.8
Deferred Income Tax, Total													-11.3	3.7	-7.8
Total income tax expense/(benefit), U.S.													0.8	3.8	4.4
Total income tax expense/(benefit), Non-U.S.													11.3	9.6	10.6
Total income tax expense/(benefit)	$0.80	$1.30	$6.20	$3.80	$3.60	$2.90	$1	$5.90	($4.60)	$8	$6.20	$5.40	$12.10	$13.40	$15

Income_Taxes_Income_Tax_Reconc

Income Taxes - Income Tax Reconciliation (Details) (USD $)	3 Months Ended												12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Income Tax Disclosure [Abstract]
Expected tax (benefit)/expense at weighted average rate													($7.30)		($7.50)		($14.40)
Prior year adjustments													-0.6	[1]	-4.2	[1]	-4.9	[1]
Valuation provision on U.S. deferred tax assets													12.7		15.1		26.7
Uncertain tax positions													5.3		8.5		9.6
Non-utilizable foreign tax credits													0		2.6		0
Disallowable expenses													1.8		1.6		1.2
Other non-taxable items													0		-0.2		-2.4
Impact of changes in statutory tax rates													0.2		-2.5		-0.8
Total income tax expense/(benefit)	$0.80	$1.30	$6.20	$3.80	$3.60	$2.90	$1	$5.90	($4.60)	$8	$6.20	$5.40	$12.10		$13.40		$15
[1]	The submission dates for filing income tax returns for the Company’s U.S. and U.K. operating subsidiaries are after the submission date of the Company’s Annual Report on Form 10-K. The final tax liabilities may differ from the estimated tax provisions included in the Annual Report on Form 10-K and may result in prior year adjustments being reported.

Income_Taxes_Schedule_of_Unrec

Income Taxes - Schedule of Unrecognized Tax Benefits (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Unrecognized tax benefits, beginning balance	$23.90	$15.40
Gross increases/(decreases) for tax positions of prior years	5.3	8.5
Gross increases/(decreases) for tax positions of current year	0	0
Unrecognized tax benefits, ending balance	$29.20	$23.90

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Narrative) (Details) (USD $)	12 Months Ended						3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Mar. 31, 2014	Dec. 31, 2014	Mar. 31, 2016
Income Taxes [Line Items]
United States corporate tax at a rate	34.00%
Accrued interest and penalties			$0
Prior year adjustments	-600,000	[1]	-4,200,000	[1]	-4,900,000	[1]
Gross increases/(decreases) for tax positions of prior years	5,300,000		8,500,000
Unrecognized Tax Benefits	29,200,000		23,900,000		15,400,000			29,200,000
Unrecognized Tax Benefits, Treatment of Interest			10,000,000
Unrecognized Tax Benefits, Valuation Allowance			13,500,000
Tax Credit Carryforward, Amount			400,000
Final Determination of Equalization Reserves [Member]
Income Taxes [Line Items]
Prior year adjustments			-2,000,000
Accumulated Net Unrealized Investment Gain (Loss)
Income Taxes [Line Items]
Prior year adjustments					-3,900,000
Employee Stock Awards
Income Taxes [Line Items]
Prior year adjustments					-1,000,000
Acquisition-related Costs
Income Taxes [Line Items]
Prior year adjustments			($2,000,000)
U.K.
Income Taxes [Line Items]
U. K. corporate tax rate reduced during the period							23.00%	21.00%
U.K. | Forecast
Income Taxes [Line Items]
U. K. corporate tax rate reduced during the period									20.00%
[1]	The submission dates for filing income tax returns for the Company’s U.S. and U.K. operating subsidiaries are after the submission date of the Company’s Annual Report on Form 10-K. The final tax liabilities may differ from the estimated tax provisions included in the Annual Report on Form 10-K and may result in prior year adjustments being reported.

Deferred_Taxation_Tax_Effects_

Deferred Taxation - Tax Effects of Deferred Tax Assets and Deferred Tax Liabilities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Deferred tax assets:
Share options	$2.80	$3.90
Operating loss carry forwards	94.4	80.9
Loss reserves	1.8	1.7
Unrealized losses on investments	0	13.3
Accrued expenses	11.2	8.4
Foreign tax credits	13	7.5
Unearned premiums	4.9	6.5
Timing differences on fixed assets	8.7	5.6
Other temporary differences	6.7	8.5
Total gross deferred tax assets	143.5	136.3
Less valuation allowance	-106.5	-93.8
Net deferred tax assets	37	42.5
Deferred tax liabilities:
Equalization provision reserves	-32.1	-34.1
Intangible assets (other)	-2.2	-1.8
Unrealized (gains) on investments	-1.5	-0.2
Deferred policy acquisition costs	-2	-4.2
Other temporary differences	-2.3	-0.6
Total gross deferred tax (liabilities)	-40.1	-40.9
Net deferred tax (liability)/asset	($3.10)	$1.60

Deferred_Taxation_Additional_I

Deferred Taxation - Additional Information (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Valuation allowance	$106,500,000	$93,800,000
Increase in valuation allowance	12,700,000
Income Statement
Valuation allowance	12,700,000
Accumulated Other Comprehensive Income
Valuation allowance	0
Internal Revenue Service (IRS)
Net operating loss carryforwards	267,400,000	230,600,000
Capital loss carryforwards	1,300,000
Charitable contribution carryforwards	600,000
Valuation allowance	$106,500,000	$93,800,000
Internal Revenue Service (IRS) | Minimum
Net operating loss carryforwards expiration year	2026
Internal Revenue Service (IRS) | Maximum
Net operating loss carryforwards expiration year	2034

Capital_Structure_Summary_of_A

Capital Structure - Summary of Authorized and Issued Share Capital (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 15, 2006
In Thousands, except Share data, unless otherwise specified
Authorized share capital:
Number of ordinary shares	969,629,030	969,629,030
Number of non-voting shares	6,787,880	6,787,880
Number of preference shares	100,000,000	100,000,000
Ordinary Shares, authorized	$1,469	$1,469
Non-Voting Shares, authorized	10	10
Preference Shares, authorized	152	152
Total authorized share capital	1,631	1,631
Issued share capital:
Ordinary shares, issued	62,017,368	65,546,976	70,753,723
Ordinary shares, value	94	99
Total issued share capital	129	134
7.401% Preference Shares
Issued share capital:
Preference shares, issued	5,327,500	5,327,500		8,000,000
Preferred Stock, Value, Subscriptions	8	8
7.250% Preference Shares
Issued share capital:
Preference shares, issued	6,400,000	6,400,000
Preferred Stock, Value, Subscriptions	10	10
Series D Preferred Stock [Member]
Issued share capital:
Preference shares, issued	11,000,000	11,000,000
Preferred Stock, Value, Subscriptions	$17	$17

Capital_Structure_Summary_of_A1

Capital Structure - Summary of Authorized and Issued Share Capital (Phantom) (Detail) (USD $)	0 Months Ended
	2-May-13	Dec. 31, 2014	Dec. 31, 2013	Apr. 25, 2013	Nov. 15, 2006	Apr. 11, 2012
Class of Stock [Line Items]
Ordinary shares, par value		$0.15	$0.15
Non-voting shares, par value		$0.15	$0.15
Preference shares, par value		$0.15	$0.15
Redemption price per share		$25	$25
Perpetual Non Cumulative Preference Shares
Class of Stock [Line Items]
Redemption price per share	$25	$25
Preferred Stock, Call or Exercise Features	No stated maturity but are callable at the option of the Company on or after the 10th anniversary of the date of issuance.
Preferred Stock, Dividend Payment Rate, Variable	3-month LIBOR
5.625% Preference Shares
Class of Stock [Line Items]
Redemption price per share				$50
Preference shares, rate		5.63%
7.401% Preference Shares
Class of Stock [Line Items]
Preference shares, par value		$0.15	$0.15
Redemption price per share		$25	$25		$25
Preference shares, rate		7.40%	7.40%
7.250% Preference Shares
Class of Stock [Line Items]
Preference shares, par value		$0.15	$0.15
Redemption price per share		$25	$25			$25
Preference shares, rate		7.25%	7.25%
5.950% Preference Shares
Class of Stock [Line Items]
Preference shares, par value		$0.15	$0.15
Redemption price per share		$25	$25
Preference shares, rate		5.95%	5.95%

Capital_Structure_Summary_of_O

Capital Structure - Summary of Ordinary Shares (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Equity [Abstract]
Ordinary shares in issue at the beginning of the year	65,546,976	70,753,723
Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or 2008 share purchase plan	756,676	1,374,567
Ordinary shares issued to non-employee directors	3,573	44,000
Ordinary shares repurchased	-4,289,857	-8,461,174
Ordinary shares issued in respect of the redemption of the PIERS	0	1,835,860
Ordinary shares in issue at the end of the year	62,017,368	65,546,976

