Aspen Insurance Holdings Limited (AHL-PC) 10-Q 30 Apr 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	Apr. 24, 2015
Document and Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	AHL
Entity Registrant Name	ASPEN INSURANCE HOLDINGS LTD
Entity Central Index Key	1267395
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		61,618,257

CONDENSED_CONSOLIDATED_BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Investments:
Fixed income securities, available for sale at fair value (amortized cost — $5,345.4 and $5,462.9)	$5,539.80	$5,630
Fixed income securities, trading at fair value (amortized cost — $779.5 and $760.9)	799.4	771
Equity securities, available for sale at fair value (cost — $Nil and $82.6)	0	109.9
Equity securities, trading at fair value (cost — $696.7 and $585.2)	719	616
Short-term investments, available for sale at fair value (amortized cost — $180.1 and $258.2)	180.2	258.3
Short-term investments, trading at fair value (amortized cost — $0.7 and $0.2)	0.7	0.2
Catastrophe bonds, trading at fair value (cost — $32.6 and $34.4)	32.8	34.8
Other investments, equity method	9.5	8.7
Total investments	7,281.40	7,428.90
Cash and cash equivalents (including $98.4 and $176.7 within consolidated variable interest entities)	1,225.90	1,178.50
Reinsurance recoverables
Unpaid losses	360.1	350
Ceded unearned premiums	276.4	206.8
Receivables
Underwriting premiums	1,264.80	1,011.70
Other	92.1	90.2
Funds withheld	46.1	46.9
Deferred policy acquisition costs	333.8	299
Derivatives at fair value	2.1	8
Receivable for securities sold	0.3	2.3
Office properties and equipment	61.9	62.2
Other assets	18.2	13.6
Intangible assets	18.2	18.2
Total assets	10,981.30	10,716.30
Insurance reserves
Losses and loss adjustment expenses	4,698.90	4,750.80
Unearned premiums	1,665.10	1,441.80
Total insurance reserves	6,364	6,192.60
Payables
Reinsurance premiums	171.5	92
Current taxation	27.9	18.3
Deferred taxation	6.9	3.1
Accrued expenses and other payables	308.6	356.9
Liabilities under derivative contracts	11.5	14.3
Total payables	526.4	484.6
Loan notes issued by variable interest entities, at fair value	76	70.7
Long-term debt	549.1	549.1
Total liabilities	7,515.50	7,297
Commitments and contingent liabilities (see Note 16)	0	0
SHAREHOLDERS’ EQUITY
Ordinary shares - 62,772,502 shares of par value 0.15144558¢ each (December 31, 2014 — 62,017,368)	0.1	0.1
Preference shares:
Non-controlling interest	0.5	0.5
Additional paid-in capital	1,106	1,134.30
Retained earnings	2,156.20	2,050.10
Accumulated other comprehensive income, net of taxes	203	234.3
Total shareholders’ equity	3,465.80	3,419.30
Total liabilities and shareholders’ equity	10,981.30	10,716.30
5.95% 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

CONDENSED_CONSOLIDATED_BALANCE1

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, except Share data, unless otherwise specified
Fixed income securities, available for sale, amortized cost	$5,345.40	$5,462.90
Fixed income securities, trading at fair value, amortized cost	779.5	760.9
Equity securities, available for sale at fair value, cost	0	82.6
Equity securities, trading at fair value, cost	696.7	585.2
Short-term investments, available for sale at fair value, amortized cost	180.1	258.2
Short-term investments, trading at fair value, amortized cost	0.7	0.2
Catastrophe bonds, trading at fair value, cost	32.6	34.4
Cash and cash equivalents (including $98.4 and $176.7 within consolidated variable interest entities)	1,225.90	1,178.50
Ordinary shares, par value (in dollars per share)	$0.15	$0.15
Ordinary shares, issued (in shares)	61,722,502	62,017,368
Preference shares, par value (in dollars per share)	$0.15	$0.15
5.95% Preference Shares
Preference shares, issued (in shares)	11,000,000	11,000,000
Preference shares, rate	5.95%	5.95%
Preference shares, par value (in dollars per share)	$0.15	$0.15
7.401% Preference Shares
Preference shares, issued (in shares)	5,327,500	5,327,500
Preference shares, rate	7.40%	7.40%
Preference shares, par value (in dollars per share)	$0.15	$0.15
7.250% preference shares
Preference shares, issued (in shares)	6,400,000	6,400,000
Preference shares, rate	7.25%	7.25%
Preference shares, par value (in dollars per share)	$0.15	$0.15
Variable Interest Entity, Primary Beneficiary
Cash and cash equivalents (including $98.4 and $176.7 within consolidated variable interest entities)	$98.40	$176.70

CONDENSED_CONSOLIDATED_STATEME

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND OTHER COMPREHENSIVE INCOME (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Revenues
Net earned premium	$593.60	$566.50
Net investment income	47.4	49.5
Realized and unrealized investment gains	57.4	17.9
Other income	3.9	0.6
Total revenues	702.3	634.5
Expenses
Losses and loss adjustment expenses	306.1	288.1
Amortization of deferred policy acquisition costs	119.3	112
General, administrative and corporate expenses	102.2	95.6
Interest on long-term debt	7.4	7.4
Change in fair value of derivatives	7.8	-1.1
Change in fair value of loan notes issued by variable interest entities	2.9	3.4
Realized and unrealized investment losses	14.5	4.3
Net realized and unrealized foreign exchange losses/(gains)	6.4	-0.1
Other expenses	2.6	0.7
Total expenses	569.2	510.3
Income from operations before income tax	133.1	124.2
Income tax expense	-5.1	-3.8
Net income	128	120.4
Add: loss attributable to non-controlling interest	0	-0.1
Net income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	128	120.3
Available for sale investments:
Reclassification adjustment for net realized (gains) on investments included in net income	-32.3	-0.4
Change in net unrealized gains on available for sale securities held	32.4	23
Net change from current period hedged transactions	-2.4	0
Change in foreign currency translation adjustment	-27.7	-4.8
Other comprehensive (loss)/income, gross of tax	-30	17.8
Tax thereon:
Reclassification adjustment for net realized gains on investments included in net income	0.4	0.2
Change in foreign currency translation adjustment	1.6	0
Change in net unrealized (losses) on available for sale securities held	-3.3	-2.5
Total tax on other comprehensive (loss)	-1.3	-2.3
Other comprehensive (loss)/income net of tax	-31.3	15.5
Total comprehensive income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	$96.70	$135.80
Weighted average number of ordinary share and share equivalents
Basic (in shares)	62,159,303	65,289,351
Diluted (in shares)	63,532,662	66,565,890
Basic earnings per ordinary share adjusted for preference share dividends	$1.91	$1.70
Diluted earnings per ordinary share adjusted for preference share dividends	$1.87	$1.66

CONDENSED_CONSOLIDATED_STATEME1

CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (USD $)	Total	Ordinary shares	Preference shares	Non-controlling interest	Additional paid-in capital	Additional paid-in capital	Retained earnings	Retained earnings	Retained earnings	Cumulative foreign currency translation adjustments, net of taxes:	Loss on derivatives, net of taxes:	Unrealized appreciation on investments, net of taxes:
In Millions, unless otherwise specified						Ordinary shares		Ordinary shares	Preference shares
Shareholder's equity at Dec. 31, 2013		$0.10	$0	($0.30)	$1,297.40		$1,783.30			$88.60	$0	$130.50
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change attributable to non-controlling interest for the period	-0.1			0.1			-0.1
Shares issued						0.8
Ordinary shares repurchased and canceled						-30.9
Share-based compensation					2.6
Net income for the period	120.4						120.4
Dividends on ordinary shares								-11.7
Net change from current period hedged transaction											0
Dividends on preference shares	-9.5								-9.5
Change for the period, net of income tax										-4.8		20.3
Total accumulated other comprehensive income, net of taxes	234.6
Shareholder's equity at Mar. 31, 2014	3,386.80	0.1	0	-0.2	1,269.90		1,882.40			83.8	0	150.8
Shareholder's equity at Dec. 31, 2014	3,419.30	0.1	0	0.5	1,134.30		2,050.10			72.7	-3.8	165.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change attributable to non-controlling interest for the period	0			0			0
Shares issued						3.5
Ordinary shares repurchased and canceled						-36.5
Share-based compensation					4.7
Net income for the period	128						128
Dividends on ordinary shares								-12.4
Net change from current period hedged transaction											-2.4
Dividends on preference shares	-9.5								-9.5
Change for the period, net of income tax										-26.1		-2.8
Total accumulated other comprehensive income, net of taxes	203
Shareholder's equity at Mar. 31, 2015	$3,465.80	$0.10	$0	$0.50	$1,106		$2,156.20			$46.60	($6.20)	$162.60

CONDENSED_CONSOLIDATED_STATEME2

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities:
Net income	$128	$120.40
Net profit attributable to non-controlling interest	0	-0.1
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	11.2	1.2
Share-based compensation	4.7	2.6
Realized and unrealized investment (gains)	-57.4	-17.9
Realized and unrealized investment losses	14.5	4.3
Change in fair value of loan notes issued by variable interest entities	2.9	3.4
Net realized and unrealized investment foreign exchange losses	21.7	-0.8
Loss on derivative contracts	-2.4	0
Insurance reserves:
Losses and loss adjustment expenses	34.1	81.6
Unearned premiums	217.8	200.4
Reinsurance recoverables:
Unpaid losses	-14.5	-19.4
Ceded unearned premiums	-70.8	-69.4
Other receivables	-1.7	-16.5
Deferred policy acquisition costs	-37.6	-27.3
Reinsurance premiums payable	80.8	66.6
Funds withheld	0.8	-2.9
Premiums receivable	-278.6	-190.5
Deferred taxes	8.7	2.1
Income tax payable	-5.2	1
Accrued expenses and other payables	16.9	76.8
Fair value of derivatives and settlement of liabilities under derivatives	3.1	-2
Other assets	-5	-1.7
Net cash generated by operating activities	72	211.9
Cash flows (used in) investing activities:
(Purchases) of fixed income securities — Available for sale	-483.7	-537.2
(Purchases) of fixed income securities — Trading	-153.4	-192.1
Proceeds from sales and maturities of fixed income securities — Available for sale	540.7	598.8
Proceeds from sales and maturities of fixed income securities — Trading	133.8	181.3
(Purchases) of equity securities — Available for sale	0	0
(Purchases) of equity securities — Trading	-186.7	-51.8
Net proceeds/(purchases) of catastrophe bonds — Trading	1.8	-12.1
Proceeds from sales of equity securities — Available for sale	108.6	4
Proceeds from sales of equity securities — Trading	83.7	5.3
Net (purchases)/sales of short-term investments — Available for sale	61.3	-61.9
Net (purchases) of short-term investments — Trading	-0.5	0
Net change in receivable for securities sold	15.9	13.6
Net proceeds from other investments	0	37.3
(Purchases)/sales of equipment	-5.6	2.7
Investment in Micro-insurance venture	-0.8	0
Net cash (used in) investing activities	115.1	-12.1
Cash flows (used in) financing activities:
Proceeds from the issuance of ordinary shares, net of issuance costs	3.5	0.8
Ordinary shares repurchased	-36.5	-30.9
Repayment of long-term debt issued by Silverton	-64.9	0
Dividends paid on ordinary shares	-12.4	-11.7
Dividends paid on preference shares	-9.5	-9.5
Net cash (used in) financing activities	-119.8	-51.3
Effect of exchange rate movements on cash and cash equivalents	-19.9	1.6
Increase in cash and cash equivalents	47.4	150.1
Cash and cash equivalents at beginning of period	1,178.50	1,293.60
Cash and cash equivalents at end of period	1,225.90	1,443.70
Supplemental disclosure of cash flow information:
Net cash (received)/paid during the period for income tax	-3.9	5.6
Cash paid during the period for interest	$0	$7.30

History_and_Organization

History and Organization	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
History and Organization	History and Organization
	Aspen Insurance Holdings Limited (“Aspen Holdings”) was incorporated on May 23, 2002 and holds subsidiaries that provide insurance and reinsurance on a worldwide basis. Its principal operating subsidiaries are Aspen Insurance UK Limited (“Aspen U.K.”), Aspen Bermuda Limited (“Aspen Bermuda”), Aspen Specialty Insurance Company (“Aspen Specialty”), Aspen American Insurance Company (“AAIC”) and Aspen Underwriting Limited (corporate member of Lloyd’s Syndicate 4711, “AUL”) (collectively, the “Operating Subsidiaries”). We also established Aspen Capital Management, Ltd and other related entities (collectively, “ACM”) to leverage our existing underwriting franchise, increase our operational flexibility in the capital markets and provide investors direct access to our underwriting expertise. In such regard, Silverton Re Ltd. (“Silverton”), a sidecar, was established in 2013 to attract third-party capital and to provide additional collateralized capacity to support Aspen Re’s global reinsurance business. References to the “Company,” “we,” “us” or “our” refer to Aspen Holdings or Aspen Holdings and its subsidiaries.

Basis_of_Preparation

Basis of Preparation	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Basis of Preparation	Basis of Preparation
	The accompanying unaudited condensed consolidated financial statements have been prepared on the basis of generally accepted accounting principles in the United States (“U.S. GAAP”) for interim financial information and in accordance with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by U.S. GAAP for complete financial statements. In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation have been included. Results for the three months ended March 31, 2015 are not necessarily indicative of the results that may be expected for the year ended December 31, 2015. The unaudited condensed consolidated financial statements include the accounts of Aspen Holdings and its subsidiaries. All intercompany transactions and balances have been eliminated on consolidation.
	The balance sheet as at December 31, 2014 has been derived from the audited consolidated financial statements at that date, but does not include all of the information and footnotes required by U.S. GAAP for complete financial statements. These unaudited condensed consolidated financial statements and notes thereto should be read in conjunction with the consolidated financial statements and notes thereto for the year ended December 31, 2014 contained in the Company’s Annual Report on Form 10-K filed with the United States Securities and Exchange Commission on February 23, 2015 (File No. 001-31909).
	Assumptions and estimates made by management have a significant effect on the amounts reported within the unaudited condensed consolidated financial statements. The most significant of these relate to losses and loss adjustment expenses, the value of investments, reinsurance recoverables and the fair value of derivatives. 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 Pronouncements Not Yet Adopted
	On January 5, 2015, the Financial Accounting Standards Board (“FASB”) issued ASU 2015-01, “Income Statement - Extraordinary and Unusual Items (Subtopic 225-20) - Simplifying Income Statement Presentation by Eliminating the Concept of Extraordinary Items” which eliminates from U.S. GAAP the requirement for an entity to separately classify, present and disclose extraordinary events and transactions as part of FASB’s initiative to reduce complexity in accounting standards. ASU 2015-01 is effective for fiscal years beginning after December 15, 2015 and the Company may apply the amendments prospectively or retrospectively to all prior periods presented in the financial statements. Early application is permitted provided the guidance is applied from the beginning of the fiscal year of adoption. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	On February 18, 2015, the FASB issued ASU 2015-02, “Consolidation (Topic 810) - Amendments to the Consolidation Analysis” which includes amendments that change the analysis that an entity must perform to determine whether it should consolidate certain types of legal entities. ASU 2015-02 is effective for fiscal years beginning after December 15, 2015. The Company does not expect this ASU to have a material impact on its consolidated financial statements.

Reclassifications_from_Accumul

Reclassifications from Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2015
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 accumulated other comprehensive income (“AOCI”) that are reclassified into the unaudited condensed consolidated statement of operations for the three months ended March 31, 2015 and 2014:
		Amount Reclassified from AOCI
	Details about the AOCI Components	Three Months Ended March 31, 2015			Three Months Ended March 31, 2014			Affected Line Item in the Unaudited
								Condensed Consolidated Statement
								of Operations
		($ in millions)
	Available for sale securities:
	Realized gains on sale of securities	$	33.5		$	3		Realized and unrealized investment gains
	Realized (losses) on sale of securities	(1.2		)	(2.6		)	Realized and unrealized investment losses
		32.3			0.4			Income from operations before income tax
	Tax on net realized gains of securities	(0.4		)	(0.2		)	Income tax expense
	Total reclassifications from AOCI to the statement of operations, net of tax	$	31.9		$	0.2		Net income

Earnings_per_Ordinary_Share

Earnings per Ordinary Share	3 Months Ended
	Mar. 31, 2015
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. 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 three months ended March 31, 2015 and 2014, respectively:
		Three Months Ended March 31,
		2015			2014
		($ in millions, except share and per share amounts)
	Net income	$	128		$	120.4
	Preference share dividends	(9.5		)	(9.5		)
	Net profit attributable to non-controlling interest	—			(0.1		)
	Basic and diluted net income available to ordinary shareholders	118.5			110.8
	Ordinary shares:
	Basic weighted average ordinary shares	62,159,303			65,289,351
	Weighted average effect of dilutive securities(1)	1,373,359			1,276,539
	Total diluted weighted average ordinary shares	63,532,662			66,565,890
	Earnings per ordinary share:
	Basic	$	1.91		$	1.7
	Diluted	$	1.87		$	1.66
	(1)	Dilutive securities comprise: employee options, 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 14.
	Dividends. On April 22, 2015, the Company’s Board of Directors (“Board of Directors”) declared the following quarterly dividends:
		Dividend			Payable on:		Record Date:
	Ordinary shares	$	0.21		May 27, 2015		May 8, 2015
	7.401% preference shares	$	0.462563		July 1, 2015		June 15, 2015
	7.250% preference shares	$	0.4531		July 1, 2015		June 15, 2015
	5.95% preference shares	$	0.3719		July 1, 2015		June 15, 2015

Segment_Reporting

Segment Reporting	3 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
Segment Reporting	Segment Reporting
	The Company has two reporting business segments: Insurance and Reinsurance. In arriving at these reporting segments, the Company considered similarities in economic characteristics, products, customers, distribution, the regulatory environment of the Company’s operating segments and quantitative thresholds. Segment profit or loss for each of the Company’s operating segments is measured by underwriting profit or loss. Underwriting profit is the excess of net earned premiums over the sum of losses and loss expenses, amortization of deferred policy acquisition costs and general and administrative expenses. Underwriting profit or loss provides a basis for management to evaluate the segment’s underwriting performance.
	Reinsurance Segment. The reinsurance segment consists of property catastrophe reinsurance, other property reinsurance (risk excess, pro rata and facultative), casualty reinsurance (U.S. treaty, international treaty and global facultative) and specialty reinsurance (credit and surety, agriculture, marine, aviation, engineering and other specialty). ACM forms part of our property catastrophe reinsurance line of business as it currently focuses 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” in the Company’s 2014 Annual Report on Form 10-K filed with the United States Securities and Exchange Commission.
	Insurance Segment. The insurance segment consists of property and casualty insurance (including the programs business which was previously reported separately), 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” in the Company’s 2014 Annual Report on Form 10-K filed with the United States Securities and Exchange Commission.
	Non-underwriting Disclosures. The Company has provided additional disclosures for corporate and other (non-underwriting) income and expenses. Corporate and other income and expenses include net investment income, net realized and unrealized investment gains or losses, expenses associated with managing the group, certain strategic and non-recurring costs, changes in fair value of derivatives and changes in fair value of the loan notes issued by variable interest entities, interest expenses, net realized and unrealized foreign exchange gains or losses and income taxes, which are not allocated to the underwriting segments. Corporate expenses are not allocated to the Company’s operating segments as they typically do not fluctuate with the levels of premiums written and are not directly related to the Company’s segment operations. The Company does not allocate its assets by segment as it evaluates underwriting results of each segment separately from the results of the Company’s investment portfolio.
	The following tables provide a summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of the Company’s business segments for the three months ended March 31, 2015 and 2014:
		Three Months Ended March 31, 2015
		Reinsurance			Insurance			Total
		($ in millions)
	Underwriting Revenues
	Gross written premiums	$	484.8		$	434.4		$	919.2
	Net written premiums	442.1			321.1			763.2
	Gross earned premiums	265.8			415.1			680.9
	Net earned premiums	249.4			344.2			593.6
	Underwriting Expenses
	Losses and loss adjustment expenses	105.5			200.6			306.1
	Amortization of deferred policy acquisition costs	53.4			65.9			119.3
	General and administrative expenses	32.4			55.3			87.7
	Underwriting income	$	58.1		$	22.4		80.5
	Corporate expenses							(14.5		)
	Net investment income							47.4
	Realized and unrealized investment gains							57.4
	Realized and unrealized investment losses							(14.5		)
	Change in fair value of loan notes issued by variable interest entities							(2.9		)
	Change in fair value of derivatives							(7.8		)
	Interest expense on long term debt							(7.4		)
	Net realized and unrealized foreign exchange (losses)							(6.4		)
	Other income							3.9
	Other expenses							(2.6		)
	Income before tax							$	133.1
	Net reserves for loss and loss adjustment expenses	$	2,410.90		$	1,927.90		$	4,338.80
	Ratios
	Loss ratio	42.3		%	58.3		%	51.6		%
	Policy acquisition expense ratio	21.4			19.1			20.1
	General and administrative expense ratio	13			16.1			17.2			(1)
	Expense ratio	34.4			35.2			37.3
	Combined ratio	76.7		%	93.5		%	88.9		%
	(1)	The general and administrative expense ratio in the total column includes corporate expenses.
		Three Months Ended March 31, 2014
		Reinsurance			Insurance			Total
		( $ in millions)
	Underwriting Revenues
	Gross written premiums	$	472.2		$	383.3		$	855.5
	Net written premiums	442.6			254.9			697.5
	Gross earned premiums	278.5			373.6			652.1
	Net earned premiums	266.7			299.8			566.5
	Underwriting Expenses
	Losses and loss adjustment expenses	110.4			177.7			288.1
	Amortization of deferred policy acquisition costs	50.4			61.6			112
	General and administrative expenses	32.8			45.9			78.7
	Underwriting income	$	73.1		$	14.6		87.7
	Corporate expenses							(16.9		)
	Net investment income							49.5
	Realized and unrealized investment gains							17.9
	Realized and unrealized investment losses							(4.3		)
	Change in fair value of loan notes issued by variable interest entities							(3.4		)
	Change in fair value of derivatives							1.1
	Interest expense on long term debt							(7.4		)
	Net realized and unrealized foreign exchange gains							0.1
	Other income							0.6
	Other expenses							(0.7		)
	Income before tax							$	124.2
	Net reserves for loss and loss adjustment expenses	$	2,626.30		$	1,782.30		$	4,408.60
	Ratios
Loss ratio	41.4		%	59.3		%	50.9		%
Policy acquisition expense ratio	18.9			20.5			19.8
General and administrative expense ratio	12.3			15.3			16.9			(1)
Expense ratio	31.2			35.8			36.7
Combined ratio	72.6		%	95.1		%	87.6		%
(1)	The general and administrative expense ratio in the total column includes corporate expenses.

