Aspen Insurance Holdings Limited (AHL-PC) 10-K 22 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Feb. 15, 2018	Jun. 30, 2017
Document Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	AHL
Entity Registrant Name	ASPEN INSURANCE HOLDINGS LTD
Entity Central Index Key	1,267,395
Current Fiscal Year End Date	--12-31
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		59,556,038
Entity Public Float			$ 3

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND OTHER COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Net earned premium	$ 2,306.6	$ 2,637.3	$ 2,473.3
Net investment income	189	187.1	185.5
Realized and unrealized investment gains	148.9	108.4	94.5
Other income	8.9	5.7	0.1
Total revenues	2,653.4	2,938.5	2,753.4
Expenses
Losses and loss adjustment expenses	1,994.7	1,576.1	1,366.2
Amortization of deferred policy acquisition costs	400.5	528.9	483.6
General, administrative and corporate expenses	502.2	490.1	424
Interest on long-term debt	29.5	29.5	29.5
Change in fair value of derivatives	(27.7)	24.6	(6.8)
Change in fair value of loan notes issued by variable interest entities	(21.2)	17.1	19.8
Realized and unrealized investment losses	28.4	63.2	77.5
Net realized and unrealized foreign exchange losses/(gains)	23.9	(1.8)	21.4
Other expenses	4.9	1.3	0.7
Total expenses	2,935.2	2,729	2,415.9
(Loss)/income from operations before income tax	(281.8)	209.5	337.5
Income tax credit/(expense)	15.4	(6.1)	(14.4)
Net (loss)/income	(266.4)	203.4	323.1
Amount attributable to non-controlling interest	(1.3)	(0.1)	(0.8)
Net (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	(267.7)	203.3	322.3
Available for sale investments:
Reclassification adjustment for net realized (losses) on investments included in net income	(4)	(9.9)	(37.9)
Change in net unrealized gains on available for sale securities held	(10.8)	(29.1)	(71.7)
Net change from current period hedged transactions	3	0.6	2.6
Change in foreign currency translation adjustment	(56.4)	(28)	(83)
Other comprehensive (loss), gross of tax	(68.2)	(66.4)	(190)
Tax thereon:
Reclassification adjustment for net realized losses on investments included in net income	0.4	1	1.2
Change in net unrealized gains on available for sale securities held	1.6	0.3	3.2
Net change from current period hedged transactions	(0.4)	0.1	0
Change in foreign currency translation adjustment	15.8	0.3	10.9
Total tax on other comprehensive income	17.4	1.7	15.3
Other comprehensive (loss)/income, net of tax	(50.8)	(64.7)	(174.7)
Total comprehensive (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	$ (318.5)	$ 138.6	$ 147.6
Earnings Per Share [Abstract]
Basic, in Shares	59,753,886	60,478,740	61,287,884
Diluted, in Shares	59,753,886	61,860,689	62,687,503
Basic (loss)/earnings per ordinary share adjusted for preference share dividends, dollars per share	$ (5.22)	$ 2.67	$ 4.64
Diluted (loss)/earnings per ordinary share adjusted for preference share dividends, dollars per share	$ (5.22)	$ 2.61	$ 4.54

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments:
Fixed income maturities, available for sale (amortized cost — $5,201.2 and $5,620.1)	$ 5,231,000	$ 5,664,600
Fixed income maturities, trading at fair value (amortized cost — $1,634.9 and $1,264.8)	1,649,300	1,265,700
Equity securities, trading at fair value (cost — $414.8 and $554.3)	491,000	584,700
Short-term investments, available for sale (amortized cost — $90.0 and $145.3)	89,900	145,300
Short-term investments, trading (amortized cost — $73.0 and $185.4)	73,000	185,400
Catastrophe bonds, trading at fair value ( cost — $33.5 and $42.5)	32,400	42,500
Other investments, equity method	66,400	12,100
Total investments	7,633,000	7,900,300
Cash and cash equivalents (including cash within consolidated variable interest entities of — $166.6 and $291.3)	1,054,800	1,273,800
Reinsurance recoverables:
Unpaid losses	1,515,200	560,700
Ceded unearned premiums	515,500	255,200
Receivables:
Underwriting premiums	1,496,500	1,399,400
Other	151,100	95,500
Funds withheld	99,800	73,100
Deferred policy acquisition costs	294,300	358,400
Derivatives at fair value	6,400	7,200
Receivables for securities sold	5,300	1,600
Office properties and equipment	75,500	83,800
Tax recoverable	2,300	500
Deferred tax assets	28,300	0
Other assets	500	1,000
Intangible assets and goodwill	27,900	79,600
Total assets	12,906,400	12,090,100
Insurance reserves
Losses and loss adjustment expenses	6,749,500	5,319,900
Unearned premiums	1,820,800	1,618,600
Total insurance reserves	8,570,300	6,938,500
Payables
Reinsurance premiums	357,500	345,300
Deferred taxation	0	6,100
Accrued expenses and other payables	455,400	469,200
Liabilities under derivative contracts	1,000	18,400
Total payables	813,900	839,000
Loan notes issued by variable interest entities, at fair value	44,200	115,000
Long-term debt	549,500	549,300
Total liabilities	9,977,900	8,441,800
Commitments and contingent liabilities (see Note 19)	0	0
Ordinary shares:
59,474,085 shares of par value 0.15144558¢ each (December 31, 2016 — 59,774,464)	90	91
Preference shares:
Non-controlling interest	2,700	1,400
Additional paid-in capital	954,700	1,259,600
Retained earnings	2,026,900	2,392,300
Accumulated other comprehensive income, net of taxes	(55,900)	(5,100)
Total shareholders’ equity	2,928,500	3,648,300
Total liabilities and shareholders’ equity	12,906,400	12,090,100
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
5.625% Preference Shares
Preference shares:
Preference shares, value	$ 0	$ 0

CONSOLIDATED BALANCE SHEETS Con

CONSOLIDATED BALANCE SHEETS Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fixed income securities, available for sale amortized cost	$ 5,201.2	$ 5,620.1
Trading Securities, Debt, Amortized Cost	1,634.9	1,264.8
Equity securities, trading, cost	414.8	554.3
Available-for-sale Securities, Short-term Investments, Amortized Cost	90	145.3
Short-term investments, trading, amortized cost	73	185.4
Cash and cash equivalents, within VIEs	166.6	291.3
Catastrophe bonds trading, at fair value	$ 33.5	$ 42.5
Ordinary shares, par value	$ 0.0015144558	$ 0.0015144558
Ordinary shares, issued	59,474,085	59,774,464
Preference shares, par value	$ 0.0015144558	$ 0.0015144558
5.950% Preference Shares
Preference shares, issued	11,000,000	11,000,000
Preference shares, rate	5.95%	5.95%
Preference shares, par value	$ 0.0015144558	$ 0.0015144558
7.401% Preference Shares
Preference shares, issued	0	5,327,500
Preference shares, rate	7.401%	7.401%
Preference shares, par value		$ 0.0015144558
7.250% Preference Shares
Preference shares, issued	0	6,400,000
Preference shares, rate	7.25%	7.25%
Preference shares, par value		$ 0.0015144558
5.625% Preference Shares
Preference shares, issued	10,000,000	10,000,000
Preference shares, rate	5.625%	5.625%
Preference shares, par value	$ 0.0015144558	$ 0.0015144558

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Millions		Total	Ordinary Shares	Preference Shares	7.401% Preference Shares	7.250% Preference Shares	Non-controlling interest	Additional paid-in capital	Additional paid-in capitalOrdinary Shares	Additional paid-in capitalPreference Shares	Retained earnings		Retained earningsOrdinary Shares	Retained earningsPreference Shares	Cumulative foreign currency translation adjustments, net of taxes:	Gain/(loss) on derivatives, net of taxes:	Unrealized appreciation on available for sale investments, net of taxes:
Preference share redemption costs		$ 0
Shareholder's equity beginning balance at Dec. 31, 2014			$ 0.1	$ 0			$ 0.5	$ 1,134.3			$ 2,050.1				$ 72.7	$ (3.8)	$ 165.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change attributable to non-controlling interest for the year		0.8					0.8
Shares issued									$ 6.8	$ 0
Ordinary shares repurchased and cancelled									(83.7)
Preference shares redeemed and cancelled										0
Preference shares redemption	[1]									0				$ 0
Share-based compensation	[2]							17.9
Net (loss)/income for the year		323.1									323.1
Dividends													$ (50.9)	(37.8)
Amount attributable to non-controlling interest		(0.8)					(0.8)
Dividends due to non-controlling interest											(0.1)
Change for the year, net of income tax															(72.1)		(105.2)
Shareholder's equity ending balance at Dec. 31, 2015		3,419.9	0.1	0			1.3	1,075.3			2,283.6				0.6	(1.2)	60.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based payment | Accounting Standards Update 2016-09	[3]										0
Net change from current period hedged transactions																2.6
Total accumulated other comprehensive (losses)/income, net of taxes		59.6
Preference share redemption costs		0
Net change attributable to non-controlling interest for the year		0.1					0.1
Shares issued									2.5	241.3
Ordinary shares repurchased and cancelled									(75)
Preference shares redeemed and cancelled										0
Preference shares redemption	[1]									0				0
Share-based compensation	[2]							15.5
Net (loss)/income for the year		203.4									203.4
Dividends													(52.7)	(41.8)
Amount attributable to non-controlling interest		(0.1)					(0.1)
Dividends due to non-controlling interest											(0.1)
Change for the year, net of income tax															(27.7)		(37.7)
Shareholder's equity ending balance at Dec. 31, 2016		3,648.3	0.1	0			1.4	1,259.6			2,392.3				(27.1)	(0.5)	22.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Dividend Rate, Percentage					7.401%	7.25%
Share-based payment | Accounting Standards Update 2016-09	[3]										0
Net change from current period hedged transactions																0.7
Total accumulated other comprehensive (losses)/income, net of taxes		(5.1)
Preference share redemption costs		(8)								8
Net change attributable to non-controlling interest for the year		1.3					1.3
Shares issued									0.5	0
Ordinary shares repurchased and cancelled									$ (30)
Preference shares redeemed and cancelled										(293.2)
Preference shares redemption	[1]									$ (8)				8
Share-based compensation	[2]							9.8
Net (loss)/income for the year		(266.4)
Dividends													$ (56.2)	$ (36.2)
Amount attributable to non-controlling interest		(1.3)					(1.3)
Dividends due to non-controlling interest											(0.1)
Change for the year, net of income tax															(40.6)		(12.8)
Shareholder's equity ending balance at Dec. 31, 2017		2,928.5	$ 0.1	$ 0			$ 2.7	954.7			2,026.9				$ (67.7)	2.1	$ 9.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Dividend Rate, Percentage					7.401%	7.25%
Share-based payment | Accounting Standards Update 2016-09								$ 7.9			$ 2.8	[3]
Net change from current period hedged transactions																$ 2.6
Total accumulated other comprehensive (losses)/income, net of taxes		$ (55.9)
[1]	The $8.0 million reclassification from additional paid-in capital to retained earnings is the difference between the capital raised upon issuance of the 7.401% and 7.250% Perpetual Non-Cumulative Preference Shares, net of issuance costs, and the final redemption costs of $293.2 million.
[2]	The balance in 2017 includes $7.9 million reclassification from accrued expenses and other payable as a result of the classification of restricted share units as equity following the adoption of ASU 2016-09 -“Compensation — Stock Compensation”.
[3]	The $2.8 million relates to the cumulative effect-adjustment to opening retained earnings as a result of the classification of restricted share units as equity following the adoption of ASU 2016-09. The adjustment has been applied using a modified retrospective approach.

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from/(used in) operating activities:
Net (loss)/income		$ (266.4)	$ 203.4	$ 323.1
Proportion due to non-controlling interest		(1.3)	(0.1)	(0.8)
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization		62.2	51.5	28.5
Share-based compensation		9.8	15.5	17.9
Realized and unrealized investment (gains)		(148.9)	(108.4)	(94.5)
Realized and unrealized investment losses		28.4	63.2	77.5
Change in fair value of loan notes issued by variable interest entities		(21.2)	17.1	19.8
Deferred taxes		(32.5)	9.4	(6.8)
Net realized and unrealized investment foreign exchange (gains)/losses		(15)	1.4	5.5
Net change from current period hedged transactions		2.6	0.7	2.6
Insurance reserves:
Losses and loss adjustment expenses		1,292.2	466.2	244.5
Unearned premiums		174.1	23.9	120.4
Reinsurance recoverables:
Unpaid losses		(943.7)	(219.9)	(8.6)
Ceded unearned premiums		(257.6)	(87.6)	37.8
Other receivables		(48.2)	1.4	0.6
Deferred policy acquisition costs		69.4	2.4	(61.9)
Reinsurance premiums payable		23.8	234	2.7
Funds withheld		(26.7)	(37.1)	10.9
Premiums receivable		(88.7)	(271.1)	(123.7)
Income tax payable		15.9	(20.1)	(7.4)
Accrued expenses and other payable		147	75.7	(11.9)
Fair value of derivatives and settlement of liabilities under derivatives		(16.6)	16.4	(11.4)
Long-term debt and loan notes issued by variable interest entities		(70.6)	12.2	0.1
Other assets		0.5	3.1	9.3
Net cash (used in)/generated by operating activities		(111.5)	453.2	574.2
Cash flows (used in)/from investing activities:
(Purchases) of fixed income securities — Available for sale		(1,573.2)	(2,236.7)	(2,131.9)
(Purchases) of fixed income securities — Trading		(1,312.9)	(973.3)	(556.9)
Proceeds from sales and maturities of fixed income securities — Available for sale		2,018.8	2,307.9	1,656.3
Proceeds from sales and maturities of fixed income securities — Trading		957.6	593.7	519.9
(Purchases) of equity securities — Trading		(131.3)	(195.3)	(392.2)
Net proceeds of catastrophe bonds — Trading		7.4	12.8	(20.9)
Proceeds from sales of equity securities — Available for sale		0	0	108.6
Proceeds from sales of equity securities — Trading		316.3	372	270.8
(Purchases) of short-term investments — Available for sale		(130.7)	(224.6)	(212.1)
Proceeds from sale of short-term investments — Available for sale		189.5	242.9	282.6
(Purchases) of short-term investments — Trading		(96)	(190.6)	(45.6)
Proceeds from sale of short-term investments — Trading		212	14.1	36.3
Net change in (payable)/receivable for securities (purchased)/sold		(9.9)	12.7	(2.1)
Net proceeds in (purchases)/sales from other investments		(0.5)	0	0
(Net) purchases of equipment		(35)	(23.7)	(13.9)
Sale of investment		9.3	0	0
Other equity investments		(2.4)	(3.3)	(0.8)
Payments for acquisitions and investments, net of cash acquired		0	(59.5)	0
Net cash from/(used in) investing activities		419	(350.9)	(501.9)
Cash flows (used in)/from financing activities:
Proceeds from the issuance of ordinary shares, net of issuance costs		0.5	2.5	6.8
Ordinary shares repurchased		(30)	(75)	(83.7)
Proceeds from the issuance of preference shares, net of issuance costs		0	241.3	0
Preference share redemption		(293.2)	0	0
Proceeds from loan notes issued by Silverton		0	105	100
Repayment of long-term debt issued by Silverton		(115.6)	(89.3)	(67.8)
Dividends paid on ordinary shares		(56.2)	(52.7)	(50.9)
Dividends paid on preference shares		(36.2)	(41.8)	(37.8)
Dividends paid to non-controlling interest		(0.1)	0	(0.1)
Cash paid for withholdings purposes	[1]	9.6	0	0
Net cash (used in)/from financing activities		(540.4)	90	(133.5)
Effect of exchange rate movements on cash and cash equivalents		13.9	(18)	(17.8)
Increase/(decrease) in cash and cash equivalents		(219)	174.3	(79)
Cash and cash equivalents at beginning of period		1,273.8	1,099.5	1,178.5
Cash and cash equivalents at end of period		1,054.8	1,273.8	1,099.5
Supplemental disclosure of cash flow information:
Net cash paid during the period for income tax		3.4	7	8.8
Cash paid during the period for interest		$ 29	$ 29	$ 29
[1]	The cash paid to the tax authority when withholding shares from employees' awards for tax-withholding purposes has been reclassified from operating activity to financing activity following the adoption of ASU 2016-09 -“Compensation — Stock Compensation”.

History and Organization

History and Organization	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
History and Organization	History and Organization Aspen Insurance Holdings Limited (“Aspen Holdings”) was incorporated on May 23, 2002 as a holding company headquartered in Bermuda. We underwrite specialty insurance and reinsurance on a global basis through our Operating Subsidiaries (as defined below) based in Bermuda, the United States and the United Kingdom: Aspen Insurance UK Limited (“Aspen U.K.”) and Aspen Underwriting Limited (corporate member of Lloyd’s Syndicate 4711, “AUL” and managed by Aspen Managing Agency Limited (“AMAL”)) (United Kingdom), Aspen Bermuda Limited (“Aspen Bermuda”) (Bermuda), Aspen Specialty Insurance Company (“Aspen Specialty”) and Aspen American Insurance Company (“AAIC”) (United States) (collectively, the “Operating Subsidiaries”). We also have branches in Australia, Canada, France, Germany, Ireland, Singapore, Switzerland and the United Arab Emirates. We established Aspen Capital Management, Ltd and other related entities (collectively, “ACM”) to leverage our existing underwriting franchise, increase our operational flexibility in the capital markets and provide investors direct access to our underwriting expertise. References to the “Company,” the “Group,” “we,” “us” or “our” refer to Aspen Holdings or Aspen Holdings and its subsidiaries.

Basis of Preparation and Signif

Basis of Preparation and Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
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 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, unrecognized tax benefits, 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 be significantly different from those expected when the assumptions or estimates were made. (b) Accounting for Insurance and Reinsurance Operations Premiums Earned. Premiums are recorded as written on the inception date of a policy. Premiums are primarily 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 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 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. Reinstatement premiums 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, which are paid when the Company purchases reinsurance or retrocessional coverage, are accounted for using the same accounting methodology as the Company uses 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. Premiums payable for retroactive reinsurance coverage and meeting the conditions of reinsurance accounting, are reported as reinsurance recoverables to the extent that those amounts do not exceed recorded liabilities relating to underlying reinsurance contracts. To the extent that recorded liabilities on an underlying reinsurance contract exceed premiums payable for retroactive coverage, a deferred gain is recognised. 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 business 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 business 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 under receivables for securities sold and accrued expenses and other payables for securities purchased, respectively. 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 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, Equity Method. 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 (i) 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 (ii) 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. 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. 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. 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) Accounting for 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 is impaired. Goodwill is assessed annually for impairment or more frequently if circumstance indicate an impairment may have occurred. (f) Accounting for 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) Accounting for 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) Accounting for 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 Taxes 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 deferred tax assets will be fully recovered, it recognizes a valuation allowance against its deferred tax assets to reduce the deferred tax assets to the amount more likely than not to be realized. 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. Furthermore, a tax benefit from a tax position may be recognized in the financial statements only if it is more-likely-than-not that the position is sustainable, based solely on its technical merits and consideration of the relevant tax authority’s widely understood administrative practices and precedents. The tax benefit recognized, when the likelihood of realization is more likely-than-not (i.e. greater than 50 percent), is measured at the largest amount that is greater than 50 percent likely of being realized upon settlement. (j) Accounting for Preference Shares The Company had at the balance sheet date in issue two classes of preference shares. The Company has no obligation to pay interest on these securities but they 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. The preference shares are therefore accounted for as equity instruments and included within total shareholders’ equity. (k) Accounting for Long-term Incentive Plans The Company operates an employee share incentive plan, a non-executive director stock incentive plan and employee share purchase plans, the terms and conditions of which are described in Note 17. 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) Accounting for Long-term Debt Issued by Variable Interest Entities Silverton, a consolidated variable interest entity, has issued debt instruments which are due to mature on September 17, 2018 and September 16, 2019 as 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 due to the potential variability over the ultimate settlement value of the debt instruments. (m) Accounting for Business Combinations The Company accounts for a transaction as a business combination where the assets acquired and liabilities assumed following a transaction constitute a business. An acquired entity must have inputs and processes that make it capable of generating a return or economic benefit to be considered a business. If the assets acquired are not a business, the Company accounts the transaction as an asset acquisition. The Company recognizes and measures at fair value 100 percent of the assets and liabilities of any acquired business. Goodwill is recognized and measured as the difference between the consideration paid or payable less the fair value of assets acquired. The Company accounts for the disposal of subsidiary undertakings when it ceases to control the subsidiary's assets and liabilities or the group of assets. A gain or loss is recognised and measured as the difference between the fair value of consideration received or receivable and the value of assets, liabilities and equity components de-recognized, related to that subsidiary or group of assets when deconsolidated. (n) New Accounting Pronouncements New Accounting Pronouncements Adopted in 2017 On March 30, 2016, the Financial Accounting Standards Board (“FASB”) issued ASU 2016-09, “ Compensation — Stock Compensation ” which provides guidance on several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. This ASU is effective for fiscal years beginning after December 15, 2016 and interim periods beginning after December 15, 2016. Following the adoption of this ASU, all excess tax benefits or expense related to stock-based compensation transactions are recognized prospectively as income tax benefits or expense in the Consolidated Income Statement and the excess tax benefits from stock-based compensation transactions previously included in “Financing activities” on the Consolidated Statements of Cash Flows are prospectively included on that statement as “Operating activities.” The cash paid to the tax authority for tax withholding purposes has also been reclassified from operating to financing activity in the Consolidated Statement of Cash Flows and the comparative period has been restated. This ASU also allows share withholding up to the maximum statutory withholding requirement while still avoiding liability accounting. As a result, the Company has applied the equity accounting method for its restricted share units retrospectively and has recorded a cumulative effect adjustment of $2.8 million through opening retained earnings and $7.9 million through additional paid-in capital. 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. This ASU was effective for fiscal years beginning after December 15, 2016 and interim periods ending after December 15, 2016. This ASU did not have a material impact on the Company's consolidated financial statements. On October 27, 2016, the FASB issued ASU 2016-17, “ Consolidation (Topic 810): Interests Held through Related Parties That Are under Common Control ” which introduces new variable interest entity guidance on evaluating indirect interests held by related parties under common control. This ASU is effective for fiscal years beginning after December 15, 2016, including interim periods within those fiscal years, and does not have an impact on the consolidation of the Company's two investments in variable interest entities as described in Note 7. Accounting Pronouncements Not Yet Adopted On August 12, 2015, the FASB issued ASU 2015-14, "Revenue from Contracts with Customers (Topic 606)" which delays the effective date of ASU 2014-09 by one year. ASU 2015-14 is effective for annual periods beginning after December 15, 2017. This ASU will not have a material impact on the Company's consolidated financial statements as insurance contracts accounted for within the scope of Topic 944, Financial Services are exempt from this ASU and the Company has immaterial other revenue. On January 5, 2016, the FASB issued ASU 2016-1, “ Financial Instruments - Overall (Subtopic 825-10) ” which enhances the reporting model for financial instruments. This ASU requires all equity investments to be measured at fair value with changes in the fair value recognized through net income (other than those accounted for under the equity method of accounting or those that result in consolidation of the investee). The amendments required as a result of this ASU are effective for fiscal years beginning after December 15, 2017. The Company does not expect this ASU to have a material impact on the Company’s consolidated financial statements as the Company’s equity portfolio is classified as held for trading with changes in fair value recognized through net income. On February 25, 2016, the FASB issued ASU 2016-2, “ Leases (Topic 842) ” which supersedes the leases requirements in Topic 840 and establishes the principles that lessees and lessors shall apply to report useful information to users of financial statements about the amount, timing, and uncertainty of cash flows arising from a lease. The amendments of this ASU are effective for fiscal years beginning after December 15, 2018. This ASU is expected to have a material impact on the Company’s consolidated financial statements, specifically increasing the Company’s assets and liabilities as all leases greater than twelve months will be recognized on the balance sheet as a right of use asset and lease liability. This ASU will not have a material impact on the Company’s consolidated income statement. On June 16, 2016, the FASB issued ASU 2016-13, “ Financial Instruments - Credit Losses (Topic 326) ” which introduces a new impairment model, known as the current expected credit loss model, which is based on expected losses rather than incurred losses. Under the new credit loss model, the Company would recognize an allowance for its estimate of expected credit losses and this would apply to most debt instruments (other than those measured at fair value), trade receivables, lease receivables, reinsurance receivables, financial guarantee contracts and loan commitments. Available-for-sale debt securities are outside the model’s scope and this ASU made limited amendments to the impairment model for available-for-sale debt securities. There are other amendments required as a result of this ASU that are effective for fiscal years beginning after December 15, 2019. The Company is currently assessing the impact the adoption of this ASU will have on future financial statements and disclosures. On August 26, 2016, the FASB issued ASU 2016-15, “ Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (a consensus of the Emerging Issues Task Force) ” which standardizes the presentation and classification of certain cash receipts and cash payments in the statement of cash flows under Topic 230 Statement of Cash Flows. This ASU is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. This ASU will not have a material impact on the Company's consolidated statement of cash flows. On October 24, 2016, the FASB issued ASU 2016-16, “ Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory ” which will require companies to account for the income tax effects of intercompany transfers of assets other than inventory (e.g., intangible assets) when the transfer occurs. This ASU is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. This ASU will not have a material impact on the Company’s future financial statements and disclosures. On March 10, 2017, FASB issued ASU 2017-7, “ Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post Retirement Benefit Cost ” which changes how employers report defined benefit pension and/or other post-retirement benefit costs in their financial statements. This ASU is effective for fiscal years beginning after December 15, 2017 and interim periods beginning after December 15, 2017. As the Company does not operate a defined benefit pension scheme, this ASU will not have a material impact on Company's consolidated financial statements and disclosures. On August 28, 2017, the FASB issued ASU 2017-12, “Derivatives and Hedging (Topic 815)” enabling entities to better align their hedge accounting and risk management activities, while also simplifying the application of hedge accounting in certain situations. This ASU is effective for fiscal years beginning after 15 December, 2018 using a modified retrospective approach for cash flow and net investment hedge relationships that exist on the date of adoption. The Company is currently evaluating the provisions of ASU 2017-12 to determine how it will be affected, but no material impact is expected on the consolidated financial statements. Other accounting pronouncements were issued during the year ended December 31, 2017 which were either not relevant to the Company or did not impact the Company's consolidated financial statements.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions There were no related party transactions for the twelve months ended December 31, 2017 or December 31, 2016 .

Earnings per Ordinary Share

Earnings per Ordinary Share	12 Months Ended
	Dec. 31, 2017
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, 2017 , 2016 , and 2015 , respectively: Twelve Months Ended December 31, 2017 2016 2015 Net (loss)/income $ (266.4 ) $ 203.4 $ 323.1 Preference share dividends (36.2 ) (41.8 ) (37.8 ) Preference share redemption costs (1) (8.0 ) — — Net profit attributable to non-controlling interest (1.3 ) (0.1 ) (0.8 ) Basic and diluted net income available to ordinary shareholders $ (311.9 ) $ 161.5 $ 284.5 Ordinary shares: Basic weighted average ordinary shares 59,753,886 60,478,740 61,287,884 Weighted average effect of dilutive securities (2) (3) — 1,381,949 1,399,619 Total diluted weighted average ordinary shares 59,753,886 61,860,689 62,687,503 Earnings per ordinary share: Basic $ (5.22 ) $ 2.67 $ 4.64 Diluted (3) $ (5.22 ) $ 2.61 $ 4.54 _______________ (1) The $8.0 million deduction from net income in 2017 is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the 7.401% Preference Shares and 7.250% Preference Shares, net of issuance costs, and the final redemption costs of $293.2 million . (2) Dilutive securities consist of employee restricted share units and performance shares associated with the Company’s long term incentive plan, employee share purchase plans and director restricted stock units and options as described in Note 17. (3) The basic and diluted number of ordinary shares for 2017 is the same, as the inclusion of dilutive securities in a loss making period would be anti-dilutive. Dividends. On February 7, 2018, the Company’s Board of Directors declared the following quarterly dividends: Dividend Payable on: Record Date: Ordinary shares $ 0.24 March 13, 2018 February 23, 2018 5.95% Preference Shares $ 0.3719 April 1, 2018 March 15, 2018 5.625% Preference Shares $ 0.3516 April 1, 2018 March 15, 2018

Segment Reporting

Segment Reporting	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Segment Reporting	Segment Reporting The Company has two reporting business segments: Insurance and Reinsurance. The Company has determined its reportable segments, Aspen Insurance and Aspen Reinsurance, by taking into account the manner in which management makes operating decisions and assesses operating performance. Profit or loss for each of the Company’s business 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, casualty reinsurance and specialty insurance and reinsurance. ACM forms part of our property catastrophe reinsurance line of business as it focuses primarily on property catastrophe business through the use of 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. 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-operating) 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, none of which are allocated to the business segments. Corporate expenses are not allocated to the Company’s business segments as they typically do not fluctuate with the levels of premiums written and are not directly related to the Company’s business segment operations. The Company does not allocate its assets by business segment as it evaluates underwriting results of each business 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, 2017 , 2016 and 2015 : Twelve Months Ended December 31, 2017 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,548.5 $ 1,812.4 $ 3,360.9 Net written premiums 1,250.0 962.5 2,212.5 Gross earned premiums 1,451.8 1,757.4 3,209.2 Net earned premiums 1,206.1 1,100.5 2,306.6 Underwriting Expenses Losses and loss adjustment expenses 1,116.4 878.3 1,994.7 Amortization of deferred policy acquisition costs 235.5 165.0 400.5 General and administrative expenses 157.3 253.9 411.2 Underwriting (loss) (303.1 ) (196.7 ) (499.8 ) Corporate expenses (58.3 ) Non-operating expenses (32.7 ) Net investment income 189.0 Realized and unrealized investment gains 148.9 Realized and unrealized investment losses (28.4 ) Change in fair value of loan notes issued by variable interest entities 21.2 Change in fair value of derivatives 27.7 Interest expense on long term debt (29.5 ) Net realized and unrealized foreign exchange (losses) (23.9 ) Other income 8.9 Other expenses (4.9 ) Income before tax (281.8 ) Income tax credit 15.4 Net income $ (266.4 ) Net reserves for loss and loss adjustment expenses $ 2,917.1 $ 2,317.2 $ 5,234.3 Ratios Loss ratio 92.6 % 79.8 % 86.5 % Policy acquisition expense ratio 19.5 15.0 17.4 General and administrative expense ratio (1) 13.0 23.1 21.8 Expense ratio 32.5 38.1 39.2 Combined ratio 125.1 % 117.9 % 125.7 % _______________ (1) The general and administrative expense ratio in the total column includes corporate and non-operating expenses. The Company uses underwriting ratios as measures of performance. The loss ratio is the ratio of losses and loss adjustment expenses to net earned premiums. The policy acquisition expense ratio is the ratio of amortization of deferred policy acquisition costs to net earned premiums. The general and administrative expense ratio is the ratio of general, administrative and corporate expenses to net earned premiums. The combined ratio is the sum of the loss ratio, the policy acquisition expense ratio and the general and administrative expense ratio. Twelve Months Ended December 31, 2016 Reinsurance Insurance Total ( $ in millions) Underwriting Revenues Gross written premiums $ 1,413.2 $ 1,733.8 $ 3,147.0 Net written premiums 1,269.2 1,324.5 2,593.7 Gross earned premiums 1,317.9 1,768.4 3,086.3 Net earned premiums 1,181.9 1,455.4 2,637.3 Underwriting Expenses Losses and loss adjustment expenses 657.9 918.2 1,576.1 Amortization of deferred policy acquisition costs 226.4 302.5 528.9 General and administrative expenses 178.2 228.4 406.6 Underwriting income 119.4 6.3 125.7 Corporate expenses (73.8 ) Non-operating expenses (1) (9.7 ) Net investment income 187.1 Realized and unrealized investment gains 108.4 Realized and unrealized investment losses (63.2 ) Change in fair value of loan notes issued by variable interest entities (17.1 ) Change in fair value of derivatives (24.6 ) Interest expense on long term debt (29.5 ) Net realized and unrealized foreign exchange gains 1.8 Other income 5.7 Other expenses (1.3 ) Income before tax 209.5 Income tax expense (6.1 ) Net income $ 203.4 Net reserves for loss and loss adjustment expenses $ 2,462.1 $ 2,297.1 $ 4,759.2 Ratios Loss ratio 55.7 % 63.1 % 59.8 % Policy acquisition expense ratio 19.2 20.8 20.1 General and administrative expense ratio (2) 15.1 15.7 18.6 Expense ratio 34.3 36.5 38.7 Combined ratio 90.0 % 99.6 % 98.5 % ________________ (1) Non-operating expenses includes amortization of intangibles acquired from the acquisition of AgriLogic. (2) The general and administrative expense ratio in the total column includes corporate and non-operating expenses. Twelve Months Ended December 31, 2015 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,248.9 $ 1,748.4 $ 2,997.3 Net written premiums 1,153.5 1,492.7 2,646.2 Gross earned premiums 1,153.5 1,703.3 2,856.8 Net earned premiums 1,072.6 1,400.7 2,473.3 Underwriting Expenses Losses and loss adjustment expenses 491.6 874.6 1,366.2 Amortization of deferred policy acquisition costs 224.7 258.9 483.6 General and administrative expenses 146.5 213.6 360.1 Underwriting income 209.8 53.6 263.4 Corporate expenses (63.9 ) Net investment income 185.5 Realized and unrealized investment gains 94.5 Realized and unrealized investment losses (77.5 ) Change in fair value of loan notes issued by variable interest entities (19.8 ) Change in fair value of derivatives 6.8 Interest expense on long term debt (29.5 ) Net realized and unrealized foreign exchange (losses) (21.4 ) Other income 0.1 Other expenses (0.7 ) Income before tax 337.5 Income tax expense (14.4 ) Net income $ 323.1 Net reserves for loss and loss adjustment expenses $ 2,409.5 $ 2,173.9 $ 4,583.4 Ratios Loss ratio 45.8 % 62.4 % 55.2 % Policy acquisition expense ratio 20.9 18.5 19.6 General and administrative expense ratio (1) 13.7 15.2 17.1 Expense ratio 34.6 33.7 36.7 Combined ratio 80.4 % 96.1 % 91.9 % _______________ (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 December 31, 2017 December 31, 2016 December 31, 2015 ($ in millions) Australia/Asia $ 167.3 $ 140.5 $ 140.0 Caribbean 17.6 14.3 20.3 Europe 94.5 109.7 113.6 United Kingdom 258.3 231.4 223.6 United States & Canada (1) 1,729.3 1,597.0 1,479.5 Worldwide excluding United States (2) 88.1 90.7 107.2 Worldwide including United States (3) 868.6 837.2 793.6 Others 137.2 126.2 119.5 Total $ 3,360.9 $ 3,147.0 $ 2,997.3 ______________ (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, 2017
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, 2017 , 2016 and 2015 : For the Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 ($ in millions) Fixed income securities — Available for sale $ 133.3 $ 141.3 $ 143.4 Fixed income securities — Trading 44.0 31.6 27.8 Short-term investments — Available for sale 0.4 0.6 1.1 Short-term investments — Trading 0.8 0.2 — Fixed term deposits (included in cash and cash equivalents) 6.2 3.4 3.0 Equity securities — Available for sale — — 0.1 Equity securities — Trading 13.6 20.4 20.0 Catastrophe bonds — Trading 1.8 1.6 1.9 Total 200.1 199.1 197.3 Investment expenses (11.1 ) (12.0 ) (11.8 ) Net investment income $ 189.0 $ 187.1 $ 185.5 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 December 31, 2017 , 2016 and 2015 : For the Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 10.2 $ 18.6 $ 11.7 Fixed income securities — gross realized (losses) (6.6 ) (8.3 ) (2.7 ) Equity securities — gross realized gains — — 31.9 Equity securities — gross realized (losses) — — (3.0 ) Short-term investments — gross realized gains 0.1 — — Cash and cash equivalents — gross realized gains 0.4 0.2 — Cash and cash equivalents — gross realized (losses) (0.1 ) (0.6 ) — Other-than-temporary impairments (0.7 ) — — Trading: Fixed income securities — gross realized gains 9.7 12.6 4.9 Fixed income securities — gross realized (losses) (4.5 ) (7.3 ) (6.1 ) Short-term investments — gross realized gains 2.7 — — Cash and cash equivalents — gross realized gains 1.3 0.1 — Cash and cash equivalents — gross realized (losses) — (0.3 ) — Equity securities — gross realized gains 59.0 54.1 46.0 Equity securities — gross realized (losses) (13.7 ) (46.3 ) (31.7 ) Catastrophe bonds — net unrealized (losses)/gains (2.4 ) — (0.3 ) Net change in gross unrealized gains (losses) 60.3 22.5 (33.1 ) Other investments, equity method: Gross realized and unrealized (loss) in MVI (0.1 ) (0.3 ) — Gross unrealized gain/(loss) in Chaspark 0.9 0.3 (0.6 ) Gross realized and unrealized (loss) in Bene (0.3 ) (0.1 ) — Gross realized gain on sale of AgriLogic 4.3 — — Total net realized and unrealized investment gains recorded in the statement of operations $ 120.5 $ 45.2 $ 17.0 Change in available for sale net unrealized gains: Fixed income securities (14.8 ) (39.0 ) (82.2 ) Short-term investments — — (0.1 ) Equity securities — — (27.3 ) Total change in pre-tax available for sale unrealized gains (14.8 ) (39.0 ) (109.6 ) Change in taxes 2.0 1.3 4.4 Total change in net unrealized gains, net of taxes recorded in other comprehensive income $ (12.8 ) $ (37.7 ) $ (105.2 ) Balance Sheet Fixed Income Securities and Short-Term Investments — 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 and short-term investments as at December 31, 2017 and December 31, 2016 : As at December 31, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,166.5 $ 4.5 $ (11.6 ) $ 1,159.4 U.S. agency 51.8 0.5 (0.2 ) 52.1 Municipal 53.0 2.1 (0.2 ) 54.9 Corporate 2,391.4 36.1 (11.8 ) 2,415.7 Non-U.S. government-backed corporate 91.5 0.3 (0.5 ) 91.3 Non-U.S government 479.7 6.4 (1.2 ) 484.9 Asset-backed 26.3 — (0.1 ) 26.2 Non-agency commercial mortgage-backed — — — — Agency mortgage-backed 941.0 13.7 (8.2 ) 946.5 Total fixed income securities — Available for sale 5,201.2 63.6 (33.8 ) 5,231.0 Total short-term investments — Available for sale 90.0 — (0.1 ) 89.9 Total $ 5,291.2 $ 63.6 $ (33.9 ) $ 5,320.9 As at December 31, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,207.9 $ 9.4 $ (11.2 ) $ 1,206.1 U.S. agency 117.7 1.9 — 119.6 Municipal 23.2 1.6 (0.4 ) 24.4 Corporate 2,566.9 39.6 (20.0 ) 2,586.5 Non-U.S. government-backed corporate 89.2 0.7 (0.1 ) 89.8 Non-U.S government 477.7 11.8 (0.8 ) 488.7 Asset-backed 62.6 0.4 — 63.0 Non-agency commercial mortgage-backed 12.3 0.3 — 12.6 Agency mortgage-backed 1,062.6 19.6 (8.3 ) 1,073.9 Total fixed income securities — Available for sale 5,620.1 85.3 (40.8 ) 5,664.6 Total short-term investments — Available for sale 145.3 — — 145.3 Total $ 5,765.4 $ 85.3 $ (40.8 ) $ 5,809.9 There were non-cash transfers of securities from available for sale to trading of $95.1 million during the twelve months ended December 31, 2016. The transfers were made following the decision by the North Dakota regulator to reduce the held capital requirement for the recognition of ceded reinsurance recoveries from Aspen Bermuda Limited from 100% to 20% . This decision resulted in the establishment of a new reduced collateral Regulation 114 trust which was funded by non-cash transfers of securities from available for sale portfolios. Fixed Income Securities, Short Term Investments, Equities and Catastrophe Bonds — Trading . The following tables present the cost or amortized cost, gross unrealized gains and losses, and estimated fair market value of trading investments in fixed income securities, short-term investments, equity securities and catastrophe bonds as at December 31, 2017 and December 31, 2016 : As at December 31, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 162.3 $ 0.4 $ (0.8 ) $ 161.9 Municipal 32.4 — (0.2 ) 32.2 Corporate 1,036.5 14.0 (4.2 ) 1,046.3 Non-U.S. government-backed corporate 1.0 — — 1.0 Non-U.S. government 196.1 6.9 (0.5 ) 202.5 Asset-backed 9.9 — — 9.9 Agency mortgage-backed 196.7 0.2 (1.4 ) 195.5 Total fixed income securities — Trading 1,634.9 21.5 (7.1 ) 1,649.3 Total short-term investments — Trading 73.0 — — 73.0 Total equity securities — Trading 414.8 83.5 (7.3 ) 491.0 Total catastrophe bonds — Trading 33.5 — (1.1 ) 32.4 Total $ 2,156.2 $ 105.0 $ (15.5 ) $ 2,245.7 As at December 31, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 82.8 $ 0.4 $ (0.8 ) $ 82.4 Municipal 15.7 — (0.2 ) 15.5 Corporate 817.8 9.9 (7.1 ) 820.6 Non-U.S government 203.4 3.5 (4.1 ) 202.8 Asset-backed 14.5 — — 14.5 Agency mortgage-backed 130.6 0.2 (0.9 ) 129.9 Total fixed income securities — Trading 1,264.8 14.0 (13.1 ) 1,265.7 Total short-term investments — Trading 185.4 — — 185.4 Total equity securities — Trading 554.3 55.4 (25.0 ) 584.7 Total catastrophe bonds — Trading 42.5 — — 42.5 Total $ 2,047.0 $ 69.4 $ (38.1 ) $ 2,078.3 The Company classifies the financial instruments listed above as held for trading as this most closely reflects the facts and circumstances of the investments held. As at December 31, 2017 , the Company had a 5.8% position in equities, a 4.0% position in BBB Emerging Market Debt and 0.2% in risk asset portfolio cash, totaling approximately 10.0% of our Managed Portfolio ( December 31, 2016 — 10.8% ). Catastrophe bonds. The Company has invested in catastrophe bonds with a total value of $32.4 million as at December 31, 2017 . The bonds receive quarterly interest payments based on variable interest rates with scheduled maturities ranging from 2018 to 2021. 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 regions of the United States, Canada, the North Atlantic, Japan or Australia. Other Investments, equity method. In January 2015, the Company established, along with seven other insurance companies, a micro-insurance venture consortium and micro-insurance incubator (“MVI”) domiciled in Bermuda. The MVI is a social impact organization that provides micro-insurance products to assist global emerging consumers. The Company’s initial investment in the MVI was $0.8 million . The Company made an additional investment of $0.1 million in the twelve months ended December 31, 2017 . 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. In the twelve months ended December 31, 2017 , the change in the value of the Company’s investment in Chaspark was an unrealized gain of $0.9 million ( December 31, 2016 — unrealized gain of $0.3 million ). On March 10, 2017, Aspen Recoveries Limited received cash of $9.3 million as settlement of its share of subrogation assets held by Chaspark. On July 26, 2016, the Company purchased through its wholly-owned subsidiary, Acorn Limited (“Acorn”), a 20.0% share of Bene Assicurazioni (“Bene”), an Italian-based motor insurer for a total consideration of $3.3 million . The investment is 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. On January 1, 2017, the Company purchased through its wholly-owned subsidiary, Aspen U.S. Holdings, Inc. (“Aspen U.S. Holdings”), a 49% share of Digital Risk Resources, LLC (“Digital Re”), a U.S.-based enterprise engaged in the business of developing, marketing and servicing turnkey information security and privacy liability insurance products for a total consideration of $2.3 million . The investment is 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. On December 18, 2017, the Company acquired through its wholly-owned subsidiary, Aspen U.S. Holdings, a 23.2% share of Crop Re Services LLC (“Crop Re”), a newly formed U.S.-based subsidiary of CGB Diversified Services, Inc (“CGB DS”) in exchange for the sale of AG Logic Holdings, LLC (“AgriLogic”), the Company’s U.S. crop insurance business. Total consideration for the sale of AgriLogic consisted of the 23.2% share of Crop Re valued at $62.5 million and cash in the amount of $5.9 million . Crop Re is responsible for directing the placement of reinsurance on behalf of CGB DS and CGB Insurance Company (“CGBIC”), an Indiana insurance company affiliate of CGB DS and an RMA licensed crop insurer. The remaining 76.8% of Crop Re is owned by CGB DS. AAIC’s primary crop insurance coverage will be run-off and AAIC, or an affiliate of AAIC, will provide quota share reinsurance to CGBIC for both federal and state regulated crop insurance as part of Aspen’s ownership in Crop Re. The investment in Crop Re represent the Company's share of the net assets of Crop Re plus a basis difference which represents the difference between the cost of the investment and the amount of underlying equity in net assets. The Company has determined that this basis difference of $62.5 million represents the value attributable to the ability of Crop Re to direct the placement of reinsurance business under the reinsurance commitment contained within the operating agreement between Crop Re and the Company. The investment in Crop Re is 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. The table below shows the Company’s investments in MVI, Chaspark, Bene, Digital Re and Crop Re for the twelve months ended December 31, 2017 and 2016 : MVI Chaspark Bene Digital Re Crop Re Total Opening undistributed value of investment as at January 1, 2017 $ 0.5 $ 8.4 $ 3.2 $ — $ — $ 12.1 Investment in the period 0.1 — — 2.3 62.5 64.9 Goodwill — — — (1.8 ) — (1.8 ) Distribution received — (9.3 ) — — — (9.3 ) Unrealized (loss)/gain for the twelve months to December 31, 2017 (0.1 ) 0.9 (0.3 ) — — 0.5 Closing value of investment as at December 31, 2017 $ 0.5 $ — $ 2.9 $ 0.5 $ 62.5 $ 66.4 Opening undistributed value of investment as at January 1, 2016 $ 0.8 $ 8.1 $ — $ — $ — $ 8.9 Investment in the period — — 3.3 — — 3.3 Unrealized (loss)/gain for the twelve months to December 31, 2016 (0.3 ) 0.3 (0.1 ) — — (0.1 ) Closing value of investment as at December 31, 2016 $ 0.5 $ 8.4 $ 3.2 $ — $ — $ 12.1 Fixed Income Securities. The scheduled maturity distribution of the Company’s available for sale fixed income securities as at December 31, 2017 and December 31, 2016 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, 2017 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 561.7 $ 562.4 AA Due after one year through five years 2,486.7 2,492.2 AA- Due after five years through ten years 1,092.2 1,097.4 A+ Due after ten years 93.3 106.3 A Total — Government and corporate 4,233.9 4,258.3 Agency mortgage-backed 941.0 946.5 AA+ Asset-backed 26.3 26.2 AAA Total fixed income securities — Available for sale $ 5,201.2 $ 5,231.0 At December 31, 2016 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 567.2 $ 570.0 AA Due after one year through five years 2,643.7 2,671.9 AA- Due after five years through ten years 1,172.3 1,168.1 A+ Due after ten years 99.4 105.1 A+ Total — Government and corporate 4,482.6 4,515.1 Non-agency commercial mortgage-backed 12.3 12.6 AAA Agency mortgage-backed 1,062.6 1,073.9 AA+ Asset-backed 62.6 63.0 AAA Total fixed income securities — Available for sale $ 5,620.1 $ 5,664.6 Guaranteed Investments. As at December 31, 2017 , the Company held no investments which are guaranteed by mono-line insurers, excluding those with explicit government guarantees. As at December 31, 2016 , the Company held one municipal security guaranteed by a mono-line insurer with fair value less than $0.1 million rated “CC” or higher. The standalone rating (rating without guarantee) is determined as the senior unsecured debt rating of the issuer. Where the credit ratings were split between the two main rating agencies, Standard & Poor’s Financial Services LLC (“S&P”) and Moody’s Investors Service Inc. (“Moody’s”), the lowest rating was used. The Company’s exposure to other third-party guaranteed debt is primarily to investments backed by non-U.S. government guaranteed issuers. Gross Unrealized Losses. The following tables summarize, by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position for the Company’s available for sale portfolio as at December 31, 2017 and December 31, 2016 : December 31, 2017 0-12 months Over 12 months Total Fair Market Value Gross Unrealized Losses Fair Market Value Gross Unrealized Losses Fair Market Value Gross Unrealized Losses Number of Securities ($ in millions) U.S. government $ 652.1 $ (5.1 ) $ 259.8 $ (6.5 ) $ 911.9 $ (11.6 ) 101 U.S. agency 20.1 (0.2 ) 6.1 — 26.2 (0.2 ) 10 Municipal 28.5 (0.2 ) — — 28.5 (0.2 ) 9 Corporate 699.3 (3.4 ) 360.7 (8.4 ) 1,060.0 (11.8 ) 412 Non-U.S. government-backed corporate 43.5 (0.3 ) 13.3 (0.2 ) 56.8 (0.5 ) 15 Non-U.S government 206.2 (0.8 ) 32.0 (0.4 ) 238.2 (1.2 ) 47 Asset-backed 11.1 — 10.5 (0.1 ) 21.6 (0.1 ) 11 Agency mortgage-backed 257.6 (1.9 ) 301.9 (6.3 ) 559.5 (8.2 ) 156 Total fixed income securities — Available for sale 1,918.4 (11.9 ) 984.3 (21.9 ) 2,902.7 (33.8 ) 761 Total short-term investments — Available for sale 46.9 (0.1 ) — — 46.9 (0.1 ) 8 Total $ 1,965.3 $ (12.0 ) $ 984.3 $ (21.9 ) $ 2,949.6 $ (33.9 ) 769 December 31, 2016 0-12 months Over 12 months Total Fair Market Value Gross Unrealized Losses Fair Market Value Gross Unrealized Losses Fair Market Value Gross Unrealized Losses Number of Securities ($ in millions) U.S. government $ 724.4 $ (11.2 ) $ — $ — $ 724.4 $ (11.2 ) 78 U.S. agency 14.1 — — — 14.1 — 4 Municipal 7.7 (0.2 ) 0.8 (0.2 ) 8.5 (0.4 ) 6 Corporate 1,044.4 (19.4 ) 6.6 (0.6 ) 1,051.0 (20.0 ) 386 Non-U.S. government-backed corporate 29.6 (0.1 ) — — 29.6 (0.1 ) 11 Non-U.S government 143.5 (0.8 ) 1.0 — 144.5 (0.8 ) 29 Asset-backed 25.8 — 1.4 — 27.2 — 15 Agency mortgage-backed 527.6 (7.6 ) 27.2 (0.7 ) 554.8 (8.3 ) 148 Total fixed income securities — Available for sale 2,517.1 (39.3 ) 37.0 (1.5 ) 2,554.1 (40.8 ) 677 Total short-term investments — Available for sale 1.1 — — — 1.1 — 2 Total $ 2,518.2 $ (39.3 ) $ 37.0 $ (1.5 ) $ 2,555.2 $ (40.8 ) 679 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, 2017 was $0.7 million ( 2016 — $ 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.