Capital_Structure_Additional_I

Capital Structure - Additional Information (Narrative) (Details) (USD $)	0 Months Ended			12 Months Ended								0 Months Ended
	Apr. 17, 2014	Apr. 25, 2013	Nov. 15, 2006	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2006	Dec. 31, 2005	Mar. 31, 2009	2-May-13	Apr. 11, 2012	Aug. 10, 2012	Mar. 23, 2012	Mar. 09, 2012	Aug. 29, 2013	Mar. 01, 2013	Mar. 04, 2013	Aug. 17, 2012	Feb. 05, 2015	Feb. 02, 2012	Dec. 31, 2011	Feb. 07, 2013	Oct. 24, 2012	Feb. 26, 2013
Class of Stock [Line Items]
Additional paid in capital				$1,134,300,000		$1,297,400,000
Aggregate Liquidation Preferences				568,200,000		568,200,000								154,500,000
Preferred Stock Issuance Cost			3,700,000	12,400,000		12,400,000
Stock Repurchase Program, Authorized Amount																							400,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount				43,300,000																				192,400,000
Stock Repurchased and Retired During Period, Shares								2,064,643
Stock Repurchased and Retired During Period, Value								59,900,000
Stock Repurchased During Period, Shares				4,289,857		8,461,174
Percentage Of Conversion Price										130.00%	130.00%
Conversion Price, Number of Trading Days in Period		20 days								20 days	20 days
Number of Consecutive Trading Days in Period										30 days	30 days
Preferred Stock, Liquidation Preference Per Share				$25		$25
Stock Issued During Period Shares For Redemption Of Preferred Income Equity Replacement Secs				0		1,835,860
Changes In Redemption Value Of Preferred Income Equity Replacement Securities				0	[1]	7,100,000	[1]	0	[1]
Preference Dividend Basis Spread On Variable Rate			3.28%
Proceeds from Issuance of Preferred Stock and Preference Stock			196,300,000	0		270,600,000		154,500,000
Preferred Stock, Redemption Date				1-Jul-23
Class of Warrant or Right, Exercise Price of Warrants or Rights	$160
Class of Warrant or Right, Rights Agreement Exercise Rule, Ordinary Share Ownership Percentage	10.00%
Passive Institutional Investors
Class of Stock [Line Items]
Class of Warrant or Right, Rights Agreement Exercise Rule, Ordinary Share Ownership Percentage	15.00%
5.625% Preference Shares
Class of Stock [Line Items]
Preferred Stock, Shares Issued										4,600,000	4,600,000
Preferred Stock, Dividend Rate, Percentage										5.63%	5.63%
Preferred Stock, Liquidation Preference Per Share		50
Conversion Rate Of Ordinary Shares		1.7121
Stock Issued During Period Shares For Redemption Of Preferred Income Equity Replacement Secs		1,835,860
Preferred Stock
Class of Stock [Line Items]
Stock Redeemed or Called During Period, Value		230,000,000		230,000,000		230,000,000
7.401% Preference Shares
Class of Stock [Line Items]
Aggregate Liquidation Preferences			200,000,000
Preferred Stock, Shares Issued			8,000,000	5,327,500		5,327,500
Preferred Stock, Dividend Rate, Percentage			7.40%	7.40%								7.40%
Preferred Stock, Liquidation Preference Per Share			$25	$25		$25
Stock Issued During Period, Shares, Purchase of Assets												2,672,500
Preferred Stock, Redemption Price Per Share												$12.50
Gain On Purchase Of Shares												31,500,000
Write Off Of Preferred Share Issuance Cost												1,200,000
Perpetual Non Cumulative Preference Shares
Class of Stock [Line Items]
Aggregate Liquidation Preferences													270,600,000
Preferred Stock Issuance Cost													4,400,000	5,500,000
Preferred Stock, Shares Issued													11,000,000	6,400,000
Preferred Stock, Dividend Rate, Percentage													5.95%	7.25%
Preferred Stock, Liquidation Preference Per Share				$25									$25
Preferred Stock, Liquidation Preference, Value													275,000,000	160,000,000
Preferred Stock Spread Variable Dividend Rate													4.06%
5.950% Preference Shares
Class of Stock [Line Items]
Preferred Stock, Shares Issued				11,000,000		11,000,000
Preferred Stock, Dividend Rate, Percentage				5.95%
Preferred Stock, Liquidation Preference Per Share				$25		$25
7.250% Preference Shares
Class of Stock [Line Items]
Preferred Stock, Shares Issued				6,400,000		6,400,000
Preferred Stock, Dividend Rate, Percentage				7.25%
Preferred Stock, Liquidation Preference Per Share				$25		$25								$25
Reclassification from Additional Paid in Capital to Retained Earnings
Class of Stock [Line Items]
Changes In Redemption Value Of Preferred Income Equity Replacement Securities				7,100,000		7,100,000
New Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount																									500,000,000	400,000,000
Names Trustee Repurchase Program
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Shares															34,151	26,708	42,578
Stock Repurchased and Retired During Period, Value															1,000,000	700,000	1,100,000
Existing Share Repurchase Program
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount																									400,000,000
Accelerated Share Repurchase Agreement
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount																											150,000,000
Stock Repurchased and Retired During Period, Shares																		4,053,276	3,348,214
Share Repurchase Agreement Commencement Date				7-Feb-15
Common Stock Additional Shares Receivable																		705,062
Stock Repurchased And Retired During Period Average Price Per Share																		$37.01
Names' Trustee
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value																				2,000,000
Stock Repurchased During Period, Shares																				54,437
Open Markets
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Shares				4,289,857
Stock Repurchased and Retired During Period, Value				180,900,000
Stock Repurchased And Retired During Period Average Price Per Share				$42.16
Senior Notes Due December 15, 2020
Class of Stock [Line Items]
Interest Rate Of Senior Notes																					6.00%
Preferred Stock
Class of Stock [Line Items]
Class of Warrant or Right, Number of Securities Called by Each Warrant or Right	1
Subsequent Event | 7.401% Preference Shares
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage																						7.40%
Subsequent Event | 5.950% Preference Shares
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage																						5.95%
Subsequent Event | 7.250% Preference Shares
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage																						7.25%
Subsequent Event | Repurchase Program 2015
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount																						500,000,000
Share Repurchase Agreement Commencement Date																						5-Feb-15
[1]	The $7.1 million deduction from net income in 2013 is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million. For more information, please refer to Note 15, “Capital Structure” of these consolidated financial statements.

Statutory_Requirements_and_Div2

Statutory Requirements and Dividends Restrictions - Summary of Statutory Requirements and Dividends Restrictions (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
U.S.
Required statutory capital and surplus	$56.90	$40.90
Statutory capital and surplus	394.1	375.3
Bermuda
Required statutory capital and surplus	1,097.60	1,068.20
Statutory capital and surplus	2,052.30	1,907.40
U.K.
Required statutory capital and surplus	202.2	218.8
Statutory capital and surplus	$989.80	$1,005.10

Statutory_Requirements_and_Div3

Statutory Requirements and Dividends Restrictions - Addtional Information (Narrative) (Details)	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2014
In Millions, unless otherwise specified	AUL	U.K.	U.K.	Bermuda	Bermuda
	USD ($)	USD ($)	GBP (£)	USD ($)	AUL
					USD ($)
Statutory Accounting Practices [Line Items]
Dividend payment made without regulatory approval		$137.50	£ 6.1	$513.10
Capital contributions reserves		470
Dividend Payments Restrictions Schedule, Statutory Capital and Surplus, Percent				25.00%
Statutory Capital and Surplus, Percent Reduction Requiring Approval				15.00%
Syndicate to maintain funds at Lloyd	395.1
Total funds held by AUL	$411.20				$377.30
Percent Amount of Enhanced Capital Required from Statutory Capital and Surplus				60.00%
Percent Warning Level of Amount of Enhanced Capital Required from Statutory Capital and Surplus				120.00%

Retirement_Plans_Additional_In

Retirement Plans - Additional Information (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Compensation and Retirement Disclosure [Abstract]
Rate of contribution on retirement plans of its employees salaries, Maximum	20.00%
Total contributions by the Company to the retirement plan	$13.70	$11.70	$11.10

ShareBased_Payments_Schedule_o

Share-Based Payments - Schedule of Number of Names Options Exercised and Number of Shares Issued (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	3,006,760	3,006,760
Options Exercised	3,006,760	3,006,760
Ordinary Shares Issued	784,001	784,001
2002
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	3,006,760	3,006,760
Options Exercised	0	0
Ordinary Shares Issued	0	0
2003
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	0	0
Options Exercised	440,144	440,144
Ordinary Shares Issued	152,583	152,583
2004
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	0	0
Options Exercised	856,218	856,218
Ordinary Shares Issued	135,321	135,321
2005
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	0	0
Options Exercised	303,321	303,321
Ordinary Shares Issued	56,982	56,982
2006
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	0	0
Options Exercised	34,155	34,155
Ordinary Shares Issued	3,757	3,757
2007
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	0	0
Options Exercised	66,759	66,759
Ordinary Shares Issued	7,381	7,381
2008
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	0	0
Options Exercised	20,641	20,641
Ordinary Shares Issued	3,369	3,369
2009
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	0	0
Options Exercised	9,342	9,342
Ordinary Shares Issued	3,056	3,056
2010
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	0	0
Options Exercised	149,895	149,895
Ordinary Shares Issued	49,538	49,538
2011
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	0	0
Options Exercised	761,037	761,037
Ordinary Shares Issued	255,504	255,504
2012
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Granted	0	0
Options Exercised	365,248	365,248
Ordinary Shares Issued	116,510	116,510

ShareBased_Payments_Informatio

Share-Based Payments - Information About Employee Options Outstanding to Purchase Ordinary Shares (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Granted	3,006,760		3,006,760
Options Exercised	3,006,760		3,006,760
Exercise Price	$25.26		$22.45
2003 Option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Granted	3,884,030
Options Forfeited	712,906
Options Exercised	3,171,124
Outstanding and Exercisable	0
Exercise Price	$16.20
Weighted Average Fair Value at Grant Date	$5.31
2004 Option grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Granted	500,113
Options Forfeited	276,082
Options Exercised	224,031
Outstanding and Exercisable	0
Exercise Price	$24.44
Weighted Average Fair Value at Grant Date	$5.74
2006 Option grants February 16
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Granted	1,072,490
Options Forfeited	450,567
Options Exercised	483,347
Outstanding and Exercisable	138,576
Exercise Price	$23.65
Weighted Average Fair Value at Grant Date	$6.99
Remaining Contractual Time	1 year 2 months
2007 Option grants May 4
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Granted	607,635	[1]
Options Forfeited	152,276
Options Exercised	376,241
Outstanding and Exercisable	79,118
Exercise Price	$27.28
Weighted Average Fair Value at Grant Date	$6.14
[1]	date for the 2007 options were extended for one year to May 4, 2015.