Investments

Investments	3 Months Ended
	Mar. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Investments	Investments
	Income Statement
	Investment Income. The following table summarizes investment income for the three months ended March 31, 2015 and 2014:
		For the Three Months Ended
		31-Mar-15			31-Mar-14
		($ in millions)
	Fixed income securities — Available for sale	$	35.2		$	38.5
	Fixed income securities — Trading	7			6.6
	Short-term investments — Available for sale	0.4			0.3
	Fixed term deposits (included in cash and cash equivalents)	1.1			0.8
	Equity securities — Available for sale	—			1.5
	Equity securities — Trading	6.2			3.9
	Catastrophe bonds — Trading	0.4			0.2
	Total	$	50.3		$	51.8
	Investment expenses	(2.9		)	(2.3		)
	Net investment income	$	47.4		$	49.5
	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 three months ended March 31, 2015 and 2014:
		For the Three Months Ended
		31-Mar-15			31-Mar-14
		($ in millions)
	Available for sale:
	Fixed income securities — gross realized gains	$	6.2		$	4.4
	Fixed income securities — gross realized (losses)	(0.5		)	(3.1		)
	Equity securities — gross realized gains	31.9			1.3
	Equity securities — gross realized (losses)	(3.0		)	—
	Trading:
	Fixed income securities — gross realized gains	2			1.9
	Fixed income securities — gross realized (losses)	(2.2		)	(1.2		)
	Equity securities — gross realized gains	16.5			1
	Equity securities — gross realized (losses)	(8.8		)	—
	Catastrophe bonds	—			0.1
	Net change in gross unrealized gains	0.8			9.2
	Total net realized and unrealized investment gains recorded in the statement of operations	$	42.9		$	13.6
	Change in available for sale net unrealized gains (losses):
	Fixed income securities	27.3			22.1
	Short-term investments	0.1			—
	Equity securities	(27.3		)	0.5
	Total change in pre-tax available for sale unrealized (losses) gains	0.1			22.6
	Change in taxes	(2.9		)	(2.3		)
	Total change in net unrealized (losses) gains, net of taxes, recorded in other comprehensive income	$	(2.8	)	$	20.3
	Other-than-temporary Impairments. A security is potentially impaired when its fair value is below its amortized cost. The Company reviews its available for sale fixed income and equity portfolios on an individual security basis for potential other-than-temporary impairment (“OTTI”) each quarter based on criteria including issuer-specific circumstances, credit ratings actions and general macro-economic conditions. For a more detailed description of OTTI, please refer to Note 2(c) of the “Notes to the Audited Consolidated Financial Statements” in the Company’s 2014 Annual Report on Form 10-K filed with the United States Securities and Exchange Commission. There was no OTTI charge recognized for the three months ended March 31, 2015 (2014 — $Nil).
	Balance Sheet
	Fixed Income Securities, Short-Term Investments and Equities — Available For Sale. The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income securities, short-term investments and equity securities as at March 31, 2015 and December 31, 2014:
		As at March 31, 2015
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	1,015.40		$	26.6		$	(0.2	)	$	1,041.80
	U.S. agency	175.4			7.5			—			182.9
	Municipal	27.9			2.8			—			30.7
	Corporate	2,242.00			93.5			(1.9		)	2,333.60
	Non-U.S. government-backed corporate	66.8			1.4			—			68.2
	Foreign government	620.3			17.5			(0.1		)	637.7
	Asset-backed	138.6			2.3			—			140.9
	Non-agency commercial mortgage-backed	37.6			2.8			—			40.4
	Agency mortgage-backed	1,021.40			43.6			(1.4		)	1,063.60
	Total fixed income securities — Available for sale	5,345.40			198			(3.6		)	5,539.80
	Total short-term investments — Available for sale	180.1			0.2			(0.1		)	180.2
	Total	$	5,525.50		$	198.2		$	(3.7	)	$	5,720.00
		As at December 31, 2014
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
	($ in millions)
U.S. government	$	1,074.20		$	21.5		$	(1.3	)	$	1,094.40
U.S. agency	190			7.5			(0.1		)	197.4
Municipal	29.1			2.4			—			31.5
Corporate	2,244.70			79.9			(5.2		)	2,319.40
Non-U.S. government-backed corporate	76.8			1.2			—			78
Foreign government	648.6			17.3			(0.2		)	665.7
Asset-backed	141.3			2.4			(0.2		)	143.5
Non-agency commercial mortgage-backed	41.5			3.3			—			44.8
Agency mortgage-backed	1,016.70			40.8			(2.2		)	1,055.30
Total fixed income securities — Available for sale	5,462.90			176.3			(9.2		)	5,630.00
Total short-term investments — Available for sale	258.2			0.1			—			258.3
Total equity securities — Available for sale	82.6			27.3			—			109.9
Total	$	5,803.70		$	203.7		$	(9.2	)	$	5,998.20
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 March 31, 2015 and December 31, 2014:
	As at March 31, 2015
	Cost or			Gross			Gross			Fair Market
	Amortized Cost			Unrealized			Unrealized			Value
				Gains			Losses
	($ in millions)
U.S. government	$	9		$	—		$	—		$	9
U.S. agency	0.2			—			—			0.2
Municipal	0.5			—			—			0.5
Corporate	539.4			15.9			(1.2		)	554.1
Foreign government	128.3			5.8			(0.4		)	133.7
Asset-backed	15.9			0.1			—			16
Bank loans	86.2			0.3			(0.6		)	85.9
Total fixed income securities — Trading	779.5			22.1			(2.2		)	799.4
Total short-term investments — Trading	0.7			—			—			0.7
Total equity securities — Trading	696.7			54.1			(31.8		)	719
Total catastrophe bonds — Trading	32.6			0.3			(0.1		)	32.8
Total	$	1,509.50		$	76.5		$	(34.1	)	$	1,551.90
	As at December 31, 2014
	Cost or			Gross			Gross			Fair Market
	Amortized Cost			Unrealized			Unrealized			Value
				Gains			Losses
	($ in millions)
U.S. government	$	—		$	—		$	—		$	—
U.S. agency	0.2			—			—			0.2
Municipal	1.1			—			—			1.1
Corporate	520.9			11.7			(2.8		)	529.8
Foreign government	137.3			4.3			(1.5		)	140.1
Asset-backed	14.6			0.1			—			14.7
Bank loans	86.8			—			(1.7		)	85.1
Total fixed income securities — Trading	760.9			16.1			(6.0		)	771
Total short-term investments — Trading	0.2			—			—			0.2
Total equity securities — Trading	585.2			55.5			(24.7		)	616
Total catastrophe bonds — Trading	34.4			0.4			—			34.8
Total	$	1,380.70		$	72		$	(30.7	)	$	1,422.00
The Company classifies these financial instruments as held for trading as this most closely reflects the facts and circumstances of the investments held.
Catastrophe Bonds. The Company has invested in catastrophe bonds with a total value of $32.8 million as of March 31, 2015.  The bonds receive quarterly interest payments based on variable interest rates with scheduled maturities ranging from 2016 to 2020.  The redemption value of the bonds will adjust based on the occurrence of a covered event, such as windstorms and earthquakes which occur in the United States, Canada, the North Atlantic, Japan or Australia.
Other Investments. On October 2, 2012, the Company established a subsidiary, Aspen Recoveries Limited, to take ownership of a 58.5% shareholding in Chaspark Maritime Holdings Ltd., a Singaporean registered company (“Chaspark”), with the remaining shareholding owned by other insurers. The shareholding in Chaspark was received as a settlement for subrogation rights associated with a contract frustration claim settlement. The Company has determined that Chaspark has the characteristics of a variable interest entity as addressed by the guidance in ASC 810-10, Consolidation. However, having considered the provisions of ASC 810-10, the Company’s investment in Chaspark does not permit the Company to direct the activities which most significantly impact Chaspark’s economic performance and the Company is not acting as principal or agent for a related party group of investors. Under these circumstances, the Company is not required to consolidate Chaspark. The investment is therefore accounted for under the equity method and adjustments to the carrying value of this investment are made based on the Company’s share of capital including share of income and expenses, which is provided in the quarterly management accounts. The adjusted carrying value approximates fair value.
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 will be a social impact organization that will provide micro-insurance products to assist global emerging consumers. The Company's initial investment in the MVI was $0.8 million.
The table below shows the Company’s investments in the Microinsurance Venture and Chaspark for the three months ended March 31, 2015:
	For the Three Months Ended March 31, 2015
	MVI			Chaspark			Total
	($ in millions)
Opening undistributed value of investment as at January 1, 2015	$	—		$	8.7		$	8.7
Initial investment	0.8			—			$	0.8
Closing value of investment as at March 31, 2015	$	0.8		$	8.7		$	9.5
Fixed Income Securities. The scheduled maturity distribution of available for sale fixed income securities as at March 31, 2015 and December 31, 2014 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 March 31, 2015
	Amortized			Fair Market			Average
	Cost or Cost			Value			S&P Ratings by
							Maturity
	($ in millions)
Due one year or less	$	512.4		$	517.4		AA
Due after one year through five years	2,559.00			2,638.40			AA-
Due after five years through ten years	987.9			1,035.50			A+
Due after ten years	88.5			103.6			A+
Subtotal	4,147.80			4,294.90
Non-agency commercial mortgage-backed	37.6			40.4			AA+
Agency mortgage-backed	1,021.40			1,063.60			AA+
Asset-backed	138.6			140.9			AAA
Total fixed income securities — Available for sale	$	5,345.40		$	5,539.80
	As at December 31, 2014
	Amortized			Fair Market			Average
	Cost or Cost			Value			S&P Ratings by
							Maturity
	($ in millions)
Due one year or less	$	590.2		$	594.7		AA
Due after one year through five years	2,552.00			2,620.80			AA-
Due after five years through ten years	1,023.50			1,059.90			A+
Due after ten years	97.7			111			A+
Subtotal	4,263.40			4,386.40
Non-agency commercial mortgage-backed	41.5			44.8			AA+
Agency mortgage-backed	1,016.70			1,055.30			AA+
Asset-backed	141.3			143.5			AAA
Total fixed income securities — Available for sale	$	5,462.90		$	5,630.00
Guaranteed Investments. As at March 31, 2015, the Company held $2.4 million (December 31, 2014 — $2.5 million) in investments which are guaranteed by mono-line insurers, excluding those with explicit government guarantees, and the Company’s holding was limited to two municipal securities, rated A3 and Caa3 (December 31, 2014 — two municipal securities, rated A3 and Caa3). The standalone rating (rating without guarantee) is determined as the senior unsecured debt rating of the issuer. Where the credit ratings were split between the two main rating agencies, Standard & Poor’s Financial Services LLC (“S&P”) and Moody’s Investors Service, Inc. (“Moody’s”), the lowest rating was used. The Company’s exposure to other third-party guaranteed debt is primarily to investments backed by non-U.S. government guaranteed issuers.
Gross Unrealized Loss. The following tables summarize as at March 31, 2015 and December 31, 2014 by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position for the Company’s available for sale portfolio:
	March 31, 2015
	0-12 months						Over 12 months						Total
	Fair			Gross			Fair			Gross			Fair		Gross			Number of
	Market			Unrealized			Market			Unrealized			Market		Unrealized			Securities
	Value			Loss			Value			Loss			Value		Loss
	($ in millions)
U.S. government	$	41.5		$	(0.1	)	$	40.6		$	(0.1	)	$	82.1	$	(0.2	)	18
U.S. agency	—			—			5			—			5		—			1
Municipal	—			—			—			—			—		—			0
Corporate	206.4			(1.3		)	77.7			(0.6		)	284.1		(1.9		)	126
Non-U.S. government-backed corporate	2.5			—			—			—			2.5		—			2
Foreign government	118.1			(0.1		)	3.9			—			122		(0.1		)	15
Asset-backed	20			—			9.5			—			29.5		—			26
Agency mortgage-backed	101.2			(0.5		)	72.2			(0.9		)	173.4		(1.4		)	52
Total fixed income securities — Available for sale	489.7			(2.0		)	208.9			(1.6		)	698.6		(3.6		)	240
Total short-term investments — Available for sale	52.2			(0.1		)	—			—			52.2		(0.1		)	9
Total	$	541.9		$	(2.1	)	$	208.9		$	(1.6	)	$	750.8	$	(3.7	)	249
	December 31, 2014
	0-12 months						Over 12 months						Total
	Fair			Gross			Fair			Gross			Fair		Gross			Number of
	Market			Unrealized			Market			Unrealized			Market		Unrealized			Securities
	Value			Loss			Value			Loss			Value		Loss
	($ in millions)
U.S. government	$	166.1		$	(0.5	)	$	79.4		$	(0.8	)	$	245.5	$	(1.3	)	39
U.S. agency	25.1			—			4.9			(0.1		)	30		(0.1		)	7
Municipal	—			—			—			—			—		—			0
Corporate	459.4			(2.1		)	171.3			(3.1		)	630.7		(5.2		)	274
Non-U.S. government-backed corporate	0.7			—			—			—			0.7		—			1
Foreign government	30.4			—			44.2			(0.2		)	74.6		(0.2		)	16
Asset-backed	43.7			(0.1		)	11.7			(0.1		)	55.4		(0.2		)	43
Agency mortgage-backed	64.7			(0.3		)	111.7			(1.9		)	176.4		(2.2		)	48
Total fixed income securities — Available for sale	790.1			(3.0		)	423.2			(6.2		)	1,213.30		(9.2		)	428
Total short-term investments — Available for sale	4.6			—			—			—			4.6		—			3
Total	$	794.7		$	(3.0	)	$	423.2		$	(6.2	)	$	1,217.90	$	(9.2	)	431
Investment Purchases and Sales. The following table summarizes investment purchases, sales and maturities for the three months ended March 31, 2015 and 2014:
	For the Three Months Ended
	31-Mar-15			31-Mar-14
	($ in millions)
(Purchases) of fixed income securities — Available for sale	$	(483.7	)	$	(537.2	)
(Purchases) of fixed income securities — Trading	(153.4		)	(192.1		)
(Purchases) of equity securities — Available for sale	—			—
(Purchases) of equity securities — Trading	(186.7		)	(51.8		)
Proceeds from sales and maturities of fixed income securities — Available for sale	540.7			598.8
Proceeds from sales and maturities of fixed income securities — Trading	133.8			181.3
Proceeds from sales of equity securities — Available for sale	108.6			4
Proceeds from sales of equity securities — Trading	83.7			5.3
Net change in receivable/(payable) for securities sold/(purchased)	15.9			13.6
Net (purchases)/sales of short-term investments — Available for sale	61.3			(61.9		)
Net (purchases)/sales of short-term investments — Trading	(0.5		)	—
Net proceeds/(purchases) of catastrophe bonds — Trading	1.8			(12.1		)
Net sales of other investments	—			37.3
Net (purchases)/sales for the period	$	121.5		$	(14.8	)

Variable_Interest_Entities_Not

Variable Interest Entities (Notes)	3 Months Ended
	Mar. 31, 2015
Business Combinations [Abstract]
Variable Interest Entities	Variable Interest Entities
	As at March 31, 2015, the Company had two investments in two variable interest entities (“VIE”), Chaspark and Silverton.
	Chaspark. On October 2, 2012, the Company established a subsidiary, Aspen Recoveries Limited, to take ownership of a 58.5% shareholding in Chaspark, with the remaining shareholding owned by other insurers. The shareholding in Chaspark was received as a settlement for subrogation rights associated with a contract frustration claim settlement. The Company has determined that Chaspark has the characteristics of a VIE as addressed by the guidance in ASC 810, Consolidation. As discussed further in Note 6 of these unaudited condensed consolidated financial statements, the investment in Chaspark is accounted for under the equity method. In the three months ended March 31, 2015, there was no change in the value of the Company’s investment in Chaspark (March 31, 2014 — $Nil). The adjusted carrying value approximates fair value.
	Silverton. On September 10, 2013, the Company established Silverton, a Bermuda domiciled special purpose insurer formed to provide additional collateralized capacity to support Aspen Re’s business through retrocession agreements which are collateralized and funded by Silverton through the issuance of a series of loan notes. Silverton is a non-rated insurer and the risks are fully collateralized by way of funds held in trust for the benefit of Aspen Bermuda.
	The proceeds of $65.0 million (of which $50.0 million was issued to third parties) from the issuance of Silverton’s Series 2014-1 Participating Notes on December 27, 2013 (“2014 Loan Notes”) were deposited into a collateral account to fund Silverton’s obligations under a retrocession property quota share agreement entered into with Aspen Bermuda effective January 1, 2014. The holders of the 2014 Loan Notes participate in any profit or loss generated by Silverton attributable to the operations of Silverton’s Series 2014-1 Segregated Account. Any existing value of the 2014 Loan Notes is being returned to the noteholders in installments after the expiration of the risk period of the retrocession agreement issued by Silverton for the related series with the final payment being contractually due on the September 16, 2016 maturity date. The fair value of the 2014 Loan Notes as at March 31, 2015 was $89.2 million (of which $68.6 million was due to external investors). During the first quarter of 2015, Silverton distributed $84.4 million (of which $64.8 million was distributed to external investors) to its noteholders. Of the remaining $3.8 million due to external investors, $0.7 million has been classified as a current liability in the Company’s consolidated financial statements with the balance of $3.1 million classified as long-term debt. The total aggregate unpaid balance of the 2014 Loan Notes held by third parties and those held by Aspen Holdings is $4.8 million. The Company’s maximum loss exposure to the 2014 Loan Notes is $1.1 million which is the fair value of its holdings as at March 31, 2015.
	The proceeds of $85.0 million (of which $70.0 million was issued to third parties) from the issuance of Silverton’s Series 2015-1 Participating Notes on December 23, 2014 (“2015 Loan Notes”) were deposited into a collateral account to fund Silverton’s obligations under a retrocession property quota share agreement entered into with Aspen Bermuda effective January 1, 2015. The holders of the 2015 Loan Notes participate in any profit or loss generated by Silverton attributable to the operations of Silverton’s Series 2015-1 Segregated Account. Any existing value of the 2015 Loan Notes will be returned to the noteholders after the expiration of the risk period of the retrocession agreement issued by Silverton for the related series with the final payment being contractually due on the September 18, 2017 maturity date. The fair value of the 2015 Loan Notes as at March 31, 2015 was $88.5 million (of which $72.9 million was held by external investors). The $72.9 million of the 2015 Loan Notes held by external investors is classified as long term debt in the Company’s consolidated financial statements. The total aggregate unpaid balance of the 2015 Loan Notes held by third parties and those held by Aspen Holdings is $88.5 million. The Company’s maximum loss exposure to the 2015 Loan Notes is $15.6 million which is the fair value of its holdings as at March 31, 2015.
	The Company has determined that Silverton has the characteristics of a VIE that are addressed by the guidance in ASC 810, Consolidation. The Company concluded that it is the primary beneficiary and has consolidated the subsidiary upon its formation as it owns 100% of the voting shares, 100% of the issued share capital and has a significant financial interest and the power to control Silverton. The Company has no other obligation to provide financial support to Silverton. Neither the creditors nor beneficial interest holders of Silverton have recourse to the Company’s general credit.
	In the event of either an extreme catastrophic property reinsurance event or severe credit-related event there is a risk that Aspen Bermuda would be unable to recover losses from Silverton. These two risks are mitigated as follows:
	i.	Silverton has collateralized the aggregate limit provided to Aspen Bermuda by way of a trust in favor of Aspen Bermuda as the beneficiary;
	ii.	the trustee is a large, well-established regulated entity; and
	iii.	all funds within the trust account are bound by investment guidelines restricting investments to one of the institutional class money market funds run by large international investment managers.
	For further information regarding the loan notes attributable to the third-party investments in Silverton, refer to Note 8 of these unaudited condensed consolidated financial statements.