Variable Interest Entities

Variable Interest Entities	12 Months Ended
	Dec. 31, 2017
Payables and Accruals [Abstract]
Variable Interest Entities	Variable Interest Entities As at December 31, 2017 , the Company had investments in two ( December 31, 2016 — three ) variable interest entities (“VIE”), Peregrine Reinsurance Ltd (“Peregrine”) and Silverton Re Ltd (“Silverton”). On March 10, 2017, Aspen Recoveries Limited received cash of $9.3 million as settlement of its share of subrogation assets held by Chaspark, discussed further in Note 6, “Investments” in these consolidated financial statements. Peregrine. In November 2016, Peregrine, a subsidiary of the Company, was registered as a segregated accounts company under the Segregated Accounts Companies Act 2000, as amended. As at December 31, 2017 , Peregrine had four segregated accounts which were funded by third party investors. The segregated accounts have not been consolidated as part of the Company’s consolidated financial statements. The Company has, however, determined that Peregrine has the characteristics of a VIE as addressed by the guidance in ASC 810, Consolidation . The Company concluded that it is not the primary beneficiary of the four segregated accounts of Peregrine but is the primary beneficiary of the Peregrine general fund and, similar to prior reporting periods, the Company has included the results of the Peregrine general fund in its consolidated financial statements. The Company’s exposure to Peregrine’s general fund is not material. 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 business through retrocession agreements which are collateralized and funded by Silverton through the issuance of one or more series of participating loan notes (collectively, the “Loan Notes”). Silverton did not issue any Loan Notes in 2017. 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 and Aspen U.K., the ceding reinsurers. All proceeds from the issuance of the Loan Notes were deposited into separate collateral accounts for each series of Loan Notes to fund Silverton’s obligations under a retrocession property quota share agreement entered into with Aspen Bermuda or Aspen Bermuda and Aspen U.K, as the case may be. The holders of the Loan Notes participate in any profit or loss generated by Silverton attributable to the operations of the respective Silverton segregated account. Any existing value of the 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 of Loan Notes with the final payment being contractually due on the respective maturity dates. The following tables show the total liability balance of the Loan Notes for the twelve months ended December 31, 2017 and 2016 : For the Twelve Months Ended December 31, 2017 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 223.4 $ 54.5 $ 277.9 Total change in fair value for the period (21.2 ) (5.3 ) (26.5 ) Total distributed in the period (115.6 ) (28.6 ) (144.2 ) Total issued in the period — — — Closing balance as at December 31, 2017 $ 86.6 $ 20.6 $ 107.2 Liability Loan notes (long-term liabilities) $ 44.2 $ 10.5 $ 54.7 Accrued expenses (current liabilities) 42.4 10.1 52.5 Total aggregate unpaid balance as at December 31, 2017 $ 86.6 $ 20.6 $ 107.2 For the Twelve Months Ended December 31, 2016 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 190.6 $ 44.4 $ 235.0 Total change in fair value for the period 17.1 4.3 21.4 Total distributed in the period (89.3 ) (19.2 ) (108.5 ) Total issued in the period 105.0 25.0 130.0 Closing balance as at December 31, 2016 $ 223.4 $ 54.5 $ 277.9 Liability Loan notes (long-term liabilities) $ 115.0 $ 27.0 $ 142.0 Accrued expenses (current liabilities) 108.4 27.5 135.9 Total aggregate unpaid balance as at December 31, 2016 $ 223.4 $ 54.5 $ 277.9 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 of Silverton as it owns all of Silverton’s voting shares and issued share capital, and has a significant financial interest in, and the power to control, Silverton. As a result, the Company consolidated Silverton upon its formation. The Company has no other obligation to provide financial support to Silverton and neither the creditors nor beneficial interest holders of Silverton have recourse to the Company’s general credit. In the event of an extreme catastrophic property reinsurance event or severe credit-related event, there is a risk that Aspen Bermuda and Aspen U.K. 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 and Aspen U.K. by way of a trust in favor of Aspen Bermuda and Aspen U.K. as beneficiaries; 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. For further information regarding the Loan Notes attributable to the third-party investments in Silverton, refer to Note 8, “Fair Value Measurements” of these unaudited condensed consolidated financial statements.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
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 as at December 31, 2017 and December 31, 2016 : As at December 31, 2017 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,159.4 $ — $ — $ 1,159.4 U.S. agency — 52.1 — 52.1 Municipal — 54.9 — 54.9 Corporate — 2,415.7 — 2,415.7 Non-U.S. government-backed corporate — 91.3 — 91.3 Non-U.S. government 341.2 143.7 — 484.9 Asset-backed — 26.2 — 26.2 Agency mortgage-backed — 946.5 — 946.5 Total fixed income securities available for sale, at fair value 1,500.6 3,730.4 — 5,231.0 Short-term investments available for sale, at fair value 87.3 2.6 — 89.9 Held for trading financial assets, at fair value U.S. government 161.9 — — 161.9 Municipal — 32.2 — 32.2 Corporate — 1,046.3 — 1,046.3 Non-U.S. government-backed corporate — 1.0 — 1.0 Non-U.S. government 24.5 178.0 — 202.5 Asset-backed — 9.9 — 9.9 Agency Mortgage-Backed — 195.5 — 195.5 Total fixed income securities trading, at fair value 186.4 1,462.9 — 1,649.3 Short-term investments trading, at fair value 73.0 — — 73.0 Equity investments trading, at fair value 491.0 — — 491.0 Catastrophe bonds trading, at fair value — 32.4 — 32.4 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 6.4 — 6.4 Liabilities under derivative contracts — foreign exchange contracts — (1.0 ) — (1.0 ) Loan notes issued by variable interest entities, at fair value — — (44.2 ) (44.2 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (42.4 ) (42.4 ) Total $ 2,338.3 $ 5,233.7 $ (86.6 ) $ 7,485.4 Transfers of assets into or out of a particular level are recorded at their fair values as of the end of each reporting period, consistent with the date of the determination of fair value. During the twelve months ended December 31, 2017 there were no transfers between Level 1, 2 and 3. The Company settled $115.6 million Level 3 liabilities in respect of the loan notes issued by the Silverton VIE for the twelve months ended December 31, 2017 . As at December 31, 2017 , there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted of the loan notes issued by the Silverton VIE. At December 31, 2016 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,206.1 $ — $ — $ 1,206.1 U.S. agency — 119.6 — 119.6 Municipal — 24.4 — 24.4 Corporate — 2,586.5 — 2,586.5 Non-U.S. government-backed corporate — 89.8 — 89.8 Non-U.S. government 343.7 145.0 — 488.7 Asset-backed — 63.0 — 63.0 Non-agency commercial mortgage-backed — 12.6 — 12.6 Agency mortgage-backed — 1,073.9 — 1,073.9 Total fixed income securities available for sale, at fair value 1,549.8 4,114.8 — 5,664.6 Short-term investments available for sale, at fair value 118.6 26.7 — 145.3 Held for trading financial assets, at fair value U.S. government 82.4 — — 82.4 Municipal — 15.5 — 15.5 Corporate — 820.6 — 820.6 Non-U.S. government — 202.8 — 202.8 Asset-backed — 14.5 — 14.5 Agency mortgage-backed — 129.9 — 129.9 Total fixed income securities trading, at fair value 82.4 1,183.3 — 1,265.7 Short-term investments trading, at fair value 76.1 109.3 — 185.4 Equity investments trading, at fair value 584.7 — — 584.7 Catastrophe bonds trading, at fair value — 42.5 — 42.5 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 7.2 — 7.2 Liabilities under derivative contracts — foreign exchange contracts — (18.4 ) — (18.4 ) Loan notes issued by variable interest entities, at fair value — — (115.0 ) (115.0 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (108.4 ) (108.4 ) Total $ 2,411.6 $ 5,465.4 $ (223.4 ) $ 7,653.6 As at December 31, 2016 , the Company transferred $83.9 million of non-U.S. government securities from Level 1 to Level 2 and there were no other transfers between Levels 1 and 2 during the twelve months ended December 31, 2016 . There were no transfers between Levels 2 and 3 during the twelve months ended December 31, 2016 . The Company settled $89.3 million Level 3 liabilities in respect to the loan notes issued by the Silverton VIE for the twelve months ended December 31, 2016 . As at December 31, 2016 , there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted of the loan notes issued by the Silverton VIE. The following table presents a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Level 3 inputs for the twelve months ended December 31, 2017 : Reconciliation of Liabilities Using Level 3 Inputs Twelve Months Ended December 31, 2017 Twelve Months Ended December 31, 2016 ($ in millions) Balance at the beginning of the period (1) $ 223.4 $ 190.6 Distributed to third party (115.6 ) (89.3 ) Loan notes issued during the period — 105.0 Total change in fair value included in the statement of operations (21.2 ) 17.1 Balance at the end of the period (1) $ 86.6 $ 223.4 ______________ (1) The amount classified as other payables was $42.4 million and $108.4 million as at December 31, 2017 and December 31, 2016 , respectively. 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. As at December 31, 2017 and December 31, 2016 , 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. As at December 31, 2017 , the Company obtained an average of 2.0 quotes per fixed income investment compared to 2.1 quotes at December 31, 2016 . Pricing sources used in pricing fixed income investments as at December 31, 2017 and December 31, 2016 were as follows: As at December 31, 2017 At December 31, 2016 Index providers 84 % 87 % Pricing services 11 7 Broker-dealers 5 6 Total 100 % 100 % Summary Pricing Information Table. A summary of securities priced using pricing information from index providers as at December 31, 2017 and December 31, 2016 is provided below: As at December 31, 2017 At December 31, 2016 Fair Market Value Determined using Prices from Index Providers % of Total Fair Value from Index Providers Fair Market Value Determined using Prices from Index Providers % of Total Fair Value from Index Providers ($ in millions, except for percentages) U.S. government $ 1,321.3 100% $ 1,288.2 100% U.S. agency 43.4 83% 110.2 92% Municipal 37.4 43% 28.8 72% Corporate 3,299.6 83% 3,275.3 96% Non-U.S. government-backed corporate 44.0 48% 44.8 50% Non-U.S. government 399.4 58% 455.6 72% Asset-backed 13.5 37% 32.1 41% Non-agency commercial mortgage-backed — —% 12.5 98% Agency mortgage-backed 605.0 53% 691.9 58% Total fixed income securities $ 5,763.6 84% $ 5,939.4 87% Equities $ 491.0 100% $ 584.7 100% Total fixed income securities and equity investments $ 6,254.6 85% $ 6,524.1 88% 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. Non-U.S. Government. The issuers for securities in this category are non-U.S. governments and their agencies. The fair values of certain 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 remaining non-U.S government bonds are classified within level 2 as they are not actively traded. 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 include a portion of the Emerging Market Debt (“EMD”) 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 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 include a portion of the EMD 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. 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 as trading and carried at fair value. As at December 31, 2017 all equity securities are classified as trading. These securities are classified within Level 1 as their fair values are based on quoted market prices in active markets from independent pricing sources. As at December 31, 2017 , the Company obtained an average of 4.0 quotes per equity investment, compared to 3.9 quotes as at December 31, 2016 . Pricing sources used in pricing equities as at December 31, 2017 and December 31, 2016 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. Loan Notes Issued by Variable Interest Entities . Silverton, a licensed special purpose insurer, is consolidated into the Company’s group accounts as a VIE. During the fourth quarter of 2015, Silverton issued an additional $125.0 million ( $100.0 million third-party funded) loan notes with a maturity date of September 17, 2018. In the fourth quarter of 2016, Silverton issued an additional $130.0 million ( $105.0 million third-party funded) of Loan Notes with a maturity date of September 16, 2019. Silverton did not issue any Loan Notes in 2017. The Company elected to account for the Loan Notes at fair value using the guidance as prescribed under ASC 825, Financial Instruments as the Company believes it represents the most meaningful measurement basis for these liabilities. The Loan Notes are recorded at fair value at each reporting period and, as they are not quoted on an active market and contain significant unobservable inputs, they have been classified as Level 3 instruments in the Company’s fair value hierarchy. The Loan Notes are unique because they are linked to the specific risks of the Company’s property catastrophe book. To determine the fair value of the Loan Notes the Company runs an internal model which considers the seasonality of the risk assumed under the retrocessional agreement between Aspen Bermuda or a combination of Aspen Bermuda and Aspen U.K., as ceding reinsurers, and Silverton. The seasonality used in the model is determined by applying the percentage of property catastrophe losses planned by the Company’s actuaries to the estimated written premium to determine earned premium for each quarter. The inputs to the internal valuation model are based on Company specific data due to the lack of observable market inputs. Reserves for losses are the most significant unobservable input. An increase in reserves for losses would normally result in a decrease in the fair value of the Loan Notes while a decrease in reserves would normally result in an increase in the fair value of the Loan Notes. The observable and unobservable inputs used to determine the fair value of the Loan Notes as at December 31, 2017 and December 31, 2016 , respectively, are presented in the tables below: At December 31, 2017 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 86.6 (1) Internal Valuation Model Gross premiums written (O) $ 50.1 $ 61.1 Reserve for losses (U) $ 4.2 $ 61.9 Contract period (O) N/A 365 days Initial value of issuance (O) $ 325.0 $ 325.0 At December 31, 2016 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 223.4 (1) Internal Valuation Model Gross premiums written (O) $ 38.9 $ 43.4 Reserve for losses (U) $ 2.7 $ 11.8 Contract period (O) N/A 365 days Initial value of issuance (O) $ 220.0 $ 220.0 ______________ (1) The amount classified as other payables was $42.4 million and $108.4 million as at December 31, 2017 and December 31, 2016 , respectively. The observable and unobservable inputs represent the potential variation around the inputs used in the valuation model. The contract period is defined in the respective Loan Notes agreement and the initial value represents the funds received from third parties.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2017
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. 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 20, “Concentrations of Credit Risk — Reinsurance recoverables” and Note 12, “Reserves for Losses and Loss Adjustment Expenses” of these consolidated financial statements. The effect of assumed and ceded reinsurance on premiums written, premiums earned and insurance losses and loss adjustment expenses for the twelve months ended December 31, 2017 , 2016 and 2015 was as follows: Twelve Months Ended December 31, 2017 2016 2015 ($ in millions) Premiums written : Direct $ 1,812.4 $ 1,733.8 $ 1,748.4 Assumed 1,548.5 1,413.2 1,248.9 Ceded (1,148.4 ) (553.3 ) (351.1 ) Net premiums written $ 2,212.5 $ 2,593.7 $ 2,646.2 Premiums earned: Direct $ 1,757.4 $ 1,768.4 $ 1,703.3 Assumed 1,451.8 1,317.9 1,153.5 Ceded (902.6 ) (449.0 ) (383.5 ) Net premiums earned $ 2,306.6 $ 2,637.3 $ 2,473.3 Insurance losses and loss adjustment expenses: Direct $ 1,673.6 $ 1,091.9 $ 980.6 Assumed 1,399.9 699.6 493.0 Ceded (1,078.8 ) (215.4 ) (107.4 ) Net insurance losses and loss adjustment expenses $ 1,994.7 $ 1,576.1 $ 1,366.2 The Company acquired retrospective reinsurance coverage during the twelve months ended December 31, 2017 as part of a loss portfolio transfer for $125.5 million which has been recognized within ceded insurance losses. For more information the loss portfolio transfer, please refer to Note 12, “Reserves for Losses and Loss Adjustment Expenses” of these consolidated financial statements.

Derivative Contracts

Derivative Contracts	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : As at December 31, 2017 As at December 31, 2016 Derivatives Not Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Derivatives at Fair Value $ 577.7 $ 5.0 (1) $ 240.2 $ 5.0 Foreign Exchange Contracts Liabilities under Derivative Contracts $ 173.9 $ (1.0 ) $ 425.4 $ (17.7 ) ______________ (1) Net of $0.6 million of cash collateral (December 31, 2016 — $ Nil ) As at December 31, 2017 As at December 31, 2016 Derivatives Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Derivatives at Fair Value $ 60.6 $ 1.4 (1) $ — $ 2.2 (1) Foreign Exchange Contracts Liabilities under Derivative Contracts — $ — $ 108.6 $ (0.7 ) ______________ (1) Net of $ Nil cash collateral (December 31, 2016 — $2.2 million ). The following table provides the unrealized and realized gains/(losses) recorded in the statements of operations and other comprehensive income for derivatives that are not designated or designated as hedging instruments under ASC 815 - " Derivatives and Hedging" for twelve months ended December 31, 2017 and 2016 : Amount of Gain/(Loss) Recognized on Derivatives For the Twelve Months Ended Location of Gain/(Loss) Recognized on Derivatives December 31, 2017 December 31, 2016 Derivatives not designated as hedges ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives 27.7 (21.5 ) Interest Rate Swaps Change in Fair Value of Derivatives — (3.1 ) Derivatives designated as hedges Foreign Exchange Contracts General, administrative and corporate expenses 4.4 (8.7 ) Foreign Exchange Contracts Net change from current period hedged transactions 3.0 0.7 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, 2017 , the Company held foreign exchange contracts that were not designated as hedging under ASC 815 with an aggregate nominal amount of $751.6 million ( 2016 — $665.6 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, 2017 , the impact of foreign exchange contracts on net income was a gain of $27.7 million ( December 31, 2016 — loss of $21.5 million ). As at December 31, 2017 , the Company held foreign exchange contracts that were designated as hedging under ASC 815 with an aggregate nominal amount of $60.6 million ( 2016 — $108.6 million ). 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 the movement in other comprehensive income representing the effective portion for the twelve months ended December 31, 2017 was a gain of $3.0 million ( December 31, 2016 — gain of $0.7 million ). As the foreign exchange contracts settle, the realized gain or loss is reclassified from other comprehensive income into general, administration and corporate expenses of the statement of operations and other comprehensive income. For the twelve months ended December 31, 2017 , the amount recognized within general, administration and corporate expenses for settled foreign exchange contracts was a realized gain of $4.4 million ( December 31, 2016 — loss of $8.7 million recognized within change in fair value of derivatives). Interest Rate Swaps. In 2014, the Company decided to let its interest rate program roll-off and not renew maturing positions. This decision was made after an extensive reassessment of the costs of maintaining an interest rate swap program in a steep yield curve environment. In addition, the continued uncertainty in the global economy and low inflation make it difficult to gauge the timing and speed of interest rate rises by the Federal Reserve. On May 9, 2016, the Company terminated all remaining outstanding interest rate swaps (notional value of $256.3 million ) under its International Swap Dealers Association agreement. As at December 31, 2017 and December 31, 2016 , the Company no longer had outstanding interest rate swaps. There was no charge in respect of the interest rate swaps for the twelve months ended December 31, 2017 ( December 31, 2016 — charge of $3.1 million ).

Deferred Policy Acquisition Cos

Deferred Policy Acquisition Costs	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : Twelve Months Ended Twelve Months Ended December 31, 2016 ($ in millions) Balance at the beginning of the period $ 358.4 $ 361.1 Acquisition costs deferred 336.4 526.2 Amortization of deferred policy acquisition costs (400.5 ) (528.9 ) Balance at the end of the period $ 294.3 $ 358.4

Reserves for Losses and Adjustm

Reserves for Losses and Adjustment Expenses	12 Months Ended
	Dec. 31, 2017
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, 2017 , 2016 and 2015 : As at December 31, 2017 2016 2015 ($ in millions) Provision for losses and LAE at the start of the year $ 5,319.9 $ 4,938.2 $ 4,750.8 Less reinsurance recoverable (560.7 ) (354.8 ) (350.0 ) Net loss and LAE at the start of the year 4,759.2 4,583.4 4,400.8 Net loss and LAE expenses (disposed) (125.5 ) (80.1 ) — Movement in provision for losses and LAE for claims incurred: Current year 2,100.1 1,705.4 1,522.7 Prior years (105.4 ) (129.3 ) (156.5 ) Total incurred 1,994.7 1,576.1 1,366.2 Losses and LAE payments for claims incurred: Current year (397.5 ) (241.0 ) (178.8 ) Prior years (1,157.6 ) (981.8 ) (929.7 ) Total paid (1,555.1 ) (1,222.8 ) (1,108.5 ) Foreign exchange losses/(gains) 161.0 (97.4 ) (75.1 ) Net losses and LAE reserves at the end of the year 5,234.3 4,759.2 4,583.4 Plus reinsurance recoverable on unpaid losses at the end of the year 1,515.2 560.7 354.8 Provision for losses and LAE at the end of the year $ 6,749.5 $ 5,319.9 $ 4,938.2 For the twelve months ended December 31, 2017 , there was a reduction of $105.4 million in the Company’s estimate of the ultimate claims to be paid in respect of prior accident years compared to $129.3 million for the twelve months ended December 31, 2016 . The Company assumed $5.7 million of additional loss reserves in 2016 as a result of its acquisition of AgriLogic in January 2016 . The Company ceded $125.5 million of reserves as part of a loss portfolio transfer agreement which includes an upper coverage limit in excess of the amount transferred, during the twelve months ended December 31, 2017 ( December 31, 2016 — $85.8 million related to an adverse development cover purchased, December 31, 2015 — $ Nil ). For additional information on 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. The following tables show an analysis of incurred claims and allocated loss adjustment expenses, net of reinsurance and cumulative paid claims and allocated claim adjustment expenses, net of reinsurance as at December 31, 2017 , 2016 , 2015 , 2014 2013 and 2012 . The loss development triangles are derived from all business written by the Company as although a limited number of contracts are written which have durations of greater than one year the contracts do not meet the definition of a long duration contract. Property Insurance Lines Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 170.7 168.6 167.3 166.2 160.6 154.7 0.3 6,872 2013 130.8 117.9 117.5 113.3 114.3 4.9 5,100 2014 166.2 157.9 134.7 135.4 3.0 8,779 2015 241.9 207.3 201.8 4.7 10,431 2016 239.9 250.7 14.7 9,514 2017 297.7 51.3 6,249 Total $ 1,154.6 Property Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 41.5 129.6 139.2 153.4 157.5 154.9 2013 39.2 76.4 89.7 101.8 106.4 2014 40.5 87.0 114.5 124.2 2015 57.5 144.0 172.1 2016 67.3 169.7 2017 97.2 $ 824.5 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 330.1 All outstanding liabilities before 2012, net of reinsurance 2.9 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 333.0 Casualty Insurance Lines Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 78.6 63.4 70.5 61.7 69.6 67.2 5.2 3,536 2013 134.1 117.5 115.9 121.4 104.4 13.6 3,852 2014 146.4 128.6 140.5 130.9 20.4 4,312 2015 205.5 225.5 188.0 64.7 4,954 2016 219.4 190.6 117.9 5,084 2017 183.4 153.0 4,447 Total $ 864.5 Casualty Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 1.3 6.7 14.3 30.0 41.3 49.7 2013 2.3 26.0 39.8 53.5 69.0 2014 2.7 13.6 33.1 60.4 2015 3.2 17.2 57.0 2016 4.2 23.1 2017 3.6 Total $ 262.8 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 601.7 All outstanding liabilities before 2012, net of reinsurance 43.0 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 644.7 Marine, Aviation and Energy Insurance Lines Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 269.8 307.2 326.7 347.3 333.0 329.0 33.8 4,593 2013 321.4 334.2 342.9 326.3 333.3 17.4 4,979 2014 310.1 314.5 299.3 310.9 45.3 4,768 2015 297.9 301.0 283.3 37.9 4,356 2016 — 219.4 231.9 49.5 4,572 2017 — 211.5 70.0 3,993 Total $ 1,699.9 Marine, Aviation and Energy Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 52.2 133.2 175.7 212.0 240.8 251.7 2013 41.6 131.8 205.3 235.7 265.0 2014 53.6 117.0 189.5 210.2 2015 45.1 123.7 174.9 2016 30.9 83.2 2017 40.4 Total $ 1,025.4 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 674.5 All outstanding liabilities before 2012, net of reinsurance 61.4 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 735.9 Financial and Professional Insurance Lines Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 88.8 90.1 94.0 97.6 94.5 90.0 9.7 593 2013 106.6 101.2 105.6 102.3 101.6 15.2 603 2014 136.4 132.6 131.3 121.4 35.2 852 2015 175.9 177.2 187.2 76.7 1,138 2016 193.0 213.6 121.1 1,365 2017 208.6 159.4 1,743 Total $ 922.4 Financial and Professional Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 22.8 39.6 50.8 59.5 65.4 70.7 2013 8.1 21.5 31.6 66.0 64.5 2014 3.0 30.9 54.1 72.8 2015 13.9 43.7 70.4 2016 15.4 71.7 2017 27.3 Total $ 377.4 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 545.0 All outstanding liabilities before 2012, net of reinsurance 27.4 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 572.4 Property Catastrophe and Other Property Reinsurance Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 281.3 304.2 287.2 280.0 283.4 280.4 19.3 797 2013 220.5 203.0 193.9 182.3 180.8 4.4 945 2014 191.7 180.4 164.0 152.4 4.1 1,016 2015 219.4 191.4 181.6 18.7 1,135 2016 277.9 277.8 35.1 1,297 2017 564.8 216.1 1,366 Total $ 1,637.8 Property Catastrophe and Other Property Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 35.7 136.2 189.5 209.9 217.5 228.6 2013 34.9 100.3 149.7 162.0 166.8 2014 37.8 102.3 129.2 139.4 2015 36.2 96.9 130.0 2016 58.0 167.2 2017 124.2 Total $ 956.2 All outstanding liabilities for 2012 and subsequent years, net of reinsurance 681.6 All outstanding liabilities before 2012, net of reinsurance 47.9 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 729.5 Casualty Reinsurance Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 236.9 235.2 246.6 238.0 234.8 235.6 66.1 2,173 2013 217.5 232.9 228.4 225.8 208.4 62.2 2,034 2014 208.7 212.0 220.8 214.0 84.5 1,842 2015 197.9 205.3 214.6 103.0 1,648 2016 237.7 250.5 156.2 1,151 2017 249.5 215.9 476 Total $ 1,372.6 Casualty Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 2.2 17.9 42.2 65.8 97.1 118.5 2013 3.5 16.0 43.2 65.5 93.7 2014 2.6 14.1 38.2 60.9 2015 3.6 18.5 39.2 2016 9.6 34.3 2017 9.1 Total $ 355.7 All outstanding liabilities for 2012 and subsequent years, net of reinsurance 1,016.9 All outstanding liabilities before 2012, net of reinsurance 575.6 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 1,592.5 Specialty Reinsurance Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 178.5 201.7 191.1 177.4 176.0 176.3 10.5 739 2013 146.9 141.9 134.5 122.5 121.7 7.7 620 2014 154.4 143.4 135.2 126.0 15.6 667 2015 169.7 174.3 168.8 29.4 826 2016 243.2 244.6 49.5 844 2017 385.2 211.6 763 Total $ 1,222.6 Specialty Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 25.7 95.8 131.2 141.4 146.9 152.2 2013 25.4 72.7 89.1 96.4 103.5 2014 17.0 57.9 83.1 91.3 2015 18.1 58.1 106.5 2016 59.6 153.1 2017 95.7 Total $ 702.3 All outstanding liabilities for 2012 and subsequent years, net of reinsurance 520.3 All outstanding liabilities before 2012, net of reinsurance 55.4 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 575.7 Reconciliation of Incurred and Paid Claims Development to total Provision for Losses and LAE Twelve Months Ended ($ in millions) Net outstanding liabilities: Insurance lines - Property insurance lines 333.0 - Casualty insurance lines 644.7 - Marine, aviation and energy insurance lines 735.9 - Financial and professional insurance lines 572.4 Total insurance lines 2,286.0 Reinsurance lines - Property catastrophe and other property reinsurance 729.5 - Casualty reinsurance 1,592.5 - Specialty reinsurance 575.7 Total reinsurance lines 2,897.7 Net loss and LAE 5,183.7 Reinsurance recoverable on unpaid losses: Insurance lines 1,245.9 Reinsurance lines 269.3 Total reinsurance recoverable on unpaid losses 1,515.2 Insurance lines other than short-duration — Unallocated claims incurred 49.2 Other 1.4 50.6 Provision for losses and LAE at the end of the year 6,749.5 Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 Insurance 15.4 % 25.3 % 16.3 % 12.2 % 7.6 % 3.4 % Reinsurance 13.2 % 25.0 % 17.0 % 8.1 % 7.1 % 5.5 %

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Aspen Holdings and Aspen Bermuda are incorporated under the laws of Bermuda. Under current Bermuda law, they are not taxed on any Bermudian income or capital gains and they have received an undertaking from the Bermudian 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. Under current Australian law, Aspen Bermuda is however subject to tax on reinsurance premium it receives from the Australian branch of Aspen U.K. and this is reflected in the tables below. The Company’s U.S. operating companies were subject to a U.S. federal income tax rate of 34% prior to January 1, 2018. Effective January 1, 2018, following the enactment of the U.S. Tax Cuts and Jobs Act of 2017 on December 22, 2017, the Company’s U.S. operating companies will be subject to a U.S. federal income tax rate of 21% . The reduction in U.S. federal income tax has been reflected in measuring the Company’s deferred taxes. The Company's U.K. operating companies are taxed at the U.K. corporate tax rate which reduced from 20% to 19% effective April 1, 2017. The U.K. corporate tax rate will be further decreased to 17% effective on April 1, 2020.The reduction in the U.K. corporate tax rate has been reflected in measuring the Company’s deferred taxes. Total income tax (benefit)/expense for the twelve months ended December 31, 2017 , 2016 and 2015 was allocated as follows: Twelve Months Ended December 31, 2017 2016 2015 ($ in millions) Income tax (benefit)/expense allocated to net income $ (15.4 ) $ 6.1 $ 14.4 Income tax (benefit) allocated to other comprehensive income (17.4 ) (1.7 ) (15.3 ) Income tax (benefit) allocated directly to shareholders’ equity — (1.0 ) (1.9 ) Total income tax (benefit)/expense $ (32.8 ) $ 3.4 $ (2.8 ) (Loss)/income from operations before income tax and income tax expense/(benefit) attributable to that income/(loss) for the twelve months ended December 31, 2017 , 2016 and 2015 is provided in the tables below: Twelve Months Ended December 31, 2017 (Loss)/income before tax Current income tax expense/(benefit) Deferred income tax (benefit)/expense Total income tax (benefit)/expense ($ in millions) Bermuda (1) $ (130.0 ) $ 0.9 $ — $ 0.9 U.S. (140.3 ) — 1.1 1.1 U.K. 15.3 14.1 (33.3 ) (19.2 ) Other (2) (26.8 ) 2.1 (0.3 ) 1.8 Total $ (281.8 ) $ 17.1 $ (32.5 ) $ (15.4 ) Twelve Months Ended December 31, 2016 (Loss)/income before tax Current Deferred income tax expense Total income tax expense ($ in millions) Bermuda $ 259.5 $ — $ — $ — U.S. (70.2 ) — 2.5 2.5 U.K. 43.7 (3.2 ) 5.4 2.2 Other (2) (23.5 ) 1.1 0.3 1.4 Total $ 209.5 $ (2.1 ) $ 8.2 $ 6.1 Twelve Months Ended December 31, 2015 (Loss)/income Current Deferred Total ($ in millions) Bermuda (1) $ 283.9 $ 1.4 $ — $ 1.4 U.S. (32.7 ) 1.6 (4.1 ) (2.5 ) U.K. 79.9 15.5 (4.2 ) 11.3 Other (2) 6.4 4.8 (0.6 ) 4.2 Total $ 337.5 $ 23.3 $ (8.9 ) $ 14.4 ______________ (1) Under Australian law, Aspen Bermuda is subject to tax on reinsurance premium it receives from the Australian branch of Aspen U.K. (2) Beginning from the twelve months ended December 31, 2017, the total income tax (benefit)/expense allocation table has been re-presented to show the branches of Aspen U.K. under the “Other” category with the exception of the U.S. branch which is reported under the “U.S.” category. The tax rate in Bermuda, the Company’s country of domicile, is zero . Application of the statutory tax rate for operations in other jurisdictions produces a differential to the expected tax (benefit)/expense as shown in the table below. The reconciliation between the income tax (benefit)/expense and the statutory rate for the Company for the twelve months ended December 31, 2017 is provided in the table below: Twelve Months Ended December 31, 2017 2016 2015 Income Tax Reconciliation ($ in millions) Expected tax (benefit)/expense $ — $ — $ — Overseas statutory tax rates differential (41.5 ) (19.3 ) 6.4 Prior year adjustments (1) 1.3 3.3 (4.5 ) Valuation allowance (37.9 ) 21.0 11.8 Impact of unrecognized tax benefits (2) 0.1 (1.9 ) — Restricted foreign tax credits 1.6 1.9 0.6 Share-based payments (0.9 ) — — Foreign exchange (2.1 ) 0.2 (0.4 ) Non-deductible expenses 0.4 0.8 1.7 Non-taxable items (0.9 ) (0.9 ) (1.3 ) Impact of changes in statutory tax rates 64.5 1.0 0.1 Total income tax (benefit)/expense $ (15.4 ) $ 6.1 $ 14.4 ________________ (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 (this “report”). Accordingly, the final tax liabilities may differ from the estimated tax expense included in this report and may result in prior year adjustments being reported. The prior period adjustments for the twelve months ended December 31, 2017 , 2016 and 2015 predominantly relate to the determination of results under U.K. GAAP, upon which the U.K. tax returns are based. These items can only be reasonably determined on an accurate basis after this report has been filed. (2) For 2017 , the $0.1 million charge relates to a $0.3 million credit following the conclusion of a tax examination in respect of tax deductions for certain expenses and accrued interest of $0.4 million in respect of the adjustment of equity reserves. For 2016 , the $1.9 million credit relates to a $3.2 million credit following the conclusion of a tax examination in respect of tax deductions for certain interest payments and accrued interest of $1.3 million in respect of the adjustment to equity reserves. For 2015 , there was $ Nil impact. Unrecognized tax benefits . As illustrated in the table below, unrecognized tax benefits totaled $11.2 million as at December 31, 2017 and relate to U.K. prior period tax positions for the year 2011. An unrecognized tax benefit of $0.3 million relating to tax deductions for certain expenses was released during the year ended December 31, 2017 following the completion of the U.K. tax authority review. If they are recognized, the unrecognized tax benefits would reduce the Company’s effective tax rate. It is possible that the unrecognized tax benefits could be eliminated following completion of tax examinations into these matters. Twelve Months Ended December 31, 2017 2016 ($ in millions) Unrecognized tax benefits balance at January 1 $ 10.5 $ 29.2 Foreign exchange re-translation $ 1.0 (3.4 ) Settlement in respect of tax position of prior years $ (0.3 ) (15.3 ) Unrecognized tax benefits balance at December 31 $ 11.2 $ 10.5 The Company accrues interest and penalties related to an underpayment of income taxes, 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 for the twelve months ended December 31, 2017 ( December 31, 2016 — $ Nil ). During the year, interest of $0.4 million was accrued in respect of the adjustment to equity reserves ( December 31, 2016 — $1.3 million ). Cumulative interest accrued as at December 31, 2017 was $1.7 million ( December 31, 2016 — $1.3 million ) Income tax returns that have been filed by the Company’s U.S. operating subsidiaries are subject to examination for 2014 and later tax years. The Company’s U.K. operating subsidiaries’ income tax returns are subject to examination for 2016 and later tax years. This is in addition to the U.K. tax returns against which tax benefits have not been recognized, as detailed above. The tax effects of temporary differences and carryforwards that give rise to deferred tax assets and deferred tax liabilities are presented in the following table as at December 31, 2017 and 2016 : As at December 31, 2017 2016 ($ in millions) Deferred tax assets: Share-based payments $ 4.0 $ 5.6 Operating loss carryforwards 102.5 124.5 Loss reserves 4.3 7.8 Accrued expenses 7.9 14.4 Foreign tax credit carryforwards 4.0 2.5 Unearned premiums 11.8 20.0 Deferred policy acquisition costs 5.9 2.1 Office properties and equipment 8.0 5.6 Other temporary differences 2.9 3.8 Total gross deferred tax assets 151.3 186.3 Less valuation allowance (104.8 ) (142.5 ) Net deferred tax assets $ 46.5 $ 43.8 Deferred tax liabilities: Equalization provision reserves $ — $ (23.0 ) Intangible assets (other) (2.0 ) (2.8 ) Deferred policy acquisition costs (14.5 ) (22.4 ) Other temporary differences (1.7 ) (1.7 ) Total gross deferred tax (liabilities) (18.2 ) (49.9 ) Net deferred tax asset/(liability) $ 28.3 $ (6.1 ) 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 carryforwards become deductible or creditable. Management considers the scheduled reversal of existing taxable temporary differences, projected future taxable income, and tax-planning strategies in making this assessment. As at December 31, 2017 , the Company had net operating losses carried forward for U.S. federal income tax purposes of $482.7 million ( 2016 — $343.2 million ) and net operating losses carried forward for U.K. corporate tax purposes of $84.9 million ( 2016 — $18.2 million ). The U.S. net operating losses are available to offset future U.S. federal taxable income, if any, with expiry periods between 2026 and 2036 and the U.K. net operating losses are available to offset future U.K. corporate income over an indefinite period. For U.S. federal income tax purposes, the Company also has capital loss carryforwards of $0.5 million ( 2016 — $0.5 million ) with expiry periods between 2020 and 2022 , and charitable contribution carryforwards of $0.7 million ( 2016 — $0.5 million ) with expiry periods between 2018 and 2022 . For U.K. corporate tax purposes, the Company has capital loss carryforwards of $3.8 million which is available to offset future U.K. capital gains over an indefinite period. A full valuation allowance of $101.1 million ( 2016 — $139.2 million ) on U.S. deferred tax assets (which includes these loss carryforwards) has been recognized at December 31, 2017 as management believes that it is more likely than not that a tax benefit will not be realized. The decrease in this portion of the valuation allowance totals $38.4 million ( 2016 — $20.7 million increase) with $38.4 million ( 2016 — $21.0 million increase) recorded in the consolidated income statement $ Nil ( 2016 — $0.3 million decrease) recorded in other comprehensive income. A valuation allowance of $3.7 million ( 2016 — $3.3 million ) has been established against U.K. deferred tax assets. The increase in this portion of the valuation allowance totals $0.4 million ( 2016 — $2.6 million increase) with $0.4 million ( 2016 — $2.7 million increase) recorded in the consolidated income statement and $ Nil ( 2016 — $ Nil ) recorded in other comprehensive income. The U.K. and U.S. valuation allowance combined total is $104.8 million ( 2016 — $142.5 million ). Included within the foreign tax credits totalling $4.0 million carried forward as at December 31, 2017 , $1.2 million is due to expire in 2021. The remainder will continue to be available to carry forward indefinitely. Aspen U.K. business includes income connected with a U.S. trade or business and therefore Aspen U.K has a branch for U.S. tax purposes (“U.S. Branch”). The U.S. Branch could become subject to an additional branch profits tax if earnings are repatriated to the Aspen U.K. head office or upon termination of the U.S. branch. However, based on the plans currently in place, the U.S. Branch profits are being, and Aspen U.K. intends they will continue to be, indefinitely reinvested in the U.S. Branch such that there is no branch profits tax liability arising in the current period or in the foreseeable future. Furthermore, based on the cumulative earnings position as at December 31, 2017 , $ Nil ( 2016 — $1.9 million ) branch profits tax liability would be expected to arise. Accordingly, the Company has determined that no deferred tax liability for branch profits tax has been recognized as permitted by ASC 740.