ShareBased_Payments_Per_share_

Share-Based Payments - Per share Weighted Average Fair Value and Related Underlying Assumptions Using Modified Black-Scholes Option Pricing Model by Date of Grant (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
22-Oct-07
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Average Fair Value at Grant Date	$5.76
Risk free interest rate	4.09%
Dividend yield	2.10%
Expected life	5 years
Share price volatility	20.28%
4-May-07
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Average Fair Value at Grant Date	$6.14
Risk free interest rate	4.55%
Dividend yield	2.20%
Expected life	5 years
Share price volatility	23.76%
4-Aug-06
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Average Fair Value at Grant Date	$4.41
Risk free interest rate	5.06%
Dividend yield	2.60%
Expected life	5 years
Share price volatility	19.33%
16-Feb-06
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Average Fair Value at Grant Date	$6.99
Risk free interest rate	4.66%
Dividend yield	2.70%
Expected life	5 years
Share price volatility	35.12%
23-Dec-04
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Average Fair Value at Grant Date	$5.74
Risk free interest rate	3.57%
Dividend yield	0.50%
Expected life	5 years
Share price volatility	19.68%
Foreign currency volatility	0.00%
20-Aug-03
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Average Fair Value at Grant Date	$5.31	[1]
Risk free interest rate	4.70%	[1]
Dividend yield	0.60%	[1]
Expected life	7 years	[1]
Foreign currency volatility	9.40%	[1]
[1]	The 2003 options had a price volatility of zero. The minimum value method was utilized because the Company was unlisted on the date that the options were issued. Foreign currency volatility of 9.4% was applied as the exercise price was initially in British Pounds and the share price of the Company is in U.S. Dollars.

ShareBased_Payments_Restricted

Share-Based Payments - Restricted Share Units (Details) (Restricted Share Units)	12 Months Ended
	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount Granted	1,883,891
Amount Vested	1,205,836
Amount Forfeited	158,943
Amount Outstanding	519,112
2004 - 2011 Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount Granted	965,911
Amount Vested	897,080
Amount Forfeited	68,831
Amount Outstanding	0
2012 Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount Granted	350,899
Amount Vested	208,744
Amount Forfeited	47,533
Amount Outstanding	94,622
2013 Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount Granted	307,441
Amount Vested	100,012
Amount Forfeited	27,511
Amount Outstanding	179,918
2014 Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount Granted	259,640
Amount Vested	0
Amount Forfeited	15,068
Amount Outstanding	244,572

ShareBased_Payments_Summary_of

Share-Based Payments - Summary of Performance Shares by Year of Grants (Details) (Performance Share Awards)	12 Months Ended
	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount Granted	5,104,224
Amount Vested	2,711,018
Amount Forfeited	2,124,788
Amount Outstanding	268,418
2004 - 2011 Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount Granted	4,194,638	[1]
Amount Vested	2,220,442	[1]
Amount Forfeited	1,974,196	[1]
Amount Outstanding	0	[1]
2012 Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount Granted	344,131
Amount Vested	241,367
Amount Forfeited	102,764
Amount Outstanding	0
2013 Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount Granted	250,066	[2]
Amount Vested	127,153	[2]
Amount Forfeited	43,728	[2]
Amount Outstanding	79,185	[2]
2014 Grants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount Granted	315,389	[2]
Amount Vested	122,056	[2]
Amount Forfeited	4,100	[2]
Amount Outstanding	189,233	[2]
[1]	The amounts vested and forfeited in respect of the 2004 - 2011 performance share awards have been updated to reflect employees leaving after the financial reporting date but before the final vesting date.
[2]	These balances could increase depending on future performance.

ShareBased_Payments_Summary_of1

Share-Based Payments - Summary of Performance and Phantom Shares Which are Vested, Roe Achieved and Vested (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Phantom Share Actual Vesting Percentage	43.00%
2011 Performance Shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Performance Shares Split, year one	33.30%
Performance Shares Split, year two	33.30%
Performance Shares Split, year three	33.30%
Performance Shares Split, Total	100.00%
ROE Achieved in year one	-5.30%			-5.30%
ROE Achieved in year two	10.00%		10.00%
ROE Achieved in year three	11.70%	11.70%
Performance Shares Banked, year one	0.00%
Performance Shares Banked, year two	33.30%
Performance Shares Banked, year three	39.00%
Performance Shares Banked, Total	72.30%
2012 Performance Shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Performance Shares Split, year one	33.30%
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Equity Securities	8.10%
Performance Shares Split, year two	33.30%
Fair Value Investments Entities That Calculate Net Asset Value Per Share Percent Equity Securities Year Two	6.20%
Performance Shares Split, year three	33.30%
Performance Shares Split, Total	100.00%
Performance Shares Banked, year one	21.90%
Performance Shares Banked, year two	10.50%
Performance Shares Banked, Total	75.40%
Fair Value Investments Entities That Calculate Net Asset Value Per Share Percent Equity Securities Year Three	13.30%
Phantom Share Actual Vesting Percentage Year Three	43.00%
2013 Performance Shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Performance Shares Split, year one	33.30%
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Equity Securities	6.20%
Performance Shares Split, year two	33.30%
Fair Value Investments Entities That Calculate Net Asset Value Per Share Percent Equity Securities Year Two	13.30%
Performance Shares Split, year three	33.30%
Performance Shares Split, Total	100.00%
Performance Shares Banked, year one	10.50%
Performance Shares Banked, year two	43.00%
Performance Shares Banked, Total	53.50%
2014 Performance Shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Performance Shares Split, year one	33.30%
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Equity Securities	13.30%
Performance Shares Split, year two	33.30%
Performance Shares Split, year three	33.30%
Performance Shares Split, Total	100.00%
Performance Shares Banked, year one	43.00%
Performance Shares Banked, Total	43.00%
2012 Phantom Shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Phantom Share Split Percentage Next Twelve Months	33.30%
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Equity Securities	8.10%
Fair Value Investments Entities That Calculate Net Asset Value Per Share Percent Equity Securities Year Two	6.20%
Phantom Share Actual Vesting Percentage Year One	21.90%
Phantom Share Split Percentage Year Two	33.30%
Phantom Share Actual Vesting Percentage Year Two	10.50%
Phantom Share Split Percentage Year Three	33.30%
Phantom Share Split Percentage	100.00%
Phantom Share Actual Vesting Percentage	75.40%
Fair Value Investments Entities That Calculate Net Asset Value Per Share Percent Equity Securities Year Three	13.30%
Phantom Share Actual Vesting Percentage Year Three	43.00%
2013 Phantom Shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Phantom Share Split Percentage Next Twelve Months	33.30%
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Equity Securities	6.20%
Fair Value Investments Entities That Calculate Net Asset Value Per Share Percent Equity Securities Year Two	13.30%
Phantom Share Actual Vesting Percentage Year One	10.50%
Phantom Share Split Percentage Year Two	33.30%
Phantom Share Actual Vesting Percentage Year Two	43.00%
Phantom Share Split Percentage Year Three	33.30%
Phantom Share Split Percentage	100.00%
Phantom Share Actual Vesting Percentage	53.50%
2014 Phantom Shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Phantom Share Split Percentage Next Twelve Months	33.30%
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Equity Securities	13.30%
Phantom Share Actual Vesting Percentage Year One	43.00%
Phantom Share Split Percentage Year Two	33.30%
Phantom Share Split Percentage Year Three	33.30%
Phantom Share Split Percentage	100.00%

ShareBased_Payments_Summary_of2

Share-Based Payments - Summary of Performance Share Activity Under Aspen's 2003 Share Incentive Plan (Details) (Performance Share Incentive Plan 2003, USD $)	12 Months Ended
	Dec. 31, 2014
Performance Share Incentive Plan 2003
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding, beginning of period	597,479
Granted, Number of Shares	315,389
Earned	-373,935
Amount Forfeited	-270,515
Number of shares outstanding, end of period	268,418
Number of shares outstanding ,weighted average grant date fair value beginning of period	$27.15
Granted, weighted average grant date fair value	$38.46
Earned, weighted average grant date fair value	$30.12
Forfeited, weighted average grant date fair value	$27.61
Number of shares outstanding, weighted average grant date fair value end of period	$25.35