Fair_Value_Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2015
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 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 March 31, 2015 and December 31, 2014, respectively:
		As at March 31, 2015
		Level 1				Level 2				Level 3				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	1,041.80			$	—			$	—			$	1,041.80
	U.S. agency	—				182.9				—				182.9
	Municipal	—				30.7				—				30.7
	Corporate	—				2,333.60				—				2,333.60
	Non-U.S. government-backed corporate	—				68.2				—				68.2
	Foreign government	462.5				175.2				—				637.7
	Asset-backed	—				140.9				—				140.9
	Non-agency commercial mortgage-backed	—				40.4				—				40.4
	Agency mortgage-backed	—				1,063.60				—				1,063.60
	Total fixed income securities available for sale, at fair value	1,504.30				4,035.50				—				5,539.80
	Short-term investments available for sale, at fair value	144				36.2				—				180.2
	Equity investments available for sale, at fair value	—				—				—				—
	Held for trading financial assets, at fair value
	U.S. government	9				—				—				9
	U.S. agency	—				0.2				—				0.2
	Municipal	—				0.5				—				0.5
	Corporate	—				554.1				—				554.1
	Foreign government	40.7				93				—				133.7
	Asset-backed	—				16				—				16
	Bank loans	—				85.9				—				85.9
	Total fixed income securities trading, at fair value	49.7				749.7				—				799.4
	Short-term investments trading, at fair value	—				0.7				—				0.7
	Equity investments trading, at fair value	719				—				—				719
	Catastrophe bonds trading, at fair value	—				32.8				—				32.8
	Other financial assets and liabilities, at fair value
	Derivatives at fair value — foreign exchange contracts	—				2				—				2
	Derivatives at fair value— interest rate swaps	—				0.1				—				0.1
	Liabilities under derivative contracts — foreign exchange contracts	—				(11.5		)		—				(11.5		)
	Loan notes issued by variable interest entities, at fair value	—				—				(76.0		)		(76.0		)
	Loan notes issued by variable interest entities, at fair value (classified as a current liability)	—				—				(0.7		)		(0.7		)
	Total	$	2,417.00			$	4,845.50			$	(76.7	)		$	7,185.80
	There were no maturities, settlements or transfers between Level 1, Level 2 and Level 3 during the three months ended March 31, 2015 and no assets or liabilities were classified as Level 3 as at March 31, 2015, except for the loan notes issued by the VIEs.
		As at December 31, 2014
		Level 1				Level 2				Level 3				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	1,094.40			$	—			$	—			$	1,094.40
	U.S. agency	—				197.4				—				197.4
	Municipal	—				31.5				—				31.5
	Corporate	—				2,319.40				—				2,319.40
	Non-U.S. government-backed corporate	—				78				—				78
	Foreign government	456.5				209.2				—				665.7
	Asset-backed	—				143.5				—				143.5
	Non-agency commercial mortgage-backed	—				44.8				—				44.8
	Agency mortgage-backed	—				1,055.30				—				1,055.30
	Total fixed income securities available for sale, at fair value	1,550.90				4,079.10				—				5,630.00
	Short-term investments available for sale, at fair value	229.3				29				—				258.3
	Equity investments available for sale, at fair value	109.9				—				—				109.9
	Held for trading financial assets, at fair value
	U.S. government	—				—				—				—
	U.S. agency	—				0.2				—				0.2
	Municipal	—				1.1				—				1.1
	Corporate	—				529.8				—				529.8
	Foreign government	36.1				104				—				140.1
Asset-backed	—				14.7				—				14.7
Bank loans	—				85.1				—				85.1
Catastrophe bonds trading, at fair value	—				34.8				—				34.8
Total fixed income securities trading, at fair value	36.1				769.7				—				805.8
Short-term investments trading, at fair value	0.1				0.1				—				0.2
Equity investments trading, at fair value	616				—				—				616
Other financial assets and liabilities, at fair value
Derivatives at fair value – foreign exchange contracts	—				7.9				—				7.9
Derivatives at fair value – interest rate swaps	—				0.1				—				0.1
Liabilities under derivative contracts – foreign exchange contracts	—				(14.3		)		—				(14.3		)
Loan notes issued by variable interest entities, at fair value	—				—				(70.7		)		(70.7		)
Loan notes issued by variable interest entities, at fair value (classified as a current liability)	—				—				(67.9		)		(67.9		)
Total	$	2,542.30			$	4,871.60			$	(138.6	)		$	7,275.30
There were no maturities, settlements or transfers between Level 1, Level 2 and Level 3 during the twelve months ended December 31, 2014. There were no assets or liabilities that were classified as Level 3 as at December 31, 2014, except for the loan notes issued by the VIEs.
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 three months ended March 31, 2015 and 2014:
Reconciliation of Liabilities Using Level 3 Inputs			Three Months Ended March 31, 2015				Three Months Ended March 31, 2014
			($ in millions)
Balance at the beginning of the period (1)			$	138.6			$	50
Distributed to third party			(64.8		)		—
Total change in fair value included in the statement of operations			2.9				3.4
Balance at the end of the period (1)			$	76.7			$	53.4
(1) The amount classified as other payables was $0.7 million and $67.9 million as at March 31, 2015 and December 31, 2014, respectively.
Valuation of Fixed Income Securities. The Company’s fixed income securities are classified as either available for sale or trading and carried at fair value. As at March 31, 2015 and December 31, 2014, 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 March 31, 2015, the Company obtained an average of 2.0 quotes per fixed income investment, consistent with 2.0 quotes as at December 31, 2014. Pricing sources used in pricing fixed income investments as at March 31, 2015 and December 31, 2014 were as follows:
	As at March 31, 2015			As at December 31, 2014
Index providers	82	%		84	%
Pricing services	13			11
Broker-dealers	5			5
Total	100	%		100	%
A summary of securities priced using pricing information from index providers as at March 31, 2015 and December 31, 2014 is provided below:
	As at March 31, 2015							As at December 31, 2014
	Fair Market				% of Total			Fair Market				% of Total
	Value Determined				Fair Value by			Value Determined				Fair Value by
	using Prices from				Security Type			using Prices from				Security Type
	Index Providers							Index Providers
	($ in millions, except for percentages)
U.S. government	$	1,039.00			99	%		$	1,044.40			95	%
U.S. agency	172.7				94	%		186.9				95	%
Municipal	13.9				45	%		13.7				42	%
Corporate	2,680.70				93	%		2,731.10				96	%
Non-U.S. government-backed corporate	33.5				49	%		48.7				62	%
Foreign government	434.6				56	%		504.4				63	%
Asset-backed	145.5				93	%		140.5				89	%
Non-agency commercial mortgage-backed	40.3				100	%		44.8				100	%
Agency mortgage-backed	651				61	%		680.6				64	%
Total fixed income securities	$	5,211.20			82	%		$	5,395.10			84	%
Equities	715.3				100	%		725.9				100	%
Total fixed income securities and equity investments	$	5,926.50			84	%		$	6,121.00			86	%
The Company, in conjunction with its third-party accounting service provider, obtains an understanding of the methods, models and inputs used by the third-party pricing service and index provider 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. The Company’s fixed income securities are traded on the over-the-counter (“OTC”) market based on prices provided by one or more market makers in each security. Securities such as U.S. Government, U.S. Agency, Foreign Government and investment grade corporate bonds have multiple market makers in addition to readily observable market value indicators such as expected credit spread, except for Treasury securities, over the yield curve. The Company uses a variety of pricing sources to value fixed income securities including those securities that have pay down/prepay features such as mortgage-backed securities and asset-backed securities in order to ensure fair and accurate pricing. The fair value estimates for the investment grade securities in the Company’s portfolio do not use significant unobservable inputs or modeling techniques.
U.S. Government and Agency. U.S. government and agency securities consist primarily of bonds issued by the U.S. Treasury and corporate debt issued by agencies such as the Federal National Mortgage Association (“FNMA”), the Federal Home Loan Mortgage Corporation (“FHLMC”) and the Federal Home Loan Bank. As the fair values of U.S. Treasury securities are based on unadjusted market prices in active markets, they are classified within Level 1. The fair values of U.S. government agency securities are priced using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of U.S. government agency securities are classified within Level 2.
Municipals. The Company’s municipal portfolio comprises bonds issued by U.S. domiciled state and municipality entities. The fair value of these securities is determined using spreads obtained from broker-dealers, trade prices and the new issue market which are Level 2 inputs in the fair value hierarchy. Consequently, these securities are classified within Level 2.
Foreign Government. The issuers for securities in this category are non-U.S. governments and their agencies. The fair values of non-U.S. government bonds, primarily sourced from international indices, are based on unadjusted market prices in active markets and are therefore classified within Level 1. The fair values of the non-U.S. agency securities, again primarily sourced from international indices, are priced using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of non-U.S. agency securities are classified within Level 2. In addition, foreign government securities include a portion of the 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 the FDIC program or other similar non-U.S. government programs which guarantee timely payment of principal and interest in the event of a default. The fair values of these securities are generally determined using the spread above the risk-free yield curve. Inputs used in the evaluation of these securities include credit data, interest rate data, market observations and sector news, broker-dealer quotes and trade volumes. In addition, corporate securities include a portion of the BBB Emerging Market Debt portfolio. The Company classifies all of these securities within Level 2.
Mortgage-backed Securities. The Company’s residential and commercial mortgage-backed securities consist of bonds issued by the Government National Mortgage Association, the FNMA and the FHLMC as well as private non-agency issuers. The fair values of these securities are determined through the use of a pricing model (including Option Adjusted Spread) which uses prepayment speeds and spreads to determine the appropriate average life of the mortgage-backed security. These spreads are generally obtained from broker-dealers, trade prices and the new issue market. As the significant inputs used to price mortgage-backed securities are observable market inputs, these securities are classified within Level 2.
Asset-backed Securities. The underlying collateral for the Company’s asset-backed securities consists mainly of student loans, automobile loans and credit card receivables. These securities are primarily priced by index providers and pricing vendors. Inputs to the valuation process include broker-dealer quotes and other available trade information, prepayment speeds, interest rate data and credit spreads. The Company classifies these securities within Level 2.
Bank Loans. These are variable rate, senior secured debt instruments issued by non-investment grade companies that are not publicly registered but are the most senior debt in a capital structure and are generally secured by company assets. Although these assets are not as liquid a market as traditional fixed income instruments, they are valued in similar fashion to other fixed maturities, using similar inputs such as yield curves, interest rates and credit spreads. These securities are primarily priced by a third-party pricing vendor. Bank loans are therefore classified within Level 2.
Short-term Investments. Short-term investments comprise highly liquid debt securities with a maturity greater than three months but less than one year from the date of purchase. Short-term investments are valued in a manner similar to the Company’s fixed maturity investments and are classified within Levels 1 and 2.
Equity Securities. Equity securities include U.S. and foreign common stocks and are classified either as trading or available for sale and carried at fair value. These securities are classified within Level 1 as their fair values are based on quoted market prices in active markets from independent pricing sources. As at March 31, 2015, the Company obtained an average of 4.0 quotes per equity investment, compared to 4.0 quotes as at December 31, 2014. Pricing sources used in pricing equities as at March 31, 2015 and December 31, 2014 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 classified as 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 deep liquid market, providing substantial price transparency and accordingly are classified as Level 2.
Interest Rate Swaps. The interest rate swaps which the Company uses to mitigate interest rate risk are also characterized as OTC and are valued by the counterparty using quantitative models with multiple market inputs. The market inputs, such as interest rates and yield curves, are observable and the valuation can be compared for reasonableness with third party pricing services. Consequently, these instruments are classified as Level 2.
Loan Notes Issued by Variable Interest Entities. Silverton, a licensed special purpose insurer, is consolidated into the Company’s group accounts as a VIE. In the fourth quarter of 2013, Silverton issued $65.0 million ($50.0 million third-party funded) loan notes with a maturity date of September 16, 2016. During the fourth quarter of 2014, Silverton issued an additional $85.0 million ($70.0 million third-party funded) loan notes with a maturity date of September 18, 2017. The Company has elected to account for the loan notes at fair value using the guidance as prescribed under ASC 825, Financial Instruments as the Company believes it represents the most meaningful measurement basis for these liabilities. The loan notes are recorded at fair value at each reporting period and, as they are not quoted on an active market and contain significant unobservable inputs, they have been classified as a Level 3 instrument in the fair value hierarchy. The loan notes are unique because they are linked to the specific risks of the Company’s property catastrophe book.
To determine the fair value of the loan notes, the Company runs an internal model which considers the seasonality of the risk assumed under the retrocessional agreement between Aspen Bermuda and Silverton. The seasonality used in the model is determined by applying the percentage of property catastrophe losses planned by the Company’s actuaries to the estimated written premium to determine earned premium for each quarter. The inputs to the internal valuation model are based on Company specific data due to the lack of availability of observable market inputs. Reserves for losses are the most significant unobservable input. An increase in reserves for losses would normally result in a decrease in the fair value of the loan notes while a decrease in reserves would normally result in an increase in the fair value of the loan notes. The observable and unobservable inputs used to determine the fair value of the 2015-1 Loan Notes and 2014-1 Loan Notes as at March 31, 2015 and December 31, 2014 are presented in the table below:
At March 31, 2015		Fair Value				Valuation Method		Observable (O) and		Low				High
		Level 3						Unobservable (U) inputs
	($ in millions)									($ in millions)
Loan notes held by third parties		$	76.7			Internal Valuation Model		Gross premiums written (O)		$	37.6			$	39.6
								Reserve for losses (U)		$	1.1			$	4.4
								Contract period (O)		N/A				365 days
								Initial value of issuance (O)		$	120			$	120
At December 31, 2014		Fair Value				Valuation Method		Observable (O) and		Low				High
		Level 3						Unobservable (U) inputs
	($ in millions)									($ in millions)
Loan notes held by third parties		$	138.6			Internal Valuation Model		Gross premiums written (O)		$	—			$	40
								Reserve for losses (U)		$	—			$	4.6
								Contract period (O)		N/A				365 days
								Initial value of issuance (O)		$	120			$	120
The observable and unobservable inputs represent the potential variation around the inputs used in the valuation model. The contract period is defined in the Silverton loan agreements and the initial value represents the funds received from third parties.

Reinsurance

Reinsurance	3 Months Ended
	Mar. 31, 2015
Insurance [Abstract]
Reinsurance	Reinsurance
	The Company purchases retrocession and reinsurance to limit and diversify the Company’s risk exposure and increase its own insurance and reinsurance underwriting capacity. These agreements provide for recovery of a portion of losses and loss adjustment expenses from reinsurers. As is the case with most reinsurance contracts, the Company remains liable to the extent that reinsurers do not meet their obligations under these agreements, and therefore, in line with its risk management objectives, the Company evaluates the financial condition of its reinsurers and monitors concentrations of credit risk. The largest concentrations of reinsurance recoverables as at March 31, 2015 were 20.0% (December 31, 2014  — 18.6%) with Munich Re rated AA- by S&P, 18.9% (December 31, 2014 — 27.3%) with Lloyd’s syndicates rated A+ by S&P and 7.9% (December 31, 2014  — 8.4%) with Arch Re which is rated A+ by S&P.

Derivative_Contracts

Derivative Contracts	3 Months Ended
	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Contracts	Derivative Contracts
	The following tables summarize information on the location and amounts of derivative fair values on the consolidated balance sheet as at March 31, 2015 and December 31, 2014:
				As at March 31, 2015							As at December 31, 2014
	Derivatives Not Designated as Hedging Instruments		Balance Sheet Location	Notional			Fair				Notional		Fair
	Under ASC 815			Amount			Value				Amount		Value
				($ in millions)							($ in millions)
	Interest Rate Swaps		Derivatives at Fair Value	$	951.3		$	0.1		(1)	$	951.3	$	0.1		(1)
	Foreign Exchange Contracts		Derivatives at Fair Value	$	220		$	2			$	165.8	$	7.9
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	142.3		$	(5.3	)		$	237.6	$	(10.5	)
	(1)	Net of $19.5 million of cash collateral provided to counterparties, Goldman Sachs International ($451.3 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2014 — $22.3 million).
				As at March 31, 2015							As at December 31, 2014
	Derivatives Designated as Hedging Instruments Under ASC 815		Balance Sheet Location	Notional			Fair				Notional		Fair
				Amount			Value				Amount		Value
				($ in millions)							($ in millions)
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	84.6		$	(6.2	)	(1)	$	135.8	$	(3.8	)	(1)
	(1)	Net of $Nil cash collateral (December 31, 2014 — $Nil).
	The following tables provide the unrealized and realized gains/(losses) recorded in the statement of operations for the three months ended March 31, 2015 and 2014, respectively:
				Amount of Income/(Loss)
				Recognized in the Statement
				of Operations and Other Comprehensive Income for the
				Three Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	31-Mar-15			31-Mar-14
	ASC 815		Statement of Operations and Other Comprehensive Income
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(4.6	)	$	3
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	(3.2	)	$	(1.9	)
				Amount of Income/(Loss)
				Recognized in the Statement
				of Operations and Other Comprehensive Income for the
				Three Months Ended
	Derivatives Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	31-Mar-15			31-Mar-14
	ASC 815		Statement of Operations and Other Comprehensive Income
				($ in millions)
	Foreign Exchange Contracts		General, administrative and corporate expenses	$	(2.8	)	$	—
	Foreign Exchange Contracts		Net change from current period hedged transactions	$	(2.4	)	$	—
	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 March 31, 2015, the Company held foreign exchange contracts that were not designated as hedging under ASC 815 with an aggregate notional value of $362.3 million (December 31, 2014 — $403.4 million). The foreign exchange contracts are recorded as derivatives at fair value with changes recorded as a change in fair value of derivatives in the statement of operations. For the three months ended March 31, 2015, the impact of foreign exchange contracts on net income was a charge of $4.6 million (March 31, 2014 — gain of $3.0 million).
	As at March 31, 2015, the Company held foreign exchange contracts that were designated as hedging under ASC 815 with an aggregate notional value of $84.6 million (December 31, 2014 — $135.8 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, for the three months ended March 31, 2015, the movement in other comprehensive income representing the effective portion was a net unrealized loss of $2.4 million (March 31, 2014 — $Nil).
	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 three months ended March 31, 2015 the realized loss recognized within general, administration and corporate expenses for settled foreign exchange contracts was $2.8 million (March 31, 2014 — $Nil).
	Interest Rate Swaps. As at March 31, 2015, the Company held fixed for floating interest rate swaps with a total notional amount of $951.3 million (December 31, 2014 — $951.3 million) that are due to mature between November 26, 2015 and November 9, 2020. The interest rate swaps are used in the ordinary course of the Company’s investment activities to partially mitigate the negative impact of rises in interest rates on the market value of the Company’s fixed income portfolio. For the three months ended March 31, 2015, there was a charge in respect of the interest rate swaps of $3.2 million (March 31, 2014 — charge of $1.9 million).
	As at March 31, 2015, cash collateral with a fair value of $19.5 million was held by the Company’s counterparties to support the current valuation of the interest rate swaps (December 31, 2014 — $22.3 million). As at March 31, 2015, no non-cash collateral was transferred to the Company by its counterparties (December 31, 2014 — $Nil). Transfers of cash collateral are recorded on the consolidated balance sheet within Derivatives at Fair Value, while transfers in respect of non-cash collateral are disclosed but not recorded. As at March 31, 2015, no amount was recorded in the consolidated balance sheet for the pledged assets.