Capital Structure

Capital Structure	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Capital Structure	Capital Structure The following table provides a summary of the Company’s authorized and issued share capital as at December 31, 2017 and 2016 : As at December 31, 2017 At December 31, 2016 Number $ in Thousands Number $ in 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 59,474,085 90 59,774,464 91 Issued 7.401% Preference Shares of 0.15144558¢ each with a liquidation preference of $25 per share — — 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 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 Issued 5.625% Preference Shares of 0.15144558¢ each with a liquidation preference of $25 per share 10,000,000 15 10,000,000 15 Total issued share capital 122 140 Additional paid-in capital as at December 31, 2017 was $954.7 million ( December 31, 2016 — $1,259.6 million ). Included within additional paid-in capital is the aggregate liquidation preferences of the Company’s preference shares of $525.0 million ( December 31, 2016 — $818.2 million ) less issue costs of $13.1 million ( December 31, 2016 — $21.1 million ). (a) Ordinary Shares The following table summarizes transactions in the Company’s ordinary shares during the years ended December 31, 2017 and 2016 : Number of Ordinary Shares 2017 2016 Ordinary shares in issue at the beginning of the year 59,774,464 60,918,373 Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or 2008 share purchase plan 309,727 419,255 Ordinary shares issued to non-employee directors 38,835 32,241 Ordinary shares repurchased (648,941 ) (1,595,405 ) Ordinary shares in issue at the end of the year 59,474,085 59,774,464 Ordinary Share Repurchases in 2015 . On February 5, 2015 , the Company’s Board of Directors approved a share repurchase authorization program of $500.0 million . The share repurchase authorization program, which was effective immediately and expired on February 6, 2017, permitted the Company to effect the repurchases of ordinary shares from time to time through a combination of transactions, including open market repurchases, privately negotiated transactions and accelerated share repurchase transactions. During the course of 2015, the Company repurchased 1,790,333 ordinary shares for a total consideration of $83.7 million at an average price of $46.74 per ordinary share. The Company had $416.3 million remaining under its share repurchase authorization program in effect as at December 31, 2015. Ordinary Share Repurchases in 2016. During the course of 2016, the Company repurchased 1,595,076 ordinary shares for a total consideration of $75.0 million at an average price of $47.02 per ordinary share. The Company had $341.3 million remaining under its $500.0 million share repurchase authorization program in effect as at December 31, 2016. Share Repurchase Authorization Program . On February 8, 2017, the Company’s Board of Directors approved a new share repurchase authorization program of $250.0 million . The new share repurchase authorization program, which was effective immediately and expires on February 8, 2019, permits the Company to effect the repurchases of its ordinary shares from time to time through a combination of transactions, including open market purchases, privately negotiated transactions and accelerated share repurchase transactions. Ordinary Share Repurchases in 2017. During the course of 2017 , the Company repurchased 648,941 ordinary shares for a total consideration of $30.0 million at an average price of $46.23 per ordinary share. The Company had $220.0 million remaining under its $250.0 million share repurchase authorization program in effect as at December 31, 2017 . (b) Preference Shares Preference Shares Issuance. On September 20, 2016, the Company issued 10,000,000 shares of 5.625% Perpetual Non-Cumulative Preference Shares (the “ 5.625% Preference Shares”). The 5.625% Preference Shares have a liquidation preference of $25 per share. Net proceeds were $241.3 million , consisting of $250.0 million of total liquidation preference less $8.7 million of issuance expenses. The Company used $133.2 million of the net proceeds from the offering to redeem all of the Company’s outstanding 7.401% Preference Shares (defined below) and the remainder were used in the redemption of the Company's 7.250% Preference Shares (defined below) for $160.0 million . The 5.625% 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 5.625% Preference Shares at a redemption price of $25 per share on or after January 1, 2027. The 5.625% Preference Shares are listed on the NYSE under the symbol “AHLPRD”. Preference Shares Redemption. On November 3, 2016, the Company issued a notice of redemption in connection with all of its issued and outstanding 7.401% Perpetual Non-Cumulative Preference Shares (the “ 7.401% Preference Shares”) (NYSE: AHLPRA). The redemption took place on January 3, 2017 and was conducted pursuant to the terms of the certificate of designation, dated November 15, 2006, governing the 7.401% Preference Shares. Each holder of a 7.401% Preference Share received $25 per 7.401% Preference Share, representing an aggregate amount of $133.2 million , plus all declared and unpaid dividends to the date of redemption. On May 8, 2017, the Company issued a notice of redemption in connection with all of its issued and outstanding 7.250% Perpetual Non-Cumulative Preference Shares (the “ 7.250% Preference Shares”) (NYSE: AHLPRB). The redemption took place on July 3, 2017 and was conducted pursuant to the terms of the certificate of designation, dated April 11, 2012, governing the 7.250% Preference Shares. Each holder of a 7.250% Preference Share received $25 per 7.250% Preference Share, representing an aggregate amount of $160.0 million , plus all declared and unpaid dividends to the date of redemption.

Statutory Requirements and Divi

Statutory Requirements and Dividends Restrictions	12 Months Ended
	Dec. 31, 2017
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 at December 31, 2017 , there were no restrictions under Bermuda 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 rules of the Prudential Regulation Authority (the “PRA”) require each insurance company within its jurisdiction to maintain its solvency margin at all times. In line with common market practice for regulated institutions, the PRA previously requested that it be afforded with the opportunity to provide a “non-objection” prior to all future dividend payments made by Aspen U.K. In 2017, the PRA stated that they no longer routinely require Aspen U.K. to apply for a non-objection to dividends provided such dividend payment and Aspen U.K.’s resulting capital position are within Aspen U.K.’s board-approved solvency capital risk appetite. As at December 31, 2017 , Aspen U.K. had an accumulated balance of retained losses of approximately $196.3 million and AUL had an accumulated balance of retained losses of approximately £35.9 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, 2017 . 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, 2017 , 25% of Aspen Bermuda’s statutory capital and surplus amounted to $448.3 million . Further, Aspen Bermuda must obtain the prior approval of the BMA before reducing by 15% or more its total statutory capital as set out in its previous year’s financial statements. Under both North Dakota and Texas law, insurance companies may only pay dividends out of earned surplus as distinguished from contributed surplus. As such, Aspen Specialty and AAIC could not pay a dividend as at December 31, 2017 . Actual and required statutory capital and surplus for the principal operating subsidiaries of the Company, excluding its Lloyd’s syndicate, as at December 31, 2017 and December 31, 2016 were estimated as follows: As at December 31, 2017 U.S. Bermuda U.K. ($ in millions) Required statutory capital and surplus $ 163.5 $ 1,146.7 $ 745.8 Statutory capital and surplus $ 645.5 $ 1,793.0 $ 888.6 As at December 31, 2016 U.S. Bermuda U.K. ($ in millions) Required statutory capital and surplus $ 77.3 $ 1,259.4 $ 558.1 Statutory capital and surplus $ 598.3 $ 2,294.4 $ 970.3 AUL, as the sole corporate member of our Lloyd’s Syndicate, is required to maintain Funds at Lloyd’s of $451.8 million . As at December 31, 2017 , AUL had total funds at Lloyd’s of $458.7 million of which $423.6 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. 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. The statutory capital requirements of our Operating Subsidiaries are set out above. To the extent that these requirements are met, we do not anticipate any dividend restrictions arising as a result of the Company's enhanced capital requirement.

Retirement Plans

Retirement Plans	12 Months Ended
	Dec. 31, 2017
Retirement Benefits [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 $16.3 million in the twelve months ended December 31, 2017 ( 2016 — $15.4 million , 2015 — $14.9 million ).

Share-Based Payments

Share-Based Payments	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share Based Payments	Share-Based Payments The Company has issued options and other equity incentives under three arrangements: the employee share incentive plan, the employee share purchase plans and the non-employee director stock incentive plan. When options are exercised or other equity awards (excluding phantom shares) vest, new ordinary shares are issued as the Company does not hold treasury shares. Phantom shares are settled in cash in lieu of ordinary shares upon vesting. (a) Employee Equity Incentives Employee equity awards were granted under the Company’s 2003 Share Incentive Plan prior to April 24, 2013 and thereafter under the Company’s 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. Restricted Share Units. Restricted share units (“RSUs”) granted to employees vest over a three -year period subject to the employee’s continued service. RSU’s granted to employees 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 RSUs will be paid one ordinary share for each RSU that vests as soon as practicable following the vesting date. Holders of RSUs generally will not be entitled to any rights of a holder of ordinary shares, including the right to vote, unless and until their RSUs vest and ordinary shares are issued but they are entitled to receive dividend equivalents. Dividend equivalents are 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, 2017 by year of grant: As at December 31, 2017 Restricted Share Units RSU Holder Amount Granted Amount Vested Amount Forfeited Amount Outstanding 2015 Grants 287,852 173,755 38,886 75,211 2016 Grants 328,550 104,042 40,579 183,929 2017 Grants 200,021 — 19,585 180,436 Total 439,576 The fair value of RSUs is based on the closing price on the date of the grant less a deduction for illiquidity. 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 $9.4 million for the twelve months ended December 31, 2017 ( 2016 — $9.8 million ; 2015 — $8.7 million ). The total tax credit recognized by the Company in relation to RSUs in the twelve months ended December 31, 2017 was $2.3 million ( 2016 — $2.4 million ; 2015 — $1.8 million ) excluding excess tax benefits. A summary of RSUs under the Company’s 2013 Share Incentive Plan for the twelve months ended December 31, 2017 , 2016 and 2015 is presented below: Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 Restricted share unit activity Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Outstanding restricted stock, beginning of period 530,340 $39.28 504,234 $40.45 519,112 $35.82 Granted 200,021 49.20 328,550 37.63 287,852 39.68 Vested (245,704 ) 39.63 (246,489 ) 34.26 (266,944 ) 34.26 Forfeited (45,081 ) 41.84 (55,955 ) 39.51 (35,786 ) 39.59 Outstanding restricted stock, end of period 439,576 $43.22 530,340 $39.28 504,234 $40.45 As at December 31, 2017 , unrecognized compensation cost related to non-vested RSUs was $11.0 million , which is expected to be recognized over a weighted-average period of 1.6 years . Performance Shares . Performance share awards are subject to a three -year service vesting period with a separate annual growth in diluted BVPS test for each calendar year during the vesting period. Accordingly, one-third of the award may be earned in each calendar year. Performance share awards are not entitled to dividends before they vest and are subject to the employee’s continued employment. If performance goals are achieved, the performance shares will vest up to a maximum of 200% of target. Notwithstanding the vesting criteria for each given year, if in any given year the shares eligible for vesting are greater than 100% or 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, 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 is due to the impact of rising interest rates and bond yields, then the Compensation Committee may, in its discretion, disapply the limitation on 100% vesting. The following table summarizes information about performance shares as at December 31, 2017 by year of grant: As at December 31, 2017 Performance Share Awards Amount Granted Amount Vested Amount Forfeited Amount Outstanding 2015 Grants 277,585 107,882 169,703 — 2016 Grants 278,477 28,566 168,224 81,687 2017 Grants 216,878 — 89,643 127,235 Total 208,922 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 illiquidity and expected dividends which would not accrue during the vesting period. Net compensation costs charged against income in the twelve months ended December 31, 2017 in respect of performance shares was a credit of $0.4 million ( 2016 — $2.2 million debit; 2015 — $6.5 million debit). The total tax charge recognized by the Company in relation to performance share awards in the twelve months ended December 31, 2017 was $0.1 million ( 2016 — $0.5 million credit; 2015 — 1.5 million credit) excluding excess tax benefits. A summary of performance share activity under the Company’s 2013 Share Incentive Plan for the twelve months ended December 31, 2017 , 2016 and 2015 is presented below: Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 Performance Share Activity Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Outstanding performance shares, beginning of period 254,988 $36.92 266,424 $24.17 268,418 $25.35 Granted 216,878 47.30 278,477 34.44 277,585 38.92 Vested — — (87,059 ) 37.30 (208,830 ) 38.11 Forfeited (262,944 ) 40.32 (202,854 ) 36.93 (70,749 ) 30.26 Outstanding performance shares, end of period 208,922 $38.71 254,988 $36.92 266,424 $24.17 As at December 31, 2017 , unrecognized compensation cost related to non-vested performance shares was $10.0 million , which is expected to be recognized over a weighted-average period of 1.7 years . Phantom Shares . Phantom share awards are subject to a three -year service vesting period with a separate annual growth in diluted BVPS test for each calendar year during the vesting period. One-third of the award may be earned in each calendar year with the vested amount being paid in cash in lieu of ordinary shares. As ordinary shares are not issued, phantom shares have no dilutive effect. The following table summarizes information about phantom shares as at December 31, 2017 by year of grant: As at December 31, 2017 Phantom Share Awards Amount Granted Amount Vested Amount Forfeited Amount Outstanding 2015 Grants 135,651 52,720 82,931 — 2016 Grants 147,513 15,132 96,978 35,403 2017 Grants 173,619 — 90,342 83,277 Total 118,680 The fair value of the phantom shares is based on the closing share price on the date of the grant less a deduction for illiquidity. The fair value is expensed through the consolidated income statement evenly over the vesting period. As the payment to beneficiaries will ultimately be in cash rather than ordinary 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, 2017 in respect of phantom shares was $0.7 million ( 2016 — $0.7 million ; 2015 — $4.3 million ) with a fair value adjustment for the twelve months ended December 31, 2017 of $1.7 million ( 2016 — $2.9 million ; 2015 — $3.2 million ). The total tax credit recognized by the Company in relation to phantom share awards in the twelve months ended December 31, 2017 was $0.2 million ( 2016 — $0.7 million ; 2015 — $1.2 million ) excluding excess tax benefits. A summary of phantom share activity under the Company’s 2013 Share Incentive Plan for the twelve months ended December 31, 2017 , 2016 and 2015 is presented below: Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 Phantom Share Activity Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Outstanding performance shares, beginning of period 131,464 $35.90 130,319 $38.75 143,248 $35.32 Granted 173,619 47.30 147,513 34.44 135,651 38.92 Vested — — (36,159 ) 36.96 (151,668 ) 34.98 Forfeited (186,403 ) 40.32 (110,209 ) 36.96 3,087 35.65 Outstanding performance shares, end of period 118,680 $38.71 131,464 $35.90 130,319 $38.75 As at December 31, 2017 , unrecognized compensation cost related to non-vested phantom shares was $5.7 million , which is expected to be recognized over a weighted-average period of 1.8 years . Vesting Criteria Performance and Phantom Shares. Both performance share and phantom share awards are subject to a three - year vesting period with a separate BVPS growth test each year, adjusted to add back ordinary shares 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. A summary of the increases in adjusted BVPS is presented below: Year ended December 31, Increase in BVPS 2013 6.2 % 2014 13.3 % 2015 10.7 % 2016 5.9 % 2017 — % Options. Stock options were last granted in 2007 with an exercise price equivalent to fair value of the share on the grant date. Stock options vested over a three -year period with a ten -year contract period, with vesting dependent on time and performance conditions established at the time of grant. During the twelve months ended December 31, 2017 no options were exercised ( 2016 — 29,222 ; 2015 — 189,215 ). No charges against income were made in respect of employee options for the twelve months ended December 31, 2017 , 2016 and 2015 . The intrinsic value of options exercised in the twelve months ended December 31, 2017 was $ Nil ( 2016 — $0.6 million ; 2015 — $4.3 million ). (b) Employee Share Purchase Plans 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 ordinary shares at a discounted price. In respect of the 2008 Sharesave Scheme, employees can save up to £500 per month over a three -year period at the end of which they will be eligible to purchase ordinary shares at a discounted price. The purchase price will be eighty-five percent 85% of the fair market value of an ordinary share on the offering date which may be adjusted upon changes in capitalization of the Company. Under the ESPP, 46,866 ordinary shares were issued during the twelve months ended December 31, 2017 ( 2016 — 21,285 shares; 2015 — 59,940 ). Compensation costs charged against income in the twelve months ended December 31, 2017 in respect of the ESPP was $0.4 million ( 2016 — $0.4 million ; 2015 — $0.3 million ). The total tax credit recognized by the Company in relation to the ESPP in the twelve months ended December 31, 2017 was $0.1 million ( 2016 — $0.1 million ; 2015 — $0.1 million ). The fair value of the employee options granted under the ESPP was estimated on the date of grant using a modified Black-Scholes option pricing model under the following assumptions: Grant Date Per share weighted average fair value Risk free interest rate Dividend yield Expected life Share price volatility ($) (%) (%) (in years) (%) March 25, 2015 8.17 0.94 1.78 3 16.00 March 25, 2015 7.08 0.60 1.78 2 16.00 March 25, 2016 7.97 1.04 1.88 3 4.21 March 25, 2016 7.00 0.87 1.88 2 2.44 April 28, 2017 6.69 1.44 1.83 3 3.67 April 28, 2017 8.70 1.44 1.83 3 3.67 April 28, 2017 8.70 1.26 1.83 2 3.52 (c) Non-Employee Director Plan On April 21, 2016, the shareholders of the Company approved the 2016 Stock Incentive Plan for Non-Employee Directors which provides for the granting of options, restricted share units or other share-based awards. Options. No options were granted during the twelve months ended December 31, 2017 , 2016 and 2015 and no options were exercised and shares issued in the twelve months ended December 31, 2017 ( 2016 — 4,447 ; 2015 — Nil ). No charges or tax charges against income were made in respect of non-employee directors options for the twelve months ended December 31, 2017 ( 2016 — $ Nil ; 2015 — $ Nil ). Restricted Share Units. RSUs granted to non-employee directors, including the Chairman, vest one-twelfth 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. The shares that are eligible to vest following final vesting date in the calendar year of the date of grant is delivered as soon as practical thereafter and the remaining shares under the RSUs are delivered on the first anniversary of the grant date. If a director leaves the Board of Directors for any reason other than “cause” (as defined in the award agreement), then the director would receive 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. The following table summarizes information about RSUs issued to non-employee directors by year of grant as at December 31, 2017 . As at December 31, 2017 Restricted Share Units Amount Granted Amount Vested Amount Forfeited Amount Outstanding Non-Employee Directors 2015 27,620 27,620 — — 2016 24,456 21,352 3,104 — 2017 22,230 17,043 1,853 3,334 Chairman 2015 12,154 12,154 — — 2016 10,952 10,952 — — 2017 8,892 7,410 — 1,482 Total 106,304 96,531 4,957 4,816 The fair value of the RSUs is based on the closing price on the date of the grant. Compensation cost charged against income was $1.5 million for the twelve months ended December 31, 2017 ( 2016 — $1.4 million ; 2015 — $1.8 million ). The total tax charge recognized by the Company in relation to non-employee RSUs in the twelve months ended December 31, 2017 was $ Nil ( 2016 — $ Nil ; 2015 — $ Nil ). A summary of RSU activity relating to non-employee directors under the Company’s 2013 Share Incentive Plan for the twelve months ended December 31, 2017 , 2016 and 2015 is presented below: Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 Restricted share unit activity Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Outstanding restricted stock, beginning of period 5,171 $41.07 6,636 $45.28 6,806 $35.83 Granted 31,122 50.18 35,408 41.07 39,774 45.25 Vested (29,624 ) 48.59 (33,769 ) 41.89 (39,944 ) 43.81 Forfeited (1,853 ) 50.18 (3,104 ) 41.07 — 0.00 Outstanding restricted stock, end of period 4,816 $50.19 5,171 $41.07 6,636 $45.28 As at December 31, 2017 , unrecognized compensation cost related to non-vested RSUs relating to non-employee directors was $0.2 million , which is expected to be recognized over a weighted-average period of 0.1 years .

Intangible Assets

Intangible Assets	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : Twelve Months Ended December 31, 2017 Beginning of the Year Additions/(Disposals) Amortization Impairment End of the Year ($ in millions) Intangible Assets Trade Mark $ 6.6 $ (3.1 ) $ (0.5 ) $ (0.1 ) $ 2.9 Insurance Licenses 16.7 — — — 16.7 Agency Relationships 26.2 (21.8 ) (2.1 ) — 2.3 Non-compete Agreements 3.3 (0.9 ) (0.7 ) (1.0 ) 0.7 Consulting Relationships 0.9 — (0.1 ) (0.8 ) — Goodwill 24.2 (18.8 ) — (1.5 ) 3.9 Renewal Rights 1.7 — (0.3 ) — 1.4 Total $ 79.6 $ (44.6 ) $ (3.7 ) $ (3.4 ) $ 27.9 Twelve Months Ended December 31, 2016 Beginning of the Year Additions Amortization Impairment End of the Year ($ in millions) Intangible Assets Trade Mark $ 1.6 $ 5.5 $ (0.5 ) $ — $ 6.6 Insurance Licenses 16.6 0.1 — — 16.7 Agency Relationships — 28.1 (1.9 ) — 26.2 Non-compete Agreements — 3.9 (0.6 ) — 3.3 Value of Business Acquired — 1.8 (1.8 ) — — Consulting Relationships — 1.0 (0.1 ) — 0.9 Goodwill — 24.2 — — 24.2 Renewal Rights — 1.9 (0.2 ) — 1.7 Total $ 18.2 $ 66.5 $ (5.1 ) $ — $ 79.6 AgriLogic On January 19, 2016, Aspen U.S. Holdings acquired 100% of the equity voting interest of AgriLogic, a specialist U.S. crop managing general agency business with an integrated agricultural consultancy, for an initial purchase price of $53.0 million . In addition, the Company recognized $14.1 million of contingent consideration, with a total maximum payable of $22.8 million , subject to the future performance of the business and $2.0 million of ceding commission. The total consideration for the acquisition was $69.1 million . A significant proportion of the acquired business was represented by intangible assets, specifically $25.0 million for agency relationships, $4.0 million for the right to use the AgriLogic trademark, $2.9 million for non-compete agreements, $1.8 million for the value of business acquired and $1.0 million for consultancy relationships. In addition, $12.0 million of software was acquired and recognized in the balance sheet under office properties and equipment along with $0.3 million of residual net assets. The total net assets acquired of $47.0 million resulted in the Company recognizing a total of $22.1 million in goodwill for the acquisition of AgriLogic, $34.0 million of intangible assets and $21.0 million of goodwill were eligible for tax deduction over the next 15 years. License to use the “AgriLogic” Trademark. The Company acquired the right to use the AgriLogic trademark in the United States. The Company valued the trademark at $4.0 million with an estimated economic useful life of 10 years. The Company planned to amortize the estimated value of the trademark over its estimated useful life. Agency Relationships. The Company valued the agency relationships at $25.0 million with an estimated economic useful life of 15 years. The Company amortized the estimated value of the agency relationships over their estimated useful life. Non-compete Agreements. The Company valued the non-compete agreements at $2.9 million with an estimated economic useful life of 5 years. The Company amortized the estimated value of the non-compete agreements over their estimated useful life. Value of Business Acquired. The Company recognized a $1.8 million asset for value of business acquired (“VOBA”) consisting of the inforce unearned premium reserve and claims reserves at fair value. The Company amortized the VOBA in line with the unwinding of the acquired unearned premium balances and loss reserves. Given the short tail nature of the book, the VOBA was fully amortized in 2016. Consulting Relationships. The Company valued the consulting relationships at $1.0 million with an estimated economic useful life of 10 years. The Company amortized the estimated value of the consulting relationships over their estimated useful life. Goodwill. The Company valued the goodwill at $22.1 million . The goodwill was deemed to have an indefinite useful life and was planned to be assessed for impairment annually. On December 18, 2017, Aspen U.S. Holdings sold its interest in specialist U.S. crop managing general agency business, AgriLogic, to CGB Diversified Service, Inc (“CGB DS”) in exchange for a 23.2% equity interest in Crop Re Services LLC. Aspen U.S. Holdings retained the agricultural consulting business previously integrated within AgriLogic. Intangible assets disposed of as part of the AgriLogic sale included $21.8 million for agency relationships, $3.1 million for the right to use the AgriLogic trademark, $0.9 million for non-compete agreements and $20.6 million for goodwill. Following the sale the Company performed its annual qualitative assessment on the residual intangible assets of the agricultural consulting business and determined that it was more likely than not that the intangible assets were impaired. The Company has therefore recognized an impairment charge of $3.4 million in the year ending December 31, 2017 . Blue Waters On October 31, 2016, Acorn acquired 100% of the equity voting interest of Blue Waters, a specialist marine insurance agency. The total consideration for the acquisition was $8.0 million . A significant proportion of the acquired business was represented by intangible assets, specifically $3.1 million for agency relationships, $1.5 million for the right to use the Blue Waters trademark, $1.0 million for non-compete agreements and $0.05 million for the value of trading licenses. In addition, $0.3 million of residual net assets were acquired. The total net assets acquired of $5.75 million resulted in the Company recognizing a total of $2.1 million in goodwill for the acquisition of Blue Waters. Agency Relationships . The Company valued the agency relationships at $3.1 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the agency relationships over their estimated useful life. License to use the “Blue Waters” Trademark . The Company acquired the right to use the Blue Waters trademark in the United States. The Company valued the trademark at $1.5 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the trademark over its estimated useful life. Non-compete Agreements . The Company valued the non-compete agreements at $1.0 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the non-compete agreements over their estimated useful life. Insurance Licenses . The Company valued the insurance licenses at $0.05 million . The insurance licenses are considered to have an indefinite useful life and are not amortized. The licenses are tested annually for impairment. Goodwill . The Company valued the goodwill at $2.1 million . The goodwill is deemed to have an indefinite useful life and will be assessed for impairment annually. Other Intangible Assets Renewal Rights. On September 22, 2016, the Company entered into a renewal rights agreement with Liberty Specialty Markets Limited (“LSML“). The Company valued the renewal rights at $1.9 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the renewal rights over the estimated useful life. In addition to the intangible assets and goodwill associated with the AgriLogic and Blue Waters acquisitions and the renewal rights agreement with LSML, the Company has the following intangible assets from prior transactions. 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, 2017 , the value of the license to use the Aspen trademark was $1.6 million ( December 31, 2016 — $1.6 million ). The trademark has an indefinite useful life and is tested for impairment annually or when events or changes in circumstances indicate that the asset might be impaired. 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, 2017 . Insurance Licenses. The value of the licenses as at December 31, 2017 was $16.6 million ( December 31, 2016 — $16.6 million ), including $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, 2017 . Goodwill. On January 1, 2017, the Company purchased through its wholly-owned subsidiary, Aspen U.S. Holdings, a 49% share of Digital Re. The Company valued the goodwill at $1.8 million . The goodwill is deemed to have an indefinite useful life and will be assessed for impairment annually under the provisions of ASC 323-10-35.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : As at December 31, 2017 At December 31, 2016 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 1,455.0 $ 1,482.8 Third party 2,425.3 2,380.8 Letters of credit / guarantees (1) 658.5 672.1 Investment commitment — real estate fund 100.0 — Total restricted assets $ 4,638.8 $ 4,535.7 Total as percent of investable assets (2) 53.4 % 49.3 % _______________ (1) As at December 31, 2017 , the Company had pledged funds of $658.5 million and £0.0 million ( December 31, 2016 — $672.0 million and £7.2 million ) as collateral for the secured letters of credit. (2) Investable assets comprise total investments, cash and cash equivalents, accrued interest, receivables for securities sold and payables for securities purchased. The Company's 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, 2017 and 2016 , these funds amounted to approximately 53.4% of the $8.7 billion and approximately 49.3% of the $9.2 billion of investable assets held by the Company, respectively. We do not consider that this unduly restricts our liquidity at this time. For more information on our credit facilities and long-term debt arrangements, please refer to Note 22, “Credit Facility and Long-term Debt” of these consolidated financial statements. Investment commitment -real estate fund. On December 20, 2017, the Company committed $100.0 million as a limited partner to a real estate fund. As of February 22, 2018 , the Company had not received a capital call. The investment objective of the fund is to achieve attractive risk-adjusted returns through the acquisition of income producing, high quality assets in gateway cities located in the U.S. and Canada in the office, retail, industrial and multifamily sectors of the real estate market. Investments in this fund may be redeemed on a quarterly basis with 90 days’ notice subject to available cash in the fund once the lock-up period ends two years after the capital call. If sufficient cash is not available then all requested redemptions will be made on a pro rata basis. If a redemption request has not been met in full, as of such calendar quarter, the remaining portion of the request will be redeemed in subsequent quarters. There are no assurances as to when the Company may be able to withdraw, in whole or in part, its redemption request from the fund. A lock-up period is the initial amount of time an investor is contractually required to remain invested before having the ability to redeem. 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, consists of investable assets as at December 31, 2017 in the amount of $458.7 million ( 2016 — $447.3 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. Aspen Managing Agency Limited, the managing agent to Syndicate 4711, is also required by Lloyd’s to maintain a minimum level of capital which as at December 31, 2017 was £0.4 million ( December 31, 2016 — £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 may 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 $1,268.4 million as at December 31, 2017 and $1,101.5 million as at December 31, 2016 . As at December 31, 2017 , the balance (including applicable letter of credit facilities) held in the trust was $1,350.9 million ( 2016 — $1,351.2 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 may 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 $895.5 million and $953.2 million as at December 31, 2017 and 2016 , respectively. As at December 31, 2017 , the balance held in the U.S. trust fund and other Aspen Bermuda trusts was $1,333.6 million ( 2016 — $1,267.4 million ). U.S. Surplus Lines Trust Fund. Aspen U.K. and Syndicate 4711 have 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 trust as at December 31, 2017 was $195.0 million ( 2016 — $185.7 million ). U.S. Regulatory Deposits. As at December 31, 2017 , Aspen Specialty had a total of $6.0 million ( 2016 — $6.0 million ) on deposit with six U.S. states in order to satisfy state regulations for writing business in those states. AAIC had a further $6.1 million ( 2016 — $6.0 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, 2017 , the balance held in trust was CAD $169.9 million ( 2016 — CAD $249.2 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, 2017 , the balance held in trust was AUD $198.7 million ( 2016 — AUD $137.8 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, 2017 , the balance held in trust was CHF 9.8 million ( 2016 — CHF 18.3 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, 2017 , the balance in the account was SGD $120.6 million ( 2016 — SGD $106.4 million ). (b) Operating leases Amounts outstanding under operating leases net of subleases as at December 31, 2017 and 2016 were: As at December 31, 2017 2018 2019 2020 2021 2022 Later Total ($ in millions) Operating Lease Obligations $ 16.2 $ 16.1 $ 15.3 $ 11.0 $ 8.7 $ 78.7 $ 146.0 As at December 31, 2016 2017 2018 2019 2020 2021 Later Total ($ in millions) Operating Lease Obligations $ 16.1 $ 15.3 $ 14.0 $ 9.9 $ 8.7 $ 86.4 $ 150.4 Total rental and premises expenses for 2017 was $26.3 million ( 2016 — $24.1 million ). For all leases, rent incentives, including reduced-rent and rent-free periods, are spread on a straight-line basis over the term of the lease. The Company believes 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 $34.6 million for the twelve months ended December 31, 2017 ( 2016 — $22.6 million ). Accumulated depreciation as at December 31, 2017 was $146.2 million ( 2016 — $111.6 million ). (c) Variable interest entities As at December 31, 2017 , the Company had investments in two ( December 31, 2016 — three ) variable interest entities, Peregrine Reinsurance Ltd and Silverton Re Ltd. Peregrine Reinsurance Ltd. For further information regarding the Company's investment in Peregrine Reinsurance Ltd, please refer to Note 7, “Variable Interest Entities” of these consolidated financial statements. Silverton Re Ltd. For further information regarding the Company’s investment in Silverton Re Ltd., please refer to Note 7, “Variable Interest Entities” of these consolidated financial statements. (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 at December 31, 2017 , it was the opinion of the Company’s management based on available information 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, 2017
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 the Risk Committee of the Board of Directors. Reinsurance recoverables The total amount recoverable by the Company from reinsurers as at December 31, 2017 was $1,515.2 million ( 2016 — $560.7 million ). As at December 31, 2017 , 6.4% of the Company’s un-collateralized reinsurance recoverables were with Markel which is rated A by A.M. Best and A by S&P, 5.5% were with Munich Re which is rated A+ by A.M. Best and AA- by S&P and 5.2% were with Lloyd’s of London Syndicates 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, 2017 . Underwriting premium receivables The total underwriting premium receivable by the Company as at December 31, 2017 was $1,496.5 million ( 2016 — $1,399.4 million ). As at December 31, 2017 , $8.7 million of the total underwriting premium receivable balance 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 $5.2 million as at December 31, 2017 ( 2016 — $5.0 million ) for underwriting premiums unlikely to be collected. Investments and cash and cash equivalents The Company’s investment policies include specific provisions that limit the allowable holdings of a single issue and issuer. As at December 31, 2017 , there were no investments in any single issuer, other than the U.S. government, U.S. government agencies, U.S. government sponsored enterprises, the Canadian government and the U.K. government in excess of 2% of the aggregate investment portfolio. Balances owed by brokers The Company underwrites a significant amount of its business through brokers and a credit risk exists should any of these brokers be unable to fulfill their contractual obligations in respect of insurance or reinsurance balances due to the Company. The following table shows the largest brokers that the Company transacted business within the three years ended December 31, 2017 and the proportion of gross written premiums from each of those brokers. Twelve Months Ended December 31, 2017 2016 2015 (in percentages) Aon Corporation 16.4 % 18.4 % 18.7 % Marsh & McLennan Companies, Inc. 16.0 14.7 15.4 Willis Group Holdings, Ltd. 13.1 13.7 14.5 Other brokers/non-broker sources (1) 54.5 53.2 51.4 Total 100.0 % 100.0 % 100.0 % Gross written premiums ($ millions) $ 3,360.9 $ 3,147.0 $ 2,997.3 ______________ (1) No other individual broker accounted for more than 10% of total gross written premiums.