ShareBased_Payments_Amounts_fo

Share-Based Payments - Amounts for Employee Options Granted were Estimated on Date of Grant Using Modified Black-Scholes Option Pricing Model (Details) (Employee Stock Purchase Plan, USD $)	12 Months Ended
	Dec. 31, 2014
4-Nov-08
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$3.18
Risk free interest rate	0.48%
Dividend yield	2.70%
Expected life	3 years
Share price volatility	68.00%
4-Dec-08
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$2.87
Risk free interest rate	-0.41%
Dividend yield	3.16%
Expected life	2 years
Share price volatility	102.00%
23-Nov-09
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$3.76
Risk free interest rate	0.01%
Dividend yield	2.28%
Expected life	3 years
Share price volatility	22.00%
21-Dec-09
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$3.82
Risk free interest rate	0.04%
Dividend yield	2.34%
Expected life	2 years
Share price volatility	18.00%
December 22, 2010 grant one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$4.24
Risk free interest rate	0.13%
Dividend yield	2.07%
Expected life	3 years
Share price volatility	14.00%
December 22, 2010 grant two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$4.46
Risk free interest rate	0.13%
Dividend yield	2.07%
Expected life	2 years
Share price volatility	14.00%
December 13, 2011 grant one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$4.20
Risk free interest rate	0.05%
Dividend yield	2.80%
Expected life	3 years
Share price volatility	26.20%
December 13, 2011 grant two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$3.85
Risk free interest rate	0.05%
Dividend yield	2.75%
Expected life	2 years
Share price volatility	26.20%
March 20, 2013 grant one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$7.79
Risk free interest rate	0.38%
Dividend yield	1.88%
Expected life	3 years
Share price volatility	2.80%
March 20, 2013 grant two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$5.75
Risk free interest rate	0.25%
Dividend yield	1.88%
Expected life	2 years
Share price volatility	3.20%
September26, 2014 grant date one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$6.61
Risk free interest rate	1.06%
Dividend yield	1.87%
Expected life	3 years
Share price volatility	6.20%
September 26, 2014 grant date two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value	$6.43
Risk free interest rate	0.58%
Dividend yield	1.87%
Expected life	2 years
Share price volatility	4.00%

ShareBased_Payments_Summary_of3

Share-Based Payments - Summary of Information About Non-employee Director Options Outstanding to Purchase Ordinary Shares (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Outstanding	224,141	302,460	1,208,787
Exercise Price	$25.26	$22.45
Non-employee directors - 2006 Option grants (May 25)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Outstanding	4,435
Options Exercisable	4,435
Exercise Price	$21.96
Fair Value at Grant Date	$4.24
Remaining Contractual Time	1 year 5 months
Non-employee directors - 2007 Option grants (July 30)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options Outstanding	2,012
Options Exercisable	2,012
Exercise Price	$24.76
Fair Value at Grant Date	$4.97
Remaining Contractual Time	2 years 7 months

ShareBased_Payments_Nonemploye

Share-Based Payments - Non-employee Director Options Granted (Details) (Stock Incentive Plan To Non Employee Directors Plan, USD $)	12 Months Ended
	Dec. 31, 2014
30-Jul-07
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Average Fair Value at Grant Date	$4.97
Risk-free interest rate	4.64%
Dividend yield	2.40%
Expected life	5 years
Share price volatility	19.55%
25-May-06
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Average Fair Value at Grant Date	$4.24
Risk-free interest rate	4.85%
Dividend yield	2.70%
Expected life	5 years
Share price volatility	20.05%

ShareBased_Payments_Restricted1

Share-Based Payments - Restricted Share Units Issued to Non-employee Directors (Details) (Restricted Share Units)	12 Months Ended
	Dec. 31, 2014
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Amount Granted	298,353
Amount Vested	282,544
Amount Forfeited	9,003
Amount Outstanding	6,806
Non-Employee Directors — 2012 and prior
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Amount Granted	138,504
Amount Vested	131,866
Amount Forfeited	6,638
Amount Outstanding	0
Non-Employee Directors — 2013
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Amount Granted	29,092
Amount Vested	26,727
Amount Forfeited	2,365
Amount Outstanding	0
Non-Employee Directors — 2014
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Amount Granted	27,180
Amount Vested	22,640
Amount Forfeited	0
Amount Outstanding	4,540
Chairman — 2012 and prior
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Amount Granted	75,799
Amount Vested	75,799
Amount Forfeited	0
Amount Outstanding	0
Chairman — 2013
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Amount Granted	14,188
Amount Vested	14,188
Amount Forfeited	0
Amount Outstanding	0
Chairman — 2014
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Amount Granted	13,590
Amount Vested	11,324
Amount Forfeited	0
Amount Outstanding	2,266

ShareBased_Payments_Summary_of4

Share-Based Payments - Summary of Option Activity and Restricted Share Units Activity (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Option Activity
Outstanding options, beginning of period	302,460	1,208,787
Exercised and issued	-74,520	-906,254
Forfeited or expired	-3,799	-73
Outstanding options, end of period	224,141	302,460
Outstanding Options, Weighted Average Exercise Price, beginning of period	$25.02	$23.07
Options Exercise Price	$25.26	$22.45
Options forfeited or expired, weighted average exercise price	$27.28	$24.21
Outstanding Options, Weighted Average Exercise Price, end of period	$24.91	$25.02
Restricted Share Units
Restricted share unit activity
Granted, Number of Shares	1,883,891
Amount Vested	-1,205,836
Amount Forfeited	-158,943
Number of shares outstanding, end of period	519,112
Restricted Share Units | Executive Officer
Restricted share unit activity
Number of shares outstanding, beginning of period	544,751	488,013
Granted, Number of Shares	300,410	350,721
Amount Vested	-291,468	-253,700
Amount Forfeited	-27,775	-40,283
Number of shares outstanding, end of period	525,918	544,751
Number of shares outstanding ,weighted average grant date fair value beginning of period	$32.13	$27.81
Granted, weighted average grant date fair value	$38.60	$35.61
Vested, weighted average grant date fair value	$32.12	$23.66
Forfeited, weighted average grant date fair value	$36.29	$31.45
Number of shares outstanding, weighted average grant date fair value end of period	$35.83	$32.13

ShareBased_Payments_Additional

Share-Based Payments - Additional Information (Narrative) (Details)

0 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

0 Months Ended

1 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

12 Months Ended

12 Months Ended

12 Months Ended

12 Months Ended

Nov. 01, 2012

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2013

Feb. 06, 2013

Dec. 31, 2014

Dec. 31, 2013

Jun. 21, 2002

Dec. 31, 2014

Dec. 31, 2014

Apr. 06, 2014

Dec. 31, 2014

Apr. 22, 2014

Jun. 11, 2013

Apr. 08, 2013

Feb. 06, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Mar. 28, 2007

Dec. 31, 2014

Dec. 31, 2007

Dec. 31, 2006

Dec. 31, 2004

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2009

Dec. 31, 2003

Dec. 31, 2014

Nov. 01, 2012

Feb. 02, 2012

2-May-11

Mar. 21, 2011

Feb. 04, 2011

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2014

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Feb. 02, 2012

Dec. 31, 2011

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2011

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2011

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

USD ($)

USD ($)

USD ($)

Employee Stock Option

Employee Stock Option

Performance Share Awards

Performance Share Awards

Performance Share Awards

Investor Options

Investor Options

International Employee Share Purchase Plan

Share Save Scheme

Share Save Scheme

Phantom Shares

Phantom Shares

Phantom Shares

Phantom Shares

Phantom Shares

Phantom Shares

Phantom Shares

Employee Share Purchase Plan

Employee Share Purchase Plan

Employee Share Purchase Plan

Minimum

Maximum

Wellington Investment

Non-employee directors - 2007 Option grants (July 30)

Investor Options

Investor Options

Investor Options

2013 Share Incentive Plan

2003 Share Incentive Plan

2003 Option grants

2003 Option grants

2003 Option grants

2011 Performance Shares

2011 Performance Shares

2011 Performance Shares

2011 Performance Shares

2011 Performance Shares

2011 Performance Shares

2011 Performance Shares

2011 Performance Shares

2011 Performance Shares

2011 Performance Shares

2012 Performance Shares

2012 Performance Shares

2013 Performance Shares

2014 Performance Shares

Employee Share Purchase Plan

Restricted Share Units

Restricted Share Units

Restricted Share Units

Non Employee Equity Incentive Plan

Non Employee Equity Incentive Plan

Non Employee Equity Incentive Plan

2012 Phantom Shares

2012 Phantom Shares

2012 Phantom Shares

2012 Phantom Shares

Performance Share Awards

Performance Share Awards

Performance Share Awards

2013 Phantom Shares

2014 Phantom Shares

Achievement One

Achievement One

Achievement One

Achievement One

Achievement One

Achievement One

Achievement Two

Achievement Two

Achievement Two

Achievement Two

Achievement Two

Achievement Two

Achievement Two

Achievement Two

Achievement Two

Achievement Two

Achievement Three

Achievement Three

Achievement Three

Achievement Three

Achievement Three

Achievement Three

Achievement Three

Achievement Three

Achievement Three

Achievement Three

Achievement Three

BVPS Growth 2012

BVPS Growth 2012

BVPS Growth 2012

BVPS Growth 2012

BVPS Growth 2013

BVPS Growth 2013

BVPS Growth 2013

BVPS Growth 2013

BVPS Growth 2013

BVPS Growth 2013

BVPS Growth 2014

BVPS Growth 2014

BVPS Growth 2014

Predecessor

USD ($)