Deferred_Policy_Acquisition_Co

Deferred Policy Acquisition Costs	3 Months Ended
	Mar. 31, 2015
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 three months ended March 31, 2015 and 2014:
			Three Months Ended March 31,
			2015			2014
			($ in millions)
	Balance at the beginning of the period		$	299		$	262.2
		Acquisition costs deferred	154.1			139.4
		Amortization of deferred policy acquisition costs	(119.3		)	(112.0		)
	Balance at the end of the period		$	333.8		$	289.6

Reserves_for_Losses_and_Loss_A

Reserves for Losses and Loss Adjustment Expenses	3 Months Ended
	Mar. 31, 2015
Insurance [Abstract]
Reserves for Losses and Loss Adjustment Expenses	Reserves for Losses and Loss Adjustment Expenses
	The following table represents a reconciliation of beginning and ending consolidated loss and loss adjustment expenses (“LAE”) reserves for the three months ended March 31, 2015 and twelve months ended December 31, 2014:
		Three Months Ended March 31, 2015			Twelve Months Ended December 31, 2014
		($ in millions)
	Provision for losses and LAE at the start of the year	$	4,750.80		$	4,678.90
	Less reinsurance recoverable	(350.0		)	(332.7		)
	Net loss and LAE at the start of the year	4,400.80			4,346.20
	Net loss and LAE expenses assumed/(disposed)	—			(24.2		)
	Provision for losses and LAE for claims incurred:
	Current year	333.6			1,411.60
	Prior years	(27.5		)	(104.1		)
	Total incurred	306.1			1,307.50
	Losses and LAE payments for claims incurred:
	Current year	(18.8		)	(112.1		)
	Prior years	(252.9		)	(995.6		)
	Total paid	(271.7		)	(1,107.7		)
	Foreign exchange (gains)	(96.4		)	(121.0		)
	Net losses and LAE reserves at period end	4,338.80			4,400.80
	Plus reinsurance recoverable on unpaid losses at period end	360.1			350
	Provision for losses and LAE at the end of the relevant period	$	4,698.90		$	4,750.80
	For the three months ended March 31, 2015, there was a reduction of $27.5 million in the Company’s estimate of the ultimate claims to be paid in respect of prior accident years compared to a reduction of $28.2 million for the three months ended March 31, 2014. The Company had no disposals relating to commutations during the three months ended March 31, 2015 (three months ended March 31, 2014 — $Nil). For additional information on the reserve releases, please refer to “Management’s Discussion and Analysis of Financial Condition and Results of Operations — Reserves for Losses and Loss Adjustment Expenses” below.

Capital_Structure

Capital Structure	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Capital Structure	Capital Structure
	The following table provides a summary of the Company’s authorized and issued share capital as at March 31, 2015 and December 31, 2014:
		As at March 31, 2015			As at December 31, 2014
		Number		$ in	Number	$ in
				Thousands		Thousands
	Authorized share capital:
	Ordinary Shares 0.15144558¢ per share	969,629,030		1,469	969,629,030	1,469
	Non-Voting Shares 0.15144558¢ per share	6,787,880		10	6,787,880	10
	Preference Shares 0.15144558¢ per share	100,000,000		152	100,000,000	152
	Total authorized share capital			1,631		1,631
	Issued share capital:
	Issued ordinary shares of 0.15144558¢ per share	61,722,502		93	62,017,368	94
	Issued 7.401% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	5,327,500		8	5,327,500	8
	Issued 7.250% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	6,400,000		10	6,400,000	10
	Issued 5.95% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	11,000,000		17	11,000,000	17
	Total issued share capital			128		129
	Additional paid-in capital as at March 31, 2015 was $1,106.0 million (December 31, 2014 — $1,134.3 million). Additional paid-in capital includes the aggregate liquidation preferences of the Company’s preference shares of $568.2 million (December 31, 2014 — $568.2 million) less issue costs of $12.4 million (December 31, 2014 — $12.4 million).
	Ordinary Shares. The following table summarizes transactions in the Company’s ordinary shares during the three months ended March 31, 2015:
		Number of Ordinary Shares
	Ordinary shares in issue at December 31, 2014	62,017,368
	Ordinary share transactions in the three months ended March 31, 2015
	Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or	485,466
	2008 share purchase plan
	Ordinary shares issued to non-employee directors	6,806
	Ordinary shares repurchased	(787,138	)
	Ordinary shares in issue at March 31, 2015	61,722,502
	Ordinary Share Repurchases. On February 5, 2015, the Company and the Board of Directors agreed a new share repurchase authorization program of $500.0 million. The total share repurchase authorization, which was effective immediately through February 6, 2017, permits the Company to effect the repurchases from time to time through a combination of transactions, including open market repurchases, privately negotiated transactions and accelerated share repurchase transactions.
	Under open market repurchases under a Rule 10b5-1 plan, the Company acquired and cancelled 787,138 ordinary shares for the three months ended March 31, 2015. The total consideration paid was $36.5 million with an average price of $46.32 per ordinary share. Under open market repurchases, the Company acquired and cancelled 770,505 ordinary shares for the three months ended March 31, 2014. The total consideration paid for the three months ended March 31, 2014 was $30.9 million with an average price of $40.08 per ordinary share.
	Rights Agreement. On April 17, 2014, the Board of Directors resolved to issue one preferred share purchase right (a “Right”) for each outstanding ordinary share, and adopted a shareholder rights plan, as set forth in the Rights Agreement dated as of April 17, 2014. Each Right would have allowed its holder to purchase from the Company one one-thousandth of a share of Series A Junior Participating Preference Shares for $160, once the Rights became exercisable. The Rights could not be exercisable until 10 business days after the public announcement that a person or group acquired the beneficial ownership of 10% or more of the outstanding ordinary shares of the Company (or 15% in the case of passive institutional investors). The Rights could have been redeemed at any time at the discretion of the Board of Directors. As of March 31, 2015, no Rights were exercisable or exercised. The Rights Agreement expired on April 16, 2015.

ShareBased_Payments

Share-Based Payments	3 Months Ended
	Mar. 31, 2015
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 incentive plans, the non-employee director plan and the employee share purchase plans. When options are exercised or other equity awards have vested, new shares are issued as the Company does not currently hold treasury shares.
	Employee and Non-Employee Director Awards. Employee options and other awards were granted under the Aspen 2003 Share Incentive Plan, as amended (the “2003 Share Incentive Plan”), prior to April 24, 2013 and thereafter under the 2013 Share Incentive Plan (the “2013 Share Incentive Plan”). The total number of ordinary shares that may be issued under the 2013 Share Incentive Plan is 2,845,683 ordinary shares, which includes 595,683 ordinary 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. The non-employee director awards are granted under the 2006 Stock Option Plan for Non-Employee Directors.
	Stock options were granted with an exercise price equivalent to the fair value of the share on the grant date. The weighted average value at grant date is determined using the Black-Scholes option pricing model. Stock options typically vest over a three-year period with a ten-year exercise period (except for options granted in 2007 which have a seven-year exercise period) with vesting dependent on time and performance conditions established at the time of grant. No options were granted during the three months ended March 31, 2015 (2014 — Nil) and 83,938 options were exercised and shares issued in the three months ended March 31, 2015 (2014 — 19,579). No charges against income were made in respect of employee options for the three months ended March 31, 2015 (2014 — $Nil).
	Restricted share units (“RSUs”) granted to employees typically vest over a three-year period based on continued service. Some of the RSU grants vest at year-end, while others vest on the anniversary of the date of grant or when the Compensation Committee of the Board of Directors agrees to deliver the RSUs. The fair value of the RSUs is based on the closing price on the date of the grant adjusted for illiquidity and is expensed through the income statement evenly over the vesting period. In the three months ended March 31, 2015, the Company granted 238,315 RSUs to its employees (2014 — Nil). Compensation costs charged against income in respect of RSUs for the three months ended March 31, 2015 were $4.7 million (2014 — $2.8 million).
	In the case of non-employee directors, generally one-twelfth of the RSUs vest on each one month anniversary of the date of grant, with 100% of the RSUs vesting on the first anniversary of the date of grant. On February 5, 2015 (with a grant date of February 9, 2015), the Board of Directors approved a total of 27,620 RSUs for the non-employee directors (2014 — 27,180 RSUs) and 12,154 RSUs to the Chairman (February 10, 2014 — 13,590). Compensation costs charged against income in respect of non-employee director RSUs for the three months ended March 31, 2015 were $0.3 million (2014 — $0.4 million).
	The total fair value adjustment for all RSUs for the three months ended March 31, 2015 was $0.3 million (2014 — $0.6 million). The total tax credit recognized by the Company in relation to RSUs in the three months ended March 31, 2015 was $1.1 million (2014 — $0.6 million).
	Performance Shares. During the three months ended March 31, 2015, the Company granted 277,585 performance shares to employees (three months ended March 31, 2014 — Nil). The performance shares will be subject to a three-year vesting period with a separate annual diluted book value per share (“BVPS”) growth test for each year, adjusted to add back ordinary dividends. One-third of the grant will be eligible for vesting each year based on a formula, and will only be issuable at the end of the three-year period.
	If the diluted BVPS growth achieved in 2015 is:
	•	less than 5.6%, then the portion of the performance shares subject to the vesting conditions in such year will be forfeited (i.e., one-third of the initial grant);
	•	between 5.6% and 11.1%, then the percentage of the performance shares eligible for vesting in such year will be between 10% and 100% on a straight-line basis; or
	•	between 11.1% and 22.2%, then the percentage of the performance shares eligible for vesting in such year will be between 100% and 200% on a straight-line basis.
	In calculating BVPS for 2015, the entire movement in AOCI will be excluded. Interest rate movements and credit spread movements in AOCI can be fairly significant and impact growth in BVPS which management does not have any control over. The Compensation Committee also agreed that it will review the impact of any capital management actions undertaken during 2015, including share repurchases and special dividends, and consider whether any further adjustments to growth in BVPS should be may be made in the context of such actions. The Compensation Committee further agreed to exclude from the calculation of BVPS for 2015 all transactional expenses incurred in connection with any transaction which, if consummated, would result in a change in control, including without limitation the cost of defending against any such transaction and any third-party legal and advisory costs. The Compensation Committee believes that it would not be appropriate for employee’s performance-related compensation to be impacted by these costs.
	The Compensation Committee will determine the vesting conditions for the 2016 and 2017 portions of the grant in such years taking into consideration the market conditions and the Company’s business plans at the commencement of the years concerned. Notwithstanding the vesting criteria for each given year, if the shares eligible for vesting in 2016 and 2017 are greater than 100% for the portion of such year’s grant and the average diluted BVPS growth over such year and the preceding year is less than the average of the minimum vesting thresholds for such year and the preceding year, then only 100% (and no more) of the ordinary 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 this limitation on 100% vesting.
	The fair value of performance share awards is based on the value of the closing share price on the date of the grant adjusted for illiquidity less a deduction for expected dividends which would not accrue during the vesting period. Compensation costs charged against income in the three months ended March 31, 2015 in respect of performance shares were $0.2 million (2014 — $1.5 million). The total tax recognized by the Company in relation to performance shares in the three months ended March 31, 2015 was a tax credit of $0.1 million (2014 — $0.4 million).
	Phantom Shares. During the three months ended March 31, 2015, the Compensation Committee approved the grant of 135,651 phantom shares to its employees (2014 — Nil). The phantom shares are subject to a three-year vesting period with a separate annual diluted BVPS growth test for each year, in accordance with the test described above for the 2015 performance shares, with the difference being that any vested amount would be paid in cash in lieu of ordinary shares. As ordinary shares are not issued, the phantom shares have no dilutive effect.
	The fair value of the phantom shares is based on the closing share price on the date of the grant adjusted for illiquidity, less estimated dividends payable over the vesting period. The fair value is expensed through the consolidated income statement evenly over the vesting period, but as the payment to beneficiaries will ultimately be in cash rather than 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 three months ended March 31, 2015 in respect of phantom shares were $0.9 million (2014 — $1.6 million) with a fair value adjustment for the three months ended March 31, 2015 of $1.1 million (2014 — $0.2 million). The total tax credit recognized by the Company in relation to phantom shares in the three months ended March 31, 2015 was $0.4 million (2014 — $0.2 million).
	Employee Share Purchase Plans. On April 30, 2008, the shareholders of the Company approved the Employee Share Purchase Plan, the 2008 Sharesave Scheme 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 Employee Share Purchase Plan, employees can save up to $500 per month over a two-year period, at the end of which they will be eligible to purchase the Company’s ordinary shares at a discounted price, subject to a further one year holding period. In respect of the 2008 Sharesave Scheme, employees can save up to £250 per month over a three-year period (and effective April 6, 2014, £500 per month), at the end of which they will be eligible to purchase the Company’s 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, 52,452 ordinary shares were issued during the three months ended March 31, 2015 (2014 — 9,994 shares). Compensation costs charged against income in the three months ended March 31, 2015 in respect of the ESPP were $0.3 million (2014 — $0.1 million).

Intangible_Assets

Intangible Assets	3 Months Ended
	Mar. 31, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets	Intangible Assets
	The following table provides a summary of the Company’s intangible assets for the three months ended March 31, 2015 and 2014:
		Three Months Ended March 31, 2015								Three Months Ended March 31, 2014
		Trade		Insurance		Other		Total		Trade		Insurance		Other			Total
		Mark		Licenses						Mark		Licenses
		($ in millions)								($ in millions)
	Intangible Assets
	Beginning of the period	$	1.6	$	16.6	$	—	$	18.2	$	1.6	$	16.6	$	0.2		$	18.4
	Amortization	—		—		—		—		—		—		(0.1		)	(0.1		)
	End of the period	$	1.6	$	16.6	$	—	$	18.2	$	1.6	$	16.6	$	0.1		$	18.3
	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 March 31, 2015, the value of the license to use the Aspen trademark was $1.6 million (December 31, 2014 — $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.
	Insurance Licenses. The total value of the licenses as at March 31, 2015 was $16.6 million (December 31, 2014 — $16.6 million). This includes $10.0 million of acquired licenses held by AAIC, $4.5 million of acquired licenses held by Aspen Specialty and $2.1 million of acquired licenses held by Aspen U.K. The insurance licenses are considered to have an indefinite life and are not being amortized. The licenses have an indefinite life and are tested for impairment annually or when events or changes in circumstances indicate that the asset might be impaired.
	Other. In 2010, the Company purchased APJ Continuation Limited and its subsidiaries (“APJ”) for an aggregate consideration of $4.8 million. The Board of Directors assessed the fair value of the net tangible and financial assets acquired at $1.2 million. The $3.6 million intangible asset represented the Company’s assessment of the value of renewal rights and distribution channels ($2.2 million) and the lock-in period for employees associated with the business ($1.4 million). The asset was amortized over a five-year period and the value as at March 31, 2015 was $Nil (December 31, 2014 — $Nil).

Commitments_and_Contingent_Lia

Commitments and Contingent Liabilities	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingent Liabilities	Commitments and Contingent Liabilities
	(a)	Restricted assets
	The Company is 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 the Company’s restricted assets as at March 31, 2015 and December 31, 2014:
		As at March 31, 2015			As at December 31, 2014
		($ in millions, except percentages)
	Regulatory trusts and deposits:
	Affiliated transactions	$	988.6		$	1,086.90
	Third party	2,121.70			2,183.40
	Letters of credit / guarantees(1)	714			778.7
	Total restricted assets	$	3,824.30		$	4,049.00
	Total as percent of cash and invested assets	45		%	47		%
	(1)	As of March 31, 2015, the Company had pledged funds of $704.8 million and £9.2 million (December 31, 2014 — $764.4 million and £9.2 million) as collateral for the secured letters of credit.
	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, comprises cash and investments as at March 31, 2015 in the amount of $417.9 million (December 31, 2014 — $411.9 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 March 31, 2015 was £0.4 million (December 31, 2014 — £0.4 million). This is not available for distribution by the Company for the payment of dividends.
	Letters of Credit Facility. On June 30, 2014, Aspen Bermuda and Citibank Europe plc (“Citi Europe”) replaced an existing letter of credit facility, dated July 30, 2012, in a maximum aggregate amount of up to $950.0 million (the “LOC Facility”) comprised of two maturity tranches (Tranche I with a limit of $650.0 million and Tranche II with a limit of $300.0 million) which expired on its own terms on June 30, 2014. The LOC Facility was replaced with a new letter of credit facility in a maximum aggregate amount of up to $575.0 million (the “New LOC Facility”). Under the New LOC Facility, which will expire on June 30, 2016, Aspen Bermuda will pay to 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 New LOC Facility. Aspen Bermuda will also pay interest on the amount drawn by any beneficiary under a credit provided under the New LOC Facility at a rate per annum of LIBOR plus 1% (plus reserve asset costs, if any) from the date of drawing until the date of reimbursement by Aspen Bermuda. The New LOC Facility is used to secure obligations of Aspen Bermuda to its policyholders. In addition to the New LOC Facility, we also use regulatory trusts to secure our obligations to policyholders.
	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.
	On December 18, 2014, Aspen Bermuda and Citi Europe entered into an amended and restated pledge agreement (“Pledge Agreement”) to, among other things, (i) change the types of securities or other assets that qualify as collateral pledged under the Pledge Agreement, (ii) provide Aspen Bermuda the right to give certain directions or entitlement orders to The Bank of New York Mellon (“BNY Mellon”), as securities intermediary, relating to the collateral without the consent of Citi Europe provided certain conditions are satisfied, (iii) provide Citi Europe, subject to the provisions set forth in the amended and restated account control agreement, dated December 18, 2014 (the “Control Agreement”), among Aspen Bermuda, Citi Europe and BNY Mellon, with the right and power to exercise exclusive control over the accounts set forth in the Control Agreement and (iv) provide a schedule of currency margins such that if the collateral is denominated in a currency other than the credit currency the collateral shall be reduced by a specified percentage.
	On December 12, 2014, Aspen Holdings and certain of its subsidiaries (the “Borrowers”) entered into a first amendment to an amended and restated credit agreement, dated June 12, 2013, with various lenders and Barclays Bank Plc, as administrative agent (“Barclays”). Aspen Holdings has recently established, and may establish additional, special purpose entities that have issued or will issue insurance-linked securities to third-party investors (each, an “ILS Entity” and collectively, the “ILS Entities”). Accordingly, the amended and restated credit agreement was amended, among other things, to (i) exclude ILS Entities from the definition of “Subsidiary”, (ii) permit the Borrowers to invest in ILS Entities and (iii) permit the Borrowers to engage in transactions with an ILS Entity.
	Aspen U.K. and Aspen Bermuda have a $100.0 million secured letter of credit facility agreement with Barclays. All letters of credit issued under the facility are used to support reinsurance obligations of the parties to the agreement and their respective subsidiaries. The Company did not extend the maturity date of the Barclays secured letter of credit facility and, as a result, it expired on January 31, 2015 and no new letters of credit can be issued under this facility. As at March 31, 2015, the Company had no outstanding collateralized letters of credit under this facility (December 31, 2014 — $5.0 million).
	For further information relating to the Company’s credit facilities, refer to Note 23 of the “Notes to Audited Consolidated Financial Statements” in the Company’s 2014 Annual Report on Form 10-K filed with the United States Securities and Exchange Commission.
	Interest Rate Swaps. As at March 31, 2015, cash collateral with a fair value of $19.5 million was held by the Company’s counterparties to support the current valuation of the interest rate swaps (December 31, 2014 — $22.3 million). For more information, please refer to Note 10 of these unaudited condensed consolidated financial statements.
	(b)	Operating leases
	Amounts outstanding under operating leases net of subleases as of March 31, 2015 were:
		2015			2016			2017		2018		2019		Later		Total
														Years
		($ in millions)
	Operating Lease Obligations	$	10.1		$	9.8		$	8.5	$	7	$	6.1	$	1.5	$	43
	(c)	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, notably the U.S., a failure to deal with such disputes or potential disputes in an appropriate manner could result in an award of “bad faith” punitive damages against the Company’s Operating Subsidiaries. In accordance with ASC 450-20-50-4b, for (a) reasonably possible losses for which no accrual is made because any of the conditions for accrual in ASC 450-20-25-2 are not met and (b) reasonably possible losses in excess of the amounts accrued pursuant to ASC 450-20-30-1, the Company will provide an estimate of the possible loss or range of possible loss or state that such an estimate cannot be made.
	As of March 31, 2015, based on available information, it was the opinion of the Company’s management that the probability of the ultimate resolution of pending or threatened litigation or arbitrations having a material effect on the Company’s financial condition, results of operations or liquidity would be remote.

