Aspen Insurance Holdings Limited (AHL-PE) 10-Q 5 Nov 15 2015 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Nov. 03, 2015
Document and Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q3
Trading Symbol	AHL
Entity Registrant Name	ASPEN INSURANCE HOLDINGS LTD
Entity Central Index Key	1,267,395
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		60,784,263

UNAUDITED CONDENSED CONSOLIDATE

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Investments:
Fixed income securities, available for sale at fair value (amortized cost — $5,570.2 and $5,462.9)	$ 5,704.5	$ 5,630
Fixed income securities, trading at fair value (amortized cost — $791.6 and $760.9)	791.9	771
Equity securities, available for sale at fair value (cost — $Nil and $82.6)	0	109.9
Equity securities, trading at fair value (cost — $717.2 and $585.2)	696.3	616
Short-term investments, available for sale at fair value (amortized cost — $176.3 and $258.2)	176.3	258.3
Short-term investments, trading at fair value (amortized cost — $6.9 and $0.2)	6.9	0.2
Catastrophe bonds, trading at fair value (cost — $36.3 and $34.4)	36.7	34.8
Other investments, equity method	9.5	8.7
Total investments	7,422.1	7,428.9
Cash and cash equivalents (including $110.6 and $176.7 within consolidated variable interest entities)	1,196.7	1,178.5
Reinsurance recoverables
Unpaid losses	348.7	350
Ceded unearned premiums	224.6	206.8
Receivables
Underwriting premiums	1,208.4	1,011.7
Other	108.7	90.2
Funds withheld	39	46.9
Deferred policy acquisition costs	346.8	299
Derivatives at fair value	9.2	8
Receivable for securities sold	6.7	2.3
Office properties and equipment	68.7	62.2
Other assets	5.9	13.6
Intangible assets	18.2	18.2
Total assets	11,003.7	10,716.3
Insurance reserves
Losses and loss adjustment expenses	4,913.9	4,750.8
Unearned premiums	1,686.9	1,441.8
Total insurance reserves	6,600.8	6,192.6
Payables
Reinsurance premiums	135.6	92
Current taxation	16.8	18.3
Deferred taxation	5.9	3.1
Accrued expenses and other payables	237.7	356.9
Liabilities under derivative contracts	1.9	14.3
Total payables	397.9	484.6
Loan notes issued by variable interest entities, at fair value	84.5	70.7
Long-term debt	549.2	549.1
Total liabilities	$ 7,632.4	$ 7,297
Commitments and contingent liabilities (see Note 16)
SHAREHOLDERS’ EQUITY
Ordinary shares, value	$ 0.1	$ 0.1
Preference shares:
Non-controlling interest	1.3	0.5
Additional paid-in capital	1,068.3	1,134.3
Retained earnings	2,188	2,050.1
Accumulated other comprehensive income, net of taxes	113.6	234.3
Total shareholders’ equity	3,371.3	3,419.3
Total liabilities and shareholders’ equity	11,003.7	10,716.3
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

UNAUDITED CONDENSED CONSOLIDAT3

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fixed income securities, available for sale, amortized cost	$ 5,570.2	$ 5,462.9
Fixed income securities, trading at fair value, amortized cost	791.6	760.9
Equity securities, available for sale at fair value, cost	0	82.6
Equity securities, trading at fair value, cost	717.2	585.2
Short-term investments, available for sale at fair value, amortized cost	176.3	258.2
Short-term investments, trading at fair value, amortized cost	6.9	0.2
Catastrophe bonds, trading at fair value, cost	36.3	34.4
Cash and cash equivalents (including $110.6 and $176.7 within consolidated variable interest entities)	$ 1,196.7	$ 1,178.5
Ordinary shares, par value (in usd per share)	$ 0.15144558	$ 0.15144558
Ordinary shares, issued (in shares)	60,781,958	62,017,368
Preference shares, par value (in usd per share)	$ 0.15144558	$ 0.15144558
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 usd per share)	$ 0.15144558	$ 0.15144558
7.401% preference shares
Preference shares, issued (in shares)	5,327,500	5,327,500
Preference shares, rate	7.401%	7.401%
Preference shares, par value (in usd per share)	$ 0.15144558	$ 0.15144558
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 usd per share)	$ 0.15144558	$ 0.15144558
Variable Interest Entity, Primary Beneficiary
Cash and cash equivalents (including $110.6 and $176.7 within consolidated variable interest entities)	$ 110.6	$ 176.7

UNAUDITED CONDENSED CONSOLIDAT4

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND OTHER COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenues
Net earned premium	$ 640.6	$ 610.4	$ 1,843.6	$ 1,793.1
Net investment income	45	48	139.1	143.6
Realized and unrealized investment gains	10.7	1.1	81.6	53.6
Other income	(2.3)	1	0.4	4.8
Total revenues	694	660.5	2,064.7	1,995.1
Expenses
Losses and loss adjustment expenses	365.6	342.7	1,032.2	967.9
Amortization of deferred policy acquisition costs	132	115.5	365.4	336.4
General, administrative and corporate expenses	100.5	119.8	298.1	324.2
Interest on long-term debt	7.4	7.4	22.1	22.1
Change in fair value of derivatives	(10.1)	5.1	(4.3)	8.6
Change in fair value of loan notes issued by variable interest entities	8.3	8.5	14.5	14.5
Realized and unrealized investment losses	51.9	1.3	95.2	8.9
Net realized and unrealized foreign exchange losses	8.4	21.2	26.4	10.4
Other expenses	0	0.3	0.8	2.2
Total expenses	664	621.8	1,850.4	1,695.2
Income from operations before income tax	30	38.7	214.3	299.9
Income tax expense	(1.8)	(1.3)	(9.1)	(11.3)
Net income	28.2	37.4	205.2	288.6
Amount attributable to non-controlling interest	(0.3)	0.1	(0.8)	0
Net income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	27.9	37.5	204.4	288.6
Available for sale investments:
Reclassification adjustment for net realized (gains) on investments included in net income	(2.1)	(3.6)	(33.8)	(5.9)
Change in net unrealized gains/(losses) on available for sale securities held	24.8	(29.4)	(21)	28.6
Net change from current period hedged transactions	(0.2)	0	2.5	0
Change in foreign currency translation adjustment	(29)	(3.3)	(72.4)	14.7
Other comprehensive (loss)/income, gross of tax	(6.5)	(36.3)	(124.7)	37.4
Tax thereon:
Reclassification adjustment for net realized gains on investments included in net income	0.3	0.1	0.7	0.2
Change in net unrealized gains/(losses) on available for sale securities held	0.5	2	1.4	(4)
Net change from current period hedged transactions	(0.3)	0	(0.3)	0
Change in foreign currency translation adjustment	1.1	0	2.2	0
Total tax on other comprehensive income/(loss)	1.6	2.1	4	(3.8)
Other comprehensive (loss)/income net of tax	(4.9)	(34.2)	(120.7)	33.6
Total comprehensive income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	$ 23	$ 3.3	$ 83.7	$ 322.2
Weighted average number of ordinary share and share equivalents
Basic (in shares)	60,779,295	65,116,463	61,442,033	65,283,681
Diluted (in shares)	62,155,125	66,513,009	62,878,436	66,598,680
Basic earnings per ordinary share adjusted for preference share dividends (in usd per share)	$ 0.30	$ 0.43	$ 2.86	$ 3.99
Diluted earnings per ordinary share adjusted for preference share dividends (in usd per share)	$ 0.30	$ 0.42	$ 2.80	$ 3.91