Reclassifications from Accumula

Reclassifications from Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2017
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 December 31, 2017 and 2016 : Amount Reclassified from AOCI Details about the AOCI Components Twelve Months Ended December 31, 2017 Twelve Months Ended December 31, 2016 Affected Line Item in the Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gain on sale of securities $ 10.7 $ 18.8 Realized and unrealized investment gains Realized (losses) on sale of securities (6.7 ) (8.9 ) Realized and unrealized investment losses 4.0 9.9 (Loss)/income from operations before income tax Tax on net realized gains of securities (0.4 ) (1.0 ) Income tax expense $ 3.6 $ 8.9 Net income Realized derivatives: Net realized (losses) on settled derivatives 4.4 (8.7 ) General, administrative and corporate expenses Tax on settled derivatives (0.8 ) — Income tax expense $ 3.6 $ (8.7 ) Net income Total reclassifications from AOCI to the statement of operations, net of income tax $ 7.2 $ 0.2 Net income

Credit Facility and Long-term D

Credit Facility and Long-term Debt	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Credit Facility and Long-term Debt	Credit Facility and Long-term Debt Letter of Credit Facilities. On March 27, 2017, Aspen Holdings and certain of its direct or indirect subsidiaries (collectively, the “Borrowers”) entered into a Second Amended and Restated Credit Agreement (the “Credit Agreement”) with various lenders and Barclays Bank plc, as administrative agent, which amends and restates the Amended and Restated Credit Agreement, dated as of June 12, 2013, among the Company, certain subsidiaries thereof, various lenders and Barclays Bank plc, as administrative agent. The credit facility will be used by the Borrowers to finance the working capital needs of the Company and its subsidiaries, for letters of credit in connection with the insurance and reinsurance businesses of the Company and its subsidiaries and for other general corporate purposes. Initial availability under the credit facility is $200,000,000 and the Company has the option (subject to obtaining commitments from acceptable lenders) to increase the credit facility by up to $100,000,000 . The credit facility will expire on March 27, 2022. As at December 31, 2017 , no borrowings were outstanding under the Credit Agreement. 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 Agreement, the Company must not permit (a) consolidated tangible net worth to be less than approximately $2,323,100,000 plus 25% of consolidated net income and 25% of aggregate net cash proceeds from the issuance by the Company of its capital stock, in each case after January 1, 2017, (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. The Credit Agreement contains other customary affirmative and negative covenants, including (subject to various exceptions) restrictions on the ability of the Company and its subsidiaries to incur indebtedness, create or permit liens on their assets, engage in mergers or consolidations, dispose of assets, pay dividends or other distributions, purchase or redeem the Company’s equity securities, make investments and enter into transactions with affiliates. In addition, the Credit Agreement 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 June 30, 2016, Aspen Bermuda and Citibank Europe plc (“Citi Europe”) amended the committed letter of credit facility, dated June 30, 2012, as amended on June 30, 2014 (the “LOC Facility”). The amendment to the LOC facility extended the term of the LOC Facility to June 30, 2018 and provides a maximum aggregate amount of up to $550.0 million . Under the LOC Facility, Aspen Bermuda will pay to Citi Europe (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 LOC Facility. Aspen Bermuda will also pay interest on the amount drawn by any beneficiary under a credit provided under the 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. In addition, Aspen Bermuda and Citi Europe entered into an uncommitted letter of credit facility whereby Aspen Bermuda has the ability to request letters of credit under this facility subject to the prior approval of Citi Europe. The fee associated with the uncommitted facility is a letter of credit fee based on the available amounts of each letter of credit issued under the uncommitted facility. Both the LOC Facility and the uncommitted facility are used to secure obligations of Aspen Bermuda to its policyholders. In addition to these facilities, we also use regulatory trusts to secure our obligations to policyholders. The terms of a pledge agreement between Aspen Bermuda and Citi Europe (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 Citi Europe, as previously filed with the United States Securities and Exchange Commission, remain in full force and effect. As at December 31, 2017 , we had $449.4 million of outstanding collateralized letters of credit under the New LOC Facility ( December 31, 2016 — $463.6 million under the LOC Facility). Long-term Debt. 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 then 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 described above remain outstanding, the Company has agreed that neither the Company nor any of its 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 . Silverton, our Bermuda-domiciled special purpose insurer, was established in December 2013 to provide additional collateralized capacity to support Aspen Re’s global reinsurance business. 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 2015 Loan Notes matured on September 18, 2017. In the fourth quarter of 2015, Silverton issued $125.0 million of participating notes (of which $100.0 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 Company’s maximum loss exposure to Silverton in relation to the 2016 Loan Notes is its $0.2 million note holdings as at December 31, 2017 due to mature on September 17, 2018. In the fourth quarter of 2016, Silverton issued $130.0 million of participating notes (of which $105.0 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 Company’s maximum loss exposure to Silverton in relation to the 2017 Loan Notes is its $20.4 million note holdings as at December 31, 2017 due to mature on September 16, 2019. The following table summarizes our contractual obligations under the long-term debts as at December 31, 2017 . Payments Due By Period Contractual Basis Less than 1-3 years 3-5 years More than 5 years Total ($ in millions) Long-term Debt Obligations $ — $ 250.0 $ — $ 300.0 $ 550.0 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 D

Unaudited Quarterly Financial Data	12 Months Ended
	Dec. 31, 2017
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, 2017 , 2016 and 2015 . 2017 Quarter Ended March 31 Quarter Ended June 30 Quarter Ended September 30 Quarter Ended December 31 Year Ended December 31 Revenues ($ in millions) Net earned premium $ 581.1 $ 562.0 $ 652.5 $ 511.0 $ 2,306.6 Net investment income 47.7 47.4 46.4 47.5 189.0 Realized and unrealized investment gains 51.2 49.0 29.9 18.8 148.9 Other income 3.6 3.6 (2.2 ) 3.9 8.9 Total revenues 683.6 662.0 726.6 581.2 2,653.4 Expenses Losses and loss adjustment expenses 328.2 346.1 776.2 544.2 1,994.7 Amortization of deferred policy acquisition costs 113.7 96.3 105.4 85.1 400.5 General, administrative and corporate expenses 121.3 119.9 110.9 150.1 502.2 Interest on long-term debt 7.4 7.4 7.4 7.3 29.5 Change in fair value of derivatives (3.1 ) (17.6 ) (4.5 ) (2.5 ) (27.7 ) Change in fair value of loan notes issued by variable interest entities 2.9 3.3 (9.8 ) (17.6 ) (21.2 ) Realized and unrealized investment losses/(gains) 5.0 7.0 12.4 4.0 28.4 Net realized and unrealized foreign exchange losses/(gains) 8.9 20.6 (8.4 ) 2.8 23.9 Other expenses — 2.0 — 2.9 4.9 Total expenses 584.3 585.0 989.6 776.3 2,935.2 Income from operations before income tax 99.3 77.0 (263.0 ) (195.1 ) (281.8 ) Income tax (expense) (2.8 ) (1.2 ) 9.2 10.2 15.4 Net income $ 96.5 $ 75.8 $ (253.8 ) $ (184.9 ) $ (266.4 ) Per Share Data Weighted average number of ordinary share and share equivalents (1) Basic 59,862,662 59,966,358 59,759,730 59,431,469 59,753,886 Diluted 61,196,772 61,022,981 59,795,730 59,431,469 59,753,886 Basic earnings per ordinary share adjusted for preference share dividends $ 1.39 $ 1.09 $ (4.48 ) $ (3.25 ) $ (5.22 ) Diluted earnings per ordinary share adjusted for preference share dividends $ 1.36 $ 1.07 $ (4.48 ) $ (3.25 ) $ (5.22 ) _______________ (1) The basic and diluted number of ordinary shares for the three months ended September 30, 2017 and the three and twelve months ended December 31, 2017 is the same, as the inclusion of dilutive securities in a loss-making period would be anti-dilutive. 2016 Quarter Ended Quarter Ended Quarter Ended September 30 Quarter Ended December 31 Year Ended December 31 Revenues ($ in millions) Net earned premium $ 663.1 $ 680.8 $ 681.0 $ 612.4 $ 2,637.3 Net investment income 49.5 48.0 46.4 43.2 187.1 Realized and unrealized investment gains 65.6 45.1 26.7 (29.0 ) 108.4 Other income 1.4 (0.5 ) 1.5 3.3 5.7 Total revenues 779.6 773.4 755.6 629.9 2,938.5 Expenses Losses and loss adjustment expenses 357.4 442.2 389.2 387.3 1,576.1 Amortization of deferred policy acquisition costs 130.2 126.7 130.9 141.1 528.9 General, administrative and corporate expenses 119.8 116.4 125.0 128.9 490.1 Interest on long-term debt 7.4 7.4 7.3 7.4 29.5 Change in fair value of derivatives 7.2 0.4 (0.6 ) 17.6 24.6 Change in fair value of loan notes issued by variable interest entities 4.4 (0.5 ) 9.8 3.4 17.1 Realized and unrealized investment losses/(gains) 20.6 8.3 5.2 29.1 63.2 Net realized and unrealized foreign exchange losses/(gains) 15.7 5.3 (10.8 ) (12.0 ) (1.8 ) Other expenses — 1.0 (0.9 ) 1.2 1.3 Total expenses 662.7 707.2 655.1 704.0 2,729.0 Income from operations before income tax 116.9 66.2 100.5 (74.1 ) 209.5 Income tax (expense) (2.5 ) (1.3 ) (4.9 ) 2.6 (6.1 ) Net income $ 114.4 $ 64.9 $ 95.6 $ (71.5 ) $ 203.4 Per Share Data Weighted average number of ordinary share and share equivalents Basic 60,867,815 60,705,028 60,225,705 60,152,420 60,478,740 Diluted 62,483,938 62,192,142 61,577,018 61,198,041 61,860,689 Basic earnings per ordinary share adjusted for preference share dividends $ 1.73 $ 0.91 $ 1.43 $ (1.41 ) $ 2.67 Diluted earnings per ordinary share adjusted for preference share dividends $ 1.68 $ 0.89 $ 1.40 $ (1.41 ) $ 2.61 2015 Quarter Ended Quarter Ended Quarter Ended September 30 Quarter Ended December 31 Year Ended December 31 Revenues ($ in millions) Net earned premium $ 593.6 $ 609.4 $ 640.6 $ 629.7 $ 2,473.3 Net investment income 47.4 46.7 45.0 46.4 185.5 Realized and unrealized investment gains/(losses) (1) 57.4 13.5 10.7 12.9 94.5 Other income 3.9 (1.2 ) (2.3 ) (0.3 ) 0.1 Total revenues 702.3 668.4 694.0 688.7 2,753.4 Expenses Losses and loss adjustment expenses 306.1 360.5 365.6 334.0 1,366.2 Amortization of deferred policy acquisition costs 119.3 114.1 132.0 118.2 483.6 General, administrative and corporate expenses 102.2 95.4 100.5 125.9 424.0 Interest on long-term debt 7.4 7.3 7.4 7.4 29.5 Change in fair value of derivatives 7.8 (2.0 ) (10.1 ) (2.5 ) (6.8 ) Change in fair value of loan notes issued by variable interest entities 2.9 3.3 8.3 5.3 19.8 Realized and unrealized investment losses/(gains) (1) 14.5 28.8 51.9 (17.7 ) 77.5 Net realized and unrealized foreign exchange (gains)/losses (1) 6.4 11.6 8.4 (5.0 ) 21.4 Other expenses 2.6 (1.8 ) — (0.1 ) 0.7 Total expenses 569.2 617.2 664.0 565.5 2,415.9 Income from operations before income tax 133.1 51.2 30.0 123.2 337.5 Income tax (expense) (5.1 ) (2.2 ) (1.8 ) (5.3 ) (14.4 ) Net income $ 128.0 $ 49.0 $ 28.2 $ 117.9 $ 323.1 Per Share Data Weighted average number of ordinary share and share equivalents Basic 62,159,303 61,408,633 60,779,295 60,784,832 61,287,884 Diluted 63,532,662 62,896,907 62,155,125 62,176,505 62,687,503 Basic earnings per ordinary share adjusted for preference share dividends $ 1.91 $ 0.64 $ 0.30 $ 1.78 $ 4.64 Diluted earnings per ordinary share adjusted for preference share dividends $ 1.87 $ 0.62 $ 0.30 $ 1.75 $ 4.54 _______________ (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).

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On January 5, 2018, we notified the Prudential Regulation Authority of our intention to close Aspen U.K.’s branches in Paris, France and Cologne, Germany. We expect these branches to be closed during the first quarter of 2018. During January 2018, we sold $206.9 million of equity securities from our investment portfolio and realized a net gain of $36.7 million .

Schedule I - Investments

Schedule I - Investments	12 Months Ended
	Dec. 31, 2017
Summary of Investments, Other than Investments in Related Parties [Abstract]
Schedule I - Investments	SCHEDULE I - INVESTMENTS For the Twelve Months Ended December 31, 2017 , 2016 and 2015 The Company’s investments comprise investments in subsidiaries.

Schedule II - Condensed Financi

Schedule II - Condensed Financial Information of Registrant	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 As at December 31, 2017 As at December 31, 2016 ($ in millions, except per share amounts) ASSETS Short-term investments (available for sale) $ — $ 25.1 Fixed income maturities (trading) 79.4 145.7 Cash and cash equivalents 32.0 156.3 Investments in subsidiaries (1) 3,394.7 3,405.8 Other investments (equity method) 3.4 11.7 Eurobond issued by subsidiary — 480.0 Long-term debt issued by Silverton 20.6 54.5 Intercompany funds due from affiliates 5.2 9.8 Other assets 8.8 8.6 Total assets $ 3,544.1 $ 4,297.5 LIABILITIES Accrued expenses and other payables 4.7 6.4 Intercompany funds due to affiliates 61.4 93.5 Long-term debt 549.5 549.3 Total liabilities $ 615.6 $ 649.2 SHAREHOLDERS’ EQUITY Ordinary Shares: 59,474,085 shares of par value 0.15144558¢ each $ 0.1 $ 0.1 Preference Shares: 11,000,000 5.950% shares of par value 0.15144558¢ each — — Nil 7.401% shares of par value 0.15144558¢ each — — Nil 7.250% shares of par value 0.15144558¢ each — — 10,000,000 5.625% shares of par value 0.15144558¢ each — — Additional paid in capital 954.7 1,259.6 Retained earnings 2,026.9 2,392.3 Non-controlling interest 2.7 1.4 Accumulated other comprehensive income, net of taxes: Unrealized gains on investments 9.7 22.5 Gain/(loss) on derivatives 2.1 (0.5 ) Gains on foreign currency translation (67.7 ) (27.1 ) Total accumulated other comprehensive (loss)/income (55.9 ) (5.1 ) Total shareholders’ equity 2,928.5 3,648.3 Total liabilities and shareholders’ equity $ 3,544.1 $ 4,297.5 ____________________ (1) The Company’s investment in subsidiaries are accounted for under the equity method and adjustments to the carrying value of these investments are made based on the Company’s share of capital, including share of income and expenses. Changes in the value were recognised in realized and unrealized investment gains and losses in the statement of operations. 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, 2017 , 2016 and 2015 Twelve Months Ended December 31, 2017 Twelve Months Ended December 31, 2016 Twelve Months Ended December 31, 2015 ($ in millions) Operating Activities: Equity in net earnings of subsidiaries and other investments, equity method $ (590.7 ) $ (85.2 ) $ 59.0 Dividend income 373.6 331.3 292.3 Interest income on Eurobond 18.7 21.1 29.5 Net realized and unrealized investment gains/(losses) (2.3 ) 3.8 4.3 Other income — — 1.9 Total revenues (200.7 ) 271.0 387.0 Expenses: General, administrative and corporate expenses (36.2 ) (38.1 ) (34.4 ) Interest expense (29.5 ) (29.5 ) (29.5 ) Income from operations before income tax (266.4 ) 203.4 323.1 Income tax — — — Net income (266.4 ) 203.4 323.1 Amount attributable to non-controlling interest (1.3 ) (0.1 ) (0.8 ) Net (loss)/income attributable to Aspen Insurance Holdings Limited ordinary shareholders (267.7 ) 203.3 322.3 Other comprehensive (loss)/income, net of taxes: Change in unrealized gains on investments (12.8 ) (37.7 ) (105.2 ) Net change from current period hedged transactions 2.6 0.7 2.6 Change in foreign currency translation adjustment (40.6 ) (27.7 ) (72.1 ) Other comprehensive (loss)/income, net of tax (50.8 ) (64.7 ) (174.7 ) Comprehensive (loss)/income $ (318.5 ) $ 138.6 $ 147.6 ASPEN INSURANCE HOLDINGS LIMITED SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - Continued STATEMENTS OF CASH FLOWS For the Twelve Months Ended December 31, 2017 , 2016 and 2015 Twelve Months Ended December 31, 2017 Twelve Months Ended December 31, 2016 Twelve Months Ended December 31, 2015 ($ in millions) Cash Flows From/(Used In) Operating Activities: Net income (1) (excluding equity in net earnings of subsidiaries) $ 323.0 $ 289.3 $ 256.4 Adjustments: Share-based compensation expenses 9.8 15.5 17.9 Realized and unrealized losses/(gains) (2.0 ) (3.2 ) (4.3 ) Loss on derivative contracts (2.6 ) 0.5 1.2 Change in other receivables — — — Change in other assets (0.2 ) 0.4 (9.0 ) Change in accrued expenses and other payables 7.0 (9.3 ) (36.8 ) Change in intercompany activities (27.5 ) (40.7 ) 71.9 Net cash generated by operating activities 307.5 252.5 297.3 Cash Flows From/(Used in) Investing Activities: Proceeds/(purchases) of short term investments 25.1 (0.1 ) (25.0 ) Proceeds/(purchases) of Fixed Income Securities 66.3 (145.7 ) — Investment in subsidiaries (111.9 ) (126.1 ) (171.5 ) Investment in long-term debt issued by Silverton — (25.0 ) (25.0 ) Repayment of loan notes issued by Silverton 13.5 19.2 20.5 Investment in Micro-insurance (0.1 ) — (0.8 ) Investment in Bene — (3.3 ) — Net proceeds from other investments, equity method — — — Net cash (used in) investing activities (7.1 ) (281.0 ) (201.8 ) Cash Flows From/(Used in) Financing Activities: Proceeds from issuance of ordinary shares, net of issuance costs 0.5 2.5 6.8 Proceeds from issuance of preference shares, net of issuance costs — 241.3 — Preference shares redeemed (293.2 ) — — Ordinary share repurchase (30.0 ) (75.0 ) (83.7 ) Ordinary and preference share dividends paid (92.4 ) (94.5 ) (88.7 ) Proceeds from maturity of Eurobond — — 573.8 Eurobond purchased from subsidiary — — (480.0 ) Cash paid for tax withholding purposes (9.6 ) — — Net cash (used in)/from financing activities (424.7 ) 74.3 (71.8 ) Increase/(decrease) in cash and cash equivalents (124.3 ) 45.8 23.7 Cash and cash equivalents — beginning of period 156.3 110.5 86.8 Cash and cash equivalents — end of period $ 32.0 $ 156.3 $ 110.5 _______________ (1) Net income has been adjusted for the proportion due to non-controlling interest.

Schedule III - Supplementary In

Schedule III - Supplementary Insurance Information	12 Months Ended
	Dec. 31, 2017
Supplementary Insurance Information [Abstract]
Schedule III - Supplementary Insurance Information	SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION For the Twelve Months Ended December 31, 2017 , 2016 and 2015 Supplementary Information ($ in millions) Year Ended December 31, 2017 Deferred Policy Acquisition Costs Net Reserves for Losses and LAE Net Reserves for Unearned Premiums Net Premiums Earned Net Investment Income Losses and LAE Expenses Policy Acquisition Expenses Net Premium Written General and Administrative Expenses Reinsurance $ 263.0 $ 2,917.1 $ 1,067.3 $ 1,206.1 $ 1,116.4 $ 235.5 $ 1,250.0 $ 157.3 Insurance 31.3 2,317.2 268.0 1,100.5 878.3 165.0 962.5 253.9 Total $ 294.3 $ 5,234.3 $ 1,335.3 $ 2,306.6 $ 189.0 $ 1,994.7 $ 400.5 $ 2,212.5 $ 411.2 Year to date December 31, 2016 Deferred Policy Acquisition Costs Net Reserves for Losses and LAE Net Reserves for Unearned Premiums Net Net Investment Income Losses and LAE Expenses Policy Acquisition Expenses Net Premium Written General and Administrative Expenses Reinsurance $ 239.6 $ 2,462.1 $ 813.3 $ 1,181.9 $ 657.9 $ 226.4 $ 1,269.2 $ 178.2 Insurance 118.8 2,297.1 550.1 1,455.4 918.2 302.5 1,324.5 228.4 Total $ 358.4 $ 4,759.2 $ 1,363.4 $ 2,637.3 $ 187.1 $ 1,576.1 $ 528.9 $ 2,593.7 $ 406.6 Year to date December 31, 2015 Deferred Policy Acquisition Costs Net Reserves for Losses and LAE Net Reserves for Unearned Premiums Net Premiums Earned Net Investment Income Losses and LAE Expenses Policy Acquisition Expenses Net Premium Written General and Administrative Expenses Reinsurance $ 235.2 $ 2,409.5 $ 484.2 $ 1,072.6 $ 491.6 $ 224.7 $ 1,153.5 $ 146.5 Insurance 125.9 2,173.9 934.1 1,400.7 874.6 258.9 1,492.7 213.6 Total $ 361.1 $ 4,583.4 $ 1,418.3 $ 2,473.3 $ 185.5 $ 1,366.2 $ 483.6 $ 2,646.2 $ 360.1

Schedule IV - Reinsurance

Schedule IV - Reinsurance	12 Months Ended
	Dec. 31, 2017
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
Schedule IV - Reinsurance	SCHEDULE IV - REINSURANCE For the Twelve Months Ended December 31, 2017 , 2016 and 2015 Premiums Written Direct Assumed Ceded Net Amount ($ in millions) 2017 $ 1,812.4 $ 1,548.5 $ (1,148.4 ) $ 2,212.5 2016 $ 1,733.8 $ 1,413.2 $ (553.3 ) $ 2,593.7 2015 $ 1,748.4 $ 1,248.9 $ (351.1 ) $ 2,646.2 Premiums Earned Gross Amount Ceded to Other Companies Assumed From Other Companies Net Amount Percentage of Amount Assumed to Net ($ in millions, except for percentages) 2017 $ 1,757.4 $ (902.6 ) $ 1,451.8 $ 2,306.6 62.9 % 2016 $ 1,768.4 $ (449.0 ) $ 1,317.9 $ 2,637.3 50.0 % 2015 $ 1,703.3 $ (383.5 ) $ 1,153.5 $ 2,473.3 46.6 %

Schedule V - Valuation and Qual

Schedule V - Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2017
Valuation and Qualifying Accounts [Abstract]
Schedule V - Valuation and Qualifying Accounts	SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS For the Twelve Months Ended December 31, 2017 , 2016 and 2015 The following table shows the movement in the Company's bad debt provision during the twelve months ended December 31, 2017 , 2016 and 2015 : Balance at Beginning of Year Charged to Costs and Expenses Charged to Other Accounts Deductions Balance at End of Year Provisions for Bad Debt ($ in millions) 2017 Premiums receivable from underwriting activities $ 5.0 $ 0.2 $ — $ — $ 5.2 Reinsurance $ — $ — $ — $ — $ — 2016 Premiums receivable from underwriting activities $ 2.6 $ 2.4 $ — $ — $ 5.0 Reinsurance $ — $ — $ — $ — $ — 2015 Premiums receivable from underwriting activities $ 4.3 $ — $ (1.7 ) $ — $ 2.6 Reinsurance $ — $ — $ — $ — $ —

Basis of Preparation and Sign36

Basis of Preparation and Significant Accounting Policies Basis of Preparation and Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
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 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, unrecognized tax benefits, 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 be 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 recorded as written on the inception date of a policy. Premiums are primarily 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 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 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. Reinstatement premiums 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, which are paid when the Company purchases reinsurance or retrocessional coverage, are accounted for using the same accounting methodology as the Company uses 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. Premiums payable for retroactive reinsurance coverage and meeting the conditions of reinsurance accounting, are reported as reinsurance recoverables to the extent that those amounts do not exceed recorded liabilities relating to underlying reinsurance contracts. To the extent that recorded liabilities on an underlying reinsurance contract exceed premiums payable for retroactive coverage, a deferred gain is recognised. 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 business 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 business 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 under receivables for securities sold and accrued expenses and other payables for securities purchased, respectively. 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 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, Equity Method. 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 (i) 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 (ii) 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. 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. 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. 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.
Accounting for Intangible Assets	Accounting for 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 is impaired.
Accounting for Office Properties and Equipment	Accounting for 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.
Accounting for Foreign Currencies Translation	Accounting for 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.
Accounting for Earnings per Ordinary Share	Accounting for 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 Taxes 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 deferred tax assets will be fully recovered, it recognizes a valuation allowance against its deferred tax assets to reduce the deferred tax assets to the amount more likely than not to be realized. 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. Furthermore, a tax benefit from a tax position may be recognized in the financial statements only if it is more-likely-than-not that the position is sustainable, based solely on its technical merits and consideration of the relevant tax authority’s widely understood administrative practices and precedents. The tax benefit recognized, when the likelihood of realization is more likely-than-not (i.e. greater than 50 percent), is measured at the largest amount that is greater than 50 percent likely of being realized upon settlement.
Accounting for Preference Shares	Accounting for Preference Shares The Company had at the balance sheet date in issue two classes of preference shares. The Company has no obligation to pay interest on these securities but they 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. The preference shares are therefore accounted for as equity instruments and included within total shareholders’ equity.
Accounting for Long-term Incentive Plans	Accounting for Long-term Incentive Plans The Company operates an employee share incentive plan, a non-executive director stock incentive plan and employee share purchase plans, the terms and conditions of which are described in Note 17. 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.
Accounting for Long-term Debt Issued by Variable Interest Entities	Accounting for Long-term Debt Issued by Variable Interest Entities Silverton, a consolidated variable interest entity, has issued debt instruments which are due to mature on September 17, 2018 and September 16, 2019 as 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 due to the potential variability over the ultimate settlement value of the debt instruments.
Accounting for Business Combinations	Accounting for Business Combinations The Company accounts for a transaction as a business combination where the assets acquired and liabilities assumed following a transaction constitute a business. An acquired entity must have inputs and processes that make it capable of generating a return or economic benefit to be considered a business. If the assets acquired are not a business, the Company accounts the transaction as an asset acquisition. The Company recognizes and measures at fair value 100 percent of the assets and liabilities of any acquired business. Goodwill is recognized and measured as the difference between the consideration paid or payable less the fair value of assets acquired.
New Accounting Pronouncements	New Accounting Pronouncements New Accounting Pronouncements Adopted in 2017 On March 30, 2016, the Financial Accounting Standards Board (“FASB”) issued ASU 2016-09, “ Compensation — Stock Compensation ” which provides guidance on several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. This ASU is effective for fiscal years beginning after December 15, 2016 and interim periods beginning after December 15, 2016. Following the adoption of this ASU, all excess tax benefits or expense related to stock-based compensation transactions are recognized prospectively as income tax benefits or expense in the Consolidated Income Statement and the excess tax benefits from stock-based compensation transactions previously included in “Financing activities” on the Consolidated Statements of Cash Flows are prospectively included on that statement as “Operating activities.” The cash paid to the tax authority for tax withholding purposes has also been reclassified from operating to financing activity in the Consolidated Statement of Cash Flows and the comparative period has been restated. This ASU also allows share withholding up to the maximum statutory withholding requirement while still avoiding liability accounting. As a result, the Company has applied the equity accounting method for its restricted share units retrospectively and has recorded a cumulative effect adjustment of $2.8 million through opening retained earnings and $7.9 million through additional paid-in capital. 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. This ASU was effective for fiscal years beginning after December 15, 2016 and interim periods ending after December 15, 2016. This ASU did not have a material impact on the Company's consolidated financial statements. On October 27, 2016, the FASB issued ASU 2016-17, “ Consolidation (Topic 810): Interests Held through Related Parties That Are under Common Control ” which introduces new variable interest entity guidance on evaluating indirect interests held by related parties under common control. This ASU is effective for fiscal years beginning after December 15, 2016, including interim periods within those fiscal years, and does not have an impact on the consolidation of the Company's two investments in variable interest entities as described in Note 7. Accounting Pronouncements Not Yet Adopted On August 12, 2015, the FASB issued ASU 2015-14, "Revenue from Contracts with Customers (Topic 606)" which delays the effective date of ASU 2014-09 by one year. ASU 2015-14 is effective for annual periods beginning after December 15, 2017. This ASU will not have a material impact on the Company's consolidated financial statements as insurance contracts accounted for within the scope of Topic 944, Financial Services are exempt from this ASU and the Company has immaterial other revenue. On January 5, 2016, the FASB issued ASU 2016-1, “ Financial Instruments - Overall (Subtopic 825-10) ” which enhances the reporting model for financial instruments. This ASU requires all equity investments to be measured at fair value with changes in the fair value recognized through net income (other than those accounted for under the equity method of accounting or those that result in consolidation of the investee). The amendments required as a result of this ASU are effective for fiscal years beginning after December 15, 2017. The Company does not expect this ASU to have a material impact on the Company’s consolidated financial statements as the Company’s equity portfolio is classified as held for trading with changes in fair value recognized through net income. On February 25, 2016, the FASB issued ASU 2016-2, “ Leases (Topic 842) ” which supersedes the leases requirements in Topic 840 and establishes the principles that lessees and lessors shall apply to report useful information to users of financial statements about the amount, timing, and uncertainty of cash flows arising from a lease. The amendments of this ASU are effective for fiscal years beginning after December 15, 2018. This ASU is expected to have a material impact on the Company’s consolidated financial statements, specifically increasing the Company’s assets and liabilities as all leases greater than twelve months will be recognized on the balance sheet as a right of use asset and lease liability. This ASU will not have a material impact on the Company’s consolidated income statement. On June 16, 2016, the FASB issued ASU 2016-13, “ Financial Instruments - Credit Losses (Topic 326) ” which introduces a new impairment model, known as the current expected credit loss model, which is based on expected losses rather than incurred losses. Under the new credit loss model, the Company would recognize an allowance for its estimate of expected credit losses and this would apply to most debt instruments (other than those measured at fair value), trade receivables, lease receivables, reinsurance receivables, financial guarantee contracts and loan commitments. Available-for-sale debt securities are outside the model’s scope and this ASU made limited amendments to the impairment model for available-for-sale debt securities. There are other amendments required as a result of this ASU that are effective for fiscal years beginning after December 15, 2019. The Company is currently assessing the impact the adoption of this ASU will have on future financial statements and disclosures. On August 26, 2016, the FASB issued ASU 2016-15, “ Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (a consensus of the Emerging Issues Task Force) ” which standardizes the presentation and classification of certain cash receipts and cash payments in the statement of cash flows under Topic 230 Statement of Cash Flows. This ASU is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. This ASU will not have a material impact on the Company's consolidated statement of cash flows. On October 24, 2016, the FASB issued ASU 2016-16, “ Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory ” which will require companies to account for the income tax effects of intercompany transfers of assets other than inventory (e.g., intangible assets) when the transfer occurs. This ASU is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. This ASU will not have a material impact on the Company’s future financial statements and disclosures. On March 10, 2017, FASB issued ASU 2017-7, “ Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post Retirement Benefit Cost ” which changes how employers report defined benefit pension and/or other post-retirement benefit costs in their financial statements. This ASU is effective for fiscal years beginning after December 15, 2017 and interim periods beginning after December 15, 2017. As the Company does not operate a defined benefit pension scheme, this ASU will not have a material impact on Company's consolidated financial statements and disclosures. On August 28, 2017, the FASB issued ASU 2017-12, “Derivatives and Hedging (Topic 815)” enabling entities to better align their hedge accounting and risk management activities, while also simplifying the application of hedge accounting in certain situations. This ASU is effective for fiscal years beginning after 15 December, 2018 using a modified retrospective approach for cash flow and net investment hedge relationships that exist on the date of adoption. The Company is currently evaluating the provisions of ASU 2017-12 to determine how it will be affected, but no material impact is expected on the consolidated financial statements. Other accounting pronouncements were issued during the year ended December 31, 2017 which were either not relevant to the Company or did not impact the Company's consolidated financial statements.

Earnings per Ordinary Share (Ta

Earnings per Ordinary Share (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 , 2016 , and 2015 , respectively: Twelve Months Ended December 31, 2017 2016 2015 Net (loss)/income $ (266.4 ) $ 203.4 $ 323.1 Preference share dividends (36.2 ) (41.8 ) (37.8 ) Preference share redemption costs (1) (8.0 ) — — Net profit attributable to non-controlling interest (1.3 ) (0.1 ) (0.8 ) Basic and diluted net income available to ordinary shareholders $ (311.9 ) $ 161.5 $ 284.5 Ordinary shares: Basic weighted average ordinary shares 59,753,886 60,478,740 61,287,884 Weighted average effect of dilutive securities (2) (3) — 1,381,949 1,399,619 Total diluted weighted average ordinary shares 59,753,886 61,860,689 62,687,503 Earnings per ordinary share: Basic $ (5.22 ) $ 2.67 $ 4.64 Diluted (3) $ (5.22 ) $ 2.61 $ 4.54 _______________ (1) The $8.0 million deduction from net income in 2017 is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the 7.401% Preference Shares and 7.250% Preference Shares, net of issuance costs, and the final redemption costs of $293.2 million . (2) Dilutive securities consist of employee restricted share units and performance shares associated with the Company’s long term incentive plan, employee share purchase plans and director restricted stock units and options as described in Note 17. (3) The basic and diluted number of ordinary shares for 2017 is the same, as the inclusion of dilutive securities in a loss making period would be anti-dilutive.
Summary of Declared Dividends	Dividends. On February 7, 2018, the Company’s Board of Directors declared the following quarterly dividends: Dividend Payable on: Record Date: Ordinary shares $ 0.24 March 13, 2018 February 23, 2018 5.95% Preference Shares $ 0.3719 April 1, 2018 March 15, 2018 5.625% Preference Shares $ 0.3516 April 1, 2018 March 15, 2018

Segment Reporting (Tables)

Segment Reporting (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 , 2016 and 2015 : Twelve Months Ended December 31, 2017 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,548.5 $ 1,812.4 $ 3,360.9 Net written premiums 1,250.0 962.5 2,212.5 Gross earned premiums 1,451.8 1,757.4 3,209.2 Net earned premiums 1,206.1 1,100.5 2,306.6 Underwriting Expenses Losses and loss adjustment expenses 1,116.4 878.3 1,994.7 Amortization of deferred policy acquisition costs 235.5 165.0 400.5 General and administrative expenses 157.3 253.9 411.2 Underwriting (loss) (303.1 ) (196.7 ) (499.8 ) Corporate expenses (58.3 ) Non-operating expenses (32.7 ) Net investment income 189.0 Realized and unrealized investment gains 148.9 Realized and unrealized investment losses (28.4 ) Change in fair value of loan notes issued by variable interest entities 21.2 Change in fair value of derivatives 27.7 Interest expense on long term debt (29.5 ) Net realized and unrealized foreign exchange (losses) (23.9 ) Other income 8.9 Other expenses (4.9 ) Income before tax (281.8 ) Income tax credit 15.4 Net income $ (266.4 ) Net reserves for loss and loss adjustment expenses $ 2,917.1 $ 2,317.2 $ 5,234.3 Ratios Loss ratio 92.6 % 79.8 % 86.5 % Policy acquisition expense ratio 19.5 15.0 17.4 General and administrative expense ratio (1) 13.0 23.1 21.8 Expense ratio 32.5 38.1 39.2 Combined ratio 125.1 % 117.9 % 125.7 % _______________ (1) The general and administrative expense ratio in the total column includes corporate and non-operating expenses. The Company uses underwriting ratios as measures of performance. The loss ratio is the ratio of losses and loss adjustment expenses to net earned premiums. The policy acquisition expense ratio is the ratio of amortization of deferred policy acquisition costs to net earned premiums. The general and administrative expense ratio is the ratio of general, administrative and corporate expenses to net earned premiums. The combined ratio is the sum of the loss ratio, the policy acquisition expense ratio and the general and administrative expense ratio. Twelve Months Ended December 31, 2016 Reinsurance Insurance Total ( $ in millions) Underwriting Revenues Gross written premiums $ 1,413.2 $ 1,733.8 $ 3,147.0 Net written premiums 1,269.2 1,324.5 2,593.7 Gross earned premiums 1,317.9 1,768.4 3,086.3 Net earned premiums 1,181.9 1,455.4 2,637.3 Underwriting Expenses Losses and loss adjustment expenses 657.9 918.2 1,576.1 Amortization of deferred policy acquisition costs 226.4 302.5 528.9 General and administrative expenses 178.2 228.4 406.6 Underwriting income 119.4 6.3 125.7 Corporate expenses (73.8 ) Non-operating expenses (1) (9.7 ) Net investment income 187.1 Realized and unrealized investment gains 108.4 Realized and unrealized investment losses (63.2 ) Change in fair value of loan notes issued by variable interest entities (17.1 ) Change in fair value of derivatives (24.6 ) Interest expense on long term debt (29.5 ) Net realized and unrealized foreign exchange gains 1.8 Other income 5.7 Other expenses (1.3 ) Income before tax 209.5 Income tax expense (6.1 ) Net income $ 203.4 Net reserves for loss and loss adjustment expenses $ 2,462.1 $ 2,297.1 $ 4,759.2 Ratios Loss ratio 55.7 % 63.1 % 59.8 % Policy acquisition expense ratio 19.2 20.8 20.1 General and administrative expense ratio (2) 15.1 15.7 18.6 Expense ratio 34.3 36.5 38.7 Combined ratio 90.0 % 99.6 % 98.5 % ________________ (1) Non-operating expenses includes amortization of intangibles acquired from the acquisition of AgriLogic. (2) The general and administrative expense ratio in the total column includes corporate and non-operating expenses. Twelve Months Ended December 31, 2015 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,248.9 $ 1,748.4 $ 2,997.3 Net written premiums 1,153.5 1,492.7 2,646.2 Gross earned premiums 1,153.5 1,703.3 2,856.8 Net earned premiums 1,072.6 1,400.7 2,473.3 Underwriting Expenses Losses and loss adjustment expenses 491.6 874.6 1,366.2 Amortization of deferred policy acquisition costs 224.7 258.9 483.6 General and administrative expenses 146.5 213.6 360.1 Underwriting income 209.8 53.6 263.4 Corporate expenses (63.9 ) Net investment income 185.5 Realized and unrealized investment gains 94.5 Realized and unrealized investment losses (77.5 ) Change in fair value of loan notes issued by variable interest entities (19.8 ) Change in fair value of derivatives 6.8 Interest expense on long term debt (29.5 ) Net realized and unrealized foreign exchange (losses) (21.4 ) Other income 0.1 Other expenses (0.7 ) Income before tax 337.5 Income tax expense (14.4 ) Net income $ 323.1 Net reserves for loss and loss adjustment expenses $ 2,409.5 $ 2,173.9 $ 4,583.4 Ratios Loss ratio 45.8 % 62.4 % 55.2 % Policy acquisition expense ratio 20.9 18.5 19.6 General and administrative expense ratio (1) 13.7 15.2 17.1 Expense ratio 34.6 33.7 36.7 Combined ratio 80.4 % 96.1 % 91.9 % _______________ (1) The general and administrative expense ratio in the total column includes corporate expenses.
Schedule of gross written premiums based on geographical areas	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 December 31, 2017 December 31, 2016 December 31, 2015 ($ in millions) Australia/Asia $ 167.3 $ 140.5 $ 140.0 Caribbean 17.6 14.3 20.3 Europe 94.5 109.7 113.6 United Kingdom 258.3 231.4 223.6 United States & Canada (1) 1,729.3 1,597.0 1,479.5 Worldwide excluding United States (2) 88.1 90.7 107.2 Worldwide including United States (3) 868.6 837.2 793.6 Others 137.2 126.2 119.5 Total $ 3,360.9 $ 3,147.0 $ 2,997.3 ______________ (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, 2017
Investments, Debt and Equity Securities [Abstract]
Summary of Investment Income	The following table summarizes investment income for the twelve months ended December 31, 2017 , 2016 and 2015 : For the Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 ($ in millions) Fixed income securities — Available for sale $ 133.3 $ 141.3 $ 143.4 Fixed income securities — Trading 44.0 31.6 27.8 Short-term investments — Available for sale 0.4 0.6 1.1 Short-term investments — Trading 0.8 0.2 — Fixed term deposits (included in cash and cash equivalents) 6.2 3.4 3.0 Equity securities — Available for sale — — 0.1 Equity securities — Trading 13.6 20.4 20.0 Catastrophe bonds — Trading 1.8 1.6 1.9 Total 200.1 199.1 197.3 Investment expenses (11.1 ) (12.0 ) (11.8 ) Net investment income $ 189.0 $ 187.1 $ 185.5
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 December 31, 2017 , 2016 and 2015 : For the Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 10.2 $ 18.6 $ 11.7 Fixed income securities — gross realized (losses) (6.6 ) (8.3 ) (2.7 ) Equity securities — gross realized gains — — 31.9 Equity securities — gross realized (losses) — — (3.0 ) Short-term investments — gross realized gains 0.1 — — Cash and cash equivalents — gross realized gains 0.4 0.2 — Cash and cash equivalents — gross realized (losses) (0.1 ) (0.6 ) — Other-than-temporary impairments (0.7 ) — — Trading: Fixed income securities — gross realized gains 9.7 12.6 4.9 Fixed income securities — gross realized (losses) (4.5 ) (7.3 ) (6.1 ) Short-term investments — gross realized gains 2.7 — — Cash and cash equivalents — gross realized gains 1.3 0.1 — Cash and cash equivalents — gross realized (losses) — (0.3 ) — Equity securities — gross realized gains 59.0 54.1 46.0 Equity securities — gross realized (losses) (13.7 ) (46.3 ) (31.7 ) Catastrophe bonds — net unrealized (losses)/gains (2.4 ) — (0.3 ) Net change in gross unrealized gains (losses) 60.3 22.5 (33.1 ) Other investments, equity method: Gross realized and unrealized (loss) in MVI (0.1 ) (0.3 ) — Gross unrealized gain/(loss) in Chaspark 0.9 0.3 (0.6 ) Gross realized and unrealized (loss) in Bene (0.3 ) (0.1 ) — Gross realized gain on sale of AgriLogic 4.3 — — Total net realized and unrealized investment gains recorded in the statement of operations $ 120.5 $ 45.2 $ 17.0 Change in available for sale net unrealized gains: Fixed income securities (14.8 ) (39.0 ) (82.2 ) Short-term investments — — (0.1 ) Equity securities — — (27.3 ) Total change in pre-tax available for sale unrealized gains (14.8 ) (39.0 ) (109.6 ) Change in taxes 2.0 1.3 4.4 Total change in net unrealized gains, net of taxes recorded in other comprehensive income $ (12.8 ) $ (37.7 ) $ (105.2 )
Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities and Short-Term Investments	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 and short-term investments as at December 31, 2017 and December 31, 2016 : As at December 31, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,166.5 $ 4.5 $ (11.6 ) $ 1,159.4 U.S. agency 51.8 0.5 (0.2 ) 52.1 Municipal 53.0 2.1 (0.2 ) 54.9 Corporate 2,391.4 36.1 (11.8 ) 2,415.7 Non-U.S. government-backed corporate 91.5 0.3 (0.5 ) 91.3 Non-U.S government 479.7 6.4 (1.2 ) 484.9 Asset-backed 26.3 — (0.1 ) 26.2 Non-agency commercial mortgage-backed — — — — Agency mortgage-backed 941.0 13.7 (8.2 ) 946.5 Total fixed income securities — Available for sale 5,201.2 63.6 (33.8 ) 5,231.0 Total short-term investments — Available for sale 90.0 — (0.1 ) 89.9 Total $ 5,291.2 $ 63.6 $ (33.9 ) $ 5,320.9 As at December 31, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,207.9 $ 9.4 $ (11.2 ) $ 1,206.1 U.S. agency 117.7 1.9 — 119.6 Municipal 23.2 1.6 (0.4 ) 24.4 Corporate 2,566.9 39.6 (20.0 ) 2,586.5 Non-U.S. government-backed corporate 89.2 0.7 (0.1 ) 89.8 Non-U.S government 477.7 11.8 (0.8 ) 488.7 Asset-backed 62.6 0.4 — 63.0 Non-agency commercial mortgage-backed 12.3 0.3 — 12.6 Agency mortgage-backed 1,062.6 19.6 (8.3 ) 1,073.9 Total fixed income securities — Available for sale 5,620.1 85.3 (40.8 ) 5,664.6 Total short-term investments — Available for sale 145.3 — — 145.3 Total $ 5,765.4 $ 85.3 $ (40.8 ) $ 5,809.9
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, 2017 and December 31, 2016 : As at December 31, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 162.3 $ 0.4 $ (0.8 ) $ 161.9 Municipal 32.4 — (0.2 ) 32.2 Corporate 1,036.5 14.0 (4.2 ) 1,046.3 Non-U.S. government-backed corporate 1.0 — — 1.0 Non-U.S. government 196.1 6.9 (0.5 ) 202.5 Asset-backed 9.9 — — 9.9 Agency mortgage-backed 196.7 0.2 (1.4 ) 195.5 Total fixed income securities — Trading 1,634.9 21.5 (7.1 ) 1,649.3 Total short-term investments — Trading 73.0 — — 73.0 Total equity securities — Trading 414.8 83.5 (7.3 ) 491.0 Total catastrophe bonds — Trading 33.5 — (1.1 ) 32.4 Total $ 2,156.2 $ 105.0 $ (15.5 ) $ 2,245.7 As at December 31, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 82.8 $ 0.4 $ (0.8 ) $ 82.4 Municipal 15.7 — (0.2 ) 15.5 Corporate 817.8 9.9 (7.1 ) 820.6 Non-U.S government 203.4 3.5 (4.1 ) 202.8 Asset-backed 14.5 — — 14.5 Agency mortgage-backed 130.6 0.2 (0.9 ) 129.9 Total fixed income securities — Trading 1,264.8 14.0 (13.1 ) 1,265.7 Total short-term investments — Trading 185.4 — — 185.4 Total equity securities — Trading 554.3 55.4 (25.0 ) 584.7 Total catastrophe bonds — Trading 42.5 — — 42.5 Total $ 2,047.0 $ 69.4 $ (38.1 ) $ 2,078.3
Other Investments	The table below shows the Company’s investments in MVI, Chaspark, Bene, Digital Re and Crop Re for the twelve months ended December 31, 2017 and 2016 : MVI Chaspark Bene Digital Re Crop Re Total Opening undistributed value of investment as at January 1, 2017 $ 0.5 $ 8.4 $ 3.2 $ — $ — $ 12.1 Investment in the period 0.1 — — 2.3 62.5 64.9 Goodwill — — — (1.8 ) — (1.8 ) Distribution received — (9.3 ) — — — (9.3 ) Unrealized (loss)/gain for the twelve months to December 31, 2017 (0.1 ) 0.9 (0.3 ) — — 0.5 Closing value of investment as at December 31, 2017 $ 0.5 $ — $ 2.9 $ 0.5 $ 62.5 $ 66.4 Opening undistributed value of investment as at January 1, 2016 $ 0.8 $ 8.1 $ — $ — $ — $ 8.9 Investment in the period — — 3.3 — — 3.3 Unrealized (loss)/gain for the twelve months to December 31, 2016 (0.3 ) 0.3 (0.1 ) — — (0.1 ) Closing value of investment as at December 31, 2016 $ 0.5 $ 8.4 $ 3.2 $ — $ — $ 12.1
Summary of Fixed Maturities	The scheduled maturity distribution of the Company’s available for sale fixed income securities as at December 31, 2017 and December 31, 2016 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, 2017 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 561.7 $ 562.4 AA Due after one year through five years 2,486.7 2,492.2 AA- Due after five years through ten years 1,092.2 1,097.4 A+ Due after ten years 93.3 106.3 A Total — Government and corporate 4,233.9 4,258.3 Agency mortgage-backed 941.0 946.5 AA+ Asset-backed 26.3 26.2 AAA Total fixed income securities — Available for sale $ 5,201.2 $ 5,231.0 At December 31, 2016 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 567.2 $ 570.0 AA Due after one year through five years 2,643.7 2,671.9 AA- Due after five years through ten years 1,172.3 1,168.1 A+ Due after ten years 99.4 105.1 A+ Total — Government and corporate 4,482.6 4,515.1 Non-agency commercial mortgage-backed 12.3 12.6 AAA Agency mortgage-backed 1,062.6 1,073.9 AA+ Asset-backed 62.6 63.0 AAA Total fixed income securities — Available for sale $ 5,620.1 $ 5,664.6
Aggregate Fair Value and Gross Unrealized Loss by Type of Security	The following tables summarize, 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 as at December 31, 2017 and December 31, 2016 : December 31, 2017 0-12 months Over 12 months Total Fair Market Value Gross Unrealized Losses Fair Market Value Gross Unrealized Losses Fair Market Value Gross Unrealized Losses Number of Securities ($ in millions) U.S. government $ 652.1 $ (5.1 ) $ 259.8 $ (6.5 ) $ 911.9 $ (11.6 ) 101 U.S. agency 20.1 (0.2 ) 6.1 — 26.2 (0.2 ) 10 Municipal 28.5 (0.2 ) — — 28.5 (0.2 ) 9 Corporate 699.3 (3.4 ) 360.7 (8.4 ) 1,060.0 (11.8 ) 412 Non-U.S. government-backed corporate 43.5 (0.3 ) 13.3 (0.2 ) 56.8 (0.5 ) 15 Non-U.S government 206.2 (0.8 ) 32.0 (0.4 ) 238.2 (1.2 ) 47 Asset-backed 11.1 — 10.5 (0.1 ) 21.6 (0.1 ) 11 Agency mortgage-backed 257.6 (1.9 ) 301.9 (6.3 ) 559.5 (8.2 ) 156 Total fixed income securities — Available for sale 1,918.4 (11.9 ) 984.3 (21.9 ) 2,902.7 (33.8 ) 761 Total short-term investments — Available for sale 46.9 (0.1 ) — — 46.9 (0.1 ) 8 Total $ 1,965.3 $ (12.0 ) $ 984.3 $ (21.9 ) $ 2,949.6 $ (33.9 ) 769 December 31, 2016 0-12 months Over 12 months Total Fair Market Value Gross Unrealized Losses Fair Market Value Gross Unrealized Losses Fair Market Value Gross Unrealized Losses Number of Securities ($ in millions) U.S. government $ 724.4 $ (11.2 ) $ — $ — $ 724.4 $ (11.2 ) 78 U.S. agency 14.1 — — — 14.1 — 4 Municipal 7.7 (0.2 ) 0.8 (0.2 ) 8.5 (0.4 ) 6 Corporate 1,044.4 (19.4 ) 6.6 (0.6 ) 1,051.0 (20.0 ) 386 Non-U.S. government-backed corporate 29.6 (0.1 ) — — 29.6 (0.1 ) 11 Non-U.S government 143.5 (0.8 ) 1.0 — 144.5 (0.8 ) 29 Asset-backed 25.8 — 1.4 — 27.2 — 15 Agency mortgage-backed 527.6 (7.6 ) 27.2 (0.7 ) 554.8 (8.3 ) 148 Total fixed income securities — Available for sale 2,517.1 (39.3 ) 37.0 (1.5 ) 2,554.1 (40.8 ) 677 Total short-term investments — Available for sale 1.1 — — — 1.1 — 2 Total $ 2,518.2 $ (39.3 ) $ 37.0 $ (1.5 ) $ 2,555.2 $ (40.8 ) 679