Basis_of_Preparation_Policies

Basis of Preparation (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Accounting Policies Not Yet Adopted	Accounting Pronouncements Not Yet Adopted
	On January 5, 2015, the Financial Accounting Standards Board (“FASB”) issued ASU 2015-01, “Income Statement - Extraordinary and Unusual Items (Subtopic 225-20) - Simplifying Income Statement Presentation by Eliminating the Concept of Extraordinary Items” which eliminates from U.S. GAAP the requirement for an entity to separately classify, present and disclose extraordinary events and transactions as part of FASB’s initiative to reduce complexity in accounting standards. ASU 2015-01 is effective for fiscal years beginning after December 15, 2015 and the Company may apply the amendments prospectively or retrospectively to all prior periods presented in the financial statements. Early application is permitted provided the guidance is applied from the beginning of the fiscal year of adoption. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	On February 18, 2015, the FASB issued ASU 2015-02, “Consolidation (Topic 810) - Amendments to the Consolidation Analysis” which includes amendments that change the analysis that an entity must perform to determine whether it should consolidate certain types of legal entities. ASU 2015-02 is effective for fiscal years beginning after December 15, 2015. The Company does not expect this ASU to have a material impact on its consolidated financial statements.

Reclassifications_from_Accumul1

Reclassifications from Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income Reclassification	The following table sets out the components of the Company’s accumulated other comprehensive income (“AOCI”) that are reclassified into the unaudited condensed consolidated statement of operations for the three months ended March 31, 2015 and 2014:
		Amount Reclassified from AOCI
	Details about the AOCI Components	Three Months Ended March 31, 2015			Three Months Ended March 31, 2014			Affected Line Item in the Unaudited
								Condensed Consolidated Statement
								of Operations
		($ in millions)
	Available for sale securities:
	Realized gains on sale of securities	$	33.5		$	3		Realized and unrealized investment gains
	Realized (losses) on sale of securities	(1.2		)	(2.6		)	Realized and unrealized investment losses
		32.3			0.4			Income from operations before income tax
	Tax on net realized gains of securities	(0.4		)	(0.2		)	Income tax expense
	Total reclassifications from AOCI to the statement of operations, net of tax	$	31.9		$	0.2		Net income

Earnings_per_Ordinary_Share_Ta

Earnings per Ordinary Share (Tables)	3 Months Ended
	Mar. 31, 2015
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 three months ended March 31, 2015 and 2014, respectively:
		Three Months Ended March 31,
		2015			2014
		($ in millions, except share and per share amounts)
	Net income	$	128		$	120.4
	Preference share dividends	(9.5		)	(9.5		)
	Net profit attributable to non-controlling interest	—			(0.1		)
	Basic and diluted net income available to ordinary shareholders	118.5			110.8
	Ordinary shares:
	Basic weighted average ordinary shares	62,159,303			65,289,351
	Weighted average effect of dilutive securities(1)	1,373,359			1,276,539
	Total diluted weighted average ordinary shares	63,532,662			66,565,890
	Earnings per ordinary share:
	Basic	$	1.91		$	1.7
	Diluted	$	1.87		$	1.66
	(1)	Dilutive securities comprise: employee options, 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 14.
Summary of Declared Dividends	Dividends. On April 22, 2015, the Company’s Board of Directors (“Board of Directors”) declared the following quarterly dividends:
		Dividend			Payable on:		Record Date:
	Ordinary shares	$	0.21		May 27, 2015		May 8, 2015
	7.401% preference shares	$	0.462563		July 1, 2015		June 15, 2015
	7.250% preference shares	$	0.4531		July 1, 2015		June 15, 2015
	5.95% preference shares	$	0.3719		July 1, 2015		June 15, 2015

Segment_Reporting_Tables

Segment Reporting (Tables)	3 Months Ended
	Mar. 31, 2015
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 three months ended March 31, 2015 and 2014:
		Three Months Ended March 31, 2015
		Reinsurance			Insurance			Total
		($ in millions)
	Underwriting Revenues
	Gross written premiums	$	484.8		$	434.4		$	919.2
	Net written premiums	442.1			321.1			763.2
	Gross earned premiums	265.8			415.1			680.9
	Net earned premiums	249.4			344.2			593.6
	Underwriting Expenses
	Losses and loss adjustment expenses	105.5			200.6			306.1
	Amortization of deferred policy acquisition costs	53.4			65.9			119.3
	General and administrative expenses	32.4			55.3			87.7
	Underwriting income	$	58.1		$	22.4		80.5
	Corporate expenses							(14.5		)
	Net investment income							47.4
	Realized and unrealized investment gains							57.4
	Realized and unrealized investment losses							(14.5		)
	Change in fair value of loan notes issued by variable interest entities							(2.9		)
	Change in fair value of derivatives							(7.8		)
	Interest expense on long term debt							(7.4		)
	Net realized and unrealized foreign exchange (losses)							(6.4		)
	Other income							3.9
	Other expenses							(2.6		)
	Income before tax							$	133.1
	Net reserves for loss and loss adjustment expenses	$	2,410.90		$	1,927.90		$	4,338.80
	Ratios
	Loss ratio	42.3		%	58.3		%	51.6		%
	Policy acquisition expense ratio	21.4			19.1			20.1
	General and administrative expense ratio	13			16.1			17.2			(1)
	Expense ratio	34.4			35.2			37.3
	Combined ratio	76.7		%	93.5		%	88.9		%
	(1)	The general and administrative expense ratio in the total column includes corporate expenses.
		Three Months Ended March 31, 2014
		Reinsurance			Insurance			Total
		( $ in millions)
	Underwriting Revenues
	Gross written premiums	$	472.2		$	383.3		$	855.5
	Net written premiums	442.6			254.9			697.5
	Gross earned premiums	278.5			373.6			652.1
	Net earned premiums	266.7			299.8			566.5
	Underwriting Expenses
	Losses and loss adjustment expenses	110.4			177.7			288.1
	Amortization of deferred policy acquisition costs	50.4			61.6			112
	General and administrative expenses	32.8			45.9			78.7
	Underwriting income	$	73.1		$	14.6		87.7
	Corporate expenses							(16.9		)
	Net investment income							49.5
	Realized and unrealized investment gains							17.9
	Realized and unrealized investment losses							(4.3		)
	Change in fair value of loan notes issued by variable interest entities							(3.4		)
	Change in fair value of derivatives							1.1
	Interest expense on long term debt							(7.4		)
	Net realized and unrealized foreign exchange gains							0.1
	Other income							0.6
	Other expenses							(0.7		)
	Income before tax							$	124.2
	Net reserves for loss and loss adjustment expenses	$	2,626.30		$	1,782.30		$	4,408.60
	Ratios
	Loss ratio	41.4		%	59.3		%	50.9		%
	Policy acquisition expense ratio	18.9			20.5			19.8
	General and administrative expense ratio	12.3			15.3			16.9			(1)
	Expense ratio	31.2			35.8			36.7
	Combined ratio	72.6		%	95.1		%	87.6		%
(1)	The general and administrative expense ratio in the total column includes corporate expenses.

Investments_Tables

Investments (Tables)	3 Months Ended
	Mar. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Summary of Investment Income	Investment Income. The following table summarizes investment income for the three months ended March 31, 2015 and 2014:
		For the Three Months Ended
		31-Mar-15			31-Mar-14
		($ in millions)
	Fixed income securities — Available for sale	$	35.2		$	38.5
	Fixed income securities — Trading	7			6.6
	Short-term investments — Available for sale	0.4			0.3
	Fixed term deposits (included in cash and cash equivalents)	1.1			0.8
	Equity securities — Available for sale	—			1.5
	Equity securities — Trading	6.2			3.9
	Catastrophe bonds — Trading	0.4			0.2
	Total	$	50.3		$	51.8
	Investment expenses	(2.9		)	(2.3		)
	Net investment income	$	47.4		$	49.5
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 three months ended March 31, 2015 and 2014:
		For the Three Months Ended
		31-Mar-15			31-Mar-14
		($ in millions)
	Available for sale:
	Fixed income securities — gross realized gains	$	6.2		$	4.4
	Fixed income securities — gross realized (losses)	(0.5		)	(3.1		)
	Equity securities — gross realized gains	31.9			1.3
	Equity securities — gross realized (losses)	(3.0		)	—
	Trading:
	Fixed income securities — gross realized gains	2			1.9
	Fixed income securities — gross realized (losses)	(2.2		)	(1.2		)
	Equity securities — gross realized gains	16.5			1
	Equity securities — gross realized (losses)	(8.8		)	—
	Catastrophe bonds	—			0.1
	Net change in gross unrealized gains	0.8			9.2
	Total net realized and unrealized investment gains recorded in the statement of operations	$	42.9		$	13.6
	Change in available for sale net unrealized gains (losses):
	Fixed income securities	27.3			22.1
	Short-term investments	0.1			—
	Equity securities	(27.3		)	0.5
	Total change in pre-tax available for sale unrealized (losses) gains	0.1			22.6
	Change in taxes	(2.9		)	(2.3		)
	Total change in net unrealized (losses) gains, net of taxes, recorded in other comprehensive income	$	(2.8	)	$	20.3
Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities, Short-Term Investments and Equities	Fixed Income Securities, Short-Term Investments and Equities — Available For Sale. The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income securities, short-term investments and equity securities as at March 31, 2015 and December 31, 2014:
		As at March 31, 2015
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	1,015.40		$	26.6		$	(0.2	)	$	1,041.80
	U.S. agency	175.4			7.5			—			182.9
	Municipal	27.9			2.8			—			30.7
	Corporate	2,242.00			93.5			(1.9		)	2,333.60
	Non-U.S. government-backed corporate	66.8			1.4			—			68.2
	Foreign government	620.3			17.5			(0.1		)	637.7
	Asset-backed	138.6			2.3			—			140.9
	Non-agency commercial mortgage-backed	37.6			2.8			—			40.4
	Agency mortgage-backed	1,021.40			43.6			(1.4		)	1,063.60
	Total fixed income securities — Available for sale	5,345.40			198			(3.6		)	5,539.80
	Total short-term investments — Available for sale	180.1			0.2			(0.1		)	180.2
	Total	$	5,525.50		$	198.2		$	(3.7	)	$	5,720.00
		As at December 31, 2014
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	1,074.20		$	21.5		$	(1.3	)	$	1,094.40
	U.S. agency	190			7.5			(0.1		)	197.4
	Municipal	29.1			2.4			—			31.5
	Corporate	2,244.70			79.9			(5.2		)	2,319.40
	Non-U.S. government-backed corporate	76.8			1.2			—			78
	Foreign government	648.6			17.3			(0.2		)	665.7
	Asset-backed	141.3			2.4			(0.2		)	143.5
	Non-agency commercial mortgage-backed	41.5			3.3			—			44.8
	Agency mortgage-backed	1,016.70			40.8			(2.2		)	1,055.30
	Total fixed income securities — Available for sale	5,462.90			176.3			(9.2		)	5,630.00
	Total short-term investments — Available for sale	258.2			0.1			—			258.3
	Total equity securities — Available for sale	82.6			27.3			—			109.9
	Total	$	5,803.70		$	203.7		$	(9.2	)	$	5,998.20
Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities	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 March 31, 2015 and December 31, 2014:
		As at March 31, 2015
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	9		$	—		$	—		$	9
	U.S. agency	0.2			—			—			0.2
	Municipal	0.5			—			—			0.5
	Corporate	539.4			15.9			(1.2		)	554.1
	Foreign government	128.3			5.8			(0.4		)	133.7
	Asset-backed	15.9			0.1			—			16
	Bank loans	86.2			0.3			(0.6		)	85.9
	Total fixed income securities — Trading	779.5			22.1			(2.2		)	799.4
	Total short-term investments — Trading	0.7			—			—			0.7
	Total equity securities — Trading	696.7			54.1			(31.8		)	719
	Total catastrophe bonds — Trading	32.6			0.3			(0.1		)	32.8
	Total	$	1,509.50		$	76.5		$	(34.1	)	$	1,551.90
		As at December 31, 2014
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	—		$	—		$	—		$	—
	U.S. agency	0.2			—			—			0.2
	Municipal	1.1			—			—			1.1
	Corporate	520.9			11.7			(2.8		)	529.8
	Foreign government	137.3			4.3			(1.5		)	140.1
	Asset-backed	14.6			0.1			—			14.7
	Bank loans	86.8			—			(1.7		)	85.1
	Total fixed income securities — Trading	760.9			16.1			(6.0		)	771
	Total short-term investments — Trading	0.2			—			—			0.2
	Total equity securities — Trading	585.2			55.5			(24.7		)	616
	Total catastrophe bonds — Trading	34.4			0.4			—			34.8
	Total	$	1,380.70		$	72		$	(30.7	)	$	1,422.00
Other Investments	The table below shows the Company’s investments in the Microinsurance Venture and Chaspark for the three months ended March 31, 2015:
		For the Three Months Ended March 31, 2015
		MVI			Chaspark			Total
		($ in millions)
	Opening undistributed value of investment as at January 1, 2015	$	—		$	8.7		$	8.7
	Initial investment	0.8			—			$	0.8
	Closing value of investment as at March 31, 2015	$	0.8		$	8.7		$	9.5
Summary of Fixed Maturities	Fixed Income Securities. The scheduled maturity distribution of available for sale fixed income securities as at March 31, 2015 and December 31, 2014 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 March 31, 2015
		Amortized			Fair Market			Average
		Cost or Cost			Value			S&P Ratings by
								Maturity
		($ in millions)
	Due one year or less	$	512.4		$	517.4		AA
	Due after one year through five years	2,559.00			2,638.40			AA-
	Due after five years through ten years	987.9			1,035.50			A+
	Due after ten years	88.5			103.6			A+
	Subtotal	4,147.80			4,294.90
	Non-agency commercial mortgage-backed	37.6			40.4			AA+
	Agency mortgage-backed	1,021.40			1,063.60			AA+
	Asset-backed	138.6			140.9			AAA
	Total fixed income securities — Available for sale	$	5,345.40		$	5,539.80
		As at December 31, 2014
		Amortized			Fair Market			Average
		Cost or Cost			Value			S&P Ratings by
								Maturity
		($ in millions)
	Due one year or less	$	590.2		$	594.7		AA
	Due after one year through five years	2,552.00			2,620.80			AA-
	Due after five years through ten years	1,023.50			1,059.90			A+
	Due after ten years	97.7			111			A+
	Subtotal	4,263.40			4,386.40
	Non-agency commercial mortgage-backed	41.5			44.8			AA+
	Agency mortgage-backed	1,016.70			1,055.30			AA+
	Asset-backed	141.3			143.5			AAA
	Total fixed income securities — Available for sale	$	5,462.90		$	5,630.00
Aggregate Fair Value and Gross Unrealized Loss by Type of Security	Gross Unrealized Loss. The following tables summarize as at March 31, 2015 and December 31, 2014 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:
		March 31, 2015
		0-12 months						Over 12 months						Total
		Fair			Gross			Fair			Gross			Fair		Gross			Number of
		Market			Unrealized			Market			Unrealized			Market		Unrealized			Securities
		Value			Loss			Value			Loss			Value		Loss
		($ in millions)
	U.S. government	$	41.5		$	(0.1	)	$	40.6		$	(0.1	)	$	82.1	$	(0.2	)	18
	U.S. agency	—			—			5			—			5		—			1
	Municipal	—			—			—			—			—		—			0
	Corporate	206.4			(1.3		)	77.7			(0.6		)	284.1		(1.9		)	126
	Non-U.S. government-backed corporate	2.5			—			—			—			2.5		—			2
	Foreign government	118.1			(0.1		)	3.9			—			122		(0.1		)	15
	Asset-backed	20			—			9.5			—			29.5		—			26
	Agency mortgage-backed	101.2			(0.5		)	72.2			(0.9		)	173.4		(1.4		)	52
	Total fixed income securities — Available for sale	489.7			(2.0		)	208.9			(1.6		)	698.6		(3.6		)	240
	Total short-term investments — Available for sale	52.2			(0.1		)	—			—			52.2		(0.1		)	9
	Total	$	541.9		$	(2.1	)	$	208.9		$	(1.6	)	$	750.8	$	(3.7	)	249
		December 31, 2014
		0-12 months						Over 12 months						Total
		Fair			Gross			Fair			Gross			Fair		Gross			Number of
		Market			Unrealized			Market			Unrealized			Market		Unrealized			Securities
		Value			Loss			Value			Loss			Value		Loss
		($ in millions)
	U.S. government	$	166.1		$	(0.5	)	$	79.4		$	(0.8	)	$	245.5	$	(1.3	)	39
	U.S. agency	25.1			—			4.9			(0.1		)	30		(0.1		)	7
	Municipal	—			—			—			—			—		—			0
	Corporate	459.4			(2.1		)	171.3			(3.1		)	630.7		(5.2		)	274
	Non-U.S. government-backed corporate	0.7			—			—			—			0.7		—			1
	Foreign government	30.4			—			44.2			(0.2		)	74.6		(0.2		)	16
	Asset-backed	43.7			(0.1		)	11.7			(0.1		)	55.4		(0.2		)	43
	Agency mortgage-backed	64.7			(0.3		)	111.7			(1.9		)	176.4		(2.2		)	48
	Total fixed income securities — Available for sale	790.1			(3.0		)	423.2			(6.2		)	1,213.30		(9.2		)	428
	Total short-term investments — Available for sale	4.6			—			—			—			4.6		—			3
	Total	$	794.7		$	(3.0	)	$	423.2		$	(6.2	)	$	1,217.90	$	(9.2	)	431
Analysis of Investment Purchases/Sales and Maturities	Investment Purchases and Sales. The following table summarizes investment purchases, sales and maturities for the three months ended March 31, 2015 and 2014:
		For the Three Months Ended
		31-Mar-15			31-Mar-14
		($ in millions)
	(Purchases) of fixed income securities — Available for sale	$	(483.7	)	$	(537.2	)
	(Purchases) of fixed income securities — Trading	(153.4		)	(192.1		)
	(Purchases) of equity securities — Available for sale	—			—
	(Purchases) of equity securities — Trading	(186.7		)	(51.8		)
	Proceeds from sales and maturities of fixed income securities — Available for sale	540.7			598.8
	Proceeds from sales and maturities of fixed income securities — Trading	133.8			181.3
	Proceeds from sales of equity securities — Available for sale	108.6			4
	Proceeds from sales of equity securities — Trading	83.7			5.3
	Net change in receivable/(payable) for securities sold/(purchased)	15.9			13.6
	Net (purchases)/sales of short-term investments — Available for sale	61.3			(61.9		)
	Net (purchases)/sales of short-term investments — Trading	(0.5		)	—
	Net proceeds/(purchases) of catastrophe bonds — Trading	1.8			(12.1		)
	Net sales of other investments	—			37.3
	Net (purchases)/sales for the period	$	121.5		$	(14.8	)