UNAUDITED CONDENSED CONSOLIDAT5

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Millions	Total	Ordinary shares	Preference shares	Non-controlling interest	Additional paid-in capital	Additional paid-in capitalOrdinary shares	Retained earnings	Retained earningsOrdinary shares	Retained earningsPreference shares	Cumulative foreign currency translation adjustments, net of taxes:	Loss on derivatives, net of taxes:	Unrealized appreciation on investments, net of taxes:
Shareholder's equity, beginning balance at Dec. 31, 2013				$ (0.3)	$ 1,297.4		$ 1,783.3			$ 88.6	$ 0	$ 130.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change attributable to non-controlling interest for the period	$ 0			0			0
New ordinary shares issued						$ 1.9
Ordinary shares repurchased and cancelled						(120.9)
Share-based compensation					8
Net income for the period	288.6						288.6
Dividends on ordinary shares								$ (37.9)
Dividends on preference shares	(28.4)								$ (28.4)
Change for the period, net of income tax										14.7		18.9
Shareholder's equity, ending balance at Sep. 30, 2014	3,444.5	$ 0.1	$ 0	(0.3)	1,186.4		2,005.6			103.3	0	149.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change from current period hedged transaction											0
Total accumulated other comprehensive income, net of taxes	252.7
Total accumulated other comprehensive income, net of taxes	234.3
Shareholder's equity, beginning balance at Dec. 31, 2014	3,419.3			0.5	1,134.3		2,050.1			72.7	(3.8)	165.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change attributable to non-controlling interest for the period	(0.8)			0.8			(0.8)
New ordinary shares issued						4.4
Ordinary shares repurchased and cancelled						$ (83.7)
Share-based compensation					13.3
Net income for the period	205.2						205.2
Dividends on ordinary shares								$ (38.1)
Dividends on preference shares	(28.4)								$ (28.4)
Change for the period, net of income tax										(70.2)		(52.7)
Shareholder's equity, ending balance at Sep. 30, 2015	3,371.3	$ 0.1	$ 0	$ 1.3	$ 1,068.3		$ 2,188			$ 2.5	(1.6)	$ 112.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change from current period hedged transaction											$ 2.2
Total accumulated other comprehensive income, net of taxes	$ 113.6

UNAUDITED CONDENSED CONSOLIDAT6

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities:
Net income	$ 205.2	$ 288.6
Proportion due to non-controlling interest	(0.8)	0
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	21.3	27.7
Share-based compensation	13.3	8
Realized and unrealized investment (gains)	(81.6)	(53.6)
Realized and unrealized investment losses	95.2	8.9
Change in fair value of loan notes issued by variable interest entities	14.5	14.5
Net realized and unrealized investment foreign exchange losses	2.3	31.3
Loss on derivative contracts	2.2	0
Insurance reserves:
Losses and loss adjustment expenses	202.2	87.3
Unearned premiums	219.1	226
Reinsurance recoverables:
Unpaid losses	(1)	(49)
Ceded unearned premiums	(17.6)	(76.7)
Other receivables	(19.7)	(10.5)
Deferred policy acquisition costs	(47.4)	(39.4)
Reinsurance premiums payable	44.3	32.8
Funds withheld	7.9	(0.9)
Premiums receivable	(209.9)	(106.7)
Deferred taxes	2.8	(1.4)
Income tax payable	(7)	10
Accrued expenses and other payables	(78.2)	76.4
Fair value of derivatives and settlement of liabilities under derivatives	(13.6)	9.8
Long-term debt	0.1	0.1
Intangible assets	0	0.1
Other assets	10.1	(3.2)
Net cash generated by operating activities	363.7	480.1
Cash flows (used in) investing activities:
(Purchases) of fixed income securities — Available for sale	(1,368.2)	(1,440.9)
(Purchases) of fixed income securities — Trading	(387)	(553.6)
Proceeds from sales and maturities of fixed income securities — Available for sale	1,211.9	1,496.9
Proceeds from sales and maturities of fixed income securities — Trading	354.4	515.7
(Purchases) of equity securities — Trading	(335.2)	(240.1)
Net (purchases) of catastrophe bonds — Trading	(2)	(26)
Proceeds from sales of equity securities — Available for sale	108.6	27.7
Proceeds from sales of equity securities — Trading	214.7	36
(Purchases) of short-term investments — Available for sale	(118.6)	(464)
Proceeds from sales of short-term investments — Available for sale	205.7	313
(Purchases) of short-term investments — Trading	(38.2)	(101.4)
Proceeds from sales of short-term investments — Trading	31.5	94.4
Net change in receivable for securities sold	14.7	12.8
Proceeds from other investments	0	37.3
(Purchases) of equipment	(10.4)	(13.4)
Investment in Micro-insurance venture	(0.8)	0
Net cash (used in) investing activities	(118.9)	(305.6)
Cash flows (used in) financing activities:
Proceeds from the issuance of ordinary shares, net of issuance costs	4.4	1.9
Ordinary shares repurchased	(83.7)	(120.9)
Repayment of long-term debt issued by Silverton	(67.8)	0
Dividends paid on ordinary shares	(38.1)	(37.9)
Dividends paid on preference shares	(28.4)	(28.4)
Net cash (used in) financing activities	(213.6)	(185.3)
Effect of exchange rate movements on cash and cash equivalents	(13)	6.3
Increase/(decrease) in cash and cash equivalents	18.2	(4.5)
Cash and cash equivalents at beginning of period	1,178.5	1,293.6
Cash and cash equivalents at end of period	1,196.7	1,289.1
Supplemental disclosure of cash flow information:
Net cash paid during the period for income tax	2.4	7.2
Cash paid during the period for interest	$ 14.5	$ 14.6