Variable Interest Entities (Tab

Variable Interest Entities (Tables)	12 Months Ended
	Dec. 31, 2017
Payables and Accruals [Abstract]
Summary of Silverton Loan Notes	The following tables show the total liability balance of the Loan Notes for the twelve months ended December 31, 2017 and 2016 : For the Twelve Months Ended December 31, 2017 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 223.4 $ 54.5 $ 277.9 Total change in fair value for the period (21.2 ) (5.3 ) (26.5 ) Total distributed in the period (115.6 ) (28.6 ) (144.2 ) Total issued in the period — — — Closing balance as at December 31, 2017 $ 86.6 $ 20.6 $ 107.2 Liability Loan notes (long-term liabilities) $ 44.2 $ 10.5 $ 54.7 Accrued expenses (current liabilities) 42.4 10.1 52.5 Total aggregate unpaid balance as at December 31, 2017 $ 86.6 $ 20.6 $ 107.2 For the Twelve Months Ended December 31, 2016 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 190.6 $ 44.4 $ 235.0 Total change in fair value for the period 17.1 4.3 21.4 Total distributed in the period (89.3 ) (19.2 ) (108.5 ) Total issued in the period 105.0 25.0 130.0 Closing balance as at December 31, 2016 $ 223.4 $ 54.5 $ 277.9 Liability Loan notes (long-term liabilities) $ 115.0 $ 27.0 $ 142.0 Accrued expenses (current liabilities) 108.4 27.5 135.9 Total aggregate unpaid balance as at December 31, 2016 $ 223.4 $ 54.5 $ 277.9

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
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 as at December 31, 2017 and December 31, 2016 : As at December 31, 2017 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,159.4 $ — $ — $ 1,159.4 U.S. agency — 52.1 — 52.1 Municipal — 54.9 — 54.9 Corporate — 2,415.7 — 2,415.7 Non-U.S. government-backed corporate — 91.3 — 91.3 Non-U.S. government 341.2 143.7 — 484.9 Asset-backed — 26.2 — 26.2 Agency mortgage-backed — 946.5 — 946.5 Total fixed income securities available for sale, at fair value 1,500.6 3,730.4 — 5,231.0 Short-term investments available for sale, at fair value 87.3 2.6 — 89.9 Held for trading financial assets, at fair value U.S. government 161.9 — — 161.9 Municipal — 32.2 — 32.2 Corporate — 1,046.3 — 1,046.3 Non-U.S. government-backed corporate — 1.0 — 1.0 Non-U.S. government 24.5 178.0 — 202.5 Asset-backed — 9.9 — 9.9 Agency Mortgage-Backed — 195.5 — 195.5 Total fixed income securities trading, at fair value 186.4 1,462.9 — 1,649.3 Short-term investments trading, at fair value 73.0 — — 73.0 Equity investments trading, at fair value 491.0 — — 491.0 Catastrophe bonds trading, at fair value — 32.4 — 32.4 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 6.4 — 6.4 Liabilities under derivative contracts — foreign exchange contracts — (1.0 ) — (1.0 ) Loan notes issued by variable interest entities, at fair value — — (44.2 ) (44.2 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (42.4 ) (42.4 ) Total $ 2,338.3 $ 5,233.7 $ (86.6 ) $ 7,485.4 Transfers of assets into or out of a particular level are recorded at their fair values as of the end of each reporting period, consistent with the date of the determination of fair value. During the twelve months ended December 31, 2017 there were no transfers between Level 1, 2 and 3. The Company settled $115.6 million Level 3 liabilities in respect of the loan notes issued by the Silverton VIE for the twelve months ended December 31, 2017 . As at December 31, 2017 , there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted of the loan notes issued by the Silverton VIE. At December 31, 2016 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,206.1 $ — $ — $ 1,206.1 U.S. agency — 119.6 — 119.6 Municipal — 24.4 — 24.4 Corporate — 2,586.5 — 2,586.5 Non-U.S. government-backed corporate — 89.8 — 89.8 Non-U.S. government 343.7 145.0 — 488.7 Asset-backed — 63.0 — 63.0 Non-agency commercial mortgage-backed — 12.6 — 12.6 Agency mortgage-backed — 1,073.9 — 1,073.9 Total fixed income securities available for sale, at fair value 1,549.8 4,114.8 — 5,664.6 Short-term investments available for sale, at fair value 118.6 26.7 — 145.3 Held for trading financial assets, at fair value U.S. government 82.4 — — 82.4 Municipal — 15.5 — 15.5 Corporate — 820.6 — 820.6 Non-U.S. government — 202.8 — 202.8 Asset-backed — 14.5 — 14.5 Agency mortgage-backed — 129.9 — 129.9 Total fixed income securities trading, at fair value 82.4 1,183.3 — 1,265.7 Short-term investments trading, at fair value 76.1 109.3 — 185.4 Equity investments trading, at fair value 584.7 — — 584.7 Catastrophe bonds trading, at fair value — 42.5 — 42.5 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 7.2 — 7.2 Liabilities under derivative contracts — foreign exchange contracts — (18.4 ) — (18.4 ) Loan notes issued by variable interest entities, at fair value — — (115.0 ) (115.0 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (108.4 ) (108.4 ) Total $ 2,411.6 $ 5,465.4 $ (223.4 ) $ 7,653.6
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, 2017 : Reconciliation of Liabilities Using Level 3 Inputs Twelve Months Ended December 31, 2017 Twelve Months Ended December 31, 2016 ($ in millions) Balance at the beginning of the period (1) $ 223.4 $ 190.6 Distributed to third party (115.6 ) (89.3 ) Loan notes issued during the period — 105.0 Total change in fair value included in the statement of operations (21.2 ) 17.1 Balance at the end of the period (1) $ 86.6 $ 223.4 ______________ (1) The amount classified as other payables was $42.4 million and $108.4 million as at December 31, 2017 and December 31, 2016 , respectively.
Pricing Sources Used in Pricing Fixed Income Investments	Pricing sources used in pricing fixed income investments as at December 31, 2017 and December 31, 2016 were as follows: As at December 31, 2017 At December 31, 2016 Index providers 84 % 87 % Pricing services 11 7 Broker-dealers 5 6 Total 100 % 100 %
Summary of Securities Priced Using Pricing Information from Index Providers	A summary of securities priced using pricing information from index providers as at December 31, 2017 and December 31, 2016 is provided below: As at December 31, 2017 At December 31, 2016 Fair Market Value Determined using Prices from Index Providers % of Total Fair Value from Index Providers Fair Market Value Determined using Prices from Index Providers % of Total Fair Value from Index Providers ($ in millions, except for percentages) U.S. government $ 1,321.3 100% $ 1,288.2 100% U.S. agency 43.4 83% 110.2 92% Municipal 37.4 43% 28.8 72% Corporate 3,299.6 83% 3,275.3 96% Non-U.S. government-backed corporate 44.0 48% 44.8 50% Non-U.S. government 399.4 58% 455.6 72% Asset-backed 13.5 37% 32.1 41% Non-agency commercial mortgage-backed — —% 12.5 98% Agency mortgage-backed 605.0 53% 691.9 58% Total fixed income securities $ 5,763.6 84% $ 5,939.4 87% Equities $ 491.0 100% $ 584.7 100% Total fixed income securities and equity investments $ 6,254.6 85% $ 6,524.1 88%
Fair Value Inputs, Assets, Quantitative Information	The observable and unobservable inputs used to determine the fair value of the Loan Notes as at December 31, 2017 and December 31, 2016 , respectively, are presented in the tables below: At December 31, 2017 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 86.6 (1) Internal Valuation Model Gross premiums written (O) $ 50.1 $ 61.1 Reserve for losses (U) $ 4.2 $ 61.9 Contract period (O) N/A 365 days Initial value of issuance (O) $ 325.0 $ 325.0 At December 31, 2016 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 223.4 (1) Internal Valuation Model Gross premiums written (O) $ 38.9 $ 43.4 Reserve for losses (U) $ 2.7 $ 11.8 Contract period (O) N/A 365 days Initial value of issuance (O) $ 220.0 $ 220.0 ______________ (1) The amount classified as other payables was $42.4 million and $108.4 million as at December 31, 2017 and December 31, 2016 , respectively.

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2017
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 for the twelve months ended December 31, 2017 , 2016 and 2015 was as follows: Twelve Months Ended December 31, 2017 2016 2015 ($ in millions) Premiums written : Direct $ 1,812.4 $ 1,733.8 $ 1,748.4 Assumed 1,548.5 1,413.2 1,248.9 Ceded (1,148.4 ) (553.3 ) (351.1 ) Net premiums written $ 2,212.5 $ 2,593.7 $ 2,646.2 Premiums earned: Direct $ 1,757.4 $ 1,768.4 $ 1,703.3 Assumed 1,451.8 1,317.9 1,153.5 Ceded (902.6 ) (449.0 ) (383.5 ) Net premiums earned $ 2,306.6 $ 2,637.3 $ 2,473.3 Insurance losses and loss adjustment expenses: Direct $ 1,673.6 $ 1,091.9 $ 980.6 Assumed 1,399.9 699.6 493.0 Ceded (1,078.8 ) (215.4 ) (107.4 ) Net insurance losses and loss adjustment expenses $ 1,994.7 $ 1,576.1 $ 1,366.2

Derivative Contracts (Tables)

Derivative Contracts (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : As at December 31, 2017 As at December 31, 2016 Derivatives Not Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Derivatives at Fair Value $ 577.7 $ 5.0 (1) $ 240.2 $ 5.0 Foreign Exchange Contracts Liabilities under Derivative Contracts $ 173.9 $ (1.0 ) $ 425.4 $ (17.7 ) ______________ (1) Net of $0.6 million of cash collateral (December 31, 2016 — $ Nil ) As at December 31, 2017 As at December 31, 2016 Derivatives Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Derivatives at Fair Value $ 60.6 $ 1.4 (1) $ — $ 2.2 (1) Foreign Exchange Contracts Liabilities under Derivative Contracts — $ — $ 108.6 $ (0.7 ) ______________ (1) Net of $ Nil cash collateral (December 31, 2016 — $2.2 million ).
Gain/(Loss) Recognized in Income on Derivative	The following table provides the unrealized and realized gains/(losses) recorded in the statements of operations and other comprehensive income for derivatives that are not designated or designated as hedging instruments under ASC 815 - " Derivatives and Hedging" for twelve months ended December 31, 2017 and 2016 : Amount of Gain/(Loss) Recognized on Derivatives For the Twelve Months Ended Location of Gain/(Loss) Recognized on Derivatives December 31, 2017 December 31, 2016 Derivatives not designated as hedges ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives 27.7 (21.5 ) Interest Rate Swaps Change in Fair Value of Derivatives — (3.1 ) Derivatives designated as hedges Foreign Exchange Contracts General, administrative and corporate expenses 4.4 (8.7 ) Foreign Exchange Contracts Net change from current period hedged transactions 3.0 0.7

Deferred Policy Acquisition C44

Deferred Policy Acquisition Costs (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : Twelve Months Ended Twelve Months Ended December 31, 2016 ($ in millions) Balance at the beginning of the period $ 358.4 $ 361.1 Acquisition costs deferred 336.4 526.2 Amortization of deferred policy acquisition costs (400.5 ) (528.9 ) Balance at the end of the period $ 294.3 $ 358.4

Reserves for Losses and Adjus45

Reserves for Losses and Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Schedule of Liability for Unpaid Claims and Claims Adjustment Expense	The following table represents a reconciliation of beginning and ending consolidated loss and LAE reserves for the twelve months ended December 31, 2017 , 2016 and 2015 : As at December 31, 2017 2016 2015 ($ in millions) Provision for losses and LAE at the start of the year $ 5,319.9 $ 4,938.2 $ 4,750.8 Less reinsurance recoverable (560.7 ) (354.8 ) (350.0 ) Net loss and LAE at the start of the year 4,759.2 4,583.4 4,400.8 Net loss and LAE expenses (disposed) (125.5 ) (80.1 ) — Movement in provision for losses and LAE for claims incurred: Current year 2,100.1 1,705.4 1,522.7 Prior years (105.4 ) (129.3 ) (156.5 ) Total incurred 1,994.7 1,576.1 1,366.2 Losses and LAE payments for claims incurred: Current year (397.5 ) (241.0 ) (178.8 ) Prior years (1,157.6 ) (981.8 ) (929.7 ) Total paid (1,555.1 ) (1,222.8 ) (1,108.5 ) Foreign exchange losses/(gains) 161.0 (97.4 ) (75.1 ) Net losses and LAE reserves at the end of the year 5,234.3 4,759.2 4,583.4 Plus reinsurance recoverable on unpaid losses at the end of the year 1,515.2 560.7 354.8 Provision for losses and LAE at the end of the year $ 6,749.5 $ 5,319.9 $ 4,938.2
Short-duration Insurance Contracts, Claims Development	The following tables show an analysis of incurred claims and allocated loss adjustment expenses, net of reinsurance and cumulative paid claims and allocated claim adjustment expenses, net of reinsurance as at December 31, 2017 , 2016 , 2015 , 2014 2013 and 2012 . The loss development triangles are derived from all business written by the Company as although a limited number of contracts are written which have durations of greater than one year the contracts do not meet the definition of a long duration contract. Property Insurance Lines Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 170.7 168.6 167.3 166.2 160.6 154.7 0.3 6,872 2013 130.8 117.9 117.5 113.3 114.3 4.9 5,100 2014 166.2 157.9 134.7 135.4 3.0 8,779 2015 241.9 207.3 201.8 4.7 10,431 2016 239.9 250.7 14.7 9,514 2017 297.7 51.3 6,249 Total $ 1,154.6 Property Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 41.5 129.6 139.2 153.4 157.5 154.9 2013 39.2 76.4 89.7 101.8 106.4 2014 40.5 87.0 114.5 124.2 2015 57.5 144.0 172.1 2016 67.3 169.7 2017 97.2 $ 824.5 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 330.1 All outstanding liabilities before 2012, net of reinsurance 2.9 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 333.0 Casualty Insurance Lines Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 78.6 63.4 70.5 61.7 69.6 67.2 5.2 3,536 2013 134.1 117.5 115.9 121.4 104.4 13.6 3,852 2014 146.4 128.6 140.5 130.9 20.4 4,312 2015 205.5 225.5 188.0 64.7 4,954 2016 219.4 190.6 117.9 5,084 2017 183.4 153.0 4,447 Total $ 864.5 Casualty Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 1.3 6.7 14.3 30.0 41.3 49.7 2013 2.3 26.0 39.8 53.5 69.0 2014 2.7 13.6 33.1 60.4 2015 3.2 17.2 57.0 2016 4.2 23.1 2017 3.6 Total $ 262.8 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 601.7 All outstanding liabilities before 2012, net of reinsurance 43.0 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 644.7 Marine, Aviation and Energy Insurance Lines Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 269.8 307.2 326.7 347.3 333.0 329.0 33.8 4,593 2013 321.4 334.2 342.9 326.3 333.3 17.4 4,979 2014 310.1 314.5 299.3 310.9 45.3 4,768 2015 297.9 301.0 283.3 37.9 4,356 2016 — 219.4 231.9 49.5 4,572 2017 — 211.5 70.0 3,993 Total $ 1,699.9 Marine, Aviation and Energy Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 52.2 133.2 175.7 212.0 240.8 251.7 2013 41.6 131.8 205.3 235.7 265.0 2014 53.6 117.0 189.5 210.2 2015 45.1 123.7 174.9 2016 30.9 83.2 2017 40.4 Total $ 1,025.4 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 674.5 All outstanding liabilities before 2012, net of reinsurance 61.4 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 735.9 Financial and Professional Insurance Lines Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 88.8 90.1 94.0 97.6 94.5 90.0 9.7 593 2013 106.6 101.2 105.6 102.3 101.6 15.2 603 2014 136.4 132.6 131.3 121.4 35.2 852 2015 175.9 177.2 187.2 76.7 1,138 2016 193.0 213.6 121.1 1,365 2017 208.6 159.4 1,743 Total $ 922.4 Financial and Professional Insurance Lines Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 22.8 39.6 50.8 59.5 65.4 70.7 2013 8.1 21.5 31.6 66.0 64.5 2014 3.0 30.9 54.1 72.8 2015 13.9 43.7 70.4 2016 15.4 71.7 2017 27.3 Total $ 377.4 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 545.0 All outstanding liabilities before 2012, net of reinsurance 27.4 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 572.4 Property Catastrophe and Other Property Reinsurance Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 281.3 304.2 287.2 280.0 283.4 280.4 19.3 797 2013 220.5 203.0 193.9 182.3 180.8 4.4 945 2014 191.7 180.4 164.0 152.4 4.1 1,016 2015 219.4 191.4 181.6 18.7 1,135 2016 277.9 277.8 35.1 1,297 2017 564.8 216.1 1,366 Total $ 1,637.8 Property Catastrophe and Other Property Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 35.7 136.2 189.5 209.9 217.5 228.6 2013 34.9 100.3 149.7 162.0 166.8 2014 37.8 102.3 129.2 139.4 2015 36.2 96.9 130.0 2016 58.0 167.2 2017 124.2 Total $ 956.2 All outstanding liabilities for 2012 and subsequent years, net of reinsurance 681.6 All outstanding liabilities before 2012, net of reinsurance 47.9 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 729.5 Casualty Reinsurance Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 236.9 235.2 246.6 238.0 234.8 235.6 66.1 2,173 2013 217.5 232.9 228.4 225.8 208.4 62.2 2,034 2014 208.7 212.0 220.8 214.0 84.5 1,842 2015 197.9 205.3 214.6 103.0 1,648 2016 237.7 250.5 156.2 1,151 2017 249.5 215.9 476 Total $ 1,372.6 Casualty Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 2.2 17.9 42.2 65.8 97.1 118.5 2013 3.5 16.0 43.2 65.5 93.7 2014 2.6 14.1 38.2 60.9 2015 3.6 18.5 39.2 2016 9.6 34.3 2017 9.1 Total $ 355.7 All outstanding liabilities for 2012 and subsequent years, net of reinsurance 1,016.9 All outstanding liabilities before 2012, net of reinsurance 575.6 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 1,592.5 Specialty Reinsurance Incurred Claims, IBNR and Loss Adjustment Expenses, Net of Reinsurance As at December 31, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 $ (in millions) 2012 178.5 201.7 191.1 177.4 176.0 176.3 10.5 739 2013 146.9 141.9 134.5 122.5 121.7 7.7 620 2014 154.4 143.4 135.2 126.0 15.6 667 2015 169.7 174.3 168.8 29.4 826 2016 243.2 244.6 49.5 844 2017 385.2 211.6 763 Total $ 1,222.6 Specialty Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 ($ in millions) 2012 25.7 95.8 131.2 141.4 146.9 152.2 2013 25.4 72.7 89.1 96.4 103.5 2014 17.0 57.9 83.1 91.3 2015 18.1 58.1 106.5 2016 59.6 153.1 2017 95.7 Total $ 702.3 All outstanding liabilities for 2012 and subsequent years, net of reinsurance 520.3 All outstanding liabilities before 2012, net of reinsurance 55.4 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 575.7
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability	Reconciliation of Incurred and Paid Claims Development to total Provision for Losses and LAE Twelve Months Ended ($ in millions) Net outstanding liabilities: Insurance lines - Property insurance lines 333.0 - Casualty insurance lines 644.7 - Marine, aviation and energy insurance lines 735.9 - Financial and professional insurance lines 572.4 Total insurance lines 2,286.0 Reinsurance lines - Property catastrophe and other property reinsurance 729.5 - Casualty reinsurance 1,592.5 - Specialty reinsurance 575.7 Total reinsurance lines 2,897.7 Net loss and LAE 5,183.7 Reinsurance recoverable on unpaid losses: Insurance lines 1,245.9 Reinsurance lines 269.3 Total reinsurance recoverable on unpaid losses 1,515.2 Insurance lines other than short-duration — Unallocated claims incurred 49.2 Other 1.4 50.6 Provision for losses and LAE at the end of the year 6,749.5
Short-duration Insurance Contracts, Schedule of Historical Claims Duration	Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 6 Insurance 15.4 % 25.3 % 16.3 % 12.2 % 7.6 % 3.4 % Reinsurance 13.2 % 25.0 % 17.0 % 8.1 % 7.1 % 5.5 %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Summary of Total Income Tax	Total income tax (benefit)/expense for the twelve months ended December 31, 2017 , 2016 and 2015 was allocated as follows: Twelve Months Ended December 31, 2017 2016 2015 ($ in millions) Income tax (benefit)/expense allocated to net income $ (15.4 ) $ 6.1 $ 14.4 Income tax (benefit) allocated to other comprehensive income (17.4 ) (1.7 ) (15.3 ) Income tax (benefit) allocated directly to shareholders’ equity — (1.0 ) (1.9 ) Total income tax (benefit)/expense $ (32.8 ) $ 3.4 $ (2.8 )
Schedule of Income Tax by Taxing Authority	(Loss)/income from operations before income tax and income tax expense/(benefit) attributable to that income/(loss) for the twelve months ended December 31, 2017 , 2016 and 2015 is provided in the tables below: Twelve Months Ended December 31, 2017 (Loss)/income before tax Current income tax expense/(benefit) Deferred income tax (benefit)/expense Total income tax (benefit)/expense ($ in millions) Bermuda (1) $ (130.0 ) $ 0.9 $ — $ 0.9 U.S. (140.3 ) — 1.1 1.1 U.K. 15.3 14.1 (33.3 ) (19.2 ) Other (2) (26.8 ) 2.1 (0.3 ) 1.8 Total $ (281.8 ) $ 17.1 $ (32.5 ) $ (15.4 ) Twelve Months Ended December 31, 2016 (Loss)/income before tax Current Deferred income tax expense Total income tax expense ($ in millions) Bermuda $ 259.5 $ — $ — $ — U.S. (70.2 ) — 2.5 2.5 U.K. 43.7 (3.2 ) 5.4 2.2 Other (2) (23.5 ) 1.1 0.3 1.4 Total $ 209.5 $ (2.1 ) $ 8.2 $ 6.1 Twelve Months Ended December 31, 2015 (Loss)/income Current Deferred Total ($ in millions) Bermuda (1) $ 283.9 $ 1.4 $ — $ 1.4 U.S. (32.7 ) 1.6 (4.1 ) (2.5 ) U.K. 79.9 15.5 (4.2 ) 11.3 Other (2) 6.4 4.8 (0.6 ) 4.2 Total $ 337.5 $ 23.3 $ (8.9 ) $ 14.4 ______________ (1) Under Australian law, Aspen Bermuda is subject to tax on reinsurance premium it receives from the Australian branch of Aspen U.K. (2) Beginning from the twelve months ended December 31, 2017, the total income tax (benefit)/expense allocation table has been re-presented to show the branches of Aspen U.K. under the “Other” category with the exception of the U.S. branch which is reported under the “U.S.” category.
Income Tax Reconciliation	The reconciliation between the income tax (benefit)/expense and the statutory rate for the Company for the twelve months ended December 31, 2017 is provided in the table below: Twelve Months Ended December 31, 2017 2016 2015 Income Tax Reconciliation ($ in millions) Expected tax (benefit)/expense $ — $ — $ — Overseas statutory tax rates differential (41.5 ) (19.3 ) 6.4 Prior year adjustments (1) 1.3 3.3 (4.5 ) Valuation allowance (37.9 ) 21.0 11.8 Impact of unrecognized tax benefits (2) 0.1 (1.9 ) — Restricted foreign tax credits 1.6 1.9 0.6 Share-based payments (0.9 ) — — Foreign exchange (2.1 ) 0.2 (0.4 ) Non-deductible expenses 0.4 0.8 1.7 Non-taxable items (0.9 ) (0.9 ) (1.3 ) Impact of changes in statutory tax rates 64.5 1.0 0.1 Total income tax (benefit)/expense $ (15.4 ) $ 6.1 $ 14.4 ________________ (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 (this “report”). Accordingly, the final tax liabilities may differ from the estimated tax expense included in this report and may result in prior year adjustments being reported. The prior period adjustments for the twelve months ended December 31, 2017 , 2016 and 2015 predominantly relate to the determination of results under U.K. GAAP, upon which the U.K. tax returns are based. These items can only be reasonably determined on an accurate basis after this report has been filed. (2) For 2017 , the $0.1 million charge relates to a $0.3 million credit following the conclusion of a tax examination in respect of tax deductions for certain expenses and accrued interest of $0.4 million in respect of the adjustment of equity reserves. For 2016 , the $1.9 million credit relates to a $3.2 million credit following the conclusion of a tax examination in respect of tax deductions for certain interest payments and accrued interest of $1.3 million in respect of the adjustment to equity reserves. For 2015 , there was $ Nil impact.
Schedule of Unrecognized Tax Benefits	As illustrated in the table below, unrecognized tax benefits totaled $11.2 million as at December 31, 2017 and relate to U.K. prior period tax positions for the year 2011. An unrecognized tax benefit of $0.3 million relating to tax deductions for certain expenses was released during the year ended December 31, 2017 following the completion of the U.K. tax authority review. If they are recognized, the unrecognized tax benefits would reduce the Company’s effective tax rate. It is possible that the unrecognized tax benefits could be eliminated following completion of tax examinations into these matters. Twelve Months Ended December 31, 2017 2016 ($ in millions) Unrecognized tax benefits balance at January 1 $ 10.5 $ 29.2 Foreign exchange re-translation $ 1.0 (3.4 ) Settlement in respect of tax position of prior years $ (0.3 ) (15.3 ) Unrecognized tax benefits balance at December 31 $ 11.2 $ 10.5
Tax Effects of Deferred Tax Assets and Deferred Tax Liabilities	The tax effects of temporary differences and carryforwards that give rise to deferred tax assets and deferred tax liabilities are presented in the following table as at December 31, 2017 and 2016 : As at December 31, 2017 2016 ($ in millions) Deferred tax assets: Share-based payments $ 4.0 $ 5.6 Operating loss carryforwards 102.5 124.5 Loss reserves 4.3 7.8 Accrued expenses 7.9 14.4 Foreign tax credit carryforwards 4.0 2.5 Unearned premiums 11.8 20.0 Deferred policy acquisition costs 5.9 2.1 Office properties and equipment 8.0 5.6 Other temporary differences 2.9 3.8 Total gross deferred tax assets 151.3 186.3 Less valuation allowance (104.8 ) (142.5 ) Net deferred tax assets $ 46.5 $ 43.8 Deferred tax liabilities: Equalization provision reserves $ — $ (23.0 ) Intangible assets (other) (2.0 ) (2.8 ) Deferred policy acquisition costs (14.5 ) (22.4 ) Other temporary differences (1.7 ) (1.7 ) Total gross deferred tax (liabilities) (18.2 ) (49.9 ) Net deferred tax asset/(liability) $ 28.3 $ (6.1 )

Capital Structure (Tables)

Capital Structure (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Summary of Authorized and Issued Share Capital	The following table provides a summary of the Company’s authorized and issued share capital as at December 31, 2017 and 2016 : As at December 31, 2017 At December 31, 2016 Number $ in Thousands Number $ in 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 59,474,085 90 59,774,464 91 Issued 7.401% Preference Shares of 0.15144558¢ each with a liquidation preference of $25 per share — — 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 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 Issued 5.625% Preference Shares of 0.15144558¢ each with a liquidation preference of $25 per share 10,000,000 15 10,000,000 15 Total issued share capital 122 140
Summary of Ordinary Shares	The following table summarizes transactions in the Company’s ordinary shares during the years ended December 31, 2017 and 2016 : Number of Ordinary Shares 2017 2016 Ordinary shares in issue at the beginning of the year 59,774,464 60,918,373 Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or 2008 share purchase plan 309,727 419,255 Ordinary shares issued to non-employee directors 38,835 32,241 Ordinary shares repurchased (648,941 ) (1,595,405 ) Ordinary shares in issue at the end of the year 59,474,085 59,774,464

Statutory Requirements and Di48

Statutory Requirements and Dividends Restrictions (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and December 31, 2016 were estimated as follows: As at December 31, 2017 U.S. Bermuda U.K. ($ in millions) Required statutory capital and surplus $ 163.5 $ 1,146.7 $ 745.8 Statutory capital and surplus $ 645.5 $ 1,793.0 $ 888.6 As at December 31, 2016 U.S. Bermuda U.K. ($ in millions) Required statutory capital and surplus $ 77.3 $ 1,259.4 $ 558.1 Statutory capital and surplus $ 598.3 $ 2,294.4 $ 970.3

Share-Based Payments (Tables)

Share-Based Payments (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Restricted Share Units by Grant Year	The following table summarizes information about RSUs as at December 31, 2017 by year of grant: As at December 31, 2017 Restricted Share Units RSU Holder Amount Granted Amount Vested Amount Forfeited Amount Outstanding 2015 Grants 287,852 173,755 38,886 75,211 2016 Grants 328,550 104,042 40,579 183,929 2017 Grants 200,021 — 19,585 180,436 Total 439,576
Summary of Restricted Share Unit Activity under 2013 Share Incentive Plan	A summary of RSUs under the Company’s 2013 Share Incentive Plan for the twelve months ended December 31, 2017 , 2016 and 2015 is presented below: Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 Restricted share unit activity Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Outstanding restricted stock, beginning of period 530,340 $39.28 504,234 $40.45 519,112 $35.82 Granted 200,021 49.20 328,550 37.63 287,852 39.68 Vested (245,704 ) 39.63 (246,489 ) 34.26 (266,944 ) 34.26 Forfeited (45,081 ) 41.84 (55,955 ) 39.51 (35,786 ) 39.59 Outstanding restricted stock, end of period 439,576 $43.22 530,340 $39.28 504,234 $40.45
Summary of Performance and Phantom Shares by Grant Year	The following table summarizes information about phantom shares as at December 31, 2017 by year of grant: As at December 31, 2017 Phantom Share Awards Amount Granted Amount Vested Amount Forfeited Amount Outstanding 2015 Grants 135,651 52,720 82,931 — 2016 Grants 147,513 15,132 96,978 35,403 2017 Grants 173,619 — 90,342 83,277 Total 118,680 The following table summarizes information about performance shares as at December 31, 2017 by year of grant: As at December 31, 2017 Performance Share Awards Amount Granted Amount Vested Amount Forfeited Amount Outstanding 2015 Grants 277,585 107,882 169,703 — 2016 Grants 278,477 28,566 168,224 81,687 2017 Grants 216,878 — 89,643 127,235 Total 208,922
Summary of Performance and Phantom Share Activity under 2013 Share Incentive Plan	A summary of performance share activity under the Company’s 2013 Share Incentive Plan for the twelve months ended December 31, 2017 , 2016 and 2015 is presented below: Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 Performance Share Activity Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Outstanding performance shares, beginning of period 254,988 $36.92 266,424 $24.17 268,418 $25.35 Granted 216,878 47.30 278,477 34.44 277,585 38.92 Vested — — (87,059 ) 37.30 (208,830 ) 38.11 Forfeited (262,944 ) 40.32 (202,854 ) 36.93 (70,749 ) 30.26 Outstanding performance shares, end of period 208,922 $38.71 254,988 $36.92 266,424 $24.17 A summary of phantom share activity under the Company’s 2013 Share Incentive Plan for the twelve months ended December 31, 2017 , 2016 and 2015 is presented below: Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 Phantom Share Activity Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Outstanding performance shares, beginning of period 131,464 $35.90 130,319 $38.75 143,248 $35.32 Granted 173,619 47.30 147,513 34.44 135,651 38.92 Vested — — (36,159 ) 36.96 (151,668 ) 34.98 Forfeited (186,403 ) 40.32 (110,209 ) 36.96 3,087 35.65 Outstanding performance shares, end of period 118,680 $38.71 131,464 $35.90 130,319 $38.75
Summary of Vesting Criteria	A summary of the increases in adjusted BVPS is presented below: Year ended December 31, Increase in BVPS 2013 6.2 % 2014 13.3 % 2015 10.7 % 2016 5.9 % 2017 — %
Fair Value of Employee Options Granted under ESPP	The fair value of the employee options granted under the ESPP was estimated on the date of grant using a modified Black-Scholes option pricing model under the following assumptions: Grant Date Per share weighted average fair value Risk free interest rate Dividend yield Expected life Share price volatility ($) (%) (%) (in years) (%) March 25, 2015 8.17 0.94 1.78 3 16.00 March 25, 2015 7.08 0.60 1.78 2 16.00 March 25, 2016 7.97 1.04 1.88 3 4.21 March 25, 2016 7.00 0.87 1.88 2 2.44 April 28, 2017 6.69 1.44 1.83 3 3.67 April 28, 2017 8.70 1.44 1.83 3 3.67 April 28, 2017 8.70 1.26 1.83 2 3.52
Summary of Non-Employee Director Restricted Share Units by Grant Year	The following table summarizes information about RSUs issued to non-employee directors by year of grant as at December 31, 2017 . As at December 31, 2017 Restricted Share Units Amount Granted Amount Vested Amount Forfeited Amount Outstanding Non-Employee Directors 2015 27,620 27,620 — — 2016 24,456 21,352 3,104 — 2017 22,230 17,043 1,853 3,334 Chairman 2015 12,154 12,154 — — 2016 10,952 10,952 — — 2017 8,892 7,410 — 1,482 Total 106,304 96,531 4,957 4,816
Summary of Restricted Share Unit, Non Employee Directors, under 2013 Share Incentive Plan	A summary of RSU activity relating to non-employee directors under the Company’s 2013 Share Incentive Plan for the twelve months ended December 31, 2017 , 2016 and 2015 is presented below: Twelve Months Ended December 31, 2017 December 31, 2016 December 31, 2015 Restricted share unit activity Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Number of Shares Weighted Average Grant Date FV Outstanding restricted stock, beginning of period 5,171 $41.07 6,636 $45.28 6,806 $35.83 Granted 31,122 50.18 35,408 41.07 39,774 45.25 Vested (29,624 ) 48.59 (33,769 ) 41.89 (39,944 ) 43.81 Forfeited (1,853 ) 50.18 (3,104 ) 41.07 — 0.00 Outstanding restricted stock, end of period 4,816 $50.19 5,171 $41.07 6,636 $45.28

Intangible Assets (Tables)

Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : Twelve Months Ended December 31, 2017 Beginning of the Year Additions/(Disposals) Amortization Impairment End of the Year ($ in millions) Intangible Assets Trade Mark $ 6.6 $ (3.1 ) $ (0.5 ) $ (0.1 ) $ 2.9 Insurance Licenses 16.7 — — — 16.7 Agency Relationships 26.2 (21.8 ) (2.1 ) — 2.3 Non-compete Agreements 3.3 (0.9 ) (0.7 ) (1.0 ) 0.7 Consulting Relationships 0.9 — (0.1 ) (0.8 ) — Goodwill 24.2 (18.8 ) — (1.5 ) 3.9 Renewal Rights 1.7 — (0.3 ) — 1.4 Total $ 79.6 $ (44.6 ) $ (3.7 ) $ (3.4 ) $ 27.9 Twelve Months Ended December 31, 2016 Beginning of the Year Additions Amortization Impairment End of the Year ($ in millions) Intangible Assets Trade Mark $ 1.6 $ 5.5 $ (0.5 ) $ — $ 6.6 Insurance Licenses 16.6 0.1 — — 16.7 Agency Relationships — 28.1 (1.9 ) — 26.2 Non-compete Agreements — 3.9 (0.6 ) — 3.3 Value of Business Acquired — 1.8 (1.8 ) — — Consulting Relationships — 1.0 (0.1 ) — 0.9 Goodwill — 24.2 — — 24.2 Renewal Rights — 1.9 (0.2 ) — 1.7 Total $ 18.2 $ 66.5 $ (5.1 ) $ — $ 79.6

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : As at December 31, 2017 At December 31, 2016 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 1,455.0 $ 1,482.8 Third party 2,425.3 2,380.8 Letters of credit / guarantees (1) 658.5 672.1 Investment commitment — real estate fund 100.0 — Total restricted assets $ 4,638.8 $ 4,535.7 Total as percent of investable assets (2) 53.4 % 49.3 % _______________ (1) As at December 31, 2017 , the Company had pledged funds of $658.5 million and £0.0 million ( December 31, 2016 — $672.0 million and £7.2 million ) as collateral for the secured letters of credit. (2) Investable assets comprise total investments, cash and cash equivalents, accrued interest, receivables for securities sold and payables for securities purchased.
Amounts Outstanding under Operating Leases	Amounts outstanding under operating leases net of subleases as at December 31, 2017 and 2016 were: As at December 31, 2017 2018 2019 2020 2021 2022 Later Total ($ in millions) Operating Lease Obligations $ 16.2 $ 16.1 $ 15.3 $ 11.0 $ 8.7 $ 78.7 $ 146.0 As at December 31, 2016 2017 2018 2019 2020 2021 Later Total ($ in millions) Operating Lease Obligations $ 16.1 $ 15.3 $ 14.0 $ 9.9 $ 8.7 $ 86.4 $ 150.4

Concentration of Credit Risk (T

Concentration of Credit Risk (Tables)	12 Months Ended
	Dec. 31, 2017
Risks and Uncertainties [Abstract]
Schedule Of Gross Written Premium From Major Brokers	The following table shows the largest brokers that the Company transacted business within the three years ended December 31, 2017 and the proportion of gross written premiums from each of those brokers. Twelve Months Ended December 31, 2017 2016 2015 (in percentages) Aon Corporation 16.4 % 18.4 % 18.7 % Marsh & McLennan Companies, Inc. 16.0 14.7 15.4 Willis Group Holdings, Ltd. 13.1 13.7 14.5 Other brokers/non-broker sources (1) 54.5 53.2 51.4 Total 100.0 % 100.0 % 100.0 % Gross written premiums ($ millions) $ 3,360.9 $ 3,147.0 $ 2,997.3 ______________ (1) No other individual broker accounted for more than 10% of total gross written premiums.