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015 and December 31, 2014, respectively:
		As at March 31, 2015
		Level 1				Level 2				Level 3				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	1,041.80			$	—			$	—			$	1,041.80
	U.S. agency	—				182.9				—				182.9
	Municipal	—				30.7				—				30.7
	Corporate	—				2,333.60				—				2,333.60
	Non-U.S. government-backed corporate	—				68.2				—				68.2
	Foreign government	462.5				175.2				—				637.7
	Asset-backed	—				140.9				—				140.9
	Non-agency commercial mortgage-backed	—				40.4				—				40.4
	Agency mortgage-backed	—				1,063.60				—				1,063.60
	Total fixed income securities available for sale, at fair value	1,504.30				4,035.50				—				5,539.80
	Short-term investments available for sale, at fair value	144				36.2				—				180.2
	Equity investments available for sale, at fair value	—				—				—				—
	Held for trading financial assets, at fair value
	U.S. government	9				—				—				9
	U.S. agency	—				0.2				—				0.2
	Municipal	—				0.5				—				0.5
	Corporate	—				554.1				—				554.1
	Foreign government	40.7				93				—				133.7
	Asset-backed	—				16				—				16
	Bank loans	—				85.9				—				85.9
	Total fixed income securities trading, at fair value	49.7				749.7				—				799.4
	Short-term investments trading, at fair value	—				0.7				—				0.7
	Equity investments trading, at fair value	719				—				—				719
	Catastrophe bonds trading, at fair value	—				32.8				—				32.8
	Other financial assets and liabilities, at fair value
	Derivatives at fair value — foreign exchange contracts	—				2				—				2
	Derivatives at fair value— interest rate swaps	—				0.1				—				0.1
	Liabilities under derivative contracts — foreign exchange contracts	—				(11.5		)		—				(11.5		)
	Loan notes issued by variable interest entities, at fair value	—				—				(76.0		)		(76.0		)
	Loan notes issued by variable interest entities, at fair value (classified as a current liability)	—				—				(0.7		)		(0.7		)
	Total	$	2,417.00			$	4,845.50			$	(76.7	)		$	7,185.80
	There were no maturities, settlements or transfers between Level 1, Level 2 and Level 3 during the three months ended March 31, 2015 and no assets or liabilities were classified as Level 3 as at March 31, 2015, except for the loan notes issued by the VIEs.
		As at December 31, 2014
		Level 1				Level 2				Level 3				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	1,094.40			$	—			$	—			$	1,094.40
	U.S. agency	—				197.4				—				197.4
	Municipal	—				31.5				—				31.5
	Corporate	—				2,319.40				—				2,319.40
	Non-U.S. government-backed corporate	—				78				—				78
	Foreign government	456.5				209.2				—				665.7
	Asset-backed	—				143.5				—				143.5
	Non-agency commercial mortgage-backed	—				44.8				—				44.8
	Agency mortgage-backed	—				1,055.30				—				1,055.30
	Total fixed income securities available for sale, at fair value	1,550.90				4,079.10				—				5,630.00
	Short-term investments available for sale, at fair value	229.3				29				—				258.3
	Equity investments available for sale, at fair value	109.9				—				—				109.9
	Held for trading financial assets, at fair value
	U.S. government	—				—				—				—
	U.S. agency	—				0.2				—				0.2
	Municipal	—				1.1				—				1.1
	Corporate	—				529.8				—				529.8
	Foreign government	36.1				104				—				140.1
	Asset-backed	—				14.7				—				14.7
	Bank loans	—				85.1				—				85.1
	Catastrophe bonds trading, at fair value	—				34.8				—				34.8
	Total fixed income securities trading, at fair value	36.1				769.7				—				805.8
	Short-term investments trading, at fair value	0.1				0.1				—				0.2
Equity investments trading, at fair value	616				—				—				616
Other financial assets and liabilities, at fair value
Derivatives at fair value – foreign exchange contracts	—				7.9				—				7.9
Derivatives at fair value – interest rate swaps	—				0.1				—				0.1
Liabilities under derivative contracts – foreign exchange contracts	—				(14.3		)		—				(14.3		)
Loan notes issued by variable interest entities, at fair value	—				—				(70.7		)		(70.7		)
Loan notes issued by variable interest entities, at fair value (classified as a current liability)	—				—				(67.9		)		(67.9		)
Total	$	2,542.30			$	4,871.60			$	(138.6	)		$	7,275.30
Reconciliation of Liabilities Using Level 3 Inputs	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 three months ended March 31, 2015 and 2014:
	Reconciliation of Liabilities Using Level 3 Inputs			Three Months Ended March 31, 2015				Three Months Ended March 31, 2014
				($ in millions)
	Balance at the beginning of the period (1)			$	138.6			$	50
	Distributed to third party			(64.8		)		—
	Total change in fair value included in the statement of operations			2.9				3.4
	Balance at the end of the period (1)			$	76.7			$	53.4
Pricing Sources Used in Pricing Fixed Income Investments	Pricing sources used in pricing fixed income investments as at March 31, 2015 and December 31, 2014 were as follows:
		As at March 31, 2015			As at December 31, 2014
	Index providers	82	%		84	%
	Pricing services	13			11
	Broker-dealers	5			5
	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 March 31, 2015 and December 31, 2014 is provided below:
		As at March 31, 2015							As at December 31, 2014
		Fair Market				% of Total			Fair Market				% of Total
		Value Determined				Fair Value by			Value Determined				Fair Value by
		using Prices from				Security Type			using Prices from				Security Type
		Index Providers							Index Providers
		($ in millions, except for percentages)
	U.S. government	$	1,039.00			99	%		$	1,044.40			95	%
	U.S. agency	172.7				94	%		186.9				95	%
	Municipal	13.9				45	%		13.7				42	%
	Corporate	2,680.70				93	%		2,731.10				96	%
	Non-U.S. government-backed corporate	33.5				49	%		48.7				62	%
	Foreign government	434.6				56	%		504.4				63	%
	Asset-backed	145.5				93	%		140.5				89	%
	Non-agency commercial mortgage-backed	40.3				100	%		44.8				100	%
	Agency mortgage-backed	651				61	%		680.6				64	%
	Total fixed income securities	$	5,211.20			82	%		$	5,395.10			84	%
	Equities	715.3				100	%		725.9				100	%
	Total fixed income securities and equity investments	$	5,926.50			84	%		$	6,121.00			86	%
Long-term Debt Issued by Silverton	The observable and unobservable inputs used to determine the fair value of the 2015-1 Loan Notes and 2014-1 Loan Notes as at March 31, 2015 and December 31, 2014 are presented in the table below:
	At March 31, 2015		Fair Value				Valuation Method		Observable (O) and		Low				High
			Level 3						Unobservable (U) inputs
		($ in millions)									($ in millions)
	Loan notes held by third parties		$	76.7			Internal Valuation Model		Gross premiums written (O)		$	37.6			$	39.6
									Reserve for losses (U)		$	1.1			$	4.4
									Contract period (O)		N/A				365 days
									Initial value of issuance (O)		$	120			$	120
	At December 31, 2014		Fair Value				Valuation Method		Observable (O) and		Low				High
			Level 3						Unobservable (U) inputs
		($ in millions)									($ in millions)
	Loan notes held by third parties		$	138.6			Internal Valuation Model		Gross premiums written (O)		$	—			$	40
									Reserve for losses (U)		$	—			$	4.6
									Contract period (O)		N/A				365 days
									Initial value of issuance (O)		$	120			$	120

Derivative_Contracts_Tables

Derivative Contracts (Tables)	3 Months Ended
	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Fair Value of Derivative Instruments	The following tables summarize information on the location and amounts of derivative fair values on the consolidated balance sheet as at March 31, 2015 and December 31, 2014:
				As at March 31, 2015							As at December 31, 2014
	Derivatives Not Designated as Hedging Instruments		Balance Sheet Location	Notional			Fair				Notional		Fair
	Under ASC 815			Amount			Value				Amount		Value
				($ in millions)							($ in millions)
	Interest Rate Swaps		Derivatives at Fair Value	$	951.3		$	0.1		(1)	$	951.3	$	0.1		(1)
	Foreign Exchange Contracts		Derivatives at Fair Value	$	220		$	2			$	165.8	$	7.9
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	142.3		$	(5.3	)		$	237.6	$	(10.5	)
	(1)	Net of $19.5 million of cash collateral provided to counterparties, Goldman Sachs International ($451.3 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2014 — $22.3 million).
				As at March 31, 2015							As at December 31, 2014
	Derivatives Designated as Hedging Instruments Under ASC 815		Balance Sheet Location	Notional			Fair				Notional		Fair
				Amount			Value				Amount		Value
				($ in millions)							($ in millions)
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	84.6		$	(6.2	)	(1)	$	135.8	$	(3.8	)	(1)
	(1)	Net of $Nil cash collateral (December 31, 2014 — $Nil).
Gain/(Loss) Recognized in Income on Derivative	The following tables provide the unrealized and realized gains/(losses) recorded in the statement of operations for the three months ended March 31, 2015 and 2014, respectively:
				Amount of Income/(Loss)
				Recognized in the Statement
				of Operations and Other Comprehensive Income for the
				Three Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	31-Mar-15			31-Mar-14
	ASC 815		Statement of Operations and Other Comprehensive Income
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(4.6	)	$	3
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	(3.2	)	$	(1.9	)
				Amount of Income/(Loss)
				Recognized in the Statement
				of Operations and Other Comprehensive Income for the
				Three Months Ended
	Derivatives Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	31-Mar-15			31-Mar-14
	ASC 815		Statement of Operations and Other Comprehensive Income
				($ in millions)
	Foreign Exchange Contracts		General, administrative and corporate expenses	$	(2.8	)	$	—
	Foreign Exchange Contracts		Net change from current period hedged transactions	$	(2.4	)	$	—

Deferred_Policy_Acquisition_Co1

Deferred Policy Acquisition Costs (Tables)	3 Months Ended
	Mar. 31, 2015
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 three months ended March 31, 2015 and 2014:
			Three Months Ended March 31,
			2015			2014
			($ in millions)
	Balance at the beginning of the period		$	299		$	262.2
		Acquisition costs deferred	154.1			139.4
		Amortization of deferred policy acquisition costs	(119.3		)	(112.0		)
	Balance at the end of the period		$	333.8		$	289.6

Reserves_for_Losses_and_Loss_A1

Reserves for Losses and Loss Adjustment Expenses (Tables)	3 Months Ended
	Mar. 31, 2015
Insurance [Abstract]
Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses ("LAE") Reserves	The following table represents a reconciliation of beginning and ending consolidated loss and loss adjustment expenses (“LAE”) reserves for the three months ended March 31, 2015 and twelve months ended December 31, 2014:
		Three Months Ended March 31, 2015			Twelve Months Ended December 31, 2014
		($ in millions)
	Provision for losses and LAE at the start of the year	$	4,750.80		$	4,678.90
	Less reinsurance recoverable	(350.0		)	(332.7		)
	Net loss and LAE at the start of the year	4,400.80			4,346.20
	Net loss and LAE expenses assumed/(disposed)	—			(24.2		)
	Provision for losses and LAE for claims incurred:
	Current year	333.6			1,411.60
	Prior years	(27.5		)	(104.1		)
	Total incurred	306.1			1,307.50
	Losses and LAE payments for claims incurred:
	Current year	(18.8		)	(112.1		)
	Prior years	(252.9		)	(995.6		)
	Total paid	(271.7		)	(1,107.7		)
	Foreign exchange (gains)	(96.4		)	(121.0		)
	Net losses and LAE reserves at period end	4,338.80			4,400.80
	Plus reinsurance recoverable on unpaid losses at period end	360.1			350
	Provision for losses and LAE at the end of the relevant period	$	4,698.90		$	4,750.80

Capital_Structure_Tables

Capital Structure (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015 and December 31, 2014:
		As at March 31, 2015			As at December 31, 2014
		Number		$ in	Number	$ in
				Thousands		Thousands
	Authorized share capital:
	Ordinary Shares 0.15144558¢ per share	969,629,030		1,469	969,629,030	1,469
	Non-Voting Shares 0.15144558¢ per share	6,787,880		10	6,787,880	10
	Preference Shares 0.15144558¢ per share	100,000,000		152	100,000,000	152
	Total authorized share capital			1,631		1,631
	Issued share capital:
	Issued ordinary shares of 0.15144558¢ per share	61,722,502		93	62,017,368	94
	Issued 7.401% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	5,327,500		8	5,327,500	8
	Issued 7.250% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	6,400,000		10	6,400,000	10
	Issued 5.95% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	11,000,000		17	11,000,000	17
	Total issued share capital			128		129
Summary of Ordinary Shares	The following table summarizes transactions in the Company’s ordinary shares during the three months ended March 31, 2015:
		Number of Ordinary Shares
	Ordinary shares in issue at December 31, 2014	62,017,368
	Ordinary share transactions in the three months ended March 31, 2015
	Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or	485,466
	2008 share purchase plan
	Ordinary shares issued to non-employee directors	6,806
	Ordinary shares repurchased	(787,138	)
	Ordinary shares in issue at March 31, 2015	61,722,502

Intangible_Assets_Tables

Intangible Assets (Tables)	3 Months Ended
	Mar. 31, 2015
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 three months ended March 31, 2015 and 2014:
		Three Months Ended March 31, 2015								Three Months Ended March 31, 2014
		Trade		Insurance		Other		Total		Trade		Insurance		Other			Total
		Mark		Licenses						Mark		Licenses
		($ in millions)								($ in millions)
	Intangible Assets
	Beginning of the period	$	1.6	$	16.6	$	—	$	18.2	$	1.6	$	16.6	$	0.2		$	18.4
	Amortization	—		—		—		—		—		—		(0.1		)	(0.1		)
	End of the period	$	1.6	$	16.6	$	—	$	18.2	$	1.6	$	16.6	$	0.1		$	18.3

Commitments_and_Contingent_Lia1

Commitments and Contingent Liabilities (Tables)	3 Months Ended
	Mar. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of Company's Restricted Assets	The following table details the forms and value of the Company’s restricted assets as at March 31, 2015 and December 31, 2014:
		As at March 31, 2015			As at December 31, 2014
		($ in millions, except percentages)
	Regulatory trusts and deposits:
	Affiliated transactions	$	988.6		$	1,086.90
	Third party	2,121.70			2,183.40
	Letters of credit / guarantees(1)	714			778.7
	Total restricted assets	$	3,824.30		$	4,049.00
	Total as percent of cash and invested assets	45		%	47		%
	(1)	As of March 31, 2015, the Company had pledged funds of $704.8 million and £9.2 million (December 31, 2014 — $764.4 million and £9.2 million) as collateral for the secured letters of credit.
Amounts Outstanding under Operating Leases	Amounts outstanding under operating leases net of subleases as of March 31, 2015 were:
		2015			2016			2017		2018		2019		Later		Total
														Years
		($ in millions)
	Operating Lease Obligations	$	10.1		$	9.8		$	8.5	$	7	$	6.1	$	1.5	$	43

Reclassifications_from_Accumul2

Reclassifications from Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income Reclassification (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Available for sale securities:
Realized gains on sale of securities	$33.50	$3
Realized (losses) on sale of securities	-1.2	-2.6
Net realized and unrealized investment gains reclassified to the statement of operations, gross of tax	32.3	0.4
Tax on net realized gains of securities	-0.4	-0.2
Total reclassifications from AOCI to the statement of operations, net of tax	$31.90	$0.20

Earnings_per_Ordinary_Share_Co

Earnings per Ordinary Share - Computation of Basic and Diluted Earnings per Share (Details) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Earnings Per Share [Abstract]
Net income	$128		$120.40
Preference share dividends	-9.5		-9.5
Net profit attributable to non-controlling interest	0		-0.1
Basic and diluted net income available to ordinary shareholders	$118.50		$110.80
Ordinary shares:
Basic weighted average ordinary shares	62,159,303		65,289,351
Weighted average effect of dilutive securities	1,373,359	[1]	1,276,539	[1]
Total diluted weighted average ordinary shares	63,532,662		66,565,890
Earnings per ordinary share:
Basic, dollars per share	$1.91		$1.70
Diluted, dollars per share	$1.87		$1.66
[1]	Dilutive securities comprise: employee options, 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 14.

Earnings_per_Ordinary_Share_Su

Earnings per Ordinary Share - Summary of Declared Dividends (Details) (USD $)	3 Months Ended
	Mar. 31, 2015
Ordinary shares
Dividends Payable [Line Items]
Dividend	$0.21
Payable on	27-May-15
Record Date	8-May-15
7.401% preference shares
Dividends Payable [Line Items]
Dividend	$0.46
Payable on	1-Jul-15
Record Date	15-Jun-15
Annual rate of dividend	7.40%
7.250% preference shares
Dividends Payable [Line Items]
Dividend	$0.45
Payable on	1-Jul-15
Record Date	15-Jun-15
Annual rate of dividend	7.25%
5.95% preference shares
Dividends Payable [Line Items]
Dividend	$0.37
Payable on	1-Jul-15
Record Date	15-Jun-15
Annual rate of dividend	5.95%

Segment_Reporting_Summary_of_G

Segment Reporting - Summary of Gross and Net Written and Earned Premiums, Underwriting Results, Ratios and Reserves for Each of Company's Business Segments (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014	Dec. 31, 2013
Underwriting Revenues
Gross written premiums	$919.20		$855.50
Net written premiums	763.2		697.5
Gross earned premiums	680.9		652.1
Net earned premiums	593.6		566.5
Underwriting Expenses
Losses and loss adjustment expenses	306.1		288.1
Amortization of deferred policy acquisition costs	119.3		112
General and administrative expenses	87.7		78.7
Underwriting income	80.5		87.7
Corporate expenses	-14.5		-16.9
Net investment income	47.4		49.5
Realized and unrealized investment gains	57.4		17.9
Realized and unrealized investment losses	-14.5		-4.3
Change in fair value of loan notes issued by variable interest entities	-2.9		-3.4
Change in fair value of derivatives	-7.8		1.1
Interest expense on long term debt	-7.4		-7.4
Net realized and unrealized foreign exchange (losses)	-6.4		0.1
Other income	3.9		0.6
Other expenses	-2.6		-0.7
Income from operations before income tax	133.1		124.2
Net reserves for loss and loss adjustment expenses	4,338.80		4,408.60		4,400.80	4,346.20
Ratios
Loss ratio	51.60%		50.90%
Policy acquisition expense ratio	20.10%		19.80%
General and administrative expense ratio	17.20%	[1]	16.90%	[1]
Expense ratio	37.30%		36.70%
Combined ratio	88.90%		87.60%
Reinsurance
Underwriting Revenues
Gross written premiums	484.8		472.2
Net written premiums	442.1		442.6
Gross earned premiums	265.8		278.5
Net earned premiums	249.4		266.7
Underwriting Expenses
Losses and loss adjustment expenses	105.5		110.4
Amortization of deferred policy acquisition costs	53.4		50.4
General and administrative expenses	32.4		32.8
Underwriting income	58.1		73.1
Net reserves for loss and loss adjustment expenses	2,410.90		2,626.30
Ratios
Loss ratio	42.30%		41.40%
Policy acquisition expense ratio	21.40%		18.90%
General and administrative expense ratio	13.00%		12.30%
Expense ratio	34.40%		31.20%
Combined ratio	76.70%		72.60%
Insurance
Underwriting Revenues
Gross written premiums	434.4		383.3
Net written premiums	321.1		254.9
Gross earned premiums	415.1		373.6
Net earned premiums	344.2		299.8
Underwriting Expenses
Losses and loss adjustment expenses	200.6		177.7
Amortization of deferred policy acquisition costs	65.9		61.6
General and administrative expenses	55.3		45.9
Underwriting income	22.4		14.6
Net reserves for loss and loss adjustment expenses	$1,927.90		$1,782.30
Ratios
Loss ratio	58.30%		59.30%
Policy acquisition expense ratio	19.10%		20.50%
General and administrative expense ratio	16.10%		15.30%
Expense ratio	35.20%		35.80%
Combined ratio	93.50%		95.10%
[1]	The general and administrative expense ratio in the total column includes corporate expenses.