History and Organization

History and Organization	9 Months Ended
	Sep. 30, 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	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 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). There have been no changes to significant accounting policies from those disclosed in the Company’s Annual Report on Form 10-K. 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 July 22, 2015, the Financial Accounting Standards Board (“FASB”) issued ASU 2015-11, “Inventory (Topic 330)” which requires an entity to measure inventory at the lower of cost and net realizable value. ASU 2015-11 is effective for fiscal years beginning after December 15, 2016. As the Company does not currently possess stock or inventory it does not expect this ASU to impact its consolidated financial statements. On July 31, 2015, the FASB issued ASU 2015-12, “Plan Accounting: Defined Benefit Pension Plans (Topic 960), Defined Contribution Pension Plans (Topic 962), Health and Welfare Benefit Plans (Topic 965): (Part I) Fully Benefit-Responsive Investment Contracts, (Part II) Plan Investment Disclosures, (Part III) Measurement Date Practical Expedient” which simplifies the benefit plan disclosures and reporting requirements. ASU 2015-12 is effective for annual periods beginning after December 15, 2015. The ASU does not impact the Company's financial statements as it does not classify any of its investment contracts as fully benefit-responsive investment contracts. On August 10, 2015, the FASB issued ASU 2015-13, “Derivatives and Hedging (Topic 815)” which applies to entities that enter into contracts for the purchase or sale of electricity. ASU 2015-13 is effective upon issuance and should be applied prospectively. The ASU does not impact the Company's financial statements as it does not purchase or sell electricity contracts. On August 12, 2015, the FASB issued ASU 2015-14, “Revenue From Contracts With Customers (Topic 606)” which delays the effective date of ASU 2014-09 by one year. ASU 2015-14 is effective for annual periods beginning after December 15, 2015. The ASU does not impact the Company’s financial statements as insurance contracts accounted for within the scope of Topic 944, Financial-Services are exempt from this ASU. On August 18, 2015, the FASB issued ASU 2015-15, “Interest – Imputation of Interest (Subtopic 835-30) Presentation and Subsequent Measurement of Debt Issuance Costs Associated with Line-of-Credit Arrangements” which updates the Accounting Standards Codification to include the SEC Staff Announcement: Presentation and Subsequent Measurement of Debt Issuance Costs Associated with Line-of-Credit Arrangements as proscribed in ASU 2015-03. The ASU does not impact the Company's financial statements. On September 25, 2015, the FASB issued ASU 2015-16, “Business Combinations (Topic 805)” which requires an acquirer to adjust retrospectively to provisional amounts recognized in a business combination. ASU 2015-16 is effective for annual periods beginning after December 15, 2016. The ASU is not expected to have a material impact on the Company’s financial statements.

Reclassifications from Accumula

Reclassifications from Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2015 and 2014 : Amount Reclassified from AOCI Details about the AOCI Components Three Months Ended September 30, 2015 Three Months Ended September 30, 2014 Affected Line Item in the Unaudited Condensed Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gains on sale of securities $ 2.1 $ 3.9 Realized and unrealized investment gains Realized (losses) on sale of securities — (0.3 ) Realized and unrealized investment losses 2.1 3.6 Income from operations before income tax Tax on net realized gains of securities (0.3 ) (0.1 ) Income tax expense $ 1.8 $ 3.5 Net income Foreign currency translation adjustments: Foreign currency translation adjustments, before tax 1.1 $ — Net realized and unrealized foreign exchange gains/(losses) $ 1.1 $ — Net income Realized derivatives: Net realized (losses) on settled derivatives (0.8 ) $ — General, administrative and corporate expenses $ (0.8 ) $ — Net income Total reclassifications from AOCI to the statement of operations, net of tax $ 2.1 $ 3.5 Net income Amount Reclassified from AOCI Details about the AOCI Components Nine Months Ended September 30, 2015 Nine Months Ended September 30, 2014 Affected Line Item in the Unaudited Condensed Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gains on sale of securities $ 37.7 $ 10.6 Realized and unrealized investment gains Realized (losses) on sale of securities (3.9 ) (4.7 ) Realized and unrealized investment losses 33.8 5.9 Income from operations before income tax Tax on net realized gains of securities (0.7 ) (0.2 ) Income tax expense Total reclassifications from AOCI to the statement of operations, net of tax $ 33.1 $ 5.7 Net income Foreign currency translation adjustments: Foreign currency translation adjustments, before tax 2.2 $ — Net realized and unrealized foreign exchange gains/(losses) $ 2.2 $ — Net income Realized derivatives: Net realized (losses) on settled derivatives (3.5 ) $ — General, administrative and corporate expenses $ (3.5 ) $ — Net income Total reclassifications from AOCI to the statement of operations, net of tax $ 31.8 $ 5.7 Net income

Earnings per Ordinary Share

Earnings per Ordinary Share	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2015 and 2014 , respectively: Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 ($ in millions, except share and per share amounts) Net income $ 28.2 $ 37.4 $ 205.2 $ 288.6 Preference share dividends (9.5 ) (9.5 ) (28.4 ) (28.4 ) Net profit attributable to non-controlling interest (0.3 ) 0.1 (0.8 ) — Basic and diluted net income available to ordinary shareholders $ 18.4 $ 28.0 $ 176.0 $ 260.2 Ordinary shares: Basic weighted average ordinary shares 60,779,295 65,116,463 61,442,033 65,283,681 Weighted average effect of dilutive securities (1) 1,375,830 1,396,546 1,436,403 1,314,999 Total diluted weighted average ordinary shares 62,155,125 66,513,009 62,878,436 66,598,680 Earnings per ordinary share: Basic $ 0.30 $ 0.43 $ 2.86 $ 3.99 Diluted $ 0.30 $ 0.42 $ 2.80 $ 3.91 (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 October 28, 2015, the Company’s Board of Directors (“Board of Directors”) declared the following quarterly dividends: Dividend Payable on: Record Date: Ordinary shares $ 0.21 December 1, 2015 November 13, 2015 7.401% preference shares $ 0.462563 January 1, 2016 December 15, 2015 7.250% preference shares $ 0.4531 January 1, 2016 December 15, 2015 5.95% preference shares $ 0.3719 January 1, 2016 December 15, 2015