Reclassifications from Accumu53

Reclassifications from Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2017
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 December 31, 2017 and 2016 : Amount Reclassified from AOCI Details about the AOCI Components Twelve Months Ended December 31, 2017 Twelve Months Ended December 31, 2016 Affected Line Item in the Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gain on sale of securities $ 10.7 $ 18.8 Realized and unrealized investment gains Realized (losses) on sale of securities (6.7 ) (8.9 ) Realized and unrealized investment losses 4.0 9.9 (Loss)/income from operations before income tax Tax on net realized gains of securities (0.4 ) (1.0 ) Income tax expense $ 3.6 $ 8.9 Net income Realized derivatives: Net realized (losses) on settled derivatives 4.4 (8.7 ) General, administrative and corporate expenses Tax on settled derivatives (0.8 ) — Income tax expense $ 3.6 $ (8.7 ) Net income Total reclassifications from AOCI to the statement of operations, net of income tax $ 7.2 $ 0.2 Net income

Credit Facility and Long-term54

Credit Facility and Long-term Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Summary of Contractual Obligations Under Long-term Debts	The following table summarizes our contractual obligations under the long-term debts as at December 31, 2017 . Payments Due By Period Contractual Basis Less than 1-3 years 3-5 years More than 5 years Total ($ in millions) Long-term Debt Obligations $ — $ 250.0 $ — $ 300.0 $ 550.0

Unaudited Quarterly Financial55

Unaudited Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 , 2016 and 2015 . 2017 Quarter Ended March 31 Quarter Ended June 30 Quarter Ended September 30 Quarter Ended December 31 Year Ended December 31 Revenues ($ in millions) Net earned premium $ 581.1 $ 562.0 $ 652.5 $ 511.0 $ 2,306.6 Net investment income 47.7 47.4 46.4 47.5 189.0 Realized and unrealized investment gains 51.2 49.0 29.9 18.8 148.9 Other income 3.6 3.6 (2.2 ) 3.9 8.9 Total revenues 683.6 662.0 726.6 581.2 2,653.4 Expenses Losses and loss adjustment expenses 328.2 346.1 776.2 544.2 1,994.7 Amortization of deferred policy acquisition costs 113.7 96.3 105.4 85.1 400.5 General, administrative and corporate expenses 121.3 119.9 110.9 150.1 502.2 Interest on long-term debt 7.4 7.4 7.4 7.3 29.5 Change in fair value of derivatives (3.1 ) (17.6 ) (4.5 ) (2.5 ) (27.7 ) Change in fair value of loan notes issued by variable interest entities 2.9 3.3 (9.8 ) (17.6 ) (21.2 ) Realized and unrealized investment losses/(gains) 5.0 7.0 12.4 4.0 28.4 Net realized and unrealized foreign exchange losses/(gains) 8.9 20.6 (8.4 ) 2.8 23.9 Other expenses — 2.0 — 2.9 4.9 Total expenses 584.3 585.0 989.6 776.3 2,935.2 Income from operations before income tax 99.3 77.0 (263.0 ) (195.1 ) (281.8 ) Income tax (expense) (2.8 ) (1.2 ) 9.2 10.2 15.4 Net income $ 96.5 $ 75.8 $ (253.8 ) $ (184.9 ) $ (266.4 ) Per Share Data Weighted average number of ordinary share and share equivalents (1) Basic 59,862,662 59,966,358 59,759,730 59,431,469 59,753,886 Diluted 61,196,772 61,022,981 59,795,730 59,431,469 59,753,886 Basic earnings per ordinary share adjusted for preference share dividends $ 1.39 $ 1.09 $ (4.48 ) $ (3.25 ) $ (5.22 ) Diluted earnings per ordinary share adjusted for preference share dividends $ 1.36 $ 1.07 $ (4.48 ) $ (3.25 ) $ (5.22 ) _______________ (1) The basic and diluted number of ordinary shares for the three months ended September 30, 2017 and the three and twelve months ended December 31, 2017 is the same, as the inclusion of dilutive securities in a loss-making period would be anti-dilutive. 2016 Quarter Ended Quarter Ended Quarter Ended September 30 Quarter Ended December 31 Year Ended December 31 Revenues ($ in millions) Net earned premium $ 663.1 $ 680.8 $ 681.0 $ 612.4 $ 2,637.3 Net investment income 49.5 48.0 46.4 43.2 187.1 Realized and unrealized investment gains 65.6 45.1 26.7 (29.0 ) 108.4 Other income 1.4 (0.5 ) 1.5 3.3 5.7 Total revenues 779.6 773.4 755.6 629.9 2,938.5 Expenses Losses and loss adjustment expenses 357.4 442.2 389.2 387.3 1,576.1 Amortization of deferred policy acquisition costs 130.2 126.7 130.9 141.1 528.9 General, administrative and corporate expenses 119.8 116.4 125.0 128.9 490.1 Interest on long-term debt 7.4 7.4 7.3 7.4 29.5 Change in fair value of derivatives 7.2 0.4 (0.6 ) 17.6 24.6 Change in fair value of loan notes issued by variable interest entities 4.4 (0.5 ) 9.8 3.4 17.1 Realized and unrealized investment losses/(gains) 20.6 8.3 5.2 29.1 63.2 Net realized and unrealized foreign exchange losses/(gains) 15.7 5.3 (10.8 ) (12.0 ) (1.8 ) Other expenses — 1.0 (0.9 ) 1.2 1.3 Total expenses 662.7 707.2 655.1 704.0 2,729.0 Income from operations before income tax 116.9 66.2 100.5 (74.1 ) 209.5 Income tax (expense) (2.5 ) (1.3 ) (4.9 ) 2.6 (6.1 ) Net income $ 114.4 $ 64.9 $ 95.6 $ (71.5 ) $ 203.4 Per Share Data Weighted average number of ordinary share and share equivalents Basic 60,867,815 60,705,028 60,225,705 60,152,420 60,478,740 Diluted 62,483,938 62,192,142 61,577,018 61,198,041 61,860,689 Basic earnings per ordinary share adjusted for preference share dividends $ 1.73 $ 0.91 $ 1.43 $ (1.41 ) $ 2.67 Diluted earnings per ordinary share adjusted for preference share dividends $ 1.68 $ 0.89 $ 1.40 $ (1.41 ) $ 2.61 2015 Quarter Ended Quarter Ended Quarter Ended September 30 Quarter Ended December 31 Year Ended December 31 Revenues ($ in millions) Net earned premium $ 593.6 $ 609.4 $ 640.6 $ 629.7 $ 2,473.3 Net investment income 47.4 46.7 45.0 46.4 185.5 Realized and unrealized investment gains/(losses) (1) 57.4 13.5 10.7 12.9 94.5 Other income 3.9 (1.2 ) (2.3 ) (0.3 ) 0.1 Total revenues 702.3 668.4 694.0 688.7 2,753.4 Expenses Losses and loss adjustment expenses 306.1 360.5 365.6 334.0 1,366.2 Amortization of deferred policy acquisition costs 119.3 114.1 132.0 118.2 483.6 General, administrative and corporate expenses 102.2 95.4 100.5 125.9 424.0 Interest on long-term debt 7.4 7.3 7.4 7.4 29.5 Change in fair value of derivatives 7.8 (2.0 ) (10.1 ) (2.5 ) (6.8 ) Change in fair value of loan notes issued by variable interest entities 2.9 3.3 8.3 5.3 19.8 Realized and unrealized investment losses/(gains) (1) 14.5 28.8 51.9 (17.7 ) 77.5 Net realized and unrealized foreign exchange (gains)/losses (1) 6.4 11.6 8.4 (5.0 ) 21.4 Other expenses 2.6 (1.8 ) — (0.1 ) 0.7 Total expenses 569.2 617.2 664.0 565.5 2,415.9 Income from operations before income tax 133.1 51.2 30.0 123.2 337.5 Income tax (expense) (5.1 ) (2.2 ) (1.8 ) (5.3 ) (14.4 ) Net income $ 128.0 $ 49.0 $ 28.2 $ 117.9 $ 323.1 Per Share Data Weighted average number of ordinary share and share equivalents Basic 62,159,303 61,408,633 60,779,295 60,784,832 61,287,884 Diluted 63,532,662 62,896,907 62,155,125 62,176,505 62,687,503 Basic earnings per ordinary share adjusted for preference share dividends $ 1.91 $ 0.64 $ 0.30 $ 1.78 $ 4.64 Diluted earnings per ordinary share adjusted for preference share dividends $ 1.87 $ 0.62 $ 0.30 $ 1.75 $ 4.54 _______________ (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).

Basis of Preparation and Sign56

Basis of Preparation and Significant Accounting Policies - Additional Information (Details) $ in Millions		12 Months Ended
		Dec. 31, 2017USD ($)periodInvestment	Dec. 31, 2016USD ($)Investment	Dec. 31, 2015USD ($)
Property, Plant and Equipment [Line Items]
Threshold Period of Value Decline in Equity Securities to be considered Other than Temporary Impairment		12 months
Non-Payment Of Dividends, Number Of Periods | period		6
Number of investments in VIEs | Investment		2	3
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
Fixed maturities
Property, Plant and Equipment [Line Items]
Percentage of Value Decline in Securities to be considered Other than Temporary Impairment		20.00%
Accounting Standards Update 2016-09 | Retained earnings
Property, Plant and Equipment [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[1]	$ 2.8	$ 0	$ 0
Accounting Standards Update 2016-09 | Additional paid-in capital
Property, Plant and Equipment [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 7.9
[1]	The $2.8 million relates to the cumulative effect-adjustment to opening retained earnings as a result of the classification of restricted share units as equity following the adoption of ASU 2016-09. The adjustment has been applied using a modified retrospective approach.

Earnings Per Ordinary Share - C

Earnings Per Ordinary Share - Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	Jan. 03, 2017	Jul. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net (loss)/income			$ (184.9)	$ (253.8)	$ 75.8	$ 96.5	$ (71.5)	$ 95.6	$ 64.9	$ 114.4	$ 117.9	$ 28.2	$ 49	$ 128	$ (266.4)	$ 203.4	$ 323.1
Preference share dividends															(36.2)	(41.8)	(37.8)
Preference share redemption costs															(8)	0	0
Net profit attributable to non-controlling interest															(1.3)	(0.1)	(0.8)
Basic net income available to ordinary shareholders															(311.9)	161.5	284.5
Diluted net income available to ordinary shareholders															$ (311.9)	$ 284.5	$ 161.5
Ordinary shares:
Basic weighted average ordinary shares			59,431,469	59,759,730	59,966,358	59,862,662	60,152,420	60,225,705	60,705,028	60,867,815	60,784,832	60,779,295	61,408,633	62,159,303	59,753,886	60,478,740	61,287,884
Weighted average effect of dilutive securities															0	1,381,949	1,399,619
Total diluted weighted average ordinary shares			59,431,469	59,795,730	61,022,981	61,196,772	61,198,041	61,577,018	62,192,142	62,483,938	62,176,505	62,155,125	62,896,907	63,532,662	59,753,886	61,860,689	62,687,503
Earnings per ordinary share:
Basic, dollars per share			$ (3.25)	$ (4.48)	$ 1.09	$ 1.39	$ (1.41)	$ 1.43	$ 0.91	$ 1.73	$ 1.78	$ 0.30	$ 0.64	$ 1.91	$ (5.22)	$ 2.67	$ 4.64
Diluted, dollars per share			$ (3.25)	$ (4.48)	$ 1.07	$ 1.36	$ (1.41)	$ 1.40	$ 0.89	$ 1.68	$ 1.75	$ 0.30	$ 0.62	$ 1.87	$ (5.22)	$ 2.61	$ 4.54
7.401% Preference Shares
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage	7.401%														7.401%	7.401%
7.250% Preference Shares
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage		7.25%													7.25%	7.25%
Additional paid-in capital | Preference Shares
Earnings Per Share [Abstract]
Preference share redemption costs															$ 8
Class of Stock [Line Items]
Preference shares redeemed and cancelled															$ 293.2	$ 0	$ 0

Earnings Per Ordinary Share - S

Earnings Per Ordinary Share - Summary of Declared Dividends (Details) - $ / shares	Feb. 07, 2018	Sep. 20, 2016	Dec. 31, 2017	Dec. 31, 2016
5.950% Preference Shares
Dividends Payable [Line Items]
Preference shares, rate			5.95%	5.95%
5.625% Preference Shares
Dividends Payable [Line Items]
Preference shares, rate		5.625%	5.625%	5.625%
Subsequent Event | Ordinary shares
Dividends Payable [Line Items]
Dividend	$ 0.24
Subsequent Event | 5.950% Preference Shares
Dividends Payable [Line Items]
Dividend	$ 0.3719
Preference shares, rate	5.95%
Subsequent Event | 5.625% Preference Shares
Dividends Payable [Line Items]
Dividend	$ 0.3516
Preference shares, rate	5.625%

Segment Reporting - Additional

Segment Reporting - Additional Information (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017segement
Segment Reporting [Abstract]
Number of business segments	2

Segment Reporting - Summary of

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 ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Gross written premiums													$ 3,360.9	$ 3,147	$ 2,997.3
Net written premiums													2,212.5	2,593.7	2,646.2
Gross earned premiums													3,209.2	3,086.3	2,856.8
Net earned premium	$ 511	$ 652.5	$ 562	$ 581.1	$ 612.4	$ 681	$ 680.8	$ 663.1	$ 629.7	$ 640.6	$ 609.4	$ 593.6	2,306.6	2,637.3	2,473.3
Losses and loss adjustment expenses	544.2	776.2	346.1	328.2	387.3	389.2	442.2	357.4	334	365.6	360.5	306.1	1,994.7	1,576.1	1,366.2
Amortization of deferred policy acquisition costs	85.1	105.4	96.3	113.7	141.1	130.9	126.7	130.2	118.2	132	114.1	119.3	400.5	528.9	483.6
General and administrative expenses													411.2	406.6	360.1
Underwriting income (loss)													(499.8)	125.7	263.4
Corporate expenses													(58.3)	(73.8)	(63.9)
Non-operating expenses													(32.7)	(9.7)
Net investment income	47.5	46.4	47.4	47.7	43.2	46.4	48	49.5	46.4	45	46.7	47.4	189	187.1	185.5
Realized and unrealized investment gains	18.8	29.9	49	51.2	(29)	26.7	45.1	65.6	12.9	10.7	13.5	57.4	148.9	108.4	94.5
Realized and unrealized investment losses	(4)	(12.4)	(7)	(5)	(29.1)	(5.2)	(8.3)	(20.6)	17.7	(51.9)	(28.8)	(14.5)	(28.4)	(63.2)	(77.5)
Change in fair value of loan notes issued by variable interest entities	17.6	9.8	(3.3)	(2.9)	(3.4)	(9.8)	0.5	(4.4)	(5.3)	(8.3)	(3.3)	(2.9)	21.2	(17.1)	(19.8)
Change in fair value of derivatives	2.5	4.5	17.6	3.1	(17.6)	0.6	(0.4)	(7.2)	2.5	10.1	2	(7.8)	27.7	(24.6)	6.8
Interest expense on long term debt	(7.3)	(7.4)	(7.4)	(7.4)	(7.4)	(7.3)	(7.4)	(7.4)	(7.4)	(7.4)	(7.3)	(7.4)	(29.5)	(29.5)	(29.5)
Net realized and unrealized foreign exchange (losses)	(2.8)	8.4	(20.6)	(8.9)	12	10.8	(5.3)	(15.7)	5	(8.4)	(11.6)	(6.4)	(23.9)	1.8	(21.4)
Other income	3.9	(2.2)	3.6	3.6	3.3	1.5	(0.5)	1.4	(0.3)	(2.3)	(1.2)	3.9	8.9	5.7	0.1
Other expenses													(4.9)	(1.3)	(0.7)
(Loss)/income from operations before income tax	(195.1)	(263)	77	99.3	(74.1)	100.5	66.2	116.9	123.2	30	51.2	133.1	(281.8)	209.5	337.5
Income tax credit/(expense)	10.2	9.2	(1.2)	(2.8)	2.6	(4.9)	(1.3)	(2.5)	(5.3)	(1.8)	(2.2)	(5.1)	15.4	(6.1)	(14.4)
Net (loss)/income	(184.9)	$ (253.8)	$ 75.8	$ 96.5	(71.5)	$ 95.6	$ 64.9	$ 114.4	117.9	$ 28.2	$ 49	$ 128	(266.4)	203.4	323.1
Losses and loss adjustment expenses	6,749.5				5,319.9				4,938.2				6,749.5	5,319.9	4,938.2	$ 4,750.8
Net Reserves for Losses and LAE	5,234.3				4,759.2				4,583.4				$ 5,234.3	$ 4,759.2	$ 4,583.4	$ 4,400.8
Ratios
Loss ratio													86.50%	59.80%	55.20%
Policy acquisition expense ratio													17.40%	20.10%	19.60%
General and administrative expense ratio													21.80%	18.60%	17.10%
Expense ratio													39.20%	38.70%	36.70%
Combined ratio													125.70%	98.50%	91.90%
Reinsurance
Segment Reporting Information [Line Items]
Net written premiums													$ 1,250	$ 1,269.2	$ 1,153.5
Net earned premium													1,206.1	1,181.9	1,072.6
Losses and loss adjustment expenses	2,917.1				2,462.1				2,409.5				2,917.1	2,462.1	2,409.5
Net Reserves for Losses and LAE	2,917.1				2,462.1				2,409.5				$ 2,917.1	$ 2,462.1	$ 2,409.5
Ratios
Loss ratio													92.60%	55.70%	45.80%
Policy acquisition expense ratio													19.50%	19.20%	20.90%
General and administrative expense ratio													13.00%	15.10%	13.70%
Expense ratio													32.50%	34.30%	34.60%
Combined ratio													125.10%	90.00%	80.40%
Insurance
Segment Reporting Information [Line Items]
Net written premiums													$ 962.5	$ 1,324.5	$ 1,492.7
Net earned premium													1,100.5	1,455.4	1,400.7
Losses and loss adjustment expenses	2,317.2				2,297.1				2,173.9				2,317.2	2,297.1	2,173.9
Net Reserves for Losses and LAE	$ 2,317.2				$ 2,297.1				$ 2,173.9				$ 2,317.2	$ 2,297.1	$ 2,173.9
Ratios
Loss ratio													79.80%	63.10%	62.40%
Policy acquisition expense ratio													15.00%	20.80%	18.50%
General and administrative expense ratio													23.10%	15.70%	15.20%
Expense ratio													38.10%	36.50%	33.70%
Combined ratio													117.90%	99.60%	96.10%
Operating Segments | Reinsurance
Segment Reporting Information [Line Items]
Gross written premiums													$ 1,548.5	$ 1,413.2	$ 1,248.9
Net written premiums													1,250	1,269.2	1,153.5
Gross earned premiums													1,451.8	1,317.9	1,153.5
Net earned premium													1,206.1	1,181.9	1,072.6
Losses and loss adjustment expenses													1,116.4	657.9	491.6
Amortization of deferred policy acquisition costs													235.5	226.4	224.7
General and administrative expenses													157.3	178.2	146.5
Underwriting income (loss)													(303.1)	119.4	209.8
Operating Segments | Insurance
Segment Reporting Information [Line Items]
Gross written premiums													1,812.4	1,733.8	1,748.4
Net written premiums													962.5	1,324.5	1,492.7
Gross earned premiums													1,757.4	1,768.4	1,703.3
Net earned premium													1,100.5	1,455.4	1,400.7
Losses and loss adjustment expenses													878.3	918.2	874.6
Amortization of deferred policy acquisition costs													165	302.5	258.9
General and administrative expenses													253.9	228.4	213.6
Underwriting income (loss)													$ (196.7)	$ 6.3	$ 53.6

Segment Reporting - Summary o61

Segment Reporting - Summary of the Schedule of the Company's gross written premiums based on the location of the insured risk (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Gross written premiums	$ 3,360.9	$ 3,147	$ 2,997.3
Australia/Asia
Segment Reporting Information [Line Items]
Gross written premiums	167.3	140.5	140
Caribbean
Segment Reporting Information [Line Items]
Gross written premiums	17.6	14.3	20.3
Europe
Segment Reporting Information [Line Items]
Gross written premiums	94.5	109.7	113.6
United Kingdom
Segment Reporting Information [Line Items]
Gross written premiums	258.3	231.4	223.6
United States & Canada
Segment Reporting Information [Line Items]
Gross written premiums	1,729.3	1,597	1,479.5
Worldwide excluding United States
Segment Reporting Information [Line Items]
Gross written premiums	88.1	90.7	107.2
Worldwide including United States
Segment Reporting Information [Line Items]
Gross written premiums	868.6	837.2	793.6
Others
Segment Reporting Information [Line Items]
Gross written premiums	$ 137.2	$ 126.2	$ 119.5

Investments - Summary of Invest

Investments - Summary of Investment Income (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													$ 200.1	$ 199.1	$ 197.3
Investment expenses													(11.1)	(12)	(11.8)
Net investment income	$ 47.5	$ 46.4	$ 47.4	$ 47.7	$ 43.2	$ 46.4	$ 48	$ 49.5	$ 46.4	$ 45	$ 46.7	$ 47.4	189	187.1	185.5
Fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													133.3	141.3	143.4
Fixed income securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													44	31.6	27.8
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													0.4	0.6	1.1
Short-term Investment, Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													0.8	0.2	0
Fixed term deposits (included in cash and cash equivalents)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													6.2	3.4	3
Equity securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													0	0	0.1
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													13.6	20.4	20
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income													$ 1.8	$ 1.6	$ 1.9

Investments - Net Realized and

Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Other-than-temporary impairments	$ (0.7)	$ 0	$ 0
Catastrophe bonds — net unrealized (losses)/gains	(2.4)	0	(0.3)
Net change in gross unrealized gains (losses)	60.3	22.5	(33.1)
Total net realized and unrealized investment gains recorded in the statement of operations	120.5	45.2	17
Change in available for sale net unrealized gains:
Fixed income securities	(14.8)	(39)	(82.2)
Short-term investments	0	0	(0.1)
Equity securities	0	0	(27.3)
Total change in pre-tax available for sale unrealized gains	(14.8)	(39)	(109.6)
Change in taxes	2	1.3	4.4
Total change in net unrealized gains, net of taxes recorded in other comprehensive income	(12.8)	(37.7)	(105.2)
Fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Available for sale securities — gross realized gains	10.2	18.6	11.7
Available for sale securities — gross realized (losses)	(6.6)	(8.3)	(2.7)
Cash and cash equivalents — gross realized gains	0.4	0.2	0
Cash and cash equivalents — gross realized (losses)	(0.1)	(0.6)	0
Equity securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Available for sale securities — gross realized gains	0	0	31.9
Available for sale securities — gross realized (losses)	0	0	(3)
Trading securities — gross realized gains	59	54.1	46
Trading securities — gross realized (losses)	(13.7)	(46.3)	(31.7)
Fixed income securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Cash and cash equivalents — gross realized gains	1.3	0.1	0
Cash and cash equivalents — gross realized (losses)	0	(0.3)	0
Trading securities — gross realized gains	9.7	12.6	4.9
Trading securities — gross realized (losses)	(4.5)	(7.3)	(6.1)
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Available for sale securities — gross realized gains	0.1	0	0
Trading securities — gross realized gains	2.7	0	0
MVI
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Loss from equity method investment	0.1	0.3	0
Chaspark
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Loss from equity method investment	(0.9)	(0.3)	0.6
Bene
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Loss from equity method investment	0.3	0.1	0
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | AgriLogic
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross realized gain on sale of AgriLogic	$ 4.3	$ 0	$ 0

Investments - Cost, Gross Unrea

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 ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	$ 5,291.2	$ 5,765.4
Gross Unrealized Gains	63.6	85.3
Gross Unrealized Losses	(33.9)	(40.8)
Fair Market Value	5,320.9	5,809.9
Total fixed income securities — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	5,201.2	5,620.1
Gross Unrealized Gains	63.6	85.3
Gross Unrealized Losses	(33.8)	(40.8)
Fair Market Value	5,231	5,664.6
Total short-term investments — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	90	145.3
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.1)	0
Fair Market Value	89.9	145.3
U.S. government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,166.5	1,207.9
Gross Unrealized Gains	4.5	9.4
Gross Unrealized Losses	(11.6)	(11.2)
Fair Market Value	1,159.4	1,206.1
U.S. agency
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	51.8	117.7
Gross Unrealized Gains	0.5	1.9
Gross Unrealized Losses	(0.2)	0
Fair Market Value	52.1	119.6
Municipal
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	53	23.2
Gross Unrealized Gains	2.1	1.6
Gross Unrealized Losses	(0.2)	(0.4)
Fair Market Value	54.9	24.4
Corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	2,391.4	2,566.9
Gross Unrealized Gains	36.1	39.6
Gross Unrealized Losses	(11.8)	(20)
Fair Market Value	2,415.7	2,586.5
Non-U.S. government-backed corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	91.5	89.2
Gross Unrealized Gains	0.3	0.7
Gross Unrealized Losses	(0.5)	(0.1)
Fair Market Value	91.3	89.8
Non-U.S government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	479.7	477.7
Gross Unrealized Gains	6.4	11.8
Gross Unrealized Losses	(1.2)	(0.8)
Fair Market Value	484.9	488.7
Asset-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	26.3	62.6
Gross Unrealized Gains	0	0.4
Gross Unrealized Losses	(0.1)	0
Fair Market Value	26.2	63
Non-agency commercial mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	0	12.3
Gross Unrealized Gains	0	0.3
Gross Unrealized Losses	0	0
Fair Market Value	0	12.6
Agency mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	941	1,062.6
Gross Unrealized Gains	13.7	19.6
Gross Unrealized Losses	(8.2)	(8.3)
Fair Market Value	$ 946.5	$ 1,073.9

Investments - Cost, Gross Unr65

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Items [Line Items]
Trading Securities	$ 1,649.3	$ 1,265.7
Fixed income securities — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,634.9	1,264.8
Gross Unrealized Gains	21.5	14
Gross Unrealized Losses	(7.1)	(13.1)
Trading Securities	1,649.3	1,265.7
Short-term Investments
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	73	185.4
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Trading Securities	73	185.4
Total equity securities — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	414.8	554.3
Gross Unrealized Gains	83.5	55.4
Gross Unrealized Losses	(7.3)	(25)
Trading Securities	491	584.7
Total catastrophe bonds — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	33.5	42.5
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1.1)	0
Trading Securities	32.4	42.5
Total
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	2,156.2	2,047
Gross Unrealized Gains	105	69.4
Gross Unrealized Losses	(15.5)	(38.1)
Trading Securities	2,245.7	2,078.3
U.S. government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	162.3	82.8
Gross Unrealized Gains	0.4	0.4
Gross Unrealized Losses	(0.8)	(0.8)
Trading Securities	161.9	82.4
Municipal
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	32.4	15.7
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.2)	(0.2)
Trading Securities	32.2	15.5
Corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,036.5	817.8
Gross Unrealized Gains	14	9.9
Gross Unrealized Losses	(4.2)	(7.1)
Trading Securities	1,046.3	820.6
Non-U.S. government-backed corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Trading Securities	1
Non-U.S government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	196.1	203.4
Gross Unrealized Gains	6.9	3.5
Gross Unrealized Losses	(0.5)	(4.1)
Trading Securities	202.5	202.8
Asset-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	9.9	14.5
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Trading Securities	9.9	14.5
Agency mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	196.7	130.6
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	(1.4)	(0.9)
Trading Securities	$ 195.5	$ 129.9

Investments - Other Investments

Investments - Other Investments (Details) - USD ($) $ in Millions	Jul. 26, 2016	Jan. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance			$ 12.1	$ 8.9
Investment in the period			64.9	3.3
Goodwill			(1.8)
Proceeds from sale of equity method investments			(9.3)	0	$ 0
Unrealized gain/(loss) for the period			0.5	(0.1)
Closing value of investment, ending balance			66.4	12.1	8.9
MVI
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance			0.5	0.8
Investment in the period		$ 0.8	0.1	0
Goodwill			0
Proceeds from sale of equity method investments			0
Unrealized gain/(loss) for the period			(0.1)	(0.3)
Closing value of investment, ending balance			0.5	0.5	0.8
Chaspark
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance			8.4	8.1
Investment in the period			0	0
Goodwill			0
Proceeds from sale of equity method investments			(9.3)
Unrealized gain/(loss) for the period			0.9	0.3
Closing value of investment, ending balance			0	8.4	8.1
Bene
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance			3.2	0
Investment in the period	$ 3.3		0	3.3
Goodwill			0
Proceeds from sale of equity method investments			0
Unrealized gain/(loss) for the period			(0.3)	(0.1)
Closing value of investment, ending balance			2.9	3.2	0
Digital Re
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance			0	0
Investment in the period			2.3	0
Goodwill			(1.8)
Proceeds from sale of equity method investments			0
Unrealized gain/(loss) for the period			0	0
Closing value of investment, ending balance			0.5	0	0
Crop Re
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment, beginning balance			0	0
Investment in the period			62.5	0
Goodwill			0
Proceeds from sale of equity method investments			0
Unrealized gain/(loss) for the period			0	0
Closing value of investment, ending balance			$ 62.5	$ 0	$ 0

Investments - Summary of Fixed

Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	$ 5,201.2	$ 5,620.1
Cost or Amortized Cost	5,291.2	5,765.4
Available for sale investments in fixed income maturities, Fair Market Value	5,231	5,664.6
Total fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
Due one year or less, Cost or Amortized Cost	561.7	567.2
Due after one year through five years, Cost or Amortized Cost	2,486.7	2,643.7
Due after five years through ten years, Cost or Amortized Cost	1,092.2	1,172.3
Due after ten years, Cost or Amortized Cost	93.3	99.4
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	4,233.9	4,482.6
Due one year or less, Fair Market Value	562.4	570
Due after one year through five years, Fair Market Value	2,492.2	2,671.9
Due after five years through ten years, Fair Market Value	1,097.4	1,168.1
Due after ten years, Fair Market Value	106.3	105.1
Available for sale investments in fixed income maturities, Fair Market Value	4,258.3	4,515.1
Non-agency commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost		12.3
Available for sale investments in fixed income maturities, Fair Market Value		12.6
Agency mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	941	1,062.6
Available for sale investments in fixed income maturities, Fair Market Value	946.5	1,073.9
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	26.3	62.6
Available for sale investments in fixed income maturities, Fair Market Value	26.2	63
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost or Amortized Cost	5,201.2	5,620.1
Available for sale investments in fixed income maturities, Fair Market Value	$ 5,231	$ 5,664.6

Investments - Aggregate Fair Va

Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions	Dec. 31, 2017USD ($)securitySecurity	Dec. 31, 2016USD ($)securitySecurity
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1,965.3	$ 2,518.2
0-12 months, Gross Unrealized Losses	(12)	(39.3)
Over 12 months, Fair Market Value	984.3	37
Over 12 months, Gross Unrealized Losses	(21.9)	(1.5)
Total, Fair Market Value	2,949.6	2,555.2
Total, Gross Unrealized Losses	$ (33.9)	$ (40.8)
Number of Securities | security	769	679
Fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1,918.4	$ 2,517.1
0-12 months, Gross Unrealized Losses	(11.9)	(39.3)
Over 12 months, Fair Market Value	984.3	37
Over 12 months, Gross Unrealized Losses	(21.9)	(1.5)
Total, Fair Market Value	2,902.7	2,554.1
Total, Gross Unrealized Losses	$ (33.8)	$ (40.8)
Number of Securities | Security	761	677
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 46.9	$ 1.1
0-12 months, Gross Unrealized Losses	(0.1)	0
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Losses	0	0
Total, Fair Market Value	46.9	1.1
Total, Gross Unrealized Losses	$ (0.1)	$ 0
Number of Securities | Security	8	2
U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 652.1	$ 724.4
0-12 months, Gross Unrealized Losses	(5.1)	(11.2)
Over 12 months, Fair Market Value	259.8	0
Over 12 months, Gross Unrealized Losses	(6.5)	0
Total, Fair Market Value	911.9	724.4
Total, Gross Unrealized Losses	$ (11.6)	$ (11.2)
Number of Securities | security	101	78
U.S. agency
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 20.1	$ 14.1
0-12 months, Gross Unrealized Losses	(0.2)	0
Over 12 months, Fair Market Value	6.1	0
Over 12 months, Gross Unrealized Losses	0	0
Total, Fair Market Value	26.2	14.1
Total, Gross Unrealized Losses	$ (0.2)	$ 0
Number of Securities | security	10	4
Municipal
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 28.5	$ 7.7
0-12 months, Gross Unrealized Losses	(0.2)	(0.2)
Over 12 months, Fair Market Value	0	0.8
Over 12 months, Gross Unrealized Losses	0	(0.2)
Total, Fair Market Value	28.5	8.5
Total, Gross Unrealized Losses	$ (0.2)	$ (0.4)
Number of Securities | security	9	6
Corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 699.3	$ 1,044.4
0-12 months, Gross Unrealized Losses	(3.4)	(19.4)
Over 12 months, Fair Market Value	360.7	6.6
Over 12 months, Gross Unrealized Losses	(8.4)	(0.6)
Total, Fair Market Value	1,060	1,051
Total, Gross Unrealized Losses	$ (11.8)	$ (20)
Number of Securities | security	412	386
Non-U.S. government-backed corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 43.5	$ 29.6
0-12 months, Gross Unrealized Losses	(0.3)	(0.1)
Over 12 months, Fair Market Value	13.3	0
Over 12 months, Gross Unrealized Losses	(0.2)	0
Total, Fair Market Value	56.8	29.6
Total, Gross Unrealized Losses	$ (0.5)	$ (0.1)
Number of Securities | security	15	11
Non-U.S government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 206.2	$ 143.5
0-12 months, Gross Unrealized Losses	(0.8)	(0.8)
Over 12 months, Fair Market Value	32	1
Over 12 months, Gross Unrealized Losses	(0.4)	0
Total, Fair Market Value	238.2	144.5
Total, Gross Unrealized Losses	$ (1.2)	$ (0.8)
Number of Securities | security	47	29
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 11.1	$ 25.8
0-12 months, Gross Unrealized Losses	0	0
Over 12 months, Fair Market Value	10.5	1.4
Over 12 months, Gross Unrealized Losses	(0.1)	0
Total, Fair Market Value	21.6	27.2
Total, Gross Unrealized Losses	$ (0.1)	$ 0
Number of Securities | security	11	15
Agency mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 257.6	$ 527.6
0-12 months, Gross Unrealized Losses	(1.9)	(7.6)
Over 12 months, Fair Market Value	301.9	27.2
Over 12 months, Gross Unrealized Losses	(6.3)	(0.7)
Total, Fair Market Value	559.5	554.8
Total, Gross Unrealized Losses	$ (8.2)	$ (8.3)
Number of Securities | security	156	148

Investments - Additional Inform

Investments - Additional Information (Narrative) (Details) $ in Millions	Dec. 18, 2017USD ($)	Jul. 26, 2016USD ($)	Jan. 31, 2015USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)municipal_security	Dec. 31, 2015USD ($)	Jan. 01, 2017USD ($)	Oct. 02, 2012
Gain (Loss) on Investments [Line Items]
Held capital requirement, percentage				20.00%	100.00%
Catastrophe bonds, trading, cost				$ 32.4	$ 42.5
Investment in the period				64.9	3.3
Proceeds from sale of equity method investments				(9.3)	0	$ 0
Equity method investment, aggregate cost				66.4	$ 12.1	8.9
Investment funded				0
Number of municipal holdings | municipal_security					1
Municipal holdings, fair value					$ 0.1
Other-than-temporary impairment charge				$ 0.7	$ 0	0
Equity securities
Gain (Loss) on Investments [Line Items]
Percentage of investment				5.80%
BBB Emerging Market Debt
Gain (Loss) on Investments [Line Items]
Percentage of investment				4.00%
Risk Asset Portfolio Cash
Gain (Loss) on Investments [Line Items]
Percentage of investment				0.20%
BBB Emerging Market Debt And Risk Portfolio Cash
Gain (Loss) on Investments [Line Items]
Percentage of investment				10.00%	10.80%
Bene
Gain (Loss) on Investments [Line Items]
Investment in the period		$ 3.3		$ 0	$ 3.3
Gross realized and unrealized gains (loss)				(0.3)	(0.1)	0
Proceeds from sale of equity method investments				0
Equity method investment, ownership percentage		20.00%
Equity method investment, aggregate cost				2.9	3.2	0
Chaspark
Gain (Loss) on Investments [Line Items]
Investment in the period				0	0
Percentage of ownership acquired								58.50%
Gross realized and unrealized gains (loss)				0.9	0.3	(0.6)
Proceeds from sale of equity method investments				(9.3)
Equity method investment, aggregate cost				0	8.4	8.1
MVI
Gain (Loss) on Investments [Line Items]
Investment in the period			$ 0.8	0.1	0
Gross realized and unrealized gains (loss)				(0.1)	(0.3)	0
Proceeds from sale of equity method investments				0
Equity method investment, aggregate cost				0.5	0.5	0.8
Digital Re
Gain (Loss) on Investments [Line Items]
Investment in the period				2.3	0
Proceeds from sale of equity method investments				0
Equity method investment, ownership percentage							49.00%
Equity method investment, aggregate cost				0.5	0	0	$ 2.3
Crop Re
Gain (Loss) on Investments [Line Items]
Investment in the period				62.5	0
Proceeds from sale of equity method investments				0
Equity method investment, ownership percentage	23.20%
Equity method investment, aggregate cost	$ 62.5			$ 62.5	0	$ 0
Equity method investment, cash payment	$ 5.9
Aspen Bermuda Limited [Member]
Gain (Loss) on Investments [Line Items]
Transfer from Investments					$ 95.1
CGB Insurance Diversified Services, Inc. | Crop Re
Gain (Loss) on Investments [Line Items]
Equity method investment, counterparty ownership percentage	76.80%

Variable Interest Entities Summ

Variable Interest Entities Summary of Silverton Loan Notes (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Silverton Loan Notes [Roll Forward]
Opening balance				$ 277.9				$ 235					$ 277.9	$ 235
Total change in fair value for the period	$ 17.6	$ 9.8	$ (3.3)	(2.9)	$ (3.4)	$ (9.8)	$ 0.5	(4.4)	$ (5.3)	$ (8.3)	$ (3.3)	$ (2.9)	21.2	(17.1)	$ (19.8)
Total distributed in the period													(144.2)	(108.5)
Total issued in the period														130
Closing balance as at December 31, 2017	107.2				277.9				235				107.2	277.9	235
Loan notes (long-term liabilities)	54.7				142								54.7	142
Accrued expenses (current liabilities)	52.5				135.9								52.5	135.9
Third party
Movement in Silverton Loan Notes [Roll Forward]
Opening balance				223.4				190.6					223.4	190.6
Total change in fair value for the period													(21.2)	17.1
Total distributed in the period													(115.6)	(89.3)
Total issued in the period														105
Closing balance as at December 31, 2017	86.6				223.4				190.6				86.6	223.4	190.6
Loan notes (long-term liabilities)	44.2				115								44.2	115
Accrued expenses (current liabilities)	42.4				108.4								42.4	108.4
Aspen Holdings
Movement in Silverton Loan Notes [Roll Forward]
Opening balance				$ 54.5				$ 44.4					54.5	44.4
Total change in fair value for the period													(5.3)	4.3
Total distributed in the period													(28.6)	(19.2)
Total issued in the period													0	25
Closing balance as at December 31, 2017	20.6				54.5				$ 44.4				20.6	54.5	$ 44.4
Loan notes (long-term liabilities)	10.5				27								10.5	27
Accrued expenses (current liabilities)	$ 10.1				$ 27.5								10.1	27.5
Aspen Holdings | Third party
Movement in Silverton Loan Notes [Roll Forward]
Total change in fair value for the period													$ (26.5)	$ 21.4

Variable Interest Entities - Na

Variable Interest Entities - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)Investment	Dec. 31, 2016USD ($)Investment	Dec. 31, 2015USD ($)
Variable Interest Entity [Line Items]
Number of investments in VIEs | Investment	2	3
Proceeds from sale of equity method investments	$ (9.3)	$ 0	$ 0
Chaspark
Variable Interest Entity [Line Items]
Proceeds from sale of equity method investments	$ (9.3)

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value	$ 33.5	$ 42.5
Derivatives at fair value	6.4	7.2
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued by variable interest entities, at fair value	(44.2)	(115)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)	(42.4)	(108.4)
Total	7,485.4	7,653.6
Recurring | Equity investments trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	491	584.7
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	6.4	7.2
Recurring | Liabilities under derivative contracts | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	(1)	(18.4)
Recurring | Total fixed income securities — Available for sale | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,159.4	1,206.1
Recurring | Total fixed income securities — Available for sale | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	52.1	119.6
Recurring | Total fixed income securities — Available for sale | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	54.9	24.4
Recurring | Total fixed income securities — Available for sale | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,415.7	2,586.5
Recurring | Total fixed income securities — Available for sale | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	91.3	89.8
Recurring | Total fixed income securities — Available for sale | Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	484.9	488.7
Recurring | Total fixed income securities — Available for sale | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	26.2	63
Recurring | Total fixed income securities — Available for sale | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		12.6
Recurring | Total fixed income securities — Available for sale | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	946.5	1,073.9
Recurring | Fixed income securities — Available for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5,231	5,664.6
Recurring | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	89.9	145.3
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	1,649.3	1,265.7
Catastrophe bonds trading, at fair value	32.4	42.5
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	161.9	82.4
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	32.2	15.5
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	1,046.3	820.6
Recurring | Held for trading 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	1
Recurring | Held for trading financial assets, at fair value | Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	202.5	202.8
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	9.9	14.5
Recurring | Held for trading financial assets, at fair value | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	195.5	129.9
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued by variable interest entities, at fair value	0	0
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)	0	0
Total	2,338.3	2,411.6
Recurring | Level 1 | Equity investments trading
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	491	584.7
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 | Liabilities under derivative 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 | Total fixed income securities — Available for sale | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,159.4	1,206.1
Recurring | Level 1 | Total fixed income securities — Available for sale | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Total fixed income securities — Available for sale | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Total fixed income securities — Available for sale | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Total fixed income securities — Available for sale | 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 | Total fixed income securities — Available for sale | Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	341.2	343.7
Recurring | Level 1 | Total fixed income securities — Available for sale | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 1 | Total fixed income securities — Available for sale | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Recurring | Level 1 | Total fixed income securities — Available for sale | 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	1,500.6	1,549.8
Recurring | Level 1 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	87.3	118.6
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	186.4	82.4
Catastrophe bonds trading, at fair value	0	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	161.9	82.4
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 | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0
Recurring | Level 1 | Held for trading financial assets, at fair value | Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	24.5	0
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 | Agency mortgage-backed
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]
Loan notes issued by variable interest entities, at fair value	0	0
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)	0	0
Total	5,233.7	5,465.4
Recurring | Level 2 | Equity investments trading
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	6.4	7.2
Recurring | Level 2 | Liabilities under derivative contracts | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value	(1)	(18.4)
Recurring | Level 2 | Total fixed income securities — Available for sale | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 2 | Total fixed income securities — Available for sale | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	52.1	119.6
Recurring | Level 2 | Total fixed income securities — Available for sale | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	54.9	24.4
Recurring | Level 2 | Total fixed income securities — Available for sale | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,415.7	2,586.5
Recurring | Level 2 | Total fixed income securities — Available for sale | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	91.3	89.8
Recurring | Level 2 | Total fixed income securities — Available for sale | Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	143.7	145
Recurring | Level 2 | Total fixed income securities — Available for sale | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	26.2	63
Recurring | Level 2 | Total fixed income securities — Available for sale | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		12.6
Recurring | Level 2 | Total fixed income securities — Available for sale | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	946.5	1,073.9
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	3,730.4	4,114.8
Recurring | Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2.6	26.7
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	1,462.9	1,183.3
Catastrophe bonds trading, at fair value	32.4	42.5
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	0
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	32.2	15.5
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	1,046.3	820.6
Recurring | Level 2 | Held for trading 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	1
Recurring | Level 2 | Held for trading financial assets, at fair value | Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	178	202.8
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	9.9	14.5
Recurring | Level 2 | Held for trading financial assets, at fair value | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	195.5	129.9
Recurring | 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	(44.2)	(115)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)	(42.4)	(108.4)
Total	(86.6)	(223.4)
Recurring | Level 3 | Equity investments trading
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 | Liabilities under derivative 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 | Total fixed income securities — Available for sale | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Total fixed income securities — Available for sale | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Total fixed income securities — Available for sale | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Total fixed income securities — Available for sale | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Total fixed income securities — Available for sale | 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 | Total fixed income securities — Available for sale | Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Total fixed income securities — Available for sale | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Recurring | Level 3 | Total fixed income securities — Available for sale | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		0
Recurring | Level 3 | Total fixed income securities — Available for sale | 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
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	0	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	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 | Non-U.S. government-backed corporate
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 | Non-U.S 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 | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0
Fixed income securities — Trading | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	73	185.4
Fixed income securities — Trading | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	73	76.1
Fixed income securities — Trading | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	109.3
Fixed income securities — Trading | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	$ 0	$ 0