USD ($)

agreement

USD ($)

GBP (£)

GBP (£)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

Performance Share Awards

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

Minimum

2012 Performance Shares

2013 Performance Shares

2013 Performance Shares

2013 Performance Shares

2014 Performance Shares

2012 Performance Shares

2012 Performance Shares

2013 Performance Shares

2013 Performance Shares

2013 Performance Shares

2013 Performance Shares

2013 Performance Shares

2013 Performance Shares

2014 Performance Shares

2014 Performance Shares

Maximum

2012 Performance Shares

2012 Performance Shares

2013 Performance Shares

2013 Performance Shares

2013 Performance Shares

2013 Performance Shares

2013 Performance Shares

2014 Performance Shares

2014 Performance Shares

2012 Phantom Shares

2011 Performance Shares

2012 Performance Shares

2012 Performance Shares

2012 Performance Shares

Performance Share Awards

Performance Share Awards

2012 Performance Shares

2012 Performance Shares

2013 Performance Shares

2013 Performance Shares

Performance Share Awards

2012 Performance Shares

2012 Performance Shares

GBP (£)

Minimum

Maximum

Minimum

Minimum

Minimum

Maximum

Maximum

Maximum

Minimum

Maximum

Minimum

Maximum

Minimum

Minimum

Maximum

Maximum

Maximum

Minimum

Maximum

Minimum

Performance Share Awards

Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]

Foreign currency volatility

9.40%

Number of arrangements under options and equity incentive plans

3

Option to subscribe for ordinary shares of Aspen Holdings (in shares)

6,787,880

ROE Achieved in year one

-5.30%

-5.30%

Ordinary shares Issued

426,083

Subscription Price Payable Under Options

£ 10

Subscription price increase percentage

5.00%

Number of Shares Available for Grant

2,845,683

595,683

Vesting period

3 years

3 years

3 years

3 years

3 years

3 years

3 years

2 years

3 years

3 years

3 years

3 years

3 years

3 years

3 years

3 years

1 year

3 years

Contract period

10 years

3 years

10 years

Exercise period

7 years

Options granted

0

0

Options exercised

74,520

906,254

84,018

904,242

0

2,012

Compensation cost

$8,900,000

$8,100,000

$6,100,000

$1,500,000

$1,300,000

$1,300,000

$200,000

$300,000

$9,300,000

$7,600,000

$7,600,000

$0

$0

$0

Portion of grant eligible for time based vesting

65.00%

Percentage of performance Shares vested

20.00%

Portion of grant eligible for performance based vesting

35.00%

Options cancelled

242,643

Number of Options vested

476,250

695,643

Restricted share units grant to employees, time period

7 years

Intrinsic value of options

1,500,000

17,200,000

18,700,000

Total tax credit

100,000

100,000

0

2,200,000

1,900,000

1,600,000

2,400,000

2,000,000

3,000,000

Fair value adjustment

3,100,000

400,000

300,000

Granted, Number of Shares

250,066

154,512

542

6,521

152,541

334,125

5,902

31,669

853,223

1,883,891

278,143

0

ROE achieved

6.00%

5.00%

5.00%

5.20%

6.00%

11.00%

5.00%

5.00%

10.00%

10.00%

5.20%

10.40%

11.00%

21.00%

10.00%

10.00%

20.00%

20.00%

10.40%

20.80%

Portion of grant eligible for vesting

33.33%

33.33%

33.33%

33.30%

33.33%

Vested, Number of Shares

315,389

303,247

169,981

1,205,836

145,425

33,012

62,930

102,152

25,001

122,056

Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period

473,228

ROE Achieved in year three

11.70%

11.70%

ROE Achieved in year two

10.00%

10.00%

Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Percent Amount Vested in Period

117.00%

100.00%

129.00%

31.60%

65.80%

129.00%

31.60%

Fair Value Investments Entities That Calculate Net Asset Value Per Share Percent Equity Securities Year Three

13.30%

13.30%

13.30%

Fair Value Investments Entities That Calculate Net Asset Value Per Share Percent Equity Securities Year Two

6.20%

13.30%

6.20%

13.30%

6.20%

13.30%

129.00%

Additional performance shares granted

10,006

Compensation cost

400,000

1,300,000

1,400,000

11,100,000

Performance share vested

16,812

9,258

61,006

64,357

59,796

Fair value adjustment on compensation cost

2,900,000

1,400,000

100,000

Tax credit recognized by the company

-2,200,000

400,000

300,000

Employee savings under ESPP

500

500

250

Shares Issued

11,821

38,915

75,066

Percentage of performance shares eligible for vesting

100.00%

10.00%

100.00%

10.00%

10.00%

10.00%

100.00%

100.00%

100.00%

100.00%

200.00%

100.00%

200.00%

200.00%

200.00%

100.00%

100.00%

Percentage of shares eligible for vesting

100.00%

Employee contribution period

2 years

3 years

Purchase Price Percentage Of Fair Market Value

85.00%

Holding period

1 year

Tax charge recognized, Total

$0

$0

$0

Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Equity Securities

8.10%

6.20%

13.30%

8.10%

6.20%

8.10%

Increase In Company's Share Price, Percent

28.80%

Vesting Adjustment Due To Share Price Increase, Percent

31.60%

Intangible_Assets_Summary_of_C

Intangible Assets - Summary of Changes in Intangible Assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Beginning of the period	$18,400,000	$19,000,000
Amortization	-200,000	-600,000
End of the period	18,200,000	18,400,000
Trade Mark
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Beginning of the period	1,600,000	1,600,000
Amortization	0	0
End of the period	1,600,000	1,600,000
Insurance Licenses
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Beginning of the period	16,600,000	16,600,000
Amortization	0	0
End of the period	16,600,000	16,600,000
Other
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Beginning of the period	200,000	800,000
Amortization	-200,000	-600,000
End of the period	$0	$200,000

Intangible_Assets_Additional_I

Intangible Assets - Additional Information (Narrative) (Details) (USD $)	0 Months Ended	12 Months Ended
	Apr. 05, 2005	Dec. 31, 2014	Dec. 31, 2010	Dec. 31, 2013	Dec. 31, 2012
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Aggregate consideration for sale and purchase agreement	$1,600,000		$4,800,000
Value of the asset		18,200,000		18,400,000	19,000,000
Acquired licenses			3,600,000
Fair value of tangible and financial assets, net			1,200,000
Asset amortization period		5 years
Trade Mark
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		1,600,000		1,600,000	1,600,000
Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		16,600,000		16,600,000	16,600,000
Insurance Licenses | AAIC
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses		10,000,000
Insurance Licenses | Aspen Specialty
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses		4,500,000
Insurance Licenses | Aspen U.K.
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses		2,100,000
Intangible Asset: Renewal Rights and Distribution Channels
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses			2,200,000
Intangible Asset: Lock-in Period
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses			1,400,000
Other
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		$0		$200,000	$800,000

Commitments_and_Contingencies_1

Commitments and Contingencies - Schedule of Company's Restricted Assets (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	$4,049		$3,752.60
Total as percent of cash and invested assets	47.00%		45.50%
Affiliated transactions
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	1,086.90		685.8
Third party
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	2,183.40		2,236.40
Letters of credit / guarantees
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	$778.70	[1]	$830.40	[1]
[1]	As of December 31, 2014, the Company had pledged funds of $764.4 million and £9.2 million (December 31, 2013 — $803.7 million and £16.1 million) as collateral for the secured letters of credit.