Segment Reporting

Segment Reporting	9 Months Ended
	Sep. 30, 2015
Segment Reporting [Abstract]
Segment Reporting	Segment Reporting The Company has two reporting business segments: Insurance and Reinsurance. In addition to the way the Company manages its business, the Company has considered similarities in economic characteristics, products, customers, distribution, the regulatory environment of the Company’s operating segments and quantitative thresholds to determine the Company’s reportable segments. Segment profit or loss for each of the Company’s operating segments is measured by underwriting profit or loss. Underwriting profit is the excess of net earned premiums over the sum of losses and loss expenses, amortization of deferred policy acquisition costs and general and administrative expenses. Underwriting profit or loss provides a basis for management to evaluate the segment’s underwriting performance. Reinsurance Segment. The reinsurance segment consists of property catastrophe reinsurance, other property reinsurance (risk excess, pro rata 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, 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 business 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 September 30, 2015 and 2014 : Three Months Ended September 30, 2015 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 316.6 $ 403.9 $ 720.5 Net written premiums 294.7 357.1 651.8 Gross earned premiums 304.6 429.0 733.6 Net earned premiums 284.6 356.0 640.6 Underwriting Expenses Losses and loss adjustment expenses 169.9 195.7 365.6 Amortization of deferred policy acquisition costs 64.8 67.2 132.0 General and administrative expenses 34.7 51.3 86.0 Underwriting income $ 15.2 $ 41.8 57.0 Corporate expenses (14.5 ) Net investment income 45.0 Realized and unrealized investment gains 10.7 Realized and unrealized investment losses (51.9 ) Change in fair value of loan notes issued by variable interest entities (8.3 ) Change in fair value of derivatives 10.1 Interest expense on long term debt (7.4 ) Net realized and unrealized foreign exchange (losses) (8.4 ) Other income (2.3 ) Other expenses — Income before tax $ 30.0 Net reserves for loss and loss adjustment expenses $ 2,469.6 $ 2,095.6 $ 4,565.2 Ratios Loss ratio 59.7 % 55.0 % 57.1 % Policy acquisition expense ratio 22.8 18.9 20.6 General and administrative expense ratio 12.2 14.4 15.7 (1) Expense ratio 35.0 33.3 36.3 Combined ratio 94.7 % 88.3 % 93.4 % (1) The general and administrative expense ratio in the total column includes corporate expenses. Three Months Ended September 30, 2014 Reinsurance Insurance Total ( $ in millions) Underwriting Revenues Gross written premiums $ 256.9 $ 395.6 $ 652.5 Net written premiums 250.9 326.4 577.3 Gross earned premiums 291.0 403.9 694.9 Net earned premiums 279.6 330.8 610.4 Underwriting Expenses Losses and loss adjustment expenses 132.0 210.7 342.7 Amortization of deferred policy acquisition costs 52.1 63.4 115.5 General and administrative expenses 38.4 45.6 84.0 Underwriting income $ 57.1 $ 11.1 68.2 Corporate expenses (35.8 ) Net investment income 48.0 Realized and unrealized investment gains 1.1 Realized and unrealized investment losses (1.3 ) Change in fair value of loan notes issued by variable interest entities (8.5 ) Change in fair value of derivatives (5.1 ) Interest expense on long term debt (7.4 ) Net realized and unrealized foreign exchange (losses) (21.2 ) Other income 1.0 Other expenses (0.3 ) Income before tax $ 38.7 Net reserves for loss and loss adjustment expenses $ 2,554.2 $ 1,848.4 $ 4,402.6 Ratios Loss ratio 47.2 % 63.7 % 56.1 % Policy acquisition expense ratio 18.6 19.2 18.9 General and administrative expense ratio 13.7 13.8 19.6 (1) Expense ratio 32.3 33.0 38.5 Combined ratio 79.5 % 96.7 % 94.6 % (1) The general and administrative expense ratio in the total column includes corporate expenses. 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 nine months ended September 30, 2015 and 2014 : Nine Months Ended September 30, 2015 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,062.1 $ 1,300.4 $ 2,362.5 Net written premiums 975.0 1,084.4 2,059.4 Gross earned premiums 857.6 1,267.3 2,124.9 Net earned premiums 802.3 1,041.3 1,843.6 Underwriting Expenses Losses and loss adjustment expenses 391.7 640.5 1,032.2 Amortization of deferred policy acquisition costs 168.6 196.8 365.4 General and administrative expenses 102.5 151.8 254.3 Underwriting income $ 139.5 $ 52.2 191.7 Corporate expenses (43.8 ) Net investment income 139.1 Realized and unrealized investment gains 81.6 Realized and unrealized investment losses (95.2 ) Change in fair value of loan notes issued by variable interest entities (14.5 ) Change in fair value of derivatives 4.3 Interest expense on long term debt (22.1 ) Net realized and unrealized foreign exchange (losses) (26.4 ) Other income 0.4 Other expenses (0.8 ) Income before tax $ 214.3 Net reserves for loss and loss adjustment expenses $ 2,469.6 $ 2,095.6 $ 4,565.2 Ratios Loss ratio 48.8 % 61.5 % 56.0 % Policy acquisition expense ratio 21.0 18.9 19.8 General and administrative expense ratio 12.8 14.6 16.2 (1) Expense ratio 33.8 33.5 36.0 Combined ratio 82.6 % 95.0 % 92.0 % (1) The general and administrative expense ratio in the total column includes corporate expenses. Nine Months Ended September 30, 2014 Reinsurance Insurance Total ( $ in millions) Underwriting Revenues Gross written premiums $ 1,027.5 $ 1,259.8 $ 2,287.3 Net written premiums 980.4 980.8 1,961.2 Gross earned premiums 859.2 1,182.0 2,041.2 Net earned premiums 825.1 968.0 1,793.1 Underwriting Expenses Losses and loss adjustment expenses 367.4 600.5 967.9 Amortization of deferred policy acquisition costs 152.3 184.1 336.4 General and administrative expenses 107.0 142.6 249.6 Underwriting income $ 198.4 $ 40.8 239.2 Corporate expenses (74.6 ) Net investment income 143.6 Realized and unrealized investment gains 53.6 Realized and unrealized investment losses (8.9 ) Change in fair value of loan notes issued by variable interest entities (14.5 ) Change in fair value of derivatives (8.6 ) Interest expense on long term debt (22.1 ) Net realized and unrealized foreign exchange (losses) (10.4 ) Other income 4.8 Other expenses (2.2 ) Income before tax $ 299.9 Net reserves for loss and loss adjustment expenses $ 2,554.2 $ 1,848.4 $ 4,402.6 Ratios Loss ratio 44.5 % 62.0 % 54.0 % Policy acquisition expense ratio 18.5 19.0 18.8 General and administrative expense ratio 13.0 14.7 18.1 (1) Expense ratio 31.5 33.7 36.9 Combined ratio 76.0 % 95.7 % 90.9 % (1) The general and administrative expense ratio in the total column includes corporate expenses.