Segment_Reporting_Narrative_De

Segment Reporting - Narrative (Details)	3 Months Ended
	Mar. 31, 2015
	Segment
Segment Reporting [Abstract]
Number of business segments	2

Investments_Summary_of_Investm

Investments - Summary of Investment Income (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	$50.30	$51.80
Investment expenses	-2.9	-2.3
Net investment income	47.4	49.5
Fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	35.2	38.5
Fixed income securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	7	6.6
Short-term investments — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	0.4	0.3
Fixed term deposits (included in cash and cash equivalents)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	1.1	0.8
Equity securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	0	1.5
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	6.2	3.9
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	$0.40	$0.20

Investments_Net_Realized_and_U

Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	$33.50	$3
Fixed income maturities — gross realized (losses)	-1.2	-2.6
Catastrophe bonds	0	0.1
Net change in gross unrealized gains	0.8	9.2
Total net realized and unrealized investment gains recorded in the statement of operations	42.9	13.6
Change in available for sale net unrealized gains (losses):
Fixed income securities	27.3	22.1
Short-term investments	0.1	0
Equity securities	-27.3	0.5
Total change in pre-tax available for sale unrealized (losses) gains	0.1	22.6
Change in taxes	-2.9	-2.3
Total change in net unrealized (losses) gains, net of taxes, recorded in other comprehensive income	-2.8	20.3
Fixed income maturities - Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	6.2	4.4
Fixed income maturities — gross realized (losses)	-0.5	-3.1
Equity Securities - Available for Sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities — gross realized gains	31.9	1.3
Equity securities — gross realized (losses)	-3	0
Fixed Income Maturities - Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	2	1.9
Fixed income maturities — gross realized (losses)	-2.2	-1.2
Equity Securities - Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities — gross realized gains	16.5	1
Equity securities — gross realized (losses)	($8.80)	$0

Investments_Cost_Gross_Unreali

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities, Short-Term Investments and Equities (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	$5,525.50	$5,803.70
Gross Unrealized Gains	198.2	203.7
Gross Unrealized Losses	-3.7	-9.2
Fair Market Value	5,720	5,998.20
Total fixed income securities — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	5,345.40	5,462.90
Gross Unrealized Gains	198	176.3
Gross Unrealized Losses	-3.6	-9.2
Fair Market Value	5,539.80	5,630
Total short-term investments — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	180.1	258.2
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Losses	-0.1	0
Fair Market Value	180.2	258.3
Total equity securities — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost		82.6
Gross Unrealized Gains		27.3
Gross Unrealized Losses		0
Fair Market Value		109.9
U.S. government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,015.40	1,074.20
Gross Unrealized Gains	26.6	21.5
Gross Unrealized Losses	-0.2	-1.3
Fair Market Value	1,041.80	1,094.40
U.S. agency
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	175.4	190
Gross Unrealized Gains	7.5	7.5
Gross Unrealized Losses	0	-0.1
Fair Market Value	182.9	197.4
Municipal
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	27.9	29.1
Gross Unrealized Gains	2.8	2.4
Gross Unrealized Losses	0	0
Fair Market Value	30.7	31.5
Corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	2,242	2,244.70
Gross Unrealized Gains	93.5	79.9
Gross Unrealized Losses	-1.9	-5.2
Fair Market Value	2,333.60	2,319.40
Non-U.S. government-backed corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	66.8	76.8
Gross Unrealized Gains	1.4	1.2
Gross Unrealized Losses	0	0
Fair Market Value	68.2	78
Foreign government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	620.3	648.6
Gross Unrealized Gains	17.5	17.3
Gross Unrealized Losses	-0.1	-0.2
Fair Market Value	637.7	665.7
Asset-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	138.6	141.3
Gross Unrealized Gains	2.3	2.4
Gross Unrealized Losses	0	-0.2
Fair Market Value	140.9	143.5
Non-agency commercial mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	37.6	41.5
Gross Unrealized Gains	2.8	3.3
Gross Unrealized Losses	0	0
Fair Market Value	40.4	44.8
Agency mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,021.40	1,016.70
Gross Unrealized Gains	43.6	40.8
Gross Unrealized Losses	-1.4	-2.2
Fair Market Value	$1,063.60	$1,055.30

Investments_Cost_Gross_Unreali1

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	$779.50	$760.90
Fair Market Value	799.4	771
U.S. government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	9	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	9	0
U.S. agency
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	0.2	0.2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	0.2	0.2
Municipal
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	0.5	1.1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	0.5	1.1
Corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	539.4	520.9
Gross Unrealized Gains	15.9	11.7
Gross Unrealized Losses	-1.2	-2.8
Fair Market Value	554.1	529.8
Foreign government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	128.3	137.3
Gross Unrealized Gains	5.8	4.3
Gross Unrealized Losses	-0.4	-1.5
Fair Market Value	133.7	140.1
Asset-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	15.9	14.6
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0	0
Fair Market Value	16	14.7
Bank loans
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	86.2	86.8
Gross Unrealized Gains	0.3	0
Gross Unrealized Losses	-0.6	-1.7
Fair Market Value	85.9	85.1
Total fixed income securities — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	779.5	760.9
Gross Unrealized Gains	22.1	16.1
Gross Unrealized Losses	-2.2	-6
Fair Market Value	799.4	771
Total short-term investments — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	0.7	0.2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	0.7	0.2
Total equity securities — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	696.7	585.2
Gross Unrealized Gains	54.1	55.5
Gross Unrealized Losses	-31.8	-24.7
Fair Market Value	719	616
Total catastrophe bonds — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	32.6	34.4
Gross Unrealized Gains	0.3	0.4
Gross Unrealized Losses	-0.1	0
Fair Market Value	32.8	34.8
Total
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,509.50	1,380.70
Gross Unrealized Gains	76.5	72
Gross Unrealized Losses	-34.1	-30.7
Fair Market Value	$1,551.90	$1,422

Investments_Other_Investments_

Investments - Other Investments (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment as at January 1, 2015	$8.70
Initial investment	0.8	0
Closing value of investment as at March 31, 2015	9.5
MVI
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment as at January 1, 2015	0
Initial investment	0.8
Closing value of investment as at March 31, 2015	0.8
Chaspark
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment as at January 1, 2015	8.7
Initial investment	0
Closing value of investment as at March 31, 2015	$8.70

Investments_Summary_of_Fixed_M

Investments - Summary of Fixed Maturities (Details) (USD $)	3 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	$5,345.40	$5,462.90
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,525.50	5,803.70
Available for sale investments in fixed income maturities, Fair Market Value	5,539.80	5,630
Total fixed income securities — Available for sale
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,345.40	5,462.90
Non-agency commercial mortgage-backed
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	37.6	41.5
Available for sale investments in fixed income maturities, Fair Market Value	40.4	44.8
Available for sale investments Average Ratings by Maturity	AA+	AA+
Agency mortgage-backed
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	1,021.40	1,016.70
Available for sale investments in fixed income maturities, Fair Market Value	1,063.60	1,055.30
Available for sale investments Average Ratings by Maturity	AA+	AA+
Asset-backed
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	138.6	141.3
Available for sale investments in fixed income maturities, Fair Market Value	140.9	143.5
Available for sale investments Average Ratings by Maturity	AAA	AAA
Total fixed income securities — Available for sale
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Due one year or less, Cost or Amortized Cost	512.4	590.2
Due after one year through five years, Cost or Amortized Cost	2,559	2,552
Due after five years through ten years, Cost or Amortized Cost	987.9	1,023.50
Due after ten years, Cost or Amortized Cost	88.5	97.7
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	4,147.80	4,263.40
Due one year or less, Fair Market Value	517.4	594.7
Due after one year through five years, Fair Market Value	2,638.40	2,620.80
Due after five years through ten years, Fair Market Value	1,035.50	1,059.90
Due after ten years, Fair Market Value	103.6	111
Available for sale investments in fixed income maturities, Fair Market Value	$4,294.90	$4,386.40
Due one year or less, Average Ratings by Maturity	AA	AA
Due after one year through five years, Average Ratings by Maturity	AA-	AA-
Due after five years through ten years, Average Ratings by Maturity	A+	A+
Due after ten years, Average Ratings by Maturity	A+	A+

Investments_Aggregate_Fair_Val

Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified	Security	Security
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$541.90	$794.70
0-12 months, Gross Unrealized Loss	-2.1	-3
Over 12 months, Fair Market Value	208.9	423.2
Over 12 months, Gross Unrealized Loss	-1.6	-6.2
Total, Fair Market Value	750.8	1,217.90
Total, Gross Unrealized Loss	-3.7	-9.2
Number of Securities	249	431
Fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	489.7	790.1
0-12 months, Gross Unrealized Loss	-2	-3
Over 12 months, Fair Market Value	208.9	423.2
Over 12 months, Gross Unrealized Loss	-1.6	-6.2
Total, Fair Market Value	698.6	1,213.30
Total, Gross Unrealized Loss	-3.6	-9.2
Number of Securities	240	428
Total short-term investments — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	52.2	4.6
0-12 months, Gross Unrealized Loss	-0.1	0
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	52.2	4.6
Total, Gross Unrealized Loss	-0.1	0
Number of Securities	9	3
U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	41.5	166.1
0-12 months, Gross Unrealized Loss	-0.1	-0.5
Over 12 months, Fair Market Value	40.6	79.4
Over 12 months, Gross Unrealized Loss	-0.1	-0.8
Total, Fair Market Value	82.1	245.5
Total, Gross Unrealized Loss	-0.2	-1.3
Number of Securities	18	39
U.S. agency
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	0	25.1
0-12 months, Gross Unrealized Loss	0	0
Over 12 months, Fair Market Value	5	4.9
Over 12 months, Gross Unrealized Loss	0	-0.1
Total, Fair Market Value	5	30
Total, Gross Unrealized Loss	0	-0.1
Number of Securities	1	7
Municipal
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	0	0
0-12 months, Gross Unrealized Loss	0	0
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	0	0
Total, Gross Unrealized Loss	0	0
Number of Securities	0	0
Corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	206.4	459.4
0-12 months, Gross Unrealized Loss	-1.3	-2.1
Over 12 months, Fair Market Value	77.7	171.3
Over 12 months, Gross Unrealized Loss	-0.6	-3.1
Total, Fair Market Value	284.1	630.7
Total, Gross Unrealized Loss	-1.9	-5.2
Number of Securities	126	274
Non-U.S. government-backed corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	2.5	0.7
0-12 months, Gross Unrealized Loss	0	0
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	2.5	0.7
Total, Gross Unrealized Loss	0	0
Number of Securities	2	1
Foreign government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	118.1	30.4
0-12 months, Gross Unrealized Loss	-0.1	0
Over 12 months, Fair Market Value	3.9	44.2
Over 12 months, Gross Unrealized Loss	0	-0.2
Total, Fair Market Value	122	74.6
Total, Gross Unrealized Loss	-0.1	-0.2
Number of Securities	15	16
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	20	43.7
0-12 months, Gross Unrealized Loss	0	-0.1
Over 12 months, Fair Market Value	9.5	11.7
Over 12 months, Gross Unrealized Loss	0	-0.1
Total, Fair Market Value	29.5	55.4
Total, Gross Unrealized Loss	0	-0.2
Number of Securities	26	43
Agency mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	101.2	64.7
0-12 months, Gross Unrealized Loss	-0.5	-0.3
Over 12 months, Fair Market Value	72.2	111.7
Over 12 months, Gross Unrealized Loss	-0.9	-1.9
Total, Fair Market Value	173.4	176.4
Total, Gross Unrealized Loss	($1.40)	($2.20)
Number of Securities	52	48

Investments_Analysis_of_Invest

Investments - Analysis of Investment Purchases/Sales and Maturities (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Gain (Loss) on Investments [Line Items]
(Purchases) of fixed income securities — Available for sale	($483.70)	($537.20)
(Purchases) of fixed income securities — Trading	-153.4	-192.1
(Purchases) of equity securities — Available for sale	0	0
(Purchases) of equity securities — Trading	-186.7	-51.8
Proceeds from sales and maturities of fixed income securities — Available for sale	540.7	598.8
Proceeds from sales and maturities of fixed income securities — Trading	133.8	181.3
Proceeds from sales of equity securities — Available for sale	108.6	4
Proceeds from sales of equity securities — Trading	83.7	5.3
Net change in receivable/(payable) for securities sold/(purchased)	15.9	13.6
Net (purchases)/sales of short-term investments — Available for sale	61.3	-61.9
Net (purchases)/sales of short-term investments — Trading	0.5	0
Net proceeds/(purchases) of catastrophe bonds — Trading	1.8	-12.1
Net sales of other investments	0	37.3
Net (purchases)/sales for the period	121.5	-14.8
Available for sale
Gain (Loss) on Investments [Line Items]
Net (purchases)/sales of short-term investments — Available for sale	61.3	-61.9
Trading
Gain (Loss) on Investments [Line Items]
Net (purchases)/sales of short-term investments — Trading	($0.50)	$0

Investments_Narrative_Details

Investments - Narrative (Details) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Oct. 02, 2012
	security		security
Investment Securities [Line Items]
Total other-than-temporary impairments	$0	$0
Fair market value	799.4		771
Catastrophe bonds	32.8		34.8
Investment funded	2.4		2.5
Number of municipal holdings	2		2
Bank Loans
Investment Securities [Line Items]
Fair market value	$85.90		$85.10
Chaspark
Investment Securities [Line Items]
Percentage of ownership acquired				58.50%

Variable_Interest_Entities_Nar

Variable Interest Entities - Narrative (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 23, 2014	Oct. 02, 2012	Dec. 27, 2013
Variable Interest Entity [Line Items]
Number Of Investments In Variable Interest Entities	2
Number of Variable Interest Entities	2
Total aggregate unpaid balance	$549,100,000		$549,100,000
Silverton Two
Variable Interest Entity [Line Items]
Loans issued				85,000,000
Loan notes held by third parties	72,900,000			70,000,000
Chaspark
Variable Interest Entity [Line Items]
Percentage of ownership acquired					58.50%
Income (Loss) from Equity Method Investments	0	0
Silverton
Variable Interest Entity [Line Items]
Loans issued						65,000,000
Percentage of interests acquired	100.00%
Silverton | Third party
Variable Interest Entity [Line Items]
Loans issued						50,000,000
Silverton | Loan Notes 2014
Variable Interest Entity [Line Items]
Debt Instrument, Fair Value Disclosure	89,200,000
Silverton | Silverton Loan Notes
Variable Interest Entity [Line Items]
Repayments of Debt	84,400,000
Silverton | Silverton Loan Notes | Aspen Holdings and Third Parties
Variable Interest Entity [Line Items]
Total aggregate unpaid balance	4,800,000
Silverton | Silverton Loan Notes | Aspen Holdings
Variable Interest Entity [Line Items]
Maximum loss exposure, amount	1,100,000
Silverton | Investor | Loan Notes 2014
Variable Interest Entity [Line Items]
Debt Instrument, Fair Value Disclosure	68,600,000
Long-term debt, current maturities	700,000
Long-term debt, excluding current maturities	3,100,000
Repayments of Debt	64,800,000
Total aggregate unpaid balance	3,800,000
Silverton Two | Silverton Loan Notes | Aspen Holdings
Variable Interest Entity [Line Items]
Maximum loss exposure, amount	15,600,000
Silverton Two | Loan Notes 2015
Variable Interest Entity [Line Items]
Debt Instrument, Fair Value Disclosure	88,500,000
Silverton Two | Loan Notes 2015 | Aspen Holdings
Variable Interest Entity [Line Items]
Debt Instrument, Fair Value Disclosure	$88,500,000

Fair_Value_Measurements_Financ

Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) (USD $)	Mar. 31, 2015		Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value	$32,600,000		$34,400,000
Derivatives at fair value	2,100,000		8,000,000
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value	32,800,000
Loan notes issued by variable interest entities, at fair value	-76,000,000		-70,700,000
Loan notes issued by variable interest entities, at fair value (classified as a current liability)	-700,000		-67,900,000
Total	7,185,800,000		7,275,300,000
Recurring | Short-term investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	700,000		200,000
Recurring | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	719,000,000		616,000,000
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	2,000,000		7,900,000
Recurring | Derivatives at fair value | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	100,000		100,000
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	-11,500,000		-14,300,000
Available for sale financial assets, at fair value | Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,041,800,000		1,094,400,000
Available for sale financial assets, at fair value | Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	182,900,000		197,400,000
Available for sale financial assets, at fair value | Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	30,700,000		31,500,000
Available for sale financial assets, at fair value | Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,333,600,000		2,319,400,000
Available for sale financial assets, at fair value | Recurring | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	68,200,000		78,000,000
Available for sale financial assets, at fair value | Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	637,700,000		665,700,000
Available for sale financial assets, at fair value | Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	140,900,000		143,500,000
Available for sale financial assets, at fair value | Recurring | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	40,400,000		44,800,000
Available for sale financial assets, at fair value | Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,063,600,000		1,055,300,000
Fixed income securities — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5,539,800,000		5,630,000,000
Short-term investments — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	180,200,000		258,300,000
Equity investments - Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		109,900,000
Held for trading financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	799,400,000		805,800,000
Catastrophe bonds trading, at fair value			34,800,000
Held for trading financial assets, at fair value | Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	9,000,000		0
Held for trading financial assets, at fair value | Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	200,000		200,000
Held for trading financial assets, at fair value | Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	500,000		1,100,000
Held for trading financial assets, at fair value | Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	554,100,000		529,800,000
Held for trading financial assets, at fair value | Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	133,700,000		140,100,000
Held for trading financial assets, at fair value | Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	16,000,000		14,700,000
Held for trading financial assets, at fair value | Recurring | Bank Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	85,900,000		85,100,000
Level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value	0
Loan notes issued by variable interest entities, at fair value	0		0
Loan notes issued by variable interest entities, at fair value (classified as a current liability)	0		0
Total	2,417,000,000		2,542,300,000
Level 1 | Recurring | Short-term investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		100,000
Level 1 | Recurring | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	719,000,000		616,000,000
Level 1 | 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	0		0
Level 1 | Recurring | Derivatives at fair value | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	0		0
Level 1 | 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	0		0
Level 1 | Available for sale financial assets, at fair value | Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,041,800,000		1,094,400,000
Level 1 | Available for sale financial assets, at fair value | Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 1 | Available for sale financial assets, at fair value | Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 1 | Available for sale financial assets, at fair value | Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 1 | Available for sale financial assets, at fair value | Recurring | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 1 | Available for sale financial assets, at fair value | Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	462,500,000		456,500,000
Level 1 | Available for sale financial assets, at fair value | Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 1 | Available for sale financial assets, at fair value | Recurring | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 1 | Available for sale financial assets, at fair value | Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 1 | Fixed income securities — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,504,300,000		1,550,900,000
Level 1 | Short-term investments — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	144,000,000		229,300,000
Level 1 | Equity investments - Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		109,900,000
Level 1 | Held for trading financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	49,700,000		36,100,000
Catastrophe bonds trading, at fair value			0
Level 1 | Held for trading financial assets, at fair value | Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	9,000,000		0
Level 1 | Held for trading financial assets, at fair value | Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 1 | Held for trading financial assets, at fair value | Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 1 | Held for trading financial assets, at fair value | Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 1 | Held for trading financial assets, at fair value | Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	40,700,000		36,100,000
Level 1 | Held for trading financial assets, at fair value | Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 1 | Held for trading financial assets, at fair value | Recurring | Bank Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 2 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value	32,800,000
Loan notes issued by variable interest entities, at fair value	0		0
Loan notes issued by variable interest entities, at fair value (classified as a current liability)	0		0
Total	4,845,500,000		4,871,600,000
Level 2 | Recurring | Short-term investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	700,000		100,000
Level 2 | Recurring | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 2 | 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	2,000,000		7,900,000
Level 2 | Recurring | Derivatives at fair value | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	100,000	[1]	100,000	[1]
Level 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	-11,500,000		-14,300,000
Level 2 | Available for sale financial assets, at fair value | Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 2 | Available for sale financial assets, at fair value | Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	182,900,000		197,400,000
Level 2 | Available for sale financial assets, at fair value | Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	30,700,000		31,500,000
Level 2 | Available for sale financial assets, at fair value | Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,333,600,000		2,319,400,000
Level 2 | Available for sale financial assets, at fair value | Recurring | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	68,200,000		78,000,000
Level 2 | Available for sale financial assets, at fair value | Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	175,200,000		209,200,000
Level 2 | Available for sale financial assets, at fair value | Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	140,900,000		143,500,000
Level 2 | Available for sale financial assets, at fair value | Recurring | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	40,400,000		44,800,000
Level 2 | Available for sale financial assets, at fair value | Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,063,600,000		1,055,300,000
Level 2 | Fixed income securities — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	4,035,500,000		4,079,100,000
Level 2 | Short-term investments — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	36,200,000		29,000,000
Level 2 | Equity investments - Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 2 | Held for trading financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	749,700,000		769,700,000
Catastrophe bonds trading, at fair value			34,800,000
Level 2 | Held for trading financial assets, at fair value | Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 2 | Held for trading financial assets, at fair value | Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	200,000		200,000
Level 2 | Held for trading financial assets, at fair value | Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	500,000		1,100,000
Level 2 | Held for trading financial assets, at fair value | Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	554,100,000		529,800,000
Level 2 | Held for trading financial assets, at fair value | Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	93,000,000		104,000,000
Level 2 | Held for trading financial assets, at fair value | Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	16,000,000		14,700,000
Level 2 | Held for trading financial assets, at fair value | Recurring | Bank Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	85,900,000		85,100,000
Level 3 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value	0
Loan notes issued by variable interest entities, at fair value	-76,000,000		-70,700,000
Loan notes issued by variable interest entities, at fair value (classified as a current liability)	-700,000		-67,900,000
Total	-76,700,000		-138,600,000
Level 3 | Recurring | Short-term investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Recurring | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | 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	0		0
Level 3 | Recurring | Derivatives at fair value | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value	0		0
Level 3 | 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	0		0
Level 3 | Available for sale financial assets, at fair value | Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Available for sale financial assets, at fair value | Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Available for sale financial assets, at fair value | Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Available for sale financial assets, at fair value | Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Available for sale financial assets, at fair value | Recurring | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Available for sale financial assets, at fair value | Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Available for sale financial assets, at fair value | Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Available for sale financial assets, at fair value | Recurring | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Available for sale financial assets, at fair value | Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Fixed income securities — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Short-term investments — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Equity investments - Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Held for trading financial assets, at fair value | Recurring
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
Level 3 | Held for trading financial assets, at fair value | Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Held for trading financial assets, at fair value | Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Held for trading financial assets, at fair value | Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Held for trading financial assets, at fair value | Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Held for trading financial assets, at fair value | Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Held for trading financial assets, at fair value | Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0		0
Level 3 | Held for trading financial assets, at fair value | Recurring | Bank Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	$0		$0
[1]	Net of $19.5 million of cash collateral provided to counterparties, Goldman Sachs International ($451.3 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2014 — $22.3 million).