Commitments_and_Contingencies_2

Commitments and Contingencies - Amounts Outstanding under Operating Leases (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Leases [Abstract]
2015	$13.40	$11.90
2016	9.3	12.1
2017	8.5	8.3
2018	7.3	7.5
2019	6.4	6.3
Later Years	1.5	6.5
Total	$46.40	$52.60

Commitments_and_Contingencies_3

Commitments and Contingencies - Additional Information (Narrative) (Details)

12 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

USD ($)

USD ($)

AUD

GBP (£)

AUD

GBP (£)

Interest Rate Swaps

Interest Rate Swaps

Us Multi Beneficiary Trust Fund

Us Multi Beneficiary Trust Fund

Us Surplus Lines Trust Fund

Us Surplus Lines Trust Fund

Us Credit And Surety Lines Trust Fund

Us Credit And Surety Lines Trust Fund

Canadian Trust Fund

Canadian Trust Fund

Swiss Trust Fund

Swiss Trust Fund

Singapore Trust Fund

Singapore Trust Fund

Bermuda

Bermuda

Investment

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

CAD

CAD

CHF

CHF

SGD

SGD

USD ($)

USD ($)

Restricted Cash And Collateral [Line Items]

Securities and cash as collateral secured letters of credit

$764,400,000

$803,700,000

£ 9,200,000

£ 16,100,000

Total as percent of cash and invested assets

47.00%

45.50%

Cash, Cash Equivalents, and Short-term Investments

8,600,000,000

8,300,000,000

Assets Held by Insurance Regulators

411,900,000

346,400,000

Minimum capital required

400,000

400,000

Minimum Trust Fund

20,000,000

20,000,000

Percentage Of Reinsurance Liabilities

100.00%

100.00%

Reinsurance Liabilities

1,071,400,000

1,035,900,000

694,800,000

581,300,000

Assets Held-in-trust

1,323,600,000

1,352,200,000

114,400,000

182,100,000

1,020,100,000

918,600,000

170,900,000

146,600,000

0

0

314,800,000

354,400,000

12,300,000

16,600,000

72,600,000

67,500,000

Regulatory Deposits

6,200,000

6,200,000

Deposit With States

7,200,000

7,200,000

Collateral provided to counterparties as security for the Company's net liability position

22,300,000

34,300,000

Operating Leases, Rent Expense

16,700,000

19,900,000

Depreciation, Depletion and Amortization, Nonproduction

8,200,000

14,400,000

Accumulated Depreciation

$97,600,000

$89,400,000

Number of investments

2

Concentration_of_Credit_Risk_S

Concentration of Credit Risk - Schedule Of Gross Written Premium From Major Brokers (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Investment Holdings [Line Items]
Gross written premiums, percentage	100.00%		100.00%		100.00%
Gross written premiums	$2,902,700,000		$2,646,700,000		$2,583,300,000
Aon Corporation
Investment Holdings [Line Items]
Gross written premiums, percentage	17.80%		16.80%		18.50%
Marsh & McLennan Companies, Inc.
Investment Holdings [Line Items]
Gross written premiums, percentage	15.10%		15.00%		15.80%
Willis Group Holdings, Ltd.
Investment Holdings [Line Items]
Gross written premiums, percentage	13.70%		14.40%		15.10%
Others
Investment Holdings [Line Items]
Gross written premiums, percentage	53.40%	[1]	53.80%	[1]	50.60%	[1]
[1]	No other individual broker accounted for more than 10% of total gross written premiums.

Concentration_of_Credit_Risk_A

Concentration of Credit Risk - Additional Information (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2012
Reinsurance [Line Items]
Unpaid losses	$332,700,000	$350,000,000	$499,000,000
Underwriting premiums	999,000,000	1,011,700,000
Due for settlement		2,300,000
Settlement period		more than one year
Premiums Receivable, Allowance for Doubtful Accounts	$0	$4,300,000
Allowable holdings of a single issue or issuer, percentage		2.00%
Percentage Of Written Premium Major Broker Accounted Minimum	10.00%
Lloyd's
Reinsurance [Line Items]
Reinsurance recoverables		27.30%
Munich Re
Reinsurance [Line Items]
Reinsurance recoverables		18.60%
Arch Re
Reinsurance [Line Items]
Reinsurance recoverables		8.40%

Reclassifications_from_Accumul2

Reclassifications from Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income Reclassification (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Foreign currency translation adjustments:
Foreign currency translation adjustments, before tax	$7.90	$0	$0
Reclassification out of Accumulated Other Comprehensive Income [Member]
Available for sale securities:
Realized gain on sale of securities	13.9	24.6
Realized (losses) on sale of securities	-6.2	-0.5
Net realized and unrealized investment gains reclassified to the statement of operations, gross of tax	7.7	24.1
Tax on realized gains and (losses) of securities	0.2	0.7
Net realized and unrealized investment gains reclassified to the statement of operations, net of tax	7.5	23.4
Foreign currency translation adjustments:
Foreign currency translation adjustments, before tax	0	-1.9
Tax on foreign currency translation adjustments	0	0.4
Net realized and unrealized foreign exchange investment gains reclassified to the statement of operations, net of tax	0	-1.5
Amortization of derivatives:
Amortization of long-term debt associated expenses, before tax	0	-0.5
Amortization of long-term debt associated expenses, net of tax	0	-0.5
Total reclassifications from AOCI to the statement of operations, net of tax	$7.50	$21.40

Recovered_Sheet1

Credit Facility and Long-Term Debt - Summary of Contractual Obligations Under Long-term Debts (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Debt Disclosure [Abstract]
Less than 1 year	$0
1-3 years	0
3-5 years	0
More than 5 years	550
Total	$550

Recovered_Sheet2

Credit Facility and Long-Term Debt - Additional Information (Narrative) (Details) (USD $)

0 Months Ended

12 Months Ended

0 Months Ended

1 Months Ended

6 Months Ended

0 Months Ended

Jun. 12, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Jul. 30, 2010

Jul. 30, 2010

Jun. 30, 2014

Dec. 16, 2013

Nov. 13, 2013

Dec. 15, 2010

Aug. 16, 2004

Dec. 27, 2013

Aug. 21, 2013

Feb. 28, 2011

Aug. 12, 2011

Apr. 29, 2009

Oct. 29, 2008

Jul. 30, 2012

tranche

Line Of Credit Facility Covenant Compliance [Line Items]

Revolving credit facility, credit agreement maximum limit

$200,000,000

Line of credit facility Increasable capacity

100,000,000

Line of credit facility borrowings outstanding

0

Minimum consolidated tangible net worth under credit facility

2,428,600,000

Percentage of consolidated net income

50.00%

Percentage of aggregate net cash proceeds from the issuance of capital stock

50.00%

Percentage of consolidated leverage ratio permitted

35.00%

Total net realized and unrealized investment gains recorded in the statement of operations

31,600,000

36,400,000

26,800,000

Silverton

Line Of Credit Facility Covenant Compliance [Line Items]

Loans issued

65,000,000

Silverton Loan Notes | Silverton | Aspen Holdings

Line Of Credit Facility Covenant Compliance [Line Items]

Maximum exposure to loss

20,600,000

Silverton Loan Notes | Silverton Two | Aspen Holdings

Line Of Credit Facility Covenant Compliance [Line Items]

Maximum exposure to loss

15,000,000

Barclays Bank Plc

Line Of Credit Facility Covenant Compliance [Line Items]

Revolving credit facility, credit agreement maximum limit

100,000,000

200,000,000

Line of credit facility borrowings outstanding

5,000,000

18,900,000

Citibank Europe

Line Of Credit Facility Covenant Compliance [Line Items]

Revolving credit facility, credit agreement maximum limit

1,050,000,000

550,000,000

450,000,000

Outstanding collateralized letters of credit facility

463,600,000

516,800,000

Three Year Revolving Credit Facility

Line Of Credit Facility Covenant Compliance [Line Items]

Debt Instrument, Term

3 years

Revolving credit facility, credit agreement maximum limit

280,000,000

280,000,000

Line of credit facility Increasable capacity

75,000,000

Citibank Europe plc | Letters of credit / guarantees

Line Of Credit Facility Covenant Compliance [Line Items]

Revolving credit facility, credit agreement maximum limit

575,000,000

950,000,000

Line of Credit Facility, Number of Maturity Tranches

2

Citibank Europe plc | Letter of Credit, Tranche I

Line Of Credit Facility Covenant Compliance [Line Items]

Revolving credit facility, credit agreement maximum limit

650,000,000

Citibank Europe plc | Letter of Credit, Tranche II

Line Of Credit Facility Covenant Compliance [Line Items]

Revolving credit facility, credit agreement maximum limit

300,000,000

Citibank Europe plc | London Interbank Offered Rate (LIBOR) | Letters of credit / guarantees

Line Of Credit Facility Covenant Compliance [Line Items]

Debt Instrument, Basis Spread on Variable Rate

1.00%

Senior Notes

Line Of Credit Facility Covenant Compliance [Line Items]

Loans issued

300,000,000

250,000,000

250,000,000

Interest rate of senior notes

4.65%

6.00%

6.00%

Net proceeds from senior notes

299,700,000

247,500,000

249,300,000

Total net realized and unrealized investment gains recorded in the statement of operations

9,300,000

Amount of indebtedness subject to default in payment

50,000,000

Annual interest payable

29,000,000

Silverton

Line Of Credit Facility Covenant Compliance [Line Items]

Loans issued

65,000,000

65,000,000

Loan notes issued by third parties

50,000,000

50,000,000

Silverton Two

Line Of Credit Facility Covenant Compliance [Line Items]