Investments

Investments	9 Months Ended
	Sep. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Investments	Investments Income Statement Investment Income. The following table summarizes investment income for the three and nine months ended September 30, 2015 and 2014 : For the Three Months Ended For the Nine Months Ended September 30, 2015 September 30, 2014 September 30, 2015 September 30, 2014 ($ in millions) ($ in millions) Fixed income securities — Available for sale $ 36.1 $ 37.6 $ 106.8 $ 113.4 Fixed income securities — Trading 7.2 6.6 21.1 19.8 Short-term investments — Available for sale 0.3 0.4 0.9 1.0 Fixed term deposits (included in cash and cash equivalents) 0.5 0.5 2.5 2.4 Equity securities — Available for sale — 1.0 0.1 3.5 Equity securities — Trading 4.6 3.5 16.5 10.5 Catastrophe bonds — Trading 0.4 0.5 1.3 0.9 Total $ 49.1 $ 50.1 $ 149.2 $ 151.5 Investment expenses (4.1 ) (2.1 ) (10.1 ) (7.9 ) Net investment income $ 45.0 $ 48.0 $ 139.1 $ 143.6 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 and nine months ended September 30, 2015 and 2014 : For the Three Months Ended For the Nine Months Ended September 30, 2015 September 30, 2014 September 30, 2015 September 30, 2014 ($ in millions) ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 2.4 $ 1.7 $ 9.7 $ 8.1 Fixed income securities — gross realized (losses) (0.3 ) (1.0 ) (1.4 ) (5.5 ) Equity securities — gross realized gains — 4.2 31.9 9.7 Equity securities — gross realized (losses) — — (3.0 ) — Other-than-temporary impairments — — — (0.7 ) Trading: Fixed income securities — gross realized gains 0.5 1.9 3.8 6.6 Fixed income securities — gross realized (losses) (1.2 ) (0.2 ) (3.8 ) (2.4 ) Equity securities — gross realized gains 7.8 2.8 36.2 4.5 Equity securities — gross realized (losses) (12.3 ) (0.1 ) (25.4 ) (0.3 ) Catastrophe bonds 0.7 0.5 (0.1 ) 0.5 Net change in gross unrealized gains (38.8 ) (10.0 ) (61.5 ) 24.2 Total net realized and unrealized investment (losses)/gains recorded in the statement of operations $ (41.2 ) $ (0.2 ) $ (13.6 ) $ 44.7 Change in available for sale net unrealized gains: Fixed income securities 22.7 (28.5 ) (27.4 ) 25.0 Equity securities — (4.5 ) (27.4 ) (2.3 ) Total change in pre-tax available for sale unrealized gains 22.7 (33.0 ) (54.8 ) 22.7 Change in taxes 0.8 8.1 2.1 (3.8 ) Total change in net unrealized gains, net of taxes, recorded in other comprehensive income $ 23.5 $ (24.9 ) $ (52.7 ) $ 18.9 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 and nine months ended September 30, 2015 ( 2014 — $ Nil and $0.7 million , respectively). Balance Sheet Fixed Income Securities, Short-Term Investments and Equities — Available For Sale. The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income securities, short-term investments and equity securities as at September 30, 2015 and December 31, 2014 : As at September 30, 2015 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,114.5 $ 23.9 $ (0.3 ) $ 1,138.1 U.S. agency 156.1 5.9 — 162.0 Municipal 26.7 1.8 — 28.5 Corporate 2,443.0 62.5 (8.8 ) 2,496.7 Non-U.S. government-backed corporate 76.1 0.9 — 77.0 Foreign government 615.1 13.6 (0.3 ) 628.4 Asset-backed 130.8 1.7 — 132.5 Non-agency commercial mortgage-backed 29.9 1.7 — 31.6 Agency mortgage-backed 978.0 34.2 (2.5 ) 1,009.7 Total fixed income securities — Available for sale 5,570.2 146.2 (11.9 ) 5,704.5 Total short-term investments — Available for sale 176.3 — — 176.3 Total $ 5,746.5 $ 146.2 $ (11.9 ) $ 5,880.8 The Company no longer holds equity investments in its available for sale portfolio. All equities are held in the trading portfolio. As at December 31, 2014 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,074.2 $ 21.5 $ (1.3 ) $ 1,094.4 U.S. agency 190.0 7.5 (0.1 ) 197.4 Municipal 29.1 2.4 — 31.5 Corporate 2,244.7 79.9 (5.2 ) 2,319.4 Non-U.S. government-backed corporate 76.8 1.2 — 78.0 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 1,016.7 40.8 (2.2 ) 1,055.3 Total fixed income securities — Available for sale 5,462.9 176.3 (9.2 ) 5,630.0 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 $ 5,803.7 $ 203.7 $ (9.2 ) $ 5,998.2 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 September 30, 2015 and December 31, 2014 : As at September 30, 2015 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 13.4 $ 0.1 $ — $ 13.5 Municipal 0.5 — — 0.5 Corporate 545.4 7.9 (6.1 ) 547.2 Foreign government 129.1 2.2 (2.4 ) 128.9 Asset-backed 18.7 0.1 (0.1 ) 18.7 Bank loans 84.5 0.1 (1.5 ) 83.1 Total fixed income securities — Trading 791.6 10.4 (10.1 ) 791.9 Total short-term investments — Trading 6.9 — — 6.9 Total equity securities — Trading 717.2 39.7 (60.6 ) 696.3 Total catastrophe bonds — Trading 36.3 0.5 (0.1 ) 36.7 Total $ 1,552.0 $ 50.6 $ (70.8 ) $ 1,531.8 As at December 31, 2014 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. agency $ 0.2 $ — $ — $ 0.2 Municipal 1.1 — — 1.1 Corporate 520.9 11.7 (2.8 ) 529.8 Foreign government 137.3 4.3 (1.5 ) 140.1 Asset-backed 14.6 0.1 — 14.7 Bank loans 86.8 — (1.7 ) 85.1 Total fixed income securities — Trading 760.9 16.1 (6.0 ) 771.0 Total short-term investments — Trading 0.2 — — 0.2 Total equity securities — Trading 585.2 55.5 (24.7 ) 616.0 Total catastrophe bonds — Trading 34.4 0.4 — 34.8 Total $ 1,380.7 $ 72.0 $ (30.7 ) $ 1,422.0 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 $36.7 million as of September 30, 2015 . The bonds receive quarterly interest payments based on variable interest rates with scheduled maturities ranging from 2016 to 2021. The redemption value of the bonds will adjust based on the occurrence of a covered event, such as windstorms and earthquakes which occur in the United States, Canada, the North Atlantic, Japan or Australia. Other Investments. In January 2015, the Company established, along with seven other insurance companies, a micro-insurance venture consortium and micro-insurance incubator (“MVI”), domiciled in Bermuda. The MVI is a social impact organization that will provide micro-insurance products to assist global emerging consumers. The Company’s initial investment in the MVI was $0.8 million . On October 2, 2012, the Company established a subsidiary, Aspen Recoveries Limited, to take ownership of a 58.5% shareholding in Chaspark Maritime Holdings Ltd., a Singaporean registered company (“Chaspark”), with the remaining shareholding owned by other insurers. The shareholding in