Fair Value Measurements - Recon

Fair Value Measurements - Reconciliation of Level 3 Inputs (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Loan notes issued during the period				$ (130)
Recurring
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities measured using level 3 inputs classified as other payables			$ (42.4)	(108.4)
Recurring | Level 3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liabilities measured using level 3 inputs classified as other payables			(42.4)	(108.4)
Silverton 2,016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Loan notes issued during the period	$ (125)
Silverton 2016 | Recurring | Level 3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the beginning of the period			223.4
Distributed to third party			(115.6)
Loan notes issued during the period			0
Total change in fair value included in the statement of operations			(21.2)
Balance at the end of the period			(86.6)	223.4
Silverton 2,015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Loan notes issued during the period		$ (85)
Silverton 2015 | Recurring | Level 3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at the beginning of the period			$ (223.4)	(190.6)
Distributed to third party				(89.3)
Loan notes issued during the period				(105)
Total change in fair value included in the statement of operations				(17.1)
Balance at the end of the period	$ (190.6)			$ (223.4)

Fair Value Measurements - Prici

Fair Value Measurements - Pricing Sources Used in Pricing Fixed Income Investments (Details)	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Index providers	84.00%	87.00%
Pricing services	11.00%	7.00%
Broker-dealers	5.00%	6.00%
Total	100.00%	100.00%

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 6,254.6	$ 6,524.1
% of Total Fair Value from Index Providers	85.00%	88.00%
U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 1,321.3	$ 1,288.2
% of Total Fair Value from Index Providers	100.00%	100.00%
U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 43.4	$ 110.2
% of Total Fair Value from Index Providers	83.00%	92.00%
Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 37.4	$ 28.8
% of Total Fair Value from Index Providers	43.00%	72.00%
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 3,299.6	$ 3,275.3
% of Total Fair Value from Index Providers	83.00%	96.00%
Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 44	$ 44.8
% of Total Fair Value from Index Providers	48.00%	50.00%
Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 399.4	$ 455.6
% of Total Fair Value from Index Providers	58.00%	72.00%
Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 13.5	$ 32.1
% of Total Fair Value from Index Providers	37.00%	41.00%
Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 0	$ 12.5
% of Total Fair Value from Index Providers	0.00%	98.00%
Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 605	$ 691.9
% of Total Fair Value from Index Providers	53.00%	58.00%
Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 5,763.6	$ 5,939.4
% of Total Fair Value from Index Providers	84.00%	87.00%
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 491	$ 584.7
% of Total Fair Value from Index Providers	100.00%	100.00%

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) $ in Millions	3 Months Ended			12 Months Ended
	Dec. 31, 2016USD ($)$ / Investment	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2017USD ($)$ / Investment	Dec. 31, 2016USD ($)$ / Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Quotes per fixed income investment | $ / Investment	2.1			2	2.1
Quotes per equity investment | $ / Investment	3.9			4	3.9
Loan notes issued during the period					$ (130)
Silverton 2,015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued during the period			$ (85)
Silverton 2,016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued during the period		$ (125)
Silverton 2016, Third-party Funded [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued during the period		(100)
Silverton 2,017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued during the period	$ (130)			$ 0
Silverton 2017, Third-party Funded [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued during the period		$ (105)		0
Recurring | Level 3 | Silverton
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repayments of debt				(115.6)
Recurring | Level 3 | Silverton 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repayments of debt					(89.3)
Loan notes issued during the period					(105)
Recurring | Level 3 | Silverton 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repayments of debt				(115.6)
Loan notes issued during the period				$ 0
Non-U.S government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 83.9				$ 83.9

Fair Value Measurements - Input

Fair Value Measurements - Inputs Used (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	$ 3,360.9	$ 3,147	$ 2,997.3
Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	50.1	38.9
Reserve for losses	4.2	2.7
Initial value of issuance	325	220
Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	61.1	43.4
Reserve for losses	$ 61.9	$ 11.8
Contract period	365 days	365 days
Initial value of issuance	$ 325	$ 220
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Initial value of issuance	44.2	115
Liabilities measured using level 3 inputs classified as other payables	42.4	108.4
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Initial value of issuance	44.2	115
Liabilities measured using level 3 inputs classified as other payables	42.4	108.4
Silverton 2015 | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of Loan Notes		223.4	$ 190.6
Silverton 2016 | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of Loan Notes	$ 86.6	$ (223.4)

Reinsurance - Summary of Assume

Reinsurance - Summary of Assumed and Ceded Reinsurance on Premiums Written, Premiums Earned and Insurance Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums written:
Direct													$ 1,812.4	$ 1,733.8	$ 1,748.4
Assumed													1,548.5	1,413.2	1,248.9
Ceded													(1,148.4)	(553.3)	(351.1)
Net premiums written													2,212.5	2,593.7	2,646.2
Premiums earned:
Direct													1,757.4	1,768.4	1,703.3
Assumed													1,451.8	1,317.9	1,153.5
Ceded													(902.6)	(449)	(383.5)
Net premiums earned	$ 511	$ 652.5	$ 562	$ 581.1	$ 612.4	$ 681	$ 680.8	$ 663.1	$ 629.7	$ 640.6	$ 609.4	$ 593.6	2,306.6	2,637.3	2,473.3
Insurance losses and loss adjustment expenses:
Direct													1,673.6	1,091.9	980.6
Assumed													1,399.9	699.6	493
Ceded													(1,078.8)	(215.4)	(107.4)
Net insurance losses and loss adjustment expenses	$ 544.2	$ 776.2	$ 346.1	$ 328.2	$ 387.3	$ 389.2	$ 442.2	$ 357.4	$ 334	$ 365.6	$ 360.5	$ 306.1	$ 1,994.7	$ 1,576.1	$ 1,366.2

Reinsurance - Additional Inform

Reinsurance - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Disclosures [Abstract]
Liability for unpaid claims and claims adjustment expense amount ceded	$ 125.5	$ 85.8	$ 0

Derivative Contracts - Fair Val

Derivative Contracts - Fair Value of Derivative Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	May 09, 2016
Not Designated as Hedging Instrument | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Notional Amount	$ 751.6	$ 665.6
Cash collateral	0.6	0
Not Designated as Hedging Instrument | Derivatives at Fair Value | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Notional Amount	577.7	240.2
Fair Value	5	5
Not Designated as Hedging Instrument | Liabilities Under Derivative Contracts [Member] | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Notional Amount	173.9	425.4
Fair Value	(1)	(17.7)
Designated as Hedging Instrument | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Cash collateral	0	2.2
Designated as Hedging Instrument | Derivatives at Fair Value | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Notional Amount	60.6	0
Fair Value	1.4	2.2
Designated as Hedging Instrument | Liabilities Under Derivative Contracts [Member] | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Notional Amount	0	108.6
Fair Value	$ 0	$ (0.7)
Goldman Sachs International
Derivatives, Fair Value [Line Items]
Notional Amount			$ 256.3

Derivative Contracts - Gain_(Lo

Derivative Contracts - Gain/(Loss) Recognized in Income on Derivative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Not Designated as Hedging Instrument | Interest Rate Swaps | Change In Fair Value Of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) to net income from derivative instruments	$ 0	$ (3.1)
Not Designated as Hedging Instrument | Foreign Exchange Contracts | Change In Fair Value Of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) to net income from derivative instruments	27.7	(21.5)
Designated as Hedging Instrument | Foreign Exchange Contracts | General, administrative and corporate expenses
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on foreign currency fair value hedge derivatives	4.4	(8.7)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net change from current period hedged transactions
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on foreign currency fair value hedge derivatives	$ 3	$ 0.7

Derivative Contracts - Addition

Derivative Contracts - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	May 09, 2016
Foreign Exchange Contracts | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange	$ 751.6	$ 665.6
Collateral provided to counterparties as security for the Company's net liability position	0.6	0
Foreign Exchange Contracts | Not Designated as Hedging Instrument | Derivative At Fair Value
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange	577.7	240.2
Foreign Exchange Contracts | Not Designated as Hedging Instrument | Liabilities Under Derivative Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange	173.9	425.4
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	2.2
Foreign Exchange Contracts | Designated as Hedging Instrument | Derivative At Fair Value
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange	60.6	0
Foreign Exchange Contracts | Designated as Hedging Instrument | Liabilities Under Derivative Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange	0	108.6
Foreign Exchange Contracts | Change In Fair Value Of Derivatives | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) to net income from derivative instruments	27.7	(21.5)
Foreign Exchange Contracts | Net change from current period hedged transactions | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on foreign currency fair value hedge derivatives	3	0.7
Foreign Exchange Contracts | General, administrative and corporate expenses | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on foreign currency fair value hedge derivatives	4.4	(8.7)
Interest Rate Swaps | Change In Fair Value Of Derivatives | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) to net income from derivative instruments	$ 0	$ (3.1)
Goldman Sachs International
Derivative Instruments, Gain (Loss) [Line Items]
Purchase of US and foreign exchange			$ 256.3

Deferred Policy Acquisition C83

Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance at the beginning of the period				$ 358.4				$ 361.1					$ 358.4	$ 361.1
Acquisition costs deferred													336.4	526.2
Amortization of deferred policy acquisition costs	$ (85.1)	$ (105.4)	$ (96.3)	$ (113.7)	$ (141.1)	$ (130.9)	$ (126.7)	$ (130.2)	$ (118.2)	$ (132)	$ (114.1)	$ (119.3)	(400.5)	(528.9)	$ (483.6)
Balance at the end of the period	$ 294.3				$ 358.4				$ 361.1				$ 294.3	$ 358.4	$ 361.1

Reserves for Losses and Adjus84

Reserves for Losses and Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses (LAE) Reserves (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Insurance [Abstract]
Provision for losses and LAE, beginning balance	$ 5,319.9	$ 4,938.2	$ 4,750.8
Less reinsurance recoverable		(560.7)	(354.8)	$ (350)
Net losses and LAE reserves, beginning balance	4,759.2	4,583.4	4,400.8
Net loss and LAE expenses (disposed)	(125.5)	(80.1)	0
Movement in provision for losses and LAE for claims incurred:
Current year	2,100.1	1,705.4	1,522.7
Prior years	(105.4)	(129.3)	(156.5)
Total incurred	1,994.7	1,576.1	1,366.2
Losses and LAE payments for claims incurred:
Current year	(397.5)	(241)	(178.8)
Prior years	(1,157.6)	(981.8)	(929.7)
Total paid	(1,555.1)	(1,222.8)	(1,108.5)
Foreign exchange losses/(gains)	161	(97.4)	(75.1)
Net losses and LAE reserves, ending balance	5,234.3	4,759.2	4,583.4
Plus reinsurance recoverable on unpaid losses at the end of the year	1,515.2	560.7	354.8
Provision for losses and LAE at the end of the year	$ 6,749.5	$ 5,319.9	$ 4,938.2

Reserves for Losses and Adjus85

Reserves for Losses and Adjustment Expenses - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Provision for losses and LAE for claims incurred	$ 105.4	$ 129.3	$ 156.5
Additional loss reserves assumed		5.7
Liability for unpaid claims and claims adjustment expense amount ceded	$ (125.5)	$ (85.8)	$ 0

Reserves for Losses and Adjus86

Reserves for Losses and Adjustment Expenses Short-duration Insurance Contracts, Claims by Accident Year (Details) $ in Millions	Dec. 31, 2017USD ($)Integer	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)
Claims Development [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	$ 5,183.7
Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,154.6
Cumulative paid claims and allocated claim adjustment expense, Net	824.5
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	330.1
All outstanding liabilities before 2012, net of reinsurance	2.9
Liability for claims and claim adjustment expenses, net of reinsurance	333
Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	864.5
Cumulative paid claims and allocated claim adjustment expense, Net	262.8
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	601.7
All outstanding liabilities before 2012, net of reinsurance	43
Liability for claims and claim adjustment expenses, net of reinsurance	644.7
Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,699.9
Cumulative paid claims and allocated claim adjustment expense, Net	1,025.4
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	674.5
All outstanding liabilities before 2012, net of reinsurance	61.4
Liability for claims and claim adjustment expenses, net of reinsurance	735.9
Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	922.4
Cumulative paid claims and allocated claim adjustment expense, Net	377.4
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	545
All outstanding liabilities before 2012, net of reinsurance	27.4
Liability for claims and claim adjustment expenses, net of reinsurance	572.4
Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,637.8
Cumulative paid claims and allocated claim adjustment expense, Net	956.2
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	681.6
All outstanding liabilities before 2012, net of reinsurance	47.9
Liability for claims and claim adjustment expenses, net of reinsurance	729.5
Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,372.6
Cumulative paid claims and allocated claim adjustment expense, Net	355.7
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	1,016.9
All outstanding liabilities before 2012, net of reinsurance	575.6
Liability for claims and claim adjustment expenses, net of reinsurance	1,592.5
Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,222.6
Cumulative paid claims and allocated claim adjustment expense, Net	702.3
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	520.3
All outstanding liabilities before 2012, net of reinsurance	55.4
Liability for claims and claim adjustment expenses, net of reinsurance	575.7
Accident Year 2012 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	154.7	$ 160.6	$ 166.2	$ 167.3	$ 168.6	$ 170.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0.3
Number of Reported Claims | Integer	6,872
Cumulative paid claims and allocated claim adjustment expense, Net	$ 154.9	157.5	153.4	139.2	129.6	41.5
Accident Year 2012 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	67.2	69.6	61.7	70.5	63.4	78.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 5.2
Number of Reported Claims | Integer	3,536
Cumulative paid claims and allocated claim adjustment expense, Net	$ 49.7	41.3	30	14.3	6.7	1.3
Accident Year 2012 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	329	333	347.3	326.7	307.2	269.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 33.8
Number of Reported Claims | Integer	4,593
Cumulative paid claims and allocated claim adjustment expense, Net	$ 251.7	240.8	212	175.7	133.2	52.2
Accident Year 2012 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	90	94.5	97.6	94	90.1	88.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 9.7
Number of Reported Claims | Integer	593
Cumulative paid claims and allocated claim adjustment expense, Net	$ 70.7	65.4	59.5	50.8	39.6	22.8
Accident Year 2012 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	280.4	283.4	280	287.2	304.2	281.3
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 19.3
Number of Reported Claims | Integer	797
Cumulative paid claims and allocated claim adjustment expense, Net	$ 228.6	217.5	209.9	189.5	136.2	35.7
Accident Year 2012 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	235.6	234.8	238	246.6	235.2	236.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 66.1
Number of Reported Claims | Integer	2,173
Cumulative paid claims and allocated claim adjustment expense, Net	$ 118.5	97.1	65.8	42.2	17.9	2.2
Accident Year 2012 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	176.3	176	177.4	191.1	201.7	178.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 10.5
Number of Reported Claims | Integer	739
Cumulative paid claims and allocated claim adjustment expense, Net	$ 152.2	146.9	141.4	131.2	95.8	$ 25.7
Accident Year 2013 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	114.3	113.3	117.5	117.9	130.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4.9
Number of Reported Claims | Integer	5,100
Cumulative paid claims and allocated claim adjustment expense, Net	$ 106.4	101.8	89.7	76.4	39.2
Accident Year 2013 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	104.4	121.4	115.9	117.5	134.1
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 13.6
Number of Reported Claims | Integer	3,852
Cumulative paid claims and allocated claim adjustment expense, Net	$ 69	53.5	39.8	26	2.3
Accident Year 2013 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	333.3	326.3	342.9	334.2	321.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 17.4
Number of Reported Claims | Integer	4,979
Cumulative paid claims and allocated claim adjustment expense, Net	$ 265	235.7	205.3	131.8	41.6
Accident Year 2013 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	101.6	102.3	105.6	101.2	106.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 15.2
Number of Reported Claims | Integer	603
Cumulative paid claims and allocated claim adjustment expense, Net	$ 64.5	66	31.6	21.5	8.1
Accident Year 2013 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	180.8	182.3	193.9	203	220.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4.4
Number of Reported Claims | Integer	945
Cumulative paid claims and allocated claim adjustment expense, Net	$ 166.8	162	149.7	100.3	34.9
Accident Year 2013 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	208.4	225.8	228.4	232.9	217.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 62.2
Number of Reported Claims | Integer	2,034
Cumulative paid claims and allocated claim adjustment expense, Net	$ 93.7	65.5	43.2	16	3.5
Accident Year 2013 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	121.7	122.5	134.5	141.9	146.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 7.7
Number of Reported Claims | Integer	620
Cumulative paid claims and allocated claim adjustment expense, Net	$ 103.5	96.4	89.1	72.7	$ 25.4
Accident Year 2014 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	135.4	134.7	157.9	166.2
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3
Number of Reported Claims | Integer	8,779
Cumulative paid claims and allocated claim adjustment expense, Net	$ 124.2	114.5	87	40.5
Accident Year 2014 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	130.9	140.5	128.6	146.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 20.4
Number of Reported Claims | Integer	4,312
Cumulative paid claims and allocated claim adjustment expense, Net	$ 60.4	33.1	13.6	2.7
Accident Year 2014 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	310.9	299.3	314.5	310.1
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 45.3
Number of Reported Claims | Integer	4,768
Cumulative paid claims and allocated claim adjustment expense, Net	$ 210.2	189.5	117	53.6
Accident Year 2014 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	121.4	131.3	132.6	136.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 35.2
Number of Reported Claims | Integer	852
Cumulative paid claims and allocated claim adjustment expense, Net	$ 72.8	54.1	30.9	3
Accident Year 2014 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	152.4	164	180.4	191.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4.1
Number of Reported Claims | Integer	1,016
Cumulative paid claims and allocated claim adjustment expense, Net	$ 139.4	129.2	102.3	37.8
Accident Year 2014 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	214	220.8	212	208.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 84.5
Number of Reported Claims | Integer	1,842
Cumulative paid claims and allocated claim adjustment expense, Net	$ 60.9	38.2	14.1	2.6
Accident Year 2014 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	126	135.2	143.4	154.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 15.6
Number of Reported Claims | Integer	667
Cumulative paid claims and allocated claim adjustment expense, Net	$ 91.3	83.1	57.9	$ 17
Accident Year 2015 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	201.8	207.3	241.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4.7
Number of Reported Claims | Integer	10,431
Cumulative paid claims and allocated claim adjustment expense, Net	$ 172.1	144	57.5
Accident Year 2015 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	188	225.5	205.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 64.7
Number of Reported Claims | Integer	4,954
Cumulative paid claims and allocated claim adjustment expense, Net	$ 57	17.2	3.2
Accident Year 2015 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	283.3	301	297.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 37.9
Number of Reported Claims | Integer	4,356
Cumulative paid claims and allocated claim adjustment expense, Net	$ 174.9	123.7	45.1
Accident Year 2015 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	187.2	177.2	175.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 76.7
Number of Reported Claims | Integer	1,138
Cumulative paid claims and allocated claim adjustment expense, Net	$ 70.4	43.7	13.9
Accident Year 2015 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	181.6	191.4	219.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 18.7
Number of Reported Claims | Integer	1,135
Cumulative paid claims and allocated claim adjustment expense, Net	$ 130	96.9	36.2
Accident Year 2015 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	214.6	205.3	197.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 103
Number of Reported Claims | Integer	1,648
Cumulative paid claims and allocated claim adjustment expense, Net	$ 39.2	18.5	3.6
Accident Year 2015 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	168.8	174.3	169.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 29.4
Number of Reported Claims | Integer	826
Cumulative paid claims and allocated claim adjustment expense, Net	$ 106.5	58.1	$ 18.1
Accident Year 2016 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	250.7	239.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 14.7
Number of Reported Claims | Integer	9,514
Cumulative paid claims and allocated claim adjustment expense, Net	$ 169.7	67.3
Accident Year 2016 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	190.6	219.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 117.9
Number of Reported Claims | Integer	5,084
Cumulative paid claims and allocated claim adjustment expense, Net	$ 23.1	4.2
Accident Year 2016 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	231.9	219.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 49.5
Number of Reported Claims | Integer	4,572
Cumulative paid claims and allocated claim adjustment expense, Net	$ 83.2	30.9
Accident Year 2016 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	213.6	193
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 121.1
Number of Reported Claims | Integer	1,365
Cumulative paid claims and allocated claim adjustment expense, Net	$ 71.7	15.4
Accident Year 2016 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	277.8	277.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 35.1
Number of Reported Claims | Integer	1,297
Cumulative paid claims and allocated claim adjustment expense, Net	$ 167.2	58
Accident Year 2016 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	250.5	237.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 156.2
Number of Reported Claims | Integer	1,151
Cumulative paid claims and allocated claim adjustment expense, Net	$ 34.3	9.6
Accident Year 2016 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	244.6	243.2
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 49.5
Number of Reported Claims | Integer	844
Cumulative paid claims and allocated claim adjustment expense, Net	$ 153.1	$ 59.6
Accident Year 2017 | Property Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	297.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 51.3
Number of Reported Claims | Integer	6,249
Cumulative paid claims and allocated claim adjustment expense, Net	$ 97.2
Accident Year 2017 | Casualty Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	183.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 153
Number of Reported Claims | Integer	4,447
Cumulative paid claims and allocated claim adjustment expense, Net	$ 3.6
Accident Year 2017 | Marine, Aviation and Energy Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	211.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 70
Number of Reported Claims | Integer	3,993
Cumulative paid claims and allocated claim adjustment expense, Net	$ 40.4
Accident Year 2017 | Financial and Professional Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	208.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 159.4
Number of Reported Claims | Integer	1,743
Cumulative paid claims and allocated claim adjustment expense, Net	$ 27.3
Accident Year 2017 | Property Catastrophe and Other Property Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	564.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 216.1
Number of Reported Claims | Integer	1,366
Cumulative paid claims and allocated claim adjustment expense, Net	$ 124.2
Accident Year 2017 | Casualty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	249.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 215.9
Number of Reported Claims | Integer	476
Cumulative paid claims and allocated claim adjustment expense, Net	$ 9.1
Accident Year 2017 | Specialty Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	385.2
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 211.6
Number of Reported Claims | Integer	763
Cumulative paid claims and allocated claim adjustment expense, Net	$ 95.7

Reserves for Losses and Adjus87

Reserves for Losses and Adjustment Expenses Short-duration insurance contracts - Reconciliation of Claims Development (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	$ 5,183.7
Reinsurance recoverable on unpaid losses	1,515.2
Unallocated claims incurred	49.2
Other	1.4
Liability for unpaid claims and claim adjustment expense, aggregate reconciling items	50.6
Losses and loss adjustment expenses	6,749.5	$ 5,319.9	$ 4,938.2	$ 4,750.8
Property Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	333
Casualty Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	644.7
Marine, Aviation and Energy Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	735.9
Financial and Professional Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	572.4
Insurance lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	2,286
Reinsurance recoverable on unpaid losses	1,245.9
Property Catastrophe and Other Property Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	729.5
Casualty Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	1,592.5
Specialty Reinsurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	575.7
Reinsurance lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expenses, net of reinsurance	2,897.7
Reinsurance recoverable on unpaid losses	269.3
Insurance lines other than short-duration
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Losses and loss adjustment expenses	$ 0

Reserves for Losses and Adjus88

Reserves for Losses and Adjustment Expenses Short-duration Contracts - Historical Claims Duration (Details)	Dec. 31, 2017
Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year One	15.40%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Two	25.30%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Three	16.30%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Four	12.20%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Five	7.60%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Six	3.40%
Reinsurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year One	13.20%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Two	25.00%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Three	17.00%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Four	8.10%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Five	7.10%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Six	5.50%

Income Taxes - Summary of Total

Income Taxes - Summary of Total Income Tax (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax (benefit)/expense allocated to net income	$ (10.2)	$ (9.2)	$ 1.2	$ 2.8	$ (2.6)	$ 4.9	$ 1.3	$ 2.5	$ 5.3	$ 1.8	$ 2.2	$ 5.1	$ (15.4)	$ 6.1	$ 14.4
Income tax (benefit) allocated to other comprehensive income													(17.4)	(1.7)	(15.3)
Income tax (benefit) allocated directly to shareholders’ equity													0	(1)	(1.9)
Total income tax (benefit)/expense													$ (32.8)	$ 3.4	$ (2.8)

Income Taxes - Income_(Loss) Be

Income Taxes - Income/(Loss) Before Tax and Income Tax Expense/(Benefit) Attributable to that Income (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Examination [Line Items]
(Loss)/income before tax, Non-U.S.													$ (26.8)	$ (23.5)	$ 6.4
(Loss)/income before tax, U.S.													(140.3)	(70.2)	(32.7)
(Loss)/income from operations before income tax	$ (195.1)	$ (263)	$ 77	$ 99.3	$ (74.1)	$ 100.5	$ 66.2	$ 116.9	$ 123.2	$ 30	$ 51.2	$ 133.1	(281.8)	209.5	337.5
Current income tax (benefit)/expense, Non-U.S.													2.1	1.1	4.8
Current income tax (benefit)/expense, U.S.													0	0	1.6
Current income tax (benefit)/expense, Total													17.1	(2.1)	23.3
Deferred income Tax (benefit), Non-U.S.													(0.3)	0.3	(0.6)
Deferred income Tax (benefit), U.S.													1.1	2.5	(4.1)
Deferred Income Tax, Total													(32.5)	8.2	(8.9)
Total income tax expense/(benefit), Non-U.S.													1.8	1.4	4.2
Total income tax expense/(benefit), U.S.													1.1	2.5	(2.5)
Total income tax expense/(benefit)	$ (10.2)	$ (9.2)	$ 1.2	$ 2.8	$ (2.6)	$ 4.9	$ 1.3	$ 2.5	$ 5.3	$ 1.8	$ 2.2	$ 5.1	(15.4)	6.1	14.4
Bermuda
Income Tax Examination [Line Items]
(Loss)/income before tax, Non-U.S.													(130)	259.5	283.9
Current income tax (benefit)/expense, Non-U.S.													0.9	0	1.4
Deferred income Tax (benefit), Non-U.S.													0	0	0
Total income tax expense/(benefit), Non-U.S.													0.9	0	1.4
U.K.
Income Tax Examination [Line Items]
(Loss)/income before tax, Non-U.S.													15.3	43.7	79.9
Current income tax (benefit)/expense, Non-U.S.													14.1	(3.2)	15.5
Deferred income Tax (benefit), Non-U.S.													(33.3)	5.4	(4.2)
Total income tax expense/(benefit), Non-U.S.													$ (19.2)	$ 2.2	$ 11.3

Income Taxes - Income Tax Recon

Income Taxes - Income Tax Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Expected tax (benefit)/expense													$ 0	$ 0	$ 0
Overseas statutory tax rates differential													(41.5)	(19.3)	6.4
Prior year adjustments													1.3	3.3	(4.5)
Valuation allowance													(37.9)	21	11.8
Impact of unrecognized tax benefits													0.1	(1.9)	0
Restricted foreign tax credits													1.6	1.9	0.6
Share-based payments													(0.9)	0	0
Foreign exchange													(2.1)	0.2	(0.4)
Non-deductible expenses													0.4	0.8	1.7
Non-taxable items													(0.9)	(0.9)	(1.3)
Impact of changes in statutory tax rates													64.5	1	0.1
Total income tax expense/(benefit)	$ (10.2)	$ (9.2)	$ 1.2	$ 2.8	$ (2.6)	$ 4.9	$ 1.3	$ 2.5	$ 5.3	$ 1.8	$ 2.2	$ 5.1	(15.4)	6.1	$ 14.4
Effective Income Rate Tax Reconciliation, Tax Settlement, Foreign Credit, Amount													0.3	3.2
Income tax examination, interest accrued	$ 0.4				$ 1.3								$ 0.4	$ 1.3

Income Taxes - Schedule of Unre

Income Taxes - Schedule of Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Unrecognized tax benefits balance at January 1	$ 10.5	$ 29.2
Foreign exchange re-translation	1	3.4
Settlement in respect of tax position of prior years	(0.3)	(15.3)
Unrecognized tax benefits balance at December 31	$ 11.2	$ 10.5

Income Taxes - Tax Effects of D

Income Taxes - Tax Effects of Deferred Tax Assets and Deferred Tax Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Share-based payments	$ 4	$ 5.6
Operating loss carryforwards	102.5	124.5
Loss reserves	4.3	7.8
Accrued expenses	7.9	14.4
Foreign tax credit carryforwards	4	2.5
Unearned premiums	11.8	20
Deferred policy acquisition costs	5.9	2.1
Office properties and equipment	8	5.6
Other temporary differences	2.9	3.8
Total gross deferred tax assets	151.3	186.3
Less valuation allowance	(104.8)	(142.5)
Net deferred tax assets	46.5	43.8
Equalization provision reserves	0	(23)
Intangible assets (other)	(2)	(2.8)
Deferred policy acquisition costs	(14.5)	(22.4)
Other temporary differences	(1.7)	(1.7)
Total gross deferred tax (liabilities)	(18.2)	(49.9)
Deferred Tax Assets, Net	28.3
Deferred tax liability	$ 0	$ (6.1)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended	8 Months Ended	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2021	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2020
Income Taxes [Line Items]
United States corporate tax at a rate					34.00%
Foreign exchange					$ (2,100,000)	$ 200,000	$ (400,000)
Prior year adjustments					1,300,000	3,300,000	(4,500,000)
Income tax examination, interest accrued					400,000	1,300,000
Income tax examination, cumulative interest accrued					1,700,000	1,300,000
Impact of unrecognized tax benefits					100,000	(1,900,000)	0
Effective Income Rate Tax Reconciliation, Tax Settlement, Foreign Credit, Amount					300,000	3,200,000
Unrecognized tax benefits					11,200,000	10,500,000	$ 29,200,000
Foreign exchange re-translation					1,000,000	3,400,000
Settlement in respect of tax position of prior years					(300,000)	(15,300,000)
Accrued interest and penalties					0	0
Less valuation allowance					(104,800,000)	(142,500,000)
Increase (decrease) in valuation allowance					38,400,000	(20,700,000)
Foreign tax credit carryforwards					4,000,000	2,500,000
US branch profits, subject to tax					0	1,900,000
Other Comprehensive Income
Income Taxes [Line Items]
Less valuation allowance					0	(300,000)
Internal Revenue Service (IRS)
Income Taxes [Line Items]
Net operating loss carryforwards					482,700,000	343,200,000
Capital loss carryforwards					500,000	500,000
Charitable contribution carryforwards					700,000	500,000
Less valuation allowance					$ (101,100,000)	(139,200,000)
Internal Revenue Service (IRS) | Minimum
Income Taxes [Line Items]
Operating loss carryforwards, date of expiration					2,026
Internal Revenue Service (IRS) | Maximum
Income Taxes [Line Items]
Operating loss carryforwards, date of expiration					2,036
U.K.
Income Taxes [Line Items]
U. K. corporate tax rate reduced during the period	20.00%
Net operating loss carryforwards					$ 84,900,000	18,200,000
Capital loss carryforwards					3,800,000
Less valuation allowance					(3,700,000)	(3,300,000)
Increase (decrease) in valuation allowance					(400,000)	(2,600,000)
U.K. | Other Comprehensive Income
Income Taxes [Line Items]
Less valuation allowance					$ 0	0
Bermuda
Income Taxes [Line Items]
Bermuda tax rate					0.00%
Forecast | U.K.
Income Taxes [Line Items]
U. K. corporate tax rate reduced during the period		17.00%
Valuation Allowance, Operating Loss Carryforwards
Income Taxes [Line Items]
Increase (decrease) in valuation allowance					$ 38,400,000	(21,000,000)
Valuation Allowance, Operating Loss Carryforwards | U.K.
Income Taxes [Line Items]
Less valuation allowance					$ (400,000)	$ (2,700,000)
Subsequent Event
Income Taxes [Line Items]
United States corporate tax at a rate			21.00%
Foreign tax credit carryforwards								$ 1,200,000
Subsequent Event | U.K.
Income Taxes [Line Items]
U. K. corporate tax rate reduced during the period				19.00%

Capital Structure - Summary of

Capital Structure - Summary of Authorized and Issued Share Capital (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	59,474,085	59,774,464	60,918,373
Ordinary shares, value	$ 90	$ 91
Total issued share capital	$ 122	$ 140
7.401% Preference Shares
Issued share capital:
Preference shares, issued	0	5,327,500
Preference shares, value	$ 0	$ 8
7.250% Preference Shares
Issued share capital:
Preference shares, issued	0	6,400,000
Preference shares, value	$ 0	$ 10
5.950% Preference Shares
Issued share capital:
Preference shares, issued	11,000,000	11,000,000
Preference shares, value	$ 17	$ 17
5.625% Preference Shares
Issued share capital:
Preference shares, issued	10,000,000	10,000,000
Preference shares, value	$ 15	$ 15

Capital Structure - Summary o96

Capital Structure - Summary of Authorized and Issued Share Capital (Phantom) (Detail) - $ / shares	Dec. 31, 2017	Jul. 03, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Ordinary shares, par value	$ 0.0015144558		$ 0.0015144558
Non-voting shares, par value	0.0015144558		0.0015144558
Preference shares, par value	0.0015144558		0.0015144558
Redemption price per share	25		25
5.950% Preference Shares
Class of Stock [Line Items]
Preference shares, par value	0.0015144558		0.0015144558
Redemption price per share	$ 25		$ 25
Preference shares, rate	5.95%		5.95%
7.401% Preference Shares
Class of Stock [Line Items]
Preference shares, par value			$ 0.0015144558
Redemption price per share		$ 25	$ 25
Preference shares, rate			7.401%
7.250% Preference Shares
Class of Stock [Line Items]
Preference shares, par value			$ 0.0015144558
Redemption price per share			$ 25
Preference shares, rate			7.25%
5.625% Preference Shares
Class of Stock [Line Items]
Preference shares, par value	$ 0.0015144558		$ 0.0015144558
Redemption price per share	$ 25		$ 25
Preference shares, rate	5.625%		5.625%

Capital Structure - Summary o97

Capital Structure - Summary of Ordinary Shares (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Ordinary shares in issue at the beginning of the year	59,774,464	60,918,373
Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or 2008 share purchase plan	309,727	419,255
Ordinary shares issued to non-employee directors	38,835	32,241
Ordinary shares repurchased	(648,941)	(1,595,405)
Ordinary shares in issue at the end of the year	59,474,085	59,774,464

Capital Structure - Additional

Capital Structure - Additional Information (Details) - USD ($)	Jan. 03, 2017	Sep. 20, 2016	Jul. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 03, 2017	Feb. 08, 2017	Feb. 05, 2015
Class of Stock [Line Items]
Additional paid in capital				$ 954,700,000	$ 1,259,600,000
Aggregate liquidation preferences				525,000,000	818,200,000
Preferred stock issuance cost				13,100,000	21,100,000
Stock repurchase program, remaining authorized repurchase amount				$ 220,000,000	$ 341,300,000	$ 416,300,000
Preferred Stock, Redemption Amount	$ 133,200,000						$ 160,000,000
Redemption price per share				$ 25	$ 25
Preference shares liquidation preference, value				$ 250,000,000
5.625% Preference Shares
Class of Stock [Line Items]
Preferred stock issuance cost				$ 8,700,000
Preference shares, issued				10,000,000	10,000,000
Preference shares, rate		5.625%		5.625%	5.625%
Redemption price per share				$ 25	$ 25
7.401% Preference Shares
Class of Stock [Line Items]
Preference shares, issued				0	5,327,500
Preference shares, rate	7.401%			7.401%	7.401%
Redemption price per share					$ 25		$ 25
7.250% Preference Shares
Class of Stock [Line Items]
Preference shares, issued				0	6,400,000
Preference shares, rate			7.25%	7.25%	7.25%
Redemption price per share					$ 25
Open Markets
Class of Stock [Line Items]
Stock repurchased during period, shares				648,941	1,595,076	1,790,333
Stock repurchased during period, value				$ 30,000,000	$ 75,000,000	$ 83,700,000
Stock repurchased during period, average price per share				$ 46.23	$ 47.02	$ 46.74
Repurchase Program 2015
Class of Stock [Line Items]
Stock repurchase program, authorized amount									$ 500,000,000
Repurchase Program 2017
Class of Stock [Line Items]
Stock repurchase program, authorized amount								$ 250,000,000
Additional paid-in capital | Preference Shares
Class of Stock [Line Items]
Shares issued				$ 0	$ 241,300,000	$ 0

Statutory Requirements and Di99

Statutory Requirements and Dividends Restrictions - Summary of Statutory Requirements and Dividends Restrictions (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
U.S.
Required statutory capital and surplus	$ 163.5	$ 77.3
Statutory capital and surplus	645.5	598.3
Bermuda
Required statutory capital and surplus	1,146.7	1,259.4
Statutory capital and surplus	1,793	2,294.4
U.K.
Required statutory capital and surplus	745.8	558.1
Statutory capital and surplus	$ 888.6	$ 970.3

Statutory Requirements and D100

Statutory Requirements and Dividends Restrictions - Additional Information (Details) - 12 months ended Dec. 31, 2017 £ in Millions, $ in Millions	USD ($)	GBP (£)
AUL
Statutory Accounting Practices [Line Items]
Syndicate to maintain funds at Lloyd	$ 451.8
Total funds held by AUL	458.7
U.K.
Statutory Accounting Practices [Line Items]
Dividend payment made without regulatory approval	(196.3)	£ (35.9)
Capital contributions reserves	470
Bermuda
Statutory Accounting Practices [Line Items]
Dividend payment made without regulatory approval	$ (448.3)
Statutory capital and surplus, percent	25.00%
Statutory capital and surplus, percent reduction requiring approval	15.00%
Percent warning level of amount of enhanced capital required from statutory capital and surplus	120.00%
Bermuda | AUL
Statutory Accounting Practices [Line Items]
Total funds held by AUL	$ 423.6

Retirement Plans (Details)

Retirement Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Rate of contribution on retirement plans of its employees salaries, Maximum	20.00%
Total contributions by the Company to the retirement plan	$ 16.3	$ 15.4	$ 14.9

Share-Based Payments - Restrict

Share-Based Payments - Restricted Share Units by Grant Year (Details) - Restricted Share Units	12 Months Ended
	Dec. 31, 2017shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding (in shares)	439,576
Grants 2,017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	200,020.707095856
Vested (in shares)	0
Forfeited (in shares)	19,585
Outstanding (in shares)	180,436
Grants 2,016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	328,550
Vested (in shares)	104,041.666666667
Forfeited (in shares)	40,579
Outstanding (in shares)	183,929
Grants 2,015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	287,852
Vested (in shares)	173,755
Forfeited (in shares)	38,886
Outstanding (in shares)	75,211

Share-Based Payments Share-Base

Share-Based Payments Share-Based Payments - Summary of Restricted Share Units under 2013 Share Incentive Plan (Details) - Restricted Share Units - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding, end of period	439,576
2013 Share Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding, beginning of period	530,340	504,234	519,112
Granted (in shares)	200,020.707095856	328,550	287,852
Vested (in shares)	(245,703.707095856)	(246,489)	(266,944)
Forfeited (in shares)	(45,081)	(55,955)	(35,786)
Number of shares outstanding, end of period	439,576	530,340	504,234
Number of shares outstanding, weighted average grant date fair value beginning of period (in dollars per share)	$ 39.28	$ 40.45	$ 35.82
Granted, weighted average grant date fair value (in dollars per share)	49.20	37.63	39.68
Vested, weighted average grant date fair value (in dollars per share)	39.63	34.26	34.26
Forfeited, weighted average grant date fair value (in dollars per share)	41.84	39.51	39.59
Number of shares outstanding, weighted average grant date fair value end of period (in dollars per share)	$ 43.22	$ 39.28	$ 40.45

Share-Based Payments - Summary

Share-Based Payments - Summary of Performance and Phantom Shares by Grant Year (Details)	12 Months Ended
	Dec. 31, 2017shares
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding (in shares)	208,922
Phantom Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding (in shares)	118,680
Grants 2015 | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	277,585
Vested (in shares)	107,882
Forfeited (in shares)	169,703
Outstanding (in shares)	0
Grants 2015 | Phantom Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	135,651
Vested (in shares)	52,719.8395718475
Forfeited (in shares)	82,931.1604281525
Outstanding (in shares)	0
Grants 2016 | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	278,477
Vested (in shares)	28,566
Forfeited (in shares)	168,224
Outstanding (in shares)	81,687
Grants 2016 | Phantom Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	147,513
Vested (in shares)	15,131.9787096774
Forfeited (in shares)	96,977.9012903226
Outstanding (in shares)	35,403
Grants 2017 | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	216,878
Vested (in shares)	0
Forfeited (in shares)	89,643
Outstanding (in shares)	127,235
Grants 2017 | Phantom Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	173,619
Vested (in shares)	0
Forfeited (in shares)	90,341.775812609
Outstanding (in shares)	83,277

Share-Based Payments Share-b105

Share-Based Payments Share-based Payments - Summary of Performance and Phantom Shares under 2013 Share Incentive Plan (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding, end of period	208,922
Phantom Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding, end of period	118,680
2013 Share Incentive Plan | Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding, beginning of period	254,988	266,424	268,418
Granted (in shares)	216,878	278,477	277,585
Vested (in shares)	0	(87,059)	(208,830)
Forfeited (in shares)	(262,944)	(202,854)	(70,749)
Number of shares outstanding, end of period	208,922	254,988	266,424
Number of shares outstanding, weighted average grant date fair value beginning of period (in dollars per share)	$ 36.92	$ 24.17	$ 25.35
Granted, weighted average grant date fair value (in dollars per share)	47.30	34.44	38.92
Vested, weighted average grant date fair value (in dollars per share)	0	37.30	38.11
Forfeited, weighted average grant date fair value (in dollars per share)	40.32	36.93	30.26
Number of shares outstanding, weighted average grant date fair value end of period (in dollars per share)	$ 38.71	$ 36.92	$ 24.17
2013 Share Incentive Plan | Phantom Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding, beginning of period	131,464	130,319	143,248.466666667
Granted (in shares)	173,619	147,513	135,651
Vested (in shares)	0	(36,159.4034422365)	(151,667.812786764)
Forfeited (in shares)	(186,403)	(110,208.856204164)	(3,087.34612009701)
Number of shares outstanding, end of period	118,680	131,464	130,319
Number of shares outstanding, weighted average grant date fair value beginning of period (in dollars per share)	$ 35.90	$ 38.75	$ 35.32
Granted, weighted average grant date fair value (in dollars per share)	47.30	34.44	38.92
Vested, weighted average grant date fair value (in dollars per share)	0	36.96	34.98
Forfeited, weighted average grant date fair value (in dollars per share)	40.32	36.96	35.65
Number of shares outstanding, weighted average grant date fair value end of period (in dollars per share)	$ 38.71	$ 35.90	$ 38.75

Share-Based Payments Share-b106

Share-Based Payments Share-based Payments - Vesting Criteria for Performance and Phantom Shares (Details)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Increase in BVPS	0.00%	5.90%	10.70%	13.30%	6.20%