Loans issued

85,000,000

Loan notes issued by third parties

$70,000,000

Unaudited_Quarterly_Financial_2

Unaudited Quarterly Financial Data - Summary of Quarterly Financial Data (Details) (USD $)	3 Months Ended																				12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Net earned premium	$612.20		$610.40		$616.20		$566.50		$572.60	$544.30	$544	$510.90	$558.50		$516.20		$513.40		$495.40		$2,405.30		$2,171.80	$2,083.50
Net investment income	46.7		48		46.1		49.5		47.2	45	45.9	48.3	51.1		48.6		52.8		52.4		190.3		186.4	204.9
Realized and unrealized investment gains	-7.3	[1]	1.1	[1]	34.6	[1]	17.9	[1]	2.6	23.6	14.4	16.3	6.1		13.2		5.4		10.7		46.3	[1]	56.9	35.4
Other income	-0.3		1		3.2		0.6		4.6	1.6	0.9	1.1	-3.4		4.8		3.5		0.7		4.5		8.2	5.6
Total Revenues	651.3		660.5		700.1		634.5		627	614.5	605.2	576.6	612.3		582.8		575.1		559.2		2,646.40		2,423.30	2,329.40
Losses and loss adjustment expenses	339.6		342.7		337.1		288.1		331.4	290.2	333.4	268.7	437.4		255		262.1		284		1,307.50		1,223.70	1,238.50
Amortization of deferred policy acquisition costs	114.8		115.5		108.9		112		99.7	110.5	107.2	104.6	80		103.1		102		96.1		451.2		422	381.2
General, administrative and corporate expenses	121.5		119.8		108.8		95.6		94.9	98.9	87.7	86.6	86.1		90.7		83.5		84.8		445.7		368.1	345.1
Interest on long-term debt	7.4		7.4		7.3		7.4		9.5	7.7	7.8	7.7	7.7		7.8		7.7		7.7		29.5		32.7	30.9
Change in fair value of derivatives	6.6		5.1		4.6		-1.1		-1.8	-6.6	2.9	4.2	4.4		4.9		11.6		7.5		15.2		-1.3	28.4
Change in fair value of loan notes issued by variable interest entities	4.1		8.5		2.6		3.4														18.6		0	0
Realized and unrealized investment losses	-14.1	[1]	21.2	[1]	3.3	[1]	4.3	[1]	-7.5	5.9	21	1.1	0.4		2.4		4.1		1.7		14.7	[1]	20.5	8.6
Net realized and unrealized foreign exchange (gains)/losses	3.9	[1]	1.3	[1]	-10.7	[1]	-0.1	[1]	6.1	-2.4	4.1	5.4	-3.9		-4.5		12.7		-7.7		-5.6	[1]	13.2	-3.4
Other expenses	-0.5		0.3		1.2		0.7		1.1	0	0	0.6	2.8		0.3		0.6		1		1.7		1.7	4.7
Total expenses	583.3		621.8		563.1		510.3		533.4	504.2	564.1	478.9	614.9		459.7		484.3		475.1		2,278.50		2,080.60	2,034
Income from operations before income tax	68		38.7		137		124.2		93.6	110.3	41.1	97.7	-2.6		123.1		90.8		84.1		367.9		342.7	295.4
Income tax expense	-0.8		-1.3		-6.2		-3.8		-3.6	-2.9	-1	-5.9	4.6		-8		-6.2		-5.4		-12.1		-13.4	-15
Net income	$67.20		$37.40		$130.80		$120.40		$90	$107.40	$40.10	$91.80	$2		$115.10		$84.60		$78.70		$355.80		$329.30	$280.40
Ordinary shares:
Basic weighted average ordinary shares	62,206,260		65,116,463		65,447,128		65,289,351		65,593,669	66,716,202	66,191,426	68,854,286	71,007,079	[2]	71,129,102	[2]	71,303,855	[2]	70,943,997	[2]	64,536,491		66,872,048	71,095,856	[2]
Diluted weighted average ordinary shares	63,605,298		66,513,009		66,700,368		66,565,890		67,051,993	68,561,515	69,291,324	72,452,705	71,007,079	[2]	73,397,796	[2]	73,845,903	[2]	73,832,734	[2]	65,872,949		69,417,903	73,689,423	[2]
Weighted average number of ordinary share and share equivalents
Basic, dollars per share	$0.92		$0.43		$1.85		$1.70		$1.23	$1.47	$0.38	$1.21	($0.09)		$1.50		$1.07		$1.03		$4.92		$4.29	$3.51
Diluted, dollars per share	$0.90		$0.42		$1.82		$1.66		$1.21	$1.43	$0.36	$1.15	($0.09)		$1.45		$1.03		$0.99		$4.82		$4.14	$3.39
[1]	Adjusted for a representation of foreign exchange in relation to investment securities from realized and unrealized exchange gains/(losses) to realized and unrealized investment gains/(losses).
[2]	The basic and diluted number of ordinary shares for the three months ended December 31, 2012 is the same, as the inclusion of dilutive securities in a loss-making period would be anti-dilutive.

Subsequent_Events_Additional_I

Subsequent Events - Additional Information (Narrative) (Details) (USD $)	0 Months Ended
	Feb. 05, 2015	Feb. 02, 2012
Subsequent Event [Line Items]
Stock Repurchase Program, Authorized Amount		$400,000,000
Repurchase Program 2015 | Subsequent Event
Subsequent Event [Line Items]
Share Repurchase Agreement Commencement Date	5-Feb-15
Stock Repurchase Program, Authorized Amount	$500,000,000

Schedule_II_Condensed_Financia1

Schedule II - Condensed Financial Information of Registrant Balance Sheets (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Nov. 15, 2006
ASSETS
Cash and cash equivalents (including cash within consolidated variable interest entities of $176.7 and $50.0)	$1,178,500,000	$1,293,600,000	$1,463,600,000	$1,239,100,000
Other assets	13,600,000	2,200,000
Total assets	10,716,300,000	10,230,500,000
LIABILITIES
Accrued expenses and other payables	356,900,000	265,600,000
Long-term debt	549,100,000	549,000,000
Total liabilities	7,297,000,000	6,930,900,000
SHAREHOLDERS’ EQUITY
62,017,368 shares of par value 0.15144558¢ each (December 31, 2013 — 65,546,976)	94,000	99,000
Additional paid in capital	1,134,300,000	1,297,400,000
Retained earnings	2,050,100,000	1,783,300,000
Non-controlling interest	500,000	-300,000
Total accumulated other comprehensive income	234,300,000	219,100,000	427,400,000
Total liabilities and shareholders’ equity	10,716,300,000	10,230,500,000
Ordinary shares, issued	62,017,368	65,546,976	70,753,723
Ordinary shares, par value	$0.15	$0.15
Preference shares, par value	$0.15	$0.15
5.950% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	0	0
Preferred Stock, Shares Issued	11,000,000	11,000,000
Preference shares, rate	5.95%	5.95%
Preference shares, par value	$0.15	$0.15
7.401% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	0	0
Preferred Stock, Shares Issued	5,327,500	5,327,500			8,000,000
Preference shares, rate	7.40%	7.40%
Preference shares, par value	$0.15	$0.15
7.250% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	0	0
Preferred Stock, Shares Issued	6,400,000	6,400,000
Preference shares, rate	7.25%	7.25%
Preference shares, par value	$0.15	$0.15
Parent
ASSETS
Cash and cash equivalents (including cash within consolidated variable interest entities of $176.7 and $50.0)	86,800,000	94,200,000	192,300,000	125,300,000
Investments in subsidiaries	3,368,500,000	3,153,700,000
Other investments	8,700,000	48,000,000
Eurobond issued by subsidiary	573,800,000	571,900,000
Long-term debt issued by subsidiaries	35,600,000	15,000,000
Intercompany funds due from affiliates	43,600,000	0
Other assets	0	600,000
Total assets	4,117,000,000	3,883,400,000
LIABILITIES
Accrued expenses and other payables	52,500,000	14,600,000
Intercompany funds due to affiliates	96,100,000	20,200,000
Long-term debt	549,100,000	549,000,000
Total liabilities	697,700,000	583,800,000
SHAREHOLDERS’ EQUITY
62,017,368 shares of par value 0.15144558¢ each (December 31, 2013 — 65,546,976)	100,000	100,000
Additional paid in capital	1,134,300,000	1,297,400,000
Retained earnings	2,050,100,000	1,783,300,000
Non-controlling interest	500,000	-300,000
Unrealized gains on investments	165,400,000	130,500,000
Loss on derivatives	-3,800,000	0
Gains on foreign currency translation	72,700,000	88,600,000
Total accumulated other comprehensive income	234,300,000	219,100,000
Total shareholders’ equity	3,419,300,000	3,299,600,000
Total liabilities and shareholders’ equity	4,117,000,000	3,883,400,000
Ordinary shares, issued	62,017,368	65,546,976
Ordinary shares, par value	$0.00	$0.00
Parent | 5.950% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	0	0
Preferred Stock, Shares Issued	11,000,000	11,000,000
Preference shares, rate	5.95%	5.95%
Preference shares, par value	$0.00	$0.00
Parent | 7.401% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	0	0
Preferred Stock, Shares Issued	5,327,500	5,327,500
Preference shares, rate	7.40%	7.40%
Preference shares, par value	$0.00	$0.00
Parent | 7.250% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	$0	$0
Preferred Stock, Shares Issued	6,400,000	6,400,000
Preference shares, rate	7.25%	7.25%
Preference shares, par value	$0.00	$0.00