Chaspark was received as a settlement for subrogation rights associated with a contract frustration claim settlement. The Company has determined that Chaspark has the characteristics of a variable interest entity as addressed by the guidance in ASC 810-10, Consolidation . However, having considered the provisions of ASC 810-10, the Company’s investment in Chaspark does not permit the Company to direct the activities which most significantly impact Chaspark’s economic performance and the Company is not acting as principal or agent for a related party group of investors. Under these circumstances, the Company is not required to consolidate Chaspark. The investment is therefore accounted for under the equity method and adjustments to the carrying value of this investment are made based on the Company’s share of capital including share of income and expenses, which is provided in the quarterly management accounts. The adjusted carrying value approximates fair value. The tables below show the Company’s investments in the MVI and Chaspark for the three and nine months ended September 30, 2015 : For the Three Months Ended September 30, 2015 MVI Chaspark Total ($ in millions) Opening and closing undistributed value of investment $ 0.8 $ 8.7 $ 9.5 For the Nine Months Ended September 30, 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 September 30, 2015 $ 0.8 $ 8.7 $ 9.5 Fixed Income Securities. The scheduled maturity distribution of available for sale fixed income securities as at September 30, 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 September 30, 2015 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 647.7 $ 651.6 AA Due after one year through five years 2,638.8 2,704.6 AA- Due after five years through ten years 1,055.3 1,077.7 A+ Due after ten years 89.7 96.8 A+ Subtotal 4,431.5 4,530.7 Non-agency commercial mortgage-backed 29.9 31.6 AA+ Agency mortgage-backed 978.0 1,009.7 AA+ Asset-backed 130.8 132.5 AAA Total fixed income securities — Available for sale $ 5,570.2 $ 5,704.5 As at December 31, 2014 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 590.2 $ 594.7 AA Due after one year through five years 2,552.0 2,620.8 AA- Due after five years through ten years 1,023.5 1,059.9 A+ Due after ten years 97.7 111.0 A+ Subtotal 4,263.4 4,386.4 Non-agency commercial mortgage-backed 41.5 44.8 AA+ Agency mortgage-backed 1,016.7 1,055.3 AA+ Asset-backed 141.3 143.5 AAA Total fixed income securities — Available for sale $ 5,462.9 $ 5,630.0 Guaranteed Investments. As at September 30, 2015 , the Company held $1.8 million ( December 31, 2014 — $2.5 million ) in investments which are guaranteed by mono-line insurers, excluding those with explicit government guarantees, and the Company’s holding was limited to two municipal securities, both rated CC ( December 31, 2014 — two municipal securities, rated A3 and Caa3). The standalone rating (rating without guarantee) is determined as the senior unsecured debt rating of the issuer. Where the credit ratings were split between the two main rating agencies, Standard & Poor’s Financial Services LLC (“S&P”) and Moody’s Investors Service, Inc. (“Moody’s”), the lowest rating was used. The Company’s exposure to other third-party guaranteed debt is primarily to investments backed by non-U.S. government guaranteed issuers. Gross Unrealized Loss. The following tables summarize as at September 30, 2015 and December 31, 2014 by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position for the Company’s available for sale portfolio: As at September 30, 2015 0-12 months Over 12 months Total Fair Market Value Gross Unrealized Loss Fair Market Value Gross Unrealized Loss Fair Market Value Gross Unrealized Loss Number of Securities ($ in millions) U.S. government $ 23.7 $ (0.3 ) $ 4.6 $ — $ 28.3 $ (0.3 ) 13 U.S. agency 1.3 — — — 1.3 — 1 Corporate 704.9 (7.4 ) 80.2 (1.4 ) 785.1 (8.8 ) 337 Non-U.S. government-backed corporate 16.1 — — — 16.1 — 6 Foreign government 124.0 (0.2 ) 2.7 (0.1 ) 126.7 (0.3 ) 18 Asset-backed 18.3 — 13.5 — 31.8 — 27 Agency mortgage-backed 150.1 (1.4 ) 61.9 (1.1 ) 212.0 (2.5 ) 62 Total fixed income securities — Available for sale 1,038.4 (9.3 ) 162.9 (2.6 ) 1,201.3 (11.9 ) 464 Total short-term investments — Available for sale 52.7 — — — 52.7 — 12 Total $ 1,091.1 $ (9.3 ) $ 162.9 $ (2.6 ) $ 1,254.0 $ (11.9 ) 476 As at December 31, 2014 0-12 months Over 12 months Total Fair Market Value Gross Unrealized Loss Fair Market Value Gross Unrealized Loss Fair Market Value Gross Unrealized Loss Number of Securities ($ 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 (0.1 ) 7 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 ) 1,213.3 (9.2 ) 428 Total short-term investments — Available for sale 4.6 — — — 4.6 — 3 Total $ 794.7 $ (3.0 ) $ 423.2 $ (6.2 ) $ 1,217.9 $ (9.2 ) 431 Investment Purchases and Sales. The following table summarizes investment purchases, sales and maturities for the three and nine months ended September 30, 2015 and 2014 : For the Three Months Ended For the Nine Months Ended September 30, 2015 September 30, 2014 September 30, 2015 September 30, 2014 ($ in millions) ($ in millions) (Purchases) of fixed income securities — Available for sale $ (428.9 ) $ (522.8 ) $ (1,368.2 ) $ (1,440.9 ) (Purchases) of fixed income securities — Trading (111.5 ) (154.7 ) (387.0 ) (553.6 ) (Purchases) of equity securities — Trading (66.3 ) (121.2 ) (335.2 ) (240.1 ) Proceeds from sales and maturities of fixed income securities — Available for sale 350.4 481.0 1,211.9 1,496.9 Proceeds from sales and maturities of fixed income securities — Trading 89.9 143.5 354.4 515.7 Proceeds from sales of equity securities — Available for sale — 12.1 108.6 27.7 Proceeds from sales of equity securities — Trading 63.6 17.8 214.7 36.0 Net change in (payable)/receivable for securities (purchased)/sold 8.9 (7.0 ) 14.7 12.8 (Purchases) of short-term investments — Available for sale (9.3 ) (195.1 ) (118.6 ) (464.0 ) Proceeds from short-term investments — Available for sale 33.3 220.1 205.7 313.0 (Purchases) of short-term investments — Trading (22.5 ) (21.2 ) (38.2 ) (101.4 ) Proceeds from short-term investments — Trading 16.7 28.2 31.5 94.4 Net proceeds/(purchases) of catastrophe bonds — Trading (3.8 ) (1.7 ) (2.0 ) (26.0 ) Proceeds from other investments — — — 37.3 Net proceeds/(purchases) for the period $ (79.5 ) $ (121.0 ) $ (107.7 ) $ (292.2 )