Fair_Value_Measurements_Reconc

Fair Value Measurements - Reconciliation of Liabilities Using Level 3 Inputs (Details) (Level 3, Fair Value, Measurements, Recurring, Silverton, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Level 3 | Fair Value, Measurements, Recurring | Silverton
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Distributed	($64.80)		$0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	138.6	[1]	50
Total change in fair value included in the statement of operations	2.9		3.4
Ending balance	$76.70	[1]	$53.40
[1]	The amount classified as other payables was $0.7 million and $67.9 million as at March 31, 2015 and December 31, 2014, respectively.

Fair_Value_Measurements_Pricin

Fair Value Measurements - Pricing Sources Used in Pricing Fixed Income Investments (Details)	Mar. 31, 2015	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Index providers	82.00%	84.00%
Pricing services	13.00%	11.00%
Broker-dealers	5.00%	5.00%
Total	100.00%	100.00%

Fair_Value_Measurements_Summar

Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$5,926.50	$6,121
% of Total Fair Value by Security Type	84.00%	86.00%
Fixed Income Maturities
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	5,211.20	5,395.10
% of Total Fair Value by Security Type	82.00%	84.00%
Fixed Income Maturities | U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	1,039	1,044.40
% of Total Fair Value by Security Type	99.00%	95.00%
Fixed Income Maturities | U.S. agency
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	172.7	186.9
% of Total Fair Value by Security Type	94.00%	95.00%
Fixed Income Maturities | Municipal
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	13.9	13.7
% of Total Fair Value by Security Type	45.00%	42.00%
Fixed Income Maturities | Corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	2,680.70	2,731.10
% of Total Fair Value by Security Type	93.00%	96.00%
Fixed Income Maturities | Non-U.S. government-backed corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	33.5	48.7
% of Total Fair Value by Security Type	49.00%	62.00%
Fixed Income Maturities | Foreign government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	434.6	504.4
% of Total Fair Value by Security Type	56.00%	63.00%
Fixed Income Maturities | Asset-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	145.5	140.5
% of Total Fair Value by Security Type	93.00%	89.00%
Fixed Income Maturities | Non-agency commercial mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	40.3	44.8
% of Total Fair Value by Security Type	100.00%	100.00%
Fixed Income Maturities | Agency mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	651	680.6
% of Total Fair Value by Security Type	61.00%	64.00%
Equities
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$715.30	$725.90
% of Total Fair Value by Security Type	100.00%	100.00%

Fair_Value_Measurements_Loan_N

Fair Value Measurements - Loan Notes Issued by Variable Interest Entities (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	$919,200,000	$855,500,000
Level 3 | Low
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	37,600,000		0
Reserve for losses	1,100,000		0
Loan notes held by third parties	120,000,000		120,000,000
Level 3 | High
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	39,600,000		40,000,000
Reserve for losses	4,400,000		0
Contract period	365 days		365 days
Loan notes held by third parties	120,000,000		120,000,000
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes held by third parties	76,000,000		70,700,000
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes held by third parties	76,700,000		138,600,000
Loan notes held by third parties	$76,000,000		$70,700,000

Fair_Value_Measurements_Narrat

Fair Value Measurements - Narrative (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 23, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Quotes per fixed income investment	2		2
Quotes per equity investment	4		4
Gross written premiums	$919,200,000	$855,500,000
Silverton
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans issued				65,000,000
Loan notes held by third parties				50,000,000
Silverton Two [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans issued					85,000,000
Loan notes held by third parties	72,900,000				70,000,000
Level 3 | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes held by third parties	120,000,000		120,000,000
Gross written premiums	37,600,000		0
Reserve for losses	$1,100,000		$0

Reinsurance_Narrative_Details

Reinsurance -Narrative (Details)	Mar. 31, 2015	Dec. 31, 2014
Munich Re
Reinsurance [Line Items]
Reinsurance recoverables	20.00%	18.60%
Lloyd's
Reinsurance [Line Items]
Reinsurance recoverables	18.90%	27.30%
Arch Re
Reinsurance [Line Items]
Reinsurance recoverables	7.90%	8.40%

Derivative_Contracts_Fair_Valu

Derivative Contracts - Fair Value of Derivative Instruments (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivatives at fair value	$2.10			$8
Interest Rate Swaps | Derivatives Not Designated as Hedging Instruments | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivative notional amount	951.3	[1]		951.3	[1]
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Derivative notional amount	362.3			403.4
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivative notional amount	220			165.8
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Liability notional amount	142.3			237.6
Derivatives at fair value | Interest Rate Swaps | Derivatives Not Designated as Hedging Instruments | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivatives at fair value	0.1	[1]		0.1	[1]
Derivatives at fair value | Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivatives at fair value	2			7.9
Derivatives at fair value | Fair Value, Measurements, Recurring | Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Derivatives at fair value	0.1			0.1
Derivatives at fair value | Fair Value, Measurements, Recurring | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Derivatives at fair value	2			7.9
Derivatives at fair value | Level 2 | Fair Value, Measurements, Recurring | Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Derivatives at fair value	0.1	[1]		0.1	[1]
Derivatives at fair value | Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Derivatives at fair value	2			7.9
Liabilities under derivative contracts | Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Liabilities at fair value	-5.3			-10.5
Liabilities under derivative contracts | Fair Value, Measurements, Recurring | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Liabilities at fair value	-11.5			-14.3
Liabilities under derivative contracts | Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Liabilities at fair value	-11.5			-14.3
Liabilities under derivative contracts | Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Liabilities at fair value	-5.3
Change In Fair Value Of Derivatives Net Foreign Exchange Gains And Losses [Member] | Interest Rate Swaps | Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	-3.2		-1.9
Change In Fair Value Of Derivatives Net Foreign Exchange Gains And Losses [Member] | Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	($4.60)		$3
[1]	Net of $19.5 million of cash collateral provided to counterparties, Goldman Sachs International ($451.3 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2014 — $22.3 million).

Derivative_Contracts_GainLoss_

Derivative Contracts - Gain/(Loss) Recognized in Income on Derivative (Details) (Change in Fair Value of Derivatives, USD $)	3 Months Ended			12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014	Dec. 31, 2014
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value	($2.40)		$0
Derivatives Not Designated as Hedging Instruments | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income for the	-4.6		3
Derivatives Not Designated as Hedging Instruments | Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income for the	-3.2		-1.9
Designated as Hedging Instrument | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income for the	-2.8		0
Fair Value	($6.20)	[1]		($3.80)	[1]
[1]	Net of $Nil cash collateral (December 31, 2014 — $Nil).

Derivative_Contracts_Narrative

Derivative Contracts - Narrative (Details) (USD $)	3 Months Ended			12 Months Ended
	Mar. 31, 2015		Mar. 31, 2014	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Collateral Already Posted, Aggregate Fair Value	$19,500,000			$22,300,000
Pledged assets	0			0
Goldman Sachs International
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	451,300,000
Credit Agricole CIB
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	500,000,000
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Realized Gain (Loss) for Settled Foreign Exchange Contracts	-2,800,000		0
Foreign Exchange Contracts | Change in Fair Value of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value	-2,400,000		0
Foreign Exchange Contracts | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	84,600,000			135,800,000
Collateral Already Posted, Aggregate Fair Value	0			0
Foreign Exchange Contracts | Designated as Hedging Instrument | Change in Fair Value of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value	-6,200,000	[1]		-3,800,000	[1]
Derivative instruments not designated as hedging instruments, gain (loss), net	-2,800,000		0
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	362,300,000			403,400,000
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments | Change in Fair Value of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	-4,600,000		3,000,000
Foreign Exchange Contracts | Derivatives at Fair Value | Derivatives Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	220,000,000			165,800,000
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Other Assets, Current	0
Interest Rate Swaps | Derivatives Not Designated as Hedging Instruments | Change in Fair Value of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	-3,200,000		-1,900,000
Interest Rate Swaps | Derivatives at Fair Value | Derivatives Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	$951,300,000	[2]		$951,300,000	[2]
[1]	Net of $Nil cash collateral (December 31, 2014 — $Nil).
[2]	Net of $19.5 million of cash collateral provided to counterparties, Goldman Sachs International ($451.3 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2014 — $22.3 million).

Deferred_Policy_Acquisition_Co2

Deferred Policy Acquisition Costs - Reconciliation of Beginning and Ending Deferred Policy Acquisition Costs (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance at the beginning of the period	$299	$262.20
Acquisition costs deferred	154.1	139.4
Amortization of deferred policy acquisition costs	-119.3	-112
Balance at the end of the period	$333.80	$289.60

Reserves_for_Losses_and_Loss_A2

Reserves for Losses and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses ("LAE") Reserves (Details) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Insurance [Abstract]
Provision for losses and LAE at the start of the year	$4,750.80	$4,678.90	$4,678.90
Less reinsurance recoverable			-350	-332.7
Net loss and LAE at the start of the year	4,400.80	4,346.20	4,346.20
Net loss and LAE expenses assumed/(disposed)	0	0	-24.2
Provision for losses and LAE for claims incurred:
Current year	333.6		1,411.60
Prior years	-27.5	-28.2	-104.1
Total incurred	306.1		1,307.50
Losses and LAE payments for claims incurred:
Current year	-18.8		-112.1
Prior years	-252.9		-995.6
Total paid	-271.7		-1,107.70
Foreign exchange (gains)	-96.4		-121
Net losses and LAE reserves at period end	4,338.80	4,408.60	4,400.80
Plus reinsurance recoverable on unpaid losses at period end	360.1		350
Provision for losses and LAE at the end of the relevant period	$4,698.90		$4,750.80

Reserves_for_Losses_and_Loss_A3

Reserves for Losses and Loss Adjustment Expenses - Narrative (Details) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Insurance [Abstract]
Reserve releases	$27.50	$28.20	$104.10
Loss portfolio transfers (disposed)	$0	$0	$24.20

Capital_Structure_Summary_of_A

Capital Structure - Summary of Authorized and Issued Share Capital (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, except Share data, unless otherwise specified
Authorized share capital:
Number of ordinary shares	969,629,030	969,629,030
Number of non-voting shares	6,787,880	6,787,880
Number of preference shares	100,000,000	100,000,000
Ordinary Shares 0.15144558¢ per share	$1,469	$1,469
Non-Voting Shares 0.15144558¢ per share	10	10
Preference Shares 0.15144558¢ per share	152	152
Total authorized share capital	1,631	1,631
Issued share capital:
Number of issued ordinary shares	61,722,502	62,017,368
Issued ordinary shares of 0.15144558¢ per share	93	94
Total issued share capital	128	129
Ordinary shares, par value (in dollars per share)	$0.15	$0.15
Non-voting shares, par value	$0.15	$0.15
Preference shares, par value (in dollars per share)	$0.15	$0.15
Preference shares, liquidation preference	$25	$25
7.401% Preference Shares
Issued share capital:
Number of issued preference shares of 0.15144558¢ each with a liquidation preference	5,327,500	5,327,500
Issued preference shares of 0.15144558¢ each with a liquidation preference	8	8
Preference shares, par value (in dollars per share)	$0.15	$0.15
Preference shares, liquidation preference	$25	$25
Preference shares, rate	7.40%	7.40%
7.250% preference shares
Issued share capital:
Number of issued preference shares of 0.15144558¢ each with a liquidation preference	6,400,000	6,400,000
Issued preference shares of 0.15144558¢ each with a liquidation preference	10	10
Preference shares, par value (in dollars per share)	$0.15	$0.15
Preference shares, liquidation preference	$25	$25
Preference shares, rate	7.25%	7.25%
5.95% Preference Shares
Issued share capital:
Number of issued preference shares of 0.15144558¢ each with a liquidation preference	11,000,000	11,000,000
Issued preference shares of 0.15144558¢ each with a liquidation preference	$17	$17
Preference shares, par value (in dollars per share)	$0.15	$0.15
Preference shares, liquidation preference	$25	$25
Preference shares, rate	5.95%	5.95%

Capital_Structure_Summary_of_O

Capital Structure - Summary of Ordinary Shares (Details)	3 Months Ended
	Mar. 31, 2015
Ordinary Shares [Roll Forward]
Ordinary shares in issue at the beginning of the year	62,017,368
Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or 2008 share purchase plan	485,466
Ordinary shares issued to non-employee directors	6,806
Ordinary shares repurchased	-787,138
Ordinary shares in issue at the end of the period	61,722,502

Capital_Structure_Narrative_De

Capital Structure - Narrative (Details) (USD $)	0 Months Ended	3 Months Ended	12 Months Ended	0 Months Ended	3 Months Ended
	Apr. 17, 2014	Mar. 31, 2015	Dec. 31, 2014	Feb. 05, 2015	Mar. 31, 2014
Class of Stock [Line Items]
Additional paid-in capital		$1,106,000,000	$1,134,300,000
Aggregate liquidation preferences		568,200,000	568,200,000
Issuance costs		12,400,000	12,400,000
Convertible preferred stock, shares issued upon conversion	0.001
Class of warrant or right, exercise price of warrants or rights	$160
Equity method investment, ownership percentage	10.00%
Minimum period before exercisable	10 days
Repurchase Program 2015 [Member]
Class of Stock [Line Items]
ASR, commenced date				5-Feb-15
Existing share repurchase authorization amount				500,000,000
Open Markets
Class of Stock [Line Items]
Shares repurchase and cancelled during period, shares		787,138			770,505
Shares repurchase and cancelled during period, value		$36,500,000			$30,900,000
Average price		$46.32			$40.08
Passive Institutional Investors
Class of Stock [Line Items]
Equity method investment, ownership percentage	15.00%
Preferred Stock [Member]
Class of Stock [Line Items]
Convertible preferred stock, shares issued upon conversion	1

ShareBased_Payments_Narrative_

Share-Based Payments - Narrative (Details)

3 Months Ended

0 Months Ended

3 Months Ended

3 Months Ended

0 Months Ended

3 Months Ended

Mar. 31, 2015

Mar. 31, 2014

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2014

Apr. 06, 2014

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2014

Mar. 31, 2015

Mar. 31, 2015

Mar. 31, 2014

Mar. 31, 2015

Mar. 31, 2014

Feb. 05, 2015

Feb. 05, 2014

Feb. 05, 2015

Feb. 10, 2014

Mar. 05, 2015

Mar. 31, 2015

Mar. 31, 2014

Mar. 31, 2015

Mar. 31, 2015

USD ($)

USD ($)

Achievement One

Achievement Two

Achievement Two

Achievement Three

Achievement Three

Employee Share Purchase Plans

Employee Share Purchase Plans

Sharesave Scheme

Sharesave Scheme

2013 Share Incentive Plan

2003 Share Incentive Plan

International Employee Share Purchase Plan

Investor Options [Member]

Performance Shares

Performance Shares

Stock Options

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Phantom Shares

Phantom Shares

Phantom Shares

Grants in 2007

Grant Year 2015 and 2016

Minimum

Maximum

Minimum

Maximum

USD ($)

USD ($)

GBP (£)

GBP (£)

USD ($)

Agreement

USD ($)

USD ($)

USD ($)

USD ($)

Director

Director

Director

Director

Board of Directors Chairman

Board of Directors Chairman

USD ($)

USD ($)

Stock Options

Performance Shares

USD ($)

USD ($)

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

Number of arrangements under options and other equity incentives

3

Shares available to grant under incentive plan

2,845,683

595,683

Vesting period

3 years

3 years

3 years

3 years

7 years

Exercise period

10 years

Options grants in period

0

0

Options exercised

83,938

19,579

Compensation costs

$0

$0

$200,000

$1,500,000

$4,700,000

$2,800,000

$300,000

$400,000

$900,000

$1,600,000

Granted, number of shares

277,585

0

238,315

0

135,651

0

Percentage of restricted share units vested and issued

100.00%

"RSUs" for non-employee directors

27,620

27,180

12,154

13,590

Achieved and required performance growth, percentage

5.60%

5.60%

11.10%

11.10%

22.20%

Percentage of initial shares granted

33.33%

Eligible for vesting, percent

10.00%

100.00%

100.00%

200.00%

Vesting rule, threshold percent of eligible shares

100.00%

Vesting rule, maximum vesting percent of eligible shares

100.00%

Employee savings under ESPP

500

250

500

Employee contribution period

3 years

2 years

Fair value adjustment capitalized amount

300,000

600,000

1,100,000

200,000

Tax benefit from compensation expense

100,000

400,000

1,100,000

600,000

400,000

200,000

Additional employee contribution period

1 year

Shares issued

52,452

9,994

Purchase price of fair market value

85.00%

Compensation costs charged in ESPP

$300,000

$100,000

Intangible_Assets_Summary_of_C

Intangible Assets - Summary of Changes in Intangible Assets (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Beginning of the period	$18,200,000	$18,400,000
Amortization	0	-100,000
End of the period	18,200,000	18,300,000
Trade Mark
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Beginning of the period	1,600,000	1,600,000
Amortization	0	0
End of the period	1,600,000	1,600,000
Insurance Licenses
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Beginning of the period	16,600,000	16,600,000
Amortization	0	0
End of the period	16,600,000	16,600,000
Other
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Beginning of the period	0	200,000
Amortization	0	-100,000
End of the period	$0	$100,000

Intangible_Assets_Narrative_De

Intangible Assets - Narrative (Details) (USD $)	3 Months Ended	0 Months Ended	12 Months Ended
	Mar. 31, 2015	Apr. 05, 2005	Dec. 31, 2010	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset	$18,200,000			$18,200,000	$18,300,000	$18,400,000
Acquired licenses			3,600,000
Fair value of tangible and financial assets, net			1,200,000
Asset amortization period	5 years
Aspen U.K.
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Aggregate consideration for sale and purchase agreement		1,600,000
APJ Continuation Limited
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Aggregate consideration for sale and purchase agreement			4,800,000
Trade Mark
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset	1,600,000			1,600,000	1,600,000	1,600,000
Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset	16,600,000			16,600,000	16,600,000	16,600,000
Insurance Licenses | AAIC
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses	10,000,000
Insurance Licenses | Aspen Specialty
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses	4,500,000
Insurance Licenses | Aspen U.K.
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses	2,100,000
Intangible Asset: Renewal Rights and Distribution Channels
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses			2,200,000
Intangible Asset: Lock-in Period
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses			1,400,000
Other
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset	$0			$0	$100,000	$200,000

Commitments_and_Contingent_Lia2

Commitments and Contingent Liabilities - Schedule of Company's Restricted Assets (Details) (USD $)	Mar. 31, 2015		Dec. 31, 2014
In Millions, unless otherwise specified
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	$3,824.30		$4,049
Total as percent of cash and invested assets	45.00%		47.00%
Letters of credit / guarantees
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	714	[1]	778.7	[1]
Affiliated transactions
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	988.6		1,086.90
Third party
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	$2,121.70		$2,183.40
[1]	As of March 31, 2015, the Company had pledged funds of $704.8 million and £9.2 million (December 31, 2014 — $764.4 million and £9.2 million) as collateral for the secured letters of credit.

Commitments_and_Contingent_Lia3

Commitments and Contingent Liabilities - Amounts Outstanding under Operating Leases (Details) (USD $)	Mar. 31, 2015
In Millions, unless otherwise specified
Leases [Abstract]
2015	$10.10
2016	9.8
2017	8.5
2018	7
2019	6.1
Later Years	1.5
Total	$43

Commitments_and_Contingent_Lia4

Commitments and Contingent Liabilities - Narrative (Details)	Mar. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2014	Jun. 30, 2014	Jul. 30, 2012	Jul. 30, 2012	Jul. 30, 2012	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2015
	USD ($)	GBP (£)	USD ($)	GBP (£)	Citi Europe	Citi Europe	Citi Europe	Citi Europe	Barclays Bank PLC	Barclays Bank PLC	LIBOR
					Letter of Credit	Letter of Credit	Letter of Credit, Tranche I	Letter of Credit, Tranche II	Letter of Credit	Letter of Credit	Citi Europe
					USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	USD ($)	Letter of Credit
						tranche
Line of Credit Facility [Line Items]
Deposit securities and cash as collateral secured letters of credit	$704,800,000	£ 9,200,000	$764,400,000	£ 9,200,000
Assets Held by Insurance Regulators	417,900,000		411,900,000
Minimum capital required		400,000		400,000
Maximum borrowing capacity					575,000,000	950,000,000	650,000,000	300,000,000	100,000,000
Line of Credit Facility, Number of Maturity Tranches						2
Debt Instrument, Basis Spread on Variable Rate											1.00%
Letters of Credit Outstanding, Amount									0	5,000,000
Collateral Already Posted, Aggregate Fair Value	$19,500,000		$22,300,000