Share-Based Payments - Amounts

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	12 Months Ended
	Dec. 31, 2017$ / shares
March 25, 2015 (1)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 8.17
Risk free interest rate	0.94%
Dividend yield	1.78%
Expected life (in years)	3
Share price volatility	16.00%
March 25, 2015 (2)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 7.08
Risk free interest rate	0.60%
Dividend yield	1.78%
Expected life (in years)	2
Share price volatility	16.00%
March 25, 2016 (1)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 7.97
Risk free interest rate	1.04%
Dividend yield	1.88%
Expected life (in years)	3
Share price volatility	4.21%
March 25, 2016 (2)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 7
Risk free interest rate	0.87%
Dividend yield	1.88%
Expected life (in years)	2
Share price volatility	2.44%
April 28, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 6.69
Risk free interest rate	1.44%
Dividend yield	1.83%
Expected life (in years)	3
Share price volatility	3.67%
April 28, 2017 (2)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 8.70
Risk free interest rate	1.44%
Dividend yield	1.83%
Expected life (in years)	3
Share price volatility	3.67%
April 28, 2017 (3)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Per share weighted average fair value (in dollars per share)	$ 8.70
Risk free interest rate	1.26%
Dividend yield	1.83%
Expected life (in years)	2
Share price volatility	3.52%

Share-Based Payments - Restr108

Share-Based Payments - Restricted Share Units Issued to Non-employee Directors (Details) - Restricted Share Units	12 Months Ended
	Dec. 31, 2017shares
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Outstanding (in shares)	439,576
Management [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Granted (in shares)	106,304
Vested (in shares)	96,531
Forfeited (in shares)	4,957
Outstanding (in shares)	4,816
Grants 2,015
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Granted (in shares)	287,852
Vested (in shares)	173,755
Forfeited (in shares)	38,886
Outstanding (in shares)	75,211
Grants 2015 | Non-Employee Directors
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Granted (in shares)	27,620
Vested (in shares)	27,620
Forfeited (in shares)	0
Outstanding (in shares)	0
Grants 2015 | Chairman
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Granted (in shares)	12,154
Vested (in shares)	12,154
Forfeited (in shares)	0
Outstanding (in shares)	0
Grants 2,016
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Granted (in shares)	328,550
Vested (in shares)	104,041.666666667
Forfeited (in shares)	40,579
Outstanding (in shares)	183,929
Grants 2016 | Non-Employee Directors
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Granted (in shares)	24,456
Vested (in shares)	21,352
Forfeited (in shares)	3,104
Outstanding (in shares)	0
Grants 2016 | Chairman
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Granted (in shares)	10,952
Vested (in shares)	10,952
Forfeited (in shares)	0
Outstanding (in shares)	0
Grants 2,017
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Granted (in shares)	200,020.707095856
Vested (in shares)	0
Forfeited (in shares)	19,585
Outstanding (in shares)	180,436
Grants 2017 | Non-Employee Directors
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Granted (in shares)	22,230
Vested (in shares)	17,042.5
Forfeited (in shares)	1,853
Outstanding (in shares)	3,334
Grants 2017 | Chairman
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Granted (in shares)	8,892
Vested (in shares)	7,410
Forfeited (in shares)	0
Outstanding (in shares)	1,482

Share-based Payments - Summa109

Share-based Payments - Summary of Restricted Share Units, Non-Employee Directors, under 2013 Share Incentive Plan (Details) - Restricted Share Units - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding, end of period	439,576
2013 Share Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding, beginning of period	530,340	504,234	519,112
Granted (in shares)	200,020.707095856	328,550	287,852
Vested (in shares)	(245,703.707095856)	(246,489)	(266,944)
Forfeited (in shares)	(45,081)	(55,955)	(35,786)
Number of shares outstanding, end of period	439,576	530,340	504,234
Number of shares outstanding, weighted average grant date fair value beginning of period (in dollars per share)	$ 39.28	$ 40.45	$ 35.82
Granted, weighted average grant date fair value (in dollars per share)	49.20	37.63	39.68
Vested, weighted average grant date fair value (in dollars per share)	39.63	34.26	34.26
Forfeited, weighted average grant date fair value (in dollars per share)	41.84	39.51	39.59
Number of shares outstanding, weighted average grant date fair value end of period (in dollars per share)	$ 43.22	$ 39.28	$ 40.45
Non-Employee Directors | 2013 Share Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares outstanding, beginning of period	5,171	6,636	6,806
Granted (in shares)	31,122	35,408	39,774
Vested (in shares)	(29,624)	(33,769)	(39,944)
Forfeited (in shares)	(1,853)	(3,104)	0
Number of shares outstanding, end of period	4,816	5,171	6,636
Number of shares outstanding, weighted average grant date fair value beginning of period (in dollars per share)	$ 41.07	$ 45.28	$ 35.83
Granted, weighted average grant date fair value (in dollars per share)	50.18	41.07	45.25
Vested, weighted average grant date fair value (in dollars per share)	48.59	41.89	43.81
Forfeited, weighted average grant date fair value (in dollars per share)	50.18	41.07	0
Number of shares outstanding, weighted average grant date fair value end of period (in dollars per share)	$ 50.19	$ 41.07	$ 45.28

Share-Based Payments - Addition

Share-Based Payments - Additional Information (Details)	Apr. 05, 2014GBP (£)	Dec. 31, 2017USD ($)agreementshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of arrangements under options and equity incentive plans | agreement		3
Vesting period (in years)		3 years
Compensation cost				$ (6,500,000)
Total tax credit		$ 100,000	$ 500,000	$ 1,500,000
Percentages for vesting		100.00%
Contract period (in years)		10 years
Options exercised | shares		0	29,222	189,215
Intrinsic value of options		$ 0	$ 600,000	$ 4,300,000
Employee Stock Ownership Plan, shares issued during period (in shares) | shares		46,866	21,285	59,940
Restricted Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost		$ (9,400,000)	$ (9,800,000)	$ (8,700,000)
Total tax credit		2,300,000	2,400,000	1,800,000
Unrecognized share compensation costs, RSUs		$ 11,000,000
Weighted average period of compensation cost recognition		1 year 7 months 6 days
Compensation cost, non employee directors		$ 1,500,000	1,400,000	1,800,000
Tax charge, non employee directors		0	0	0
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost		(400,000)	(2,200,000)
Unrecognized share compensation costs, RSUs		$ 10,000,000
Weighted average period of compensation cost recognition		1 year 8 months 12 days
Percentages for vesting		33.33%
Phantom Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost		$ (700,000)	(700,000)	(4,300,000)
Unrecognized share compensation costs, RSUs		$ 5,700,000
Weighted average period of compensation cost recognition		1 year 9 months 18 days
Fair value adjustment on compensation cost		$ (1,700,000)	(2,900,000)	(3,200,000)
Tax credit recognized by the company		$ 200,000	(700,000)	(1,200,000)
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)		3 years
Percentages for vesting		200.00%
2013 Share Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant | shares		2,845,683
2003 Share Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant | shares		595,683
International Employee Share Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee savings under ESPP		$ 500
Employee contribution period (in years)		2 years
2008 Share Save Scheme
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee savings under ESPP | £	£ 500
Employee contribution period (in years)		3 years
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost		$ (400,000)	(400,000)	(300,000)
Total tax credit		$ 100,000	100,000
Purchase price percentage of fair market value		85.00%
Director Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost		$ 0	$ 0	$ 0
Options granted (in shares) | shares		0	4,447	0
Non-Employee Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentages for vesting		8.33%
Non-Employee Directors | Restricted Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized share compensation costs, RSUs		$ 200,000
Weighted average period of compensation cost recognition		1 month 6 days

Intangible Assets - Summary of

Intangible Assets - Summary of Changes in Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 18, 2017	Dec. 31, 2017	Dec. 31, 2016
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Amortization		$ (3,700)	$ (5,100)
Goodwill [Roll Forward]
Goodwill, Beginning of the period		24,200	0
Goodwill, Additions (Disposals)		(18,800)	24,200
Goodwill, Impairment Loss		(1,500)	0
Goodwill, End of the Period		3,900	24,200
Intangible Assets and goodwill, Beginning of the period		79,600	18,200
Intangible Assets and Goodwill, Additions (Disposals)		(44,600)	66,500
Goodwill and Intangible Asset Impairment		(3,400)	0
Intangible Assets and goodwill, End of the period		27,900	79,600
Trade Mark
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		6,600	1,600
Intangible Assets acquired (disposed)	$ (3,100)		5,500
Amortization		(500)	(500)
Impairment of intangible assets (excluding goodwill)		(100)	0
Intangible Assets, End of the period		2,900	6,600
Insurance Licenses
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		16,700	16,600
Intangible Assets acquired (disposed)		0	100
Amortization		0	0
Impairment of intangible assets (excluding goodwill)		0	0
Intangible Assets, End of the period		16,700	16,700
Agency Relationships
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		26,200	0
Intangible Assets acquired (disposed)	(21,800)		28,100
Amortization		(2,100)	(1,900)
Impairment of intangible assets (excluding goodwill)		0	0
Intangible Assets, End of the period		2,300	26,200
Non-compete Agreements
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		3,300	0
Intangible Assets acquired (disposed)	$ (900)		3,900
Amortization		(700)	(600)
Impairment of intangible assets (excluding goodwill)		(1,000)	0
Intangible Assets, End of the period		700	3,300
Value of Business Acquired
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		0	0
Intangible Assets acquired (disposed)			1,800
Amortization			(1,800)
Impairment of intangible assets (excluding goodwill)			0
Intangible Assets, End of the period			0
Consulting Relationships
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		900	0
Intangible Assets acquired (disposed)		0	1,000
Amortization		(100)	(100)
Impairment of intangible assets (excluding goodwill)		(800)	0
Intangible Assets, End of the period		0	900
Renewal Rights
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		1,700	0
Intangible Assets acquired (disposed)		0	1,900
Amortization		(300)	(200)
Impairment of intangible assets (excluding goodwill)		0	0
Intangible Assets, End of the period		$ 1,400	$ 1,700

Intangible Assets - Additional

Intangible Assets - Additional Information (Narrative) (Details) - USD ($) $ in Thousands	Dec. 18, 2017	Oct. 31, 2016	Sep. 22, 2016	Jan. 19, 2016	Apr. 05, 2005	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2017	Dec. 31, 2015
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Goodwill, acquired during period						$ 1,800
Goodwill and intangible asset impairment						(3,400)	$ 0
AgriLogic
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Percentage of interests acquired				100.00%
Initial purchase price				$ 53,000
Business combination, contingent consideration, liability				14,100
Business combination, contingent consideration, maximum payable				22,800
Business combination, ceding commission				2,000
Aggregate consideration for sale and purchase agreement				69,100
Business combination, office properties and equipments				12,000
Business combination, residual assets				300
Business combination, net assets acquired				47,000
Goodwill, acquired during period				22,100
Intangible assets acquired eligible tax deductible amount				34,000
Goodwill acquired eligible tax deductible amount				$ 21,000
Business acquisition, eligible tax deduction period				15 years
Blue Waters
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Percentage of interests acquired		100.00%
Aggregate consideration for sale and purchase agreement		$ 8,000
Business combination, residual assets		300
Business combination, net assets acquired		5,750
Goodwill, acquired during period		2,100
Trade Mark
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset						$ 2,900	6,600		$ 1,600
Intangible Assets acquired (disposed)	$ (3,100)						5,500
Trade Mark | AgriLogic
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset				$ 4,000
Asset amortization period						10 years
Trade Mark | Blue Waters
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		1,500
Asset amortization period						5 years
Aspen trademark
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset						$ 1,600
Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset						16,700	16,700		16,600
Intangible Assets acquired (disposed)						0	100
Insurance Licenses | Blue Waters
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses		50
Insurance Licenses | AAIC, Aspen Specialty, AIUK [Member]
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset						16,600
Insurance Licenses | Aspen U.K.
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses						2,100
Insurance Licenses | Aspen Specialty
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses						4,500
Insurance Licenses | AAIC
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses						10,000
Agency Relationships
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset						$ 2,300	26,200		0
Intangible Assets acquired (disposed)	(21,800)						28,100
Agency Relationships | AgriLogic
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset				25,000
Asset amortization period						15 years
Agency Relationships | Blue Waters
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		3,100
Asset amortization period						5 years
Non-compete Agreements
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset						$ 700	3,300		0
Intangible Assets acquired (disposed)	$ (900)						3,900
Non-compete Agreements | AgriLogic
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite-lived and finite-lived intangible assets acquired				2,900
Asset amortization period						5 years
Non-compete Agreements | Blue Waters
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		$ 1,000
Asset amortization period						5 years
Value of Business Acquired
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset							0		0
Intangible Assets acquired (disposed)							1,800
Value of Business Acquired | AgriLogic
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Indefinite-lived and finite-lived intangible assets acquired				1,800
Consulting Relationships
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset						$ 0	900		0
Asset amortization period						10 years
Intangible Assets acquired (disposed)						$ 0	1,000
Consulting Relationships | AgriLogic
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset				$ 1,000
Renewal Rights
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset						1,400	1,700		$ 0
Intangible Assets acquired (disposed)						0	1,900
AAIC, Aspen Specialty, AIUK [Member] | Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset							16,600
Liberty Specialty | Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset			$ 1,900
Liberty Specialty | Renewal Rights
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Asset amortization period			5 years
Aspen U.K.
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Initial purchase price					$ 1,600
Aspen U.K. | Trade Mark
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset							$ 1,600
Crop Re
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Goodwill, acquired during period						0
Equity method investment, ownership percentage	23.20%
Digital Re
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Goodwill, acquired during period						$ 1,800
Equity method investment, ownership percentage								49.00%
AgriLogic | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Goodwill, disposed	$ (20,600)
AgriLogic | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Trade Mark
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Intangible Assets acquired (disposed)	(3,100)
AgriLogic | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Agency Relationships
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Intangible Assets acquired (disposed)	(21,800)
AgriLogic | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Non-compete Agreements
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Intangible Assets acquired (disposed)	$ (900)

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Company's Restricted Assets (Details) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2017GBP (£)	Dec. 31, 2016GBP (£)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	$ 4,638.8	$ 4,535.7
Total as percent of investable assets	53.40%	49.30%
Securities and cash as collateral secured letters of credit	$ 658.5	$ 672	£ 0	£ 7.2
Affiliated transactions
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	1,455	1,482.8
Third party
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	2,425.3	2,380.8
Letters of credit / guarantees
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	658.5	672.1
Real estate fund
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	$ 100	$ 0

Commitments and Contingencie114

Commitments and Contingencies - Amounts Outstanding under Operating Leases (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Leases [Abstract]
Year one	$ 16.2	$ 16.1
Year two	16.1	15.3
Year three	15.3	14
Year four	11	9.9
Year five	8.7	8.7
Later Years	78.7	86.4
Total	$ 146	$ 150.4

Commitments and Contingencie115

Commitments and Contingencies - Additional Information (Details) £ in Millions, SGD in Millions, SFr in Millions, CAD in Millions, AUD in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)Investment	Dec. 31, 2016USD ($)Investment	Dec. 31, 2017SGDInvestment	Dec. 31, 2017AUDInvestment	Dec. 31, 2017CADInvestment	Dec. 31, 2017CHF (SFr)Investment	Dec. 31, 2017GBP (£)Investment	Dec. 31, 2016SGDInvestment	Dec. 31, 2016AUDInvestment	Dec. 31, 2016CADInvestment	Dec. 31, 2016CHF (SFr)Investment	Dec. 31, 2016GBP (£)Investment
Restricted Cash And Collateral [Line Items]
Total as percent of investable assets	53.40%	49.30%
Investable assets held by the Company	$ 8,700	$ 9,200
Restricted assets	4,638.8	4,535.7
Investable assets held by insurance regulators	$ 458.7	447.3
Minimum capital required | £							£ 0.4					£ 0.4
Percentage of reinsurance liabilities	100.00%		100.00%	100.00%	100.00%	100.00%	100.00%
Reinsurance liabilities	$ 1,268.4	1,101.5
Assets held-in-trust	1,350.9	1,351.2
Regulatory deposits	6	6
Deposit with states	6.1	6
Rental and premises expenses	26.3	24.1
Depreciation for fixed assets	34.6	22.6
Accumulated depreciation	$ 146.2	$ 111.6
Number of investments | Investment	2	3	2	2	2	2	2	3	3	3	3	3
Us Multi Beneficiary Trust Fund
Restricted Cash And Collateral [Line Items]
Minimum trust fund	$ 20
Assets held-in-trust	1,333.6	$ 1,267.4
Us Surplus Lines Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust	$ 195	185.7
Canadian Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust | CAD					CAD 169.9					CAD 249.2
Australian Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust | AUD				AUD 198.7					AUD 137.8
Swiss Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust | SFr						SFr 9.8					SFr 18.3
Singapore Trust Fund
Restricted Cash And Collateral [Line Items]
Assets held-in-trust | SGD			SGD 120.6					SGD 106.4
Bermuda
Restricted Cash And Collateral [Line Items]
Percentage of reinsurance liabilities	100.00%		100.00%	100.00%	100.00%	100.00%	100.00%
Reinsurance liabilities	$ 895.5	953.2
Real estate fund
Restricted Cash And Collateral [Line Items]
Restricted assets	$ 100	$ 0

Concentration of Credit Risk -

Concentration of Credit Risk - Schedule Of Gross Written Premium From Major Brokers (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Gross written premiums, percentage	100.00%	100.00%	100.00%
Gross written premiums	$ 3,360.9	$ 3,147	$ 2,997.3
Aon Corporation
Investment Holdings [Line Items]
Gross written premiums, percentage	16.40%	18.40%	18.70%
Marsh & McLennan Companies, Inc.
Investment Holdings [Line Items]
Gross written premiums, percentage	16.00%	14.70%	15.40%
Willis Group Holdings, Ltd.
Investment Holdings [Line Items]
Gross written premiums, percentage	13.10%	13.70%	14.50%
Others
Investment Holdings [Line Items]
Gross written premiums, percentage	54.50%	53.20%	51.40%

Concentration of Credit Risk117

Concentration of Credit Risk - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance [Line Items]
Unpaid losses	$ 1,515.2	$ 560.7	$ 354.8
Underwriting premiums	1,496.5	1,399.4
Due for settlement	8.7
Premiums Receivable, Allowance for Doubtful Accounts	$ 5.2	$ 5
Allowable holdings of a single issue or issuer, percentage	2.00%
Percentage of written premium major broker accounted minimum	10.00%
Markel
Reinsurance [Line Items]
Concentration Risk, Percentage	6.40%
Munich Re
Reinsurance [Line Items]
Concentration Risk, Percentage	5.50%
Lloyds
Reinsurance [Line Items]
Concentration Risk, Percentage	5.20%

Reclassifications from Accum118

Reclassifications from Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income Reclassification (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
(Loss)/income from operations before income tax	$ (195.1)	$ (263)	$ 77	$ 99.3	$ (74.1)	$ 100.5	$ 66.2	$ 116.9	$ 123.2	$ 30	$ 51.2	$ 133.1	$ (281.8)	$ 209.5	$ 337.5
Income tax expense	10.2	9.2	(1.2)	(2.8)	2.6	(4.9)	(1.3)	(2.5)	(5.3)	(1.8)	(2.2)	(5.1)	15.4	(6.1)	(14.4)
General, administrative and corporate expenses	(150.1)	(110.9)	(119.9)	(121.3)	(128.9)	(125)	(116.4)	(119.8)	(125.9)	(100.5)	(95.4)	(102.2)	(502.2)	(490.1)	(424)
Income (Loss), Including Portion Attributable to Noncontrolling Interest, before Tax													(266.4)	203.4	323.1
Net (loss)/income	$ (184.9)	$ (253.8)	$ 75.8	$ 96.5	$ (71.5)	$ 95.6	$ 64.9	$ 114.4	$ 117.9	$ 28.2	$ 49	$ 128	(266.4)	203.4	$ 323.1
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (Loss), Including Portion Attributable to Noncontrolling Interest, before Tax													7.2	0.2
Available for sale securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Available for sale securities — gross realized gains													10.7	18.8
Realized (losses) on sale of securities													(6.7)	(8.9)
(Loss)/income from operations before income tax													4	9.9
Income tax expense													(0.4)	(1)
Net (loss)/income													3.6	8.9
Foreign currency translation adjustments | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense													(0.8)	0
General, administrative and corporate expenses													4.4	(8.7)
Income (Loss), Including Portion Attributable to Noncontrolling Interest, before Tax													$ 3.6	$ (8.7)

Credit Facility and Long-Ter119

Credit Facility and Long-Term Debt - Summary of Contractual Obligations Under Long-term Debts (Details) $ in Millions	Dec. 31, 2017USD ($)
Debt Disclosure [Abstract]
Less than 1 year	$ 0
1-3 years	250
3-5 years	0
More than 5 years	300
Total	$ 550

Credit Facility and Long-Ter120

Credit Facility and Long-Term Debt - Additional Information (Details) - USD ($)	Mar. 27, 2017	Nov. 13, 2013	Dec. 15, 2010	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2016
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							$ 2,323,100,000
Percentage of consolidated net income							25.00%
Percentage of aggregate net cash proceeds from the issuance of capital stock							25.00%
Percentage of consolidated leverage ratio permitted							35.00%
Loan notes issued by third parties								$ 130,000,000
Citibank Europe
Line Of Credit Facility Covenant Compliance [Line Items]
Outstanding collateralized letters of credit facility							$ 449,400,000	$ 463,600,000
London Interbank Offered Rate (LIBOR)
Line Of Credit Facility Covenant Compliance [Line Items]
Line of Credit Facility, Interest Rate During Period							1.00%
Citibank Europe plc | Letters of credit / guarantees
Line Of Credit Facility Covenant Compliance [Line Items]
Revolving credit facility, credit agreement maximum limit									$ 550,000,000
Senior Notes
Line Of Credit Facility Covenant Compliance [Line Items]
Loans issued		$ 300,000,000	$ 250,000,000
Debt stated interest rate		4.65%	6.00%
Net proceeds from senior notes		$ 299,700,000	$ 247,500,000
Amount of indebtedness subject to default in payment							$ 50,000,000
Annual interest payable							29,000,000
Silverton 2,015
Line Of Credit Facility Covenant Compliance [Line Items]
Loan notes issued by third parties						$ 85,000,000
Silverton 2015, Third-party Funded
Line Of Credit Facility Covenant Compliance [Line Items]
Loan notes issued by third parties						$ 70,000,000
Silverton 2,016
Line Of Credit Facility Covenant Compliance [Line Items]
Loan notes issued by third parties					$ 125,000,000
Silverton 2016 | Silverton Loan Notes | Aspen Holdings
Line Of Credit Facility Covenant Compliance [Line Items]
Maximum exposure to loss							200,000
Silverton 2,017
Line Of Credit Facility Covenant Compliance [Line Items]
Loan notes issued by third parties				$ 130,000,000			0
Silverton 2017 | Silverton Loan Notes | Aspen Holdings
Line Of Credit Facility Covenant Compliance [Line Items]
Maximum exposure to loss							20,400,000
Silverton 2017, Third-party Funded [Member]
Line Of Credit Facility Covenant Compliance [Line Items]
Loan notes issued by third parties					105,000,000		$ 0
Silverton 2016, Third-party Funded [Member]
Line Of Credit Facility Covenant Compliance [Line Items]
Loan notes issued by third parties					$ 100,000,000

Unaudited Quarterly Financia121

Unaudited Quarterly Financial Data - Summary of Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net earned premium	$ 511	$ 652.5	$ 562	$ 581.1	$ 612.4	$ 681	$ 680.8	$ 663.1	$ 629.7	$ 640.6	$ 609.4	$ 593.6	$ 2,306.6	$ 2,637.3	$ 2,473.3
Net investment income	47.5	46.4	47.4	47.7	43.2	46.4	48	49.5	46.4	45	46.7	47.4	189	187.1	185.5
Realized and unrealized investment gains	18.8	29.9	49	51.2	(29)	26.7	45.1	65.6	12.9	10.7	13.5	57.4	148.9	108.4	94.5
Other income	3.9	(2.2)	3.6	3.6	3.3	1.5	(0.5)	1.4	(0.3)	(2.3)	(1.2)	3.9	8.9	5.7	0.1
Total revenues	581.2	726.6	662	683.6	629.9	755.6	773.4	779.6	688.7	694	668.4	702.3	2,653.4	2,938.5	2,753.4
Losses and loss adjustment expenses	544.2	776.2	346.1	328.2	387.3	389.2	442.2	357.4	334	365.6	360.5	306.1	1,994.7	1,576.1	1,366.2
Amortization of deferred policy acquisition costs	85.1	105.4	96.3	113.7	141.1	130.9	126.7	130.2	118.2	132	114.1	119.3	400.5	528.9	483.6
General, administrative and corporate expenses	150.1	110.9	119.9	121.3	128.9	125	116.4	119.8	125.9	100.5	95.4	102.2	502.2	490.1	424
Interest on long-term debt	7.3	7.4	7.4	7.4	7.4	7.3	7.4	7.4	7.4	7.4	7.3	7.4	29.5	29.5	29.5
Change in fair value of derivatives	(2.5)	(4.5)	(17.6)	(3.1)	17.6	(0.6)	0.4	7.2	(2.5)	(10.1)	(2)	7.8	(27.7)	24.6	(6.8)
Change in fair value of loan notes issued by variable interest entities	(17.6)	(9.8)	3.3	2.9	3.4	9.8	(0.5)	4.4	5.3	8.3	3.3	2.9	(21.2)	17.1	19.8
Realized and unrealized investment losses	4	12.4	7	5	29.1	5.2	8.3	20.6	(17.7)	51.9	28.8	14.5	28.4	63.2	77.5
Net realized and unrealized foreign exchange (gains)/losses(1)	2.8	(8.4)	20.6	8.9	(12)	(10.8)	5.3	15.7	(5)	8.4	11.6	6.4	23.9	(1.8)	21.4
Other (expense)	2.9	0	2	0	1.2	(0.9)	1	0	(0.1)	0	(1.8)	2.6	4.9	1.3	0.7
Total expenses	776.3	989.6	585	584.3	704	655.1	707.2	662.7	565.5	664	617.2	569.2	2,935.2	2,729	2,415.9
(Loss)/income from operations before income tax	(195.1)	(263)	77	99.3	(74.1)	100.5	66.2	116.9	123.2	30	51.2	133.1	(281.8)	209.5	337.5
Income tax credit	10.2	9.2	(1.2)	(2.8)	2.6	(4.9)	(1.3)	(2.5)	(5.3)	(1.8)	(2.2)	(5.1)	15.4	(6.1)	(14.4)
Net (loss)/income	$ (184.9)	$ (253.8)	$ 75.8	$ 96.5	$ (71.5)	$ 95.6	$ 64.9	$ 114.4	$ 117.9	$ 28.2	$ 49	$ 128	$ (266.4)	$ 203.4	$ 323.1
Ordinary shares:
Basic weighted average ordinary shares	59,431,469	59,759,730	59,966,358	59,862,662	60,152,420	60,225,705	60,705,028	60,867,815	60,784,832	60,779,295	61,408,633	62,159,303	59,753,886	60,478,740	61,287,884
Diluted weighted average ordinary shares	59,431,469	59,795,730	61,022,981	61,196,772	61,198,041	61,577,018	62,192,142	62,483,938	62,176,505	62,155,125	62,896,907	63,532,662	59,753,886	61,860,689	62,687,503
Weighted average number of ordinary share and share equivalents
Basic, dollars per share	$ (3.25)	$ (4.48)	$ 1.09	$ 1.39	$ (1.41)	$ 1.43	$ 0.91	$ 1.73	$ 1.78	$ 0.30	$ 0.64	$ 1.91	$ (5.22)	$ 2.67	$ 4.64
Diluted, dollars per share	$ (3.25)	$ (4.48)	$ 1.07	$ 1.36	$ (1.41)	$ 1.40	$ 0.89	$ 1.68	$ 1.75	$ 0.30	$ 0.62	$ 1.87	$ (5.22)	$ 2.61	$ 4.54

Subsequent Events Subsequent Ev

Subsequent Events Subsequent Events - Narrative (Details) - Equity securities - Subsequent Event $ in Millions	3 Months Ended
	Mar. 30, 2018USD ($)
Subsequent Event [Line Items]
Proceeds from sale of equity securities	$ 206.9
Gain on sale of equity securities	$ 36.7

Schedule II - Condensed Fina123

Schedule II - Condensed Financial Information of Registrant Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
ASSETS
Trading Securities	$ 1,649,300	$ 1,265,700
Cash and cash equivalents (including cash within consolidated variable interest entities of — $166.6 and $291.3)	1,054,800	1,273,800	$ 1,099,500	$ 1,178,500
Other assets	500	1,000
Total assets	12,906,400	12,090,100
LIABILITIES
Accrued expenses and other payables	455,400	469,200
Long-term debt	549,500	549,300
Total liabilities	9,977,900	8,441,800
SHAREHOLDERS’ EQUITY
59,474,085 shares of par value 0.15144558¢ each (December 31, 2016 — 59,774,464)	90	91
Additional paid in capital	954,700	1,259,600
Retained earnings	2,026,900	2,392,300
Non-controlling interest	2,700	1,400
Total accumulated other comprehensive (loss)/income	(55,900)	(5,100)	$ 59,600
Total liabilities and shareholders’ equity	$ 12,906,400	$ 12,090,100
Ordinary shares, issued	59,474,085	59,774,464	60,918,373
Ordinary shares, par value	$ 0.0015144558	$ 0.0015144558
Preference shares, par value	$ 0.0015144558	$ 0.0015144558
5.950% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	$ 0	$ 0
Preference shares, issued	11,000,000	11,000,000
Preference shares, rate	5.95%	5.95%
Preference shares, par value	$ 0.0015144558	$ 0.0015144558
7.401% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	$ 0	$ 0
Preference shares, issued	0	5,327,500
Preference shares, rate		7.401%
Preference shares, par value		$ 0.0015144558
7.250% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	$ 0	$ 0
Preference shares, issued	0	6,400,000
Preference shares, rate		7.25%
Preference shares, par value		$ 0.0015144558
5.625% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	$ 0	$ 0
Preference shares, issued	10,000,000	10,000,000
Preference shares, rate	5.625%	5.625%
Preference shares, par value	$ 0.0015144558	$ 0.0015144558
Parent
ASSETS
Short-term investments (available for sale)	$ 0	$ 25,100
Trading Securities	79,400	145,700
Cash and cash equivalents (including cash within consolidated variable interest entities of — $166.6 and $291.3)	32,000	156,300	$ 110,500		$ 86,800
Investments in subsidiaries (1)	3,394,700	3,405,800
Other investments (equity method)	3,400	11,700
Eurobond issued by subsidiary	0	480,000
Long-term debt issued by Silverton	20,600	54,500
Intercompany funds due from affiliates	5,200	9,800
Other assets	8,800	8,600
Total assets	3,544,100	4,297,500
LIABILITIES
Accrued expenses and other payables	4,700	6,400
Intercompany funds due to affiliates	61,400	93,500
Long-term debt	549,500	549,300
Total liabilities	615,600	649,200
SHAREHOLDERS’ EQUITY
59,474,085 shares of par value 0.15144558¢ each (December 31, 2016 — 59,774,464)	100	100
Additional paid in capital	954,700	1,259,600
Retained earnings	2,026,900	2,392,300
Non-controlling interest	2,700	1,400
Gain/(loss) on derivatives	9,700	22,500
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) on Derivatives, Effect Net of Tax	2,100	(500)
Gains on foreign currency translation	(67,700)	(27,100)
Total accumulated other comprehensive (loss)/income	(55,900)	(5,100)
Total shareholders’ equity	2,928,500	3,648,300
Total liabilities and shareholders’ equity	$ 3,544,100	$ 4,297,500
Ordinary shares, issued	597,474,085	59,774,464
Ordinary shares, par value	$ 0.0015144558	$ 0.0015144558
Parent | 5.950% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	$ 0	$ 0
Preference shares, issued	11,000,000	11,000,000
Preference shares, rate	5.95%	5.95%
Preference shares, par value	$ 0.0015144558	$ 0.0015144558
Parent | 7.401% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	$ 0	$ 0
Preference shares, issued		5,327,500
Preference shares, rate		7.401%
Preference shares, par value		$ 0.0015144558
Parent | 7.250% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	0	$ 0
Preference shares, issued		6,400,000
Preference shares, rate		7.25%
Preference shares, par value		$ 0.0015144558
Parent | 5.625% Preference Shares
SHAREHOLDERS’ EQUITY
Preference shares, value	$ 0	$ 0
Preference shares, issued	10,000,000	10,000,000
Preference shares, rate	5.625%	5.625%
Preference shares, par value	$ 0.0015144558	$ 0.15144558

Schedule II - Condensed Fina124

Schedule II - Condensed Financial Information of Registrant Statements of Operations and Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Dividend income													$ 200.1	$ 199.1	$ 197.3
Other income	$ 3.9	$ (2.2)	$ 3.6	$ 3.6	$ 3.3	$ 1.5	$ (0.5)	$ 1.4	$ (0.3)	$ (2.3)	$ (1.2)	$ 3.9	8.9	5.7	0.1
Total revenues	581.2	726.6	662	683.6	629.9	755.6	773.4	779.6	688.7	694	668.4	702.3	2,653.4	2,938.5	2,753.4
Expenses
General, administrative and corporate expenses	(150.1)	(110.9)	(119.9)	(121.3)	(128.9)	(125)	(116.4)	(119.8)	(125.9)	(100.5)	(95.4)	(102.2)	(502.2)	(490.1)	(424)
(Loss)/income from operations before income tax	(195.1)	(263)	77	99.3	(74.1)	100.5	66.2	116.9	123.2	30	51.2	133.1	(281.8)	209.5	337.5
Income tax credit	10.2	9.2	(1.2)	(2.8)	2.6	(4.9)	(1.3)	(2.5)	(5.3)	(1.8)	(2.2)	(5.1)	15.4	(6.1)	(14.4)
Net (loss)/income	$ (184.9)	$ (253.8)	$ 75.8	$ 96.5	$ (71.5)	$ 95.6	$ 64.9	$ 114.4	$ 117.9	$ 28.2	$ 49	$ 128	(266.4)	203.4	323.1
Amount attributable to non-controlling interest													(1.3)	(0.1)	(0.8)
Net (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders													(267.7)	203.3	322.3
Other Comprehensive Income:
Change in unrealized gains on investments													(12.8)	(37.7)	(105.2)
Other comprehensive (loss)/income, net of tax													(50.8)	(64.7)	(174.7)
Total comprehensive (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders													(318.5)	138.6	147.6
Parent
Condensed Financial Statements, Captions [Line Items]
Equity in net earnings of subsidiaries and other investments, equity method													(590.7)	(85.2)	59
Dividend income													373.6	331.3	292.3
Interest income on Eurobond													18.7	21.1	29.5
Net realized and unrealized investment gains/(losses)													(2.3)	3.8	4.3
Other income													0	0	1.9
Total revenues													(200.7)	271	387
Expenses
General, administrative and corporate expenses													(36.2)	(38.1)	(34.4)
Interest expense													(29.5)	(29.5)	(29.5)
(Loss)/income from operations before income tax													(266.4)	203.4	323.1
Income tax credit													0	0	0
Net (loss)/income													(266.4)	203.4	323.1
Amount attributable to non-controlling interest													(1.3)	(0.1)	(0.8)
Net (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders													(267.7)	203.3	322.3
Other Comprehensive Income:
Change in unrealized gains on investments													(12.8)	(37.7)	(105.2)
Net change from current period hedged transactions													2.6	0.7	2.6
Change in foreign currency translation adjustment													(40.6)	(27.7)	(72.1)
Other comprehensive (loss)/income, net of tax													(50.8)	(64.7)	(174.7)
Total comprehensive (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders													$ (318.5)	$ 138.6	$ 147.6

Schedule II - Condensed Fina125

Schedule II - Condensed Financial Information of Registrant Statements of Cash Flows (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from/(used in) operating activities:
Share-based compensation		$ 9.8	$ 15.5	$ 17.9
Net cash (used in)/generated by operating activities		(111.5)	453.2	574.2
Cash flows (used in)/from investing activities:
Proceeds/(purchases) of short term investments		96	190.6	45.6
Repayment of loan notes issued by Silverton		(115.6)	(89.3)	(67.8)
Payments for acquisitions and investments, net of cash acquired		(64.9)	(3.3)
Other equity investments		(2.4)	(3.3)	(0.8)
Net cash from/(used in) investing activities		419	(350.9)	(501.9)
Cash flows (used in)/from financing activities:
Proceeds from the issuance of ordinary shares, net of issuance costs		0.5	2.5	6.8
Proceeds from the issuance of preference shares, net of issuance costs		0	241.3	0
Preference share redemption		(293.2)	0	0
Ordinary share repurchase		(30)	(75)	(83.7)
Proceeds from long term debt		0	105	100
Cash paid for withholdings purposes	[1]	9.6	0	0
Net cash (used in)/from financing activities		(540.4)	90	(133.5)
Increase/(decrease) in cash and cash equivalents		(219)	174.3	(79)
Cash and cash equivalents at beginning of period		1,273.8	1,099.5	1,178.5
Cash and cash equivalents at end of period		1,054.8	1,273.8	1,099.5
Parent
Cash flows from/(used in) operating activities:
Net income (1) (excluding equity in net earnings of subsidiaries)		323	289.3	256.4
Share-based compensation		9.8	15.5	17.9
Realized and unrealized losses/(gains)		(2)	(3.2)	(4.3)
Loss on derivative contracts		(2.6)	0.5	1.2
Increase (Decrease) in Accounts and Other Receivables		0	0	0
Change in other assets		(0.2)	0.4	(9)
Change in accrued expenses and other payables		7	(9.3)	(36.8)
Change in intercompany activities		(27.5)	(40.7)	71.9
Net cash (used in)/generated by operating activities		307.5	252.5	297.3
Cash flows (used in)/from investing activities:
Proceeds/(purchases) of short term investments		(25.1)	0.1	25
Payments to Acquire Marketable Securities		66.3	(145.7)	0
Investment in subsidiaries		(111.9)	(126.1)	(171.5)
Investment in long-term debt issued by Silverton		0	(25)	(25)
Repayment of loan notes issued by Silverton		13.5	19.2	20.5
Other equity investments		0	0	0
Net cash from/(used in) investing activities		(7.1)	(281)	(201.8)
Cash flows (used in)/from financing activities:
Proceeds from the issuance of ordinary shares, net of issuance costs		0.5	2.5	6.8
Proceeds from the issuance of preference shares, net of issuance costs		0	241.3	0
Preference share redemption		(293.2)	0	0
Ordinary share repurchase		(30)	(75)	(83.7)
Ordinary and preference share dividends paid		(92.4)	(94.5)	(88.7)
Proceeds from maturity of Eurobond		0	0	573.8
Eurobond purchased from subsidiary		0	0	(480)
Cash paid for withholdings purposes		9.6	0	0
Net cash (used in)/from financing activities		(424.7)	74.3	(71.8)
Increase/(decrease) in cash and cash equivalents		(124.3)	45.8	23.7
Cash and cash equivalents at beginning of period		156.3	110.5
Cash and cash equivalents at end of period		32	156.3	110.5
MVI | Parent
Cash flows (used in)/from investing activities:
Payments for acquisitions and investments, net of cash acquired		(0.1)	0	(0.8)
Bene | Parent
Cash flows (used in)/from investing activities:
Payments for acquisitions and investments, net of cash acquired		$ 0	$ (3.3)	$ 0
[1]	The cash paid to the tax authority when withholding shares from employees' awards for tax-withholding purposes has been reclassified from operating activity to financing activity following the adoption of ASU 2016-09 -“Compensation — Stock Compensation”.

Schedule III - Supplementary126

Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred Policy Acquisition Costs	$ 294.3				$ 358.4				$ 361.1				$ 294.3	$ 358.4	$ 361.1
Net Reserves for Losses and LAE	5,234.3				4,759.2				4,583.4				5,234.3	4,759.2	4,583.4	$ 4,400.8
Net Reserves for Unearned Premiums	1,335.3				1,363.4				1,418.3				1,335.3	1,363.4	1,418.3
Net earned premium	511	$ 652.5	$ 562	$ 581.1	612.4	$ 681	$ 680.8	$ 663.1	629.7	$ 640.6	$ 609.4	$ 593.6	2,306.6	2,637.3	2,473.3
Net Investment Income													189	187.1	185.5
Losses and LAE Expenses													1,994.7	1,576.1	1,366.2
Policy Acquisition Expenses													400.5	528.9	483.6
Net written premiums													2,212.5	2,593.7	2,646.2
General and Administrative Expenses													411.2	406.6	360.1
Reinsurance
Deferred Policy Acquisition Costs	263				239.6				235.2				263	239.6	235.2
Net Reserves for Losses and LAE	2,917.1				2,462.1				2,409.5				2,917.1	2,462.1	2,409.5
Net Reserves for Unearned Premiums	1,067.3				813.3				484.2				1,067.3	813.3	484.2
Net earned premium													1,206.1	1,181.9	1,072.6
Losses and LAE Expenses													1,116.4	657.9	491.6
Policy Acquisition Expenses													235.5	226.4	224.7
Net written premiums													1,250	1,269.2	1,153.5
General and Administrative Expenses													157.3	178.2	146.5
Insurance
Deferred Policy Acquisition Costs	31.3				118.8				125.9				31.3	118.8	125.9
Net Reserves for Losses and LAE	2,317.2				2,297.1				2,173.9				2,317.2	2,297.1	2,173.9
Net Reserves for Unearned Premiums	$ 268				$ 550.1				$ 934.1				268	550.1	934.1
Net earned premium													1,100.5	1,455.4	1,400.7
Losses and LAE Expenses													878.3	918.2	874.6
Policy Acquisition Expenses													165	302.5	258.9
Net written premiums													962.5	1,324.5	1,492.7
General and Administrative Expenses													$ 253.9	$ 228.4	$ 213.6

Schedule IV - Reinsurance Premi

Schedule IV - Reinsurance Premiums Written and Premiums Earned (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
Direct													$ 1,812.4	$ 1,733.8	$ 1,748.4
Assumed													1,548.5	1,413.2	1,248.9
Ceded													(1,148.4)	(553.3)	(351.1)
Net written premiums													2,212.5	2,593.7	2,646.2
Gross Amount													1,757.4	1,768.4	1,703.3
Ceded to Other Companies													(902.6)	(449)	(383.5)
Assumed From Other Companies													1,451.8	1,317.9	1,153.5
Net premiums earned	$ 511	$ 652.5	$ 562	$ 581.1	$ 612.4	$ 681	$ 680.8	$ 663.1	$ 629.7	$ 640.6	$ 609.4	$ 593.6	$ 2,306.6	$ 2,637.3	$ 2,473.3
Percentage of Amount Assumed to Net													62.90%	50.00%	46.60%

Schedule V - Valuation and Q128

Schedule V - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Premiums receivable from underwriting activities
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ 5	$ 2.6	$ 4.3
Charged to Costs and Expenses	0.2	2.4	0
Charged to Other Accounts	0	0	(1.7)
Deductions	0	0	0
Balance at End of Year	5.2	5	2.6
Reinsurance
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	0	0	0
Charged to Costs and Expenses	0
Charged to Other Accounts	0	0	0
Deductions	0	0	0
Balance at End of Year	$ 0	$ 0	$ 0