Variable Interest Entities

Variable Interest Entities	9 Months Ended
	Sep. 30, 2015
Business Combinations [Abstract]
Variable Interest Entities	Variable Interest Entities As at September 30, 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 and nine months ended September 30, 2015 , there was no change in the value of the Company’s investment in Chaspark ( September 30, 2014 — $ Nil and $ 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 remaining 2014 Loan Notes as at September 30, 2015 was $1.0 million (of which $0.8 million was due to external investors). During the first nine months of 2015, Silverton distributed $88.1 million (of which $67.8 million was distributed to external investors) to its noteholders. Of the remaining $0.8 million due to external investors, $0.8 million has been classified as a current liability in the Company’s consolidated financial statements. The total aggregate unpaid balance of the 2014 Loan Notes held by third parties and those held by Aspen Holdings is $1.0 million . The Company’s maximum loss exposure to the 2014 Loan Notes is $0.2 million which is the fair value of its holdings as at September 30, 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” and, together with the 2014 Loan Notes, the "Silverton 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 September 30, 2015 was $103.0 million (of which $84.5 million was held by external investors). The $84.5 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 $103.0 million . The Company’s maximum loss exposure to the 2015 Loan Notes is $18.5 million which is the fair value of its holdings as at September 30, 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	9 Months Ended
	Sep. 30, 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 September 30, 2015 and December 31, 2014 , respectively: As at September 30, 2015 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,138.1 $ — $ — $ 1,138.1 U.S. agency — 162.0 — 162.0 Municipal — 28.5 — 28.5 Corporate — 2,496.7 — 2,496.7 Non-U.S. government-backed corporate — 77.0 — 77.0 Foreign government 447.2 181.2 — 628.4 Asset-backed — 132.5 — 132.5 Non-agency commercial mortgage-backed — 31.6 — 31.6 Agency mortgage-backed — 1,009.7 — 1,009.7 Total fixed income securities available for sale, at fair value 1,585.3 4,119.2 — 5,704.5 Short-term investments available for sale, at fair value 143.9 32.4 — 176.3 Held for trading financial assets, at fair value U.S. government 13.5 — — 13.5 Municipal — 0.5 — 0.5 Corporate — 547.2 — 547.2 Foreign government 41.2 87.7 — 128.9 Asset-backed — 18.7 — 18.7 Bank loans — 83.1 — 83.1 Total fixed income securities trading, at fair value 54.7 737.2 — 791.9 Short-term investments trading, at fair value — 6.9 — 6.9 Equity investments trading, at fair value 696.3 — — 696.3 Catastrophe bonds trading, at fair value — 36.7 — 36.7 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 9.2 — 9.2 Liabilities under derivative contracts — interest rate swaps — — — — Liabilities under derivative contracts — foreign exchange contracts — (1.9 ) — (1.9 ) Loan notes issued by variable interest entities, at fair value — — (84.5 ) (84.5 ) Loan notes issued by variable interest entities, at fair value (classified as a current liability) — — (0.8 ) (0.8 ) Total $ 2,480.2 $ 4,939.7 $ (85.3 ) $ 7,334.6 There were no maturities or transfers between Level 1, Level 2 and Level 3 during the three and nine months ended September 30, 2015 . The Company settled $0.8 million and $67.8 million Level 3 liabilities in respect to the loan notes issued by the VIEs for the three and nine months ended September 30, 2015 , respectively. As at September 30, 2015 , there were no assets classified as Level 3 and the Company's Level 3 liabilities consisted of the loan notes issued by the VIEs. As at December 31, 2014 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,094.4 $ — $ — $ 1,094.4 U.S. agency — 197.4 — 197.4 Municipal — 31.5 — 31.5 Corporate — 2,319.4 — 2,319.4 Non-U.S. government-backed corporate — 78.0 — 78.0 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 — 1,055.3 — 1,055.3 Total fixed income securities available for sale, at fair value 1,550.9 4,079.1 — 5,630.0 Short-term investments available for sale, at fair value 229.3 29.0 — 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 36.1 104.0 — 140.1 Asset-backed — 14.7 — 14.7 Bank loans — 85.1 — 85.1 Total fixed income securities trading, at fair value 36.1 734.9 — 771.0 Short-term investments trading, at fair value 0.1 0.1 — 0.2 Equity investments trading, at fair value 616.0 — — 616.0 Catastrophe bonds trading, at fair value — 34.8 — 34.8 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 $ 2,542.3 $ 4,871.6 $ (138.6 ) $ 7,275.3 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 tables present 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 and nine months ended September 30, 2015 and 2014 : Reconciliation of Liabilities Using Level 3 Inputs Three Months Ended September 30, 2015 Three Months Ended September 30, 2014 ($ in millions) Balance at the beginning of the period (1) $ 77.8 $ 56.0 Distributed to third party (0.8 ) — Total change in fair value included in the statement of operations 8.3 8.5 Balance at the end of the period (1) $ 85.3 $ 64.5 Reconciliation of Liabilities Using Level 3 Inputs Nine Months Ended September 30, 2015 Nine Months Ended September 30, 2014 ($ in millions) Balance at the beginning of the period (1) $ 138.6 $ 50.0 Distributed to third party (67.8 ) — Total change in fair value included in the statement of operations 14.5 14.5 Balance at the end of the period (1) $ 85.3 $ 64.5 (1) The amount classified as other payables was $0.8 million and $67.9 million as at September 30, 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 September 30, 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 September 30, 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 September 30, 2015 and December 31, 2014 were as follows: As at September 30, 2015 As at December 31, 2014 Index providers 84 % 84 % Pricing services 11 11 Broker-dealers 5 5 Total 100 % 100 % A summary of securities priced using pricing information from index providers as at September 30, 2015 and December 31, 2014 is provided below: As at September 30, 2015 As at December 31, 2014 Fair Market Value Determined using Prices from Index Providers % of Total Fair Value by Security Type Fair Market Value Determined using Prices from Index Providers % of Total Fair Value by Security Type ($ in millions, except for percentages) U.S. government $ 1,144.1 99 % $ 1,044.4 95 % U.S. agency 149.8 92 % 186.9 95 % Municipal 11.4 39 % 13.7 42 % Corporate 2,921.4 96 % 2,731.1 96 % Non-U.S. government-backed corporate 34.0 44 % 48.7 62 % Foreign government 432.4 57 % 504.4 63 % Asset-backed 100.3 66 % 140.5 89 % Non-agency commercial mortgage-backed 24.8 79 % 44.8 100 % Agency mortgage-backed 629.7 62 % 680.6 64 % Total fixed income securities $ 5,447.9 84 % $ 5,395.1 84 % Equities 696.3 100 % 725.9 100 % Total fixed income securities and equity investments $ 6,144.2 85 % $ 6,121.0 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 Emerging Market Debt (“EMD”) portfolio which is also classified within Level 2. Corporate. Corporate securities consist primarily of U.S. and foreign corporations covering a variety of industries and are for the most part priced by index providers and pricing vendors. Some issuers may participate in government programs which guarantee timely payment of principal and interest in the event of a default. The fair values of these securities are generally determined using the spread above the risk-free yield curve. Inputs used in the evaluation of these securities include credit data, interest rate data, market observations and sector news, broker-dealer quotes and trade volumes. In addition, corporate securities include a portion of the EMD portfolio. The Company classifies all of these securities within Level 2. Mortgage-backed Securities. 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 September 30, 2015 , the Company obtained an average of 2.0 quotes per equity investment, compared to 4.0 quotes as at December 31, 2014 . Pricing sources used in pricing equities as at September 30, 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. In the fourth quarter of 2014, Silverton issued an additional $85.0 million ( $70.0 million third-party funded) loan notes with a maturity date of September 18, 2017. The Company has elected to account for the Silverton Loan Notes at fair value using the guidance as prescribed under ASC 825, Financial Instruments as the Company believes it represents the most meaningful measurement basis for these liabilities. The Silverton Loan Notes are recorded at fair value at each reporting period and, as they are not quoted on an active market and contain significant unobservable inputs, they have been classified as a Level 3 instrument in the Company's fair value hierarchy. The Silverton Loan Notes are unique because they are linked to the specific risks of the Company’s property catastrophe book. To determine the fair value of the Silverton Loan Notes, the Company runs an internal model which considers the seasonality of the risk assumed under the retrocessional agreement between Aspen Bermuda and Silverton. The seasonality used in the model is determined by applying the percentage of property catastrophe losses planned by the Company’s actuaries to the estimated written premium to determine earned premium for each quarter. The inputs to the internal valuation model are based on Company specific data due to the lack of availability of observable market inputs. Reserves for losses are the most significant unobservable input. An increase in reserves for losses would normally result in a decrease in the fair value of the Silverton Loan Notes while a decrease in reserves would normally result in an increase in the fair value of the Silverton Loan Notes. The observable and unobservable inputs used to determine the fair value of the 2015 Loan Notes and 2014 Loan Notes as at September 30, 2015 and December 31, 2014 are presented in the table below: At September 30, 2015 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan notes held by third parties $ 85.3 (1) Internal Valuation Model Gross premiums written (O) $ 37.7 $ 39.8 Reserve for losses (U) $ 4.9 $ 4.9 Contract period (O) N/A 365 days Initial value of issuance (O) $ 70.2 $ 120.0 At December 31, 2014 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan notes held by third parties $ 138.6 (1) Internal Valuation Model Gross premiums written (O) $ — $ 40.0 Reserve for losses (U) $ — $ 4.6 Contract period (O) N/A 365 days Initial value of issuance (O) $ 120.0 $ 120.0 (1) The amount classified as other payables was $0.8 million and $67.9 million as at September 30, 2015 and December 31, 2014, respectively. The observable and unobservable inputs represent the potential variation around the inputs used in the valuation model. The contract period is defined in the Silverton loan agreements and the initial value represents the funds received from third parties.