Schedule_II_Condensed_Financia2

Schedule II - Condensed Financial Information of Registrant Statements of Operations and Comprehensive Income (Details) (USD $)	3 Months Ended												12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Dividend income													$200.90	$195.90	$213.60
Other income	-0.3	1	3.2	0.6	4.6	1.6	0.9	1.1	-3.4	4.8	3.5	0.7	4.5	8.2	5.6
Total Revenues	651.3	660.5	700.1	634.5	627	614.5	605.2	576.6	612.3	582.8	575.1	559.2	2,646.40	2,423.30	2,329.40
Expenses
Operating and administrative expenses	-121.5	-119.8	-108.8	-95.6	-94.9	-98.9	-87.7	-86.6	-86.1	-90.7	-83.5	-84.8	-445.7	-368.1	-345.1
Income from operations before income tax	68	38.7	137	124.2	93.6	110.3	41.1	97.7	-2.6	123.1	90.8	84.1	367.9	342.7	295.4
Income tax expense	-0.8	-1.3	-6.2	-3.8	-3.6	-2.9	-1	-5.9	4.6	-8	-6.2	-5.4	-12.1	-13.4	-15
Net income	67.2	37.4	130.8	120.4	90	107.4	40.1	91.8	2	115.1	84.6	78.7	355.8	329.3	280.4
Add: Loss attributable to non-controlling interest													-0.8	0.5	0.2
Net income attributable to Aspen Insurance Holdings Limited ordinary shareholders													355	329.8	280.6
Other Comprehensive Income:
Change in unrealized losses on investments													34.9	-184.7	9.8
Other comprehensive income													15.2	-208.3	-1.5
Total comprehensive income attributable to Aspen Holdings’ ordinary shareholders													370.2	121.5	279.1
Parent
Condensed Financial Statements, Captions [Line Items]
Equity in net earnings/(loss) of subsidiaries and other investments													146.9	40.6	160.6
Dividend income													258.5	301.8	100
Interest income on Eurobond													29.5	44.6	56.5
Realized investment gains													5.6	-6.3	3.2
Other income													1.9	1.9	3.4
Total Revenues													442.4	382.6	323.7
Expenses
Operating and administrative expenses													-57.1	-20.6	-12.4
Interest expense													-29.5	-32.7	-30.9
Income from operations before income tax													355.8	329.3	280.4
Income tax expense													0	0	0
Net income													355.8	329.3	280.4
Add: Loss attributable to non-controlling interest													-0.8	0.5	0.2
Net income attributable to Aspen Insurance Holdings Limited ordinary shareholders													355	329.8	280.6
Other Comprehensive Income:
Change in unrealized losses on investments													34.9	-184.7	9.8
Loss on derivatives reclassified to interest expense													0	0.5	0.2
Net change from current period hedged transactions													-3.8
Change in unrealized gains on foreign currency translation													-15.9	-24.1	-11.5
Other comprehensive income													15.2	-208.3	-1.5
Total comprehensive income attributable to Aspen Holdings’ ordinary shareholders													$370.20	$121.50	$279.10

Schedule_II_Condensed_Financia3

Schedule II - Condensed Financial Information of Registrant Statements of Cash Flows (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Share-based compensation	$15.10	$21.40	$17.80
Net cash generated by operating activities	607.4	566.4	496.4
Cash flows (used in) investing activities:
Net proceeds from other investments	39.3	0	0
Net cash (used in)/from investing activities	-515	-497.9	-317.2
Cash flows (used in)/from financing activities:
Proceeds from the issuance of ordinary shares, net of issuance costs	2.7	21.2	22.1
Proceeds from the issuance of preference shares, net of issuance costs	0	270.6	154.5
PIERS redeemed and cancelled	0	-230	0
Ordinary share repurchase	-180.9	-309.6	-62.7
Make whole payment	0	-9.3	0
Proceeds from long term debt	70	50	0
Debt redemption	0	-250	0
Net cash (used in)/from financing activities	-196.4	-240.8	35.7
Increase in cash and cash equivalents	-115.1	-170	224.5
Cash and cash equivalents at beginning of period	1,293.60	1,463.60	1,239.10
Cash and cash equivalents at end of period	1,178.50	1,293.60	1,463.60
Parent
Cash flows from operating activities:
Net income (excluding equity in net earnings of subsidiaries)	209.8	288.8	119.8
Share-based compensation	15.1	21.4	17.8
Realized and unrealized losses/(gains)	-5.6	6.3	-3.2
Loss on derivative reclassified to interest expense	0	0.5	0.2
Increase (Decrease) in Accounts and Other Receivables	1.1	0	0
Change in other assets	0.6	-2.8	1.1
Change in accrued expenses and other payables	37.8	-5.5	4
Change in intercompany activities	32.3	104.3	-58.5
Net cash generated by operating activities	291.1	413	81.2
Cash flows (used in) investing activities:
Investment in subsidiaries	-56.6	-605.4
Investment in long-term debt issued by subsidiary	-15	-15	0
Net proceeds from other investments	39.3	0	0
Net cash (used in)/from investing activities	-32.3	-620.4
Cash flows (used in)/from financing activities:
Proceeds from the issuance of ordinary shares, net of issuance costs	2.7	21.2	22.1
Proceeds from the issuance of preference shares, net of issuance costs	0	270.6	154.5
PIERS redeemed and cancelled	0	-230	0
Ordinary share repurchase	-180.9	-309.6	-62.7
Make whole payment	0	-9.3	0
Proceeds from long term debt	0	299.7	0
Debt redemption	0	-250	0
Ordinary and preference share dividends paid	-88.1	-83.3	-78.1
Proceeds from maturity of Eurobond	0	400	50
Eurobond purchased from subsidiary	0	0	-100
Net cash (used in)/from financing activities	-266.3	109.3	-14.2
Increase in cash and cash equivalents	-7.4	-98.1	67
Cash and cash equivalents at beginning of period	94.2	192.3	125.3
Cash and cash equivalents at end of period	$86.80	$94.20	$192.30

Schedule_III_Supplementary_Ins1

Schedule III - Supplementary Insurance Information (Details) (USD $)	3 Months Ended												12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Deferred Policy Acquisition Costs	$299				$262.20				$223				$299	$262.20	$223
Liability for Unpaid Claims and Claims Adjustment Expense, Net	4,400.80				4,346.20				4,280.70				4,400.80	4,346.20	4,280.70	4,098.60
Losses and loss adjustment expenses	4,750.80				4,678.90				4,779.70				4,750.80	4,678.90	4,779.70	4,525.20
Net Reserves for Unearned Premiums	1,235				1,128.70				998.2				1,235	1,128.70	998.2
Net earned premium	612.2	610.4	616.2	566.5	572.6	544.3	544	510.9	558.5	516.2	513.4	495.4	2,405.30	2,171.80	2,083.50
Net Investment Income													190.3	186.4	204.9
Losses and LAE Expenses													1,307.50	1,223.70	1,238.50
Policy Acquisition Expenses													451.2	422	381.2
Net written premiums													2,515.20	2,299.70	2,246.90
General and Administrative Expenses													351.9	316.9	292.1
Reinsurance
Deferred Policy Acquisition Costs	156.4				131.9				109.4				156.4	131.9	109.4
Losses and loss adjustment expenses	2,493.30				2,646.80				2,811.30				2,493.30	2,646.80	2,811.30
Net Reserves for Unearned Premiums	680.1				529.9				469.1				680.1	529.9	469.1
Net earned premium													1,088.20	1,073	1,132.40
Losses and LAE Expenses													497.8	481.7	635.3
Policy Acquisition Expenses													200	207.2	207.8
Net written premiums													1,124	1,082	1,156.90
General and Administrative Expenses													146.4	131	123.9
Insurance
Deferred Policy Acquisition Costs	142.6				130.3				113.6				142.6	130.3	113.6
Losses and loss adjustment expenses	1,907.50				1,699.40				1,469.40				1,907.50	1,699.40	1,469.40
Net Reserves for Unearned Premiums	554.9				598.8				529.1				554.9	598.8	529.1
Net earned premium													1,317.10	1,098.80	951.1
Losses and LAE Expenses													809.7	742	603.2
Policy Acquisition Expenses													251.2	214.8	173.4
Net written premiums													1,391.20	1,217.70	1,090
General and Administrative Expenses													$205.50	$185.90	$168.20

Schedule_IV_Reinsurance_Premiu

Schedule IV - Reinsurance Premiums Written and Premiums Earned (Details) (USD $)	3 Months Ended												12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
Direct													$1,729.90	$1,512.80	$1,355.40
Assumed													1,172.80	1,133.90	1,227.90
Ceded													-387.5	-347	-336.4
Net premiums written													2,515.20	2,299.70	2,246.90
Gross Amount Premiums Earned													1,599	1,366.80	1,177
Ceded to Other Companies Premiums Earned													-331.3	-321.6	-301.5
Assumed From Other Companies Premiums Earned													1,137.60	1,126.60	1,208
Net premiums earned	$612.20	$610.40	$616.20	$566.50	$572.60	$544.30	$544	$510.90	$558.50	$516.20	$513.40	$495.40	$2,405.30	$2,171.80	$2,083.50
Percentage of Amount Assumed to Net Premiums Earned													47.30%	51.90%	58.00%

Schedule_V_Valuation_and_Quali1

Schedule V - Valuation and Qualifying Accounts (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Premiums Receivable
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$1,100,000	$100,000	$0
Charged to Costs and Expenses	3,200,000	1,000,000	100,000
Charged to Other Accounts	0	0	0
Deductions	0	0	0
Balance at End of Year	4,300,000	1,100,000	100,000
Reinsurance
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	0	200,000	200,000
Charged to Costs and Expenses		-200,000
Charged to Other Accounts	0	0	0
Deductions	0	0	0
Balance at End of Year	$0	$0	$200,000