Reinsurance

Reinsurance	9 Months Ended
	Sep. 30, 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 September 30, 2015 were 22.9% ( December 31, 2014 — 18.6% ) with Munich Re rated AA- by S&P, 17.1% ( December 31, 2014 — 27.3% ) with Lloyd’s syndicates rated A+ by S&P and 8.2% ( December 31, 2014 — 8.4% ) with Arch Re which is rated A+ by S&P.

Derivative Contracts

Derivative Contracts	9 Months Ended
	Sep. 30, 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 September 30, 2015 and December 31, 2014 : As at September 30, 2015 As at December 31, 2014 Derivatives Not Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Interest Rate Swaps Liabilities under Derivative Contracts $ 951.3 $ — (1) $ 951.3 $ 0.1 (1) Foreign Exchange Contracts Derivatives at Fair Value $ 363.3 $ 9.2 $ 165.8 $ 7.9 Foreign Exchange Contracts Liabilities under Derivative Contracts $ 34.2 $ (1.1 ) $ 237.6 $ (10.5 ) (1) Net of $13.7 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 — $22.3 million ). As at September 30, 2015 As at December 31, 2014 Derivatives Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Liabilities under Derivative Contracts $ 28.2 $ (0.8 ) (1) $ 135.8 $ (3.8 ) (1) (1) Net of $0.8 million 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 and nine months ended September 30, 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 ASC 815 Location of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income September 30, 2015 September 30, 2014 ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives $ 12.9 $ (6.4 ) Interest Rate Swaps Change in Fair Value of Derivatives $ (2.8 ) $ 1.3 Amount of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income for the Nine Months Ended Derivatives Not Designated as Hedging Instruments Under ASC 815 Location of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income September 30, 2015 September 30, 2014 ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives $ 10.5 $ (4.5 ) Interest Rate Swaps Change in Fair Value of Derivatives $ (6.2 ) $ (4.1 ) 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 ASC 815 Location of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income September 30, 2015 September 30, 2014 ($ in millions) Foreign Exchange Contracts General, administrative and corporate expenses $ (0.8 ) $ — Foreign Exchange Contracts Net change from current period hedged transactions $ (0.5 ) $ — Amount of Income/(Loss) Nine Months Ended Derivatives Designated as Hedging Instruments Under ASC 815 Location of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income September 30, 2015 September 30, 2014 ($ in millions) Foreign Exchange Contracts General, administrative and corporate expenses $ (3.5 ) $ — Foreign Exchange Contracts Net change from current period hedged transactions $ 2.2 $ — 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 September 30, 2015 , the Company held foreign exchange contracts that were not designated as hedging under ASC 815 with an aggregate notional value of $397.5 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 and nine months ended September 30, 2015 , the impact of foreign exchange contracts on net income was a gain of $12.9 million ( September 30, 2014 — charge of $6.4 million ) and a gain of $10.5 million ( September 30, 2014 — charge of $4.5 million ), respectively. As at September 30, 2015 , the Company held foreign exchange contracts that were designated as hedging under ASC 815 with an aggregate notional value of $28.2 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 and nine months ended September 30, 2015 , the movement in other comprehensive income representing the effective portion was a net unrealized loss of $0.5 million ( September 30, 2014 — $ Nil ) and a net unrealized gain of $2.2 million ( September 30, 2014 — $ Nil ), respectively. As the foreign exchange contracts settle, the realized gain or loss is reclassified from other comprehensive income into general, administrative and corporate expenses of the statement of operations and other comprehensive income. For the three and nine months ended September 30, 2015 , the amount recognized within general, administrative and corporate expenses for settled foreign exchange contracts was a realized loss of $0.8 million ( September 30, 2014 — $ Nil ) and a realized loss of $3.5 million ( September 30, 2014 — $ Nil ), respectively. Interest Rate Swaps. As at September 30, 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 and nine months ended September 30, 2015 , there was a charge in respect of the interest rate swaps of $2.8 million ( September 30, 2014 — gain of $1.3 million ) and a charge of $6.2 million ( September 30, 2014 — charge of $4.1 million ), respectively. As at September 30, 2015 , cash collateral with a fair value of $13.7 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 September 30, 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 September 30, 2015 , no amount was recorded in the consolidated balance sheet for the pledged assets.

Deferred Policy Acquisition Cos

Deferred Policy Acquisition Costs	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2015 and 2014 : Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 ($ in millions) ($ in millions) Balance at the beginning of the period $ 349.0 $ 306.4 $ 299.0 $ 262.2 Acquisition costs deferred 129.8 110.7 413.2 375.8 Amortization of deferred policy acquisition costs (132.0 ) (115.5 ) (365.4 ) (336.4 ) Balance at the end of the period $ 346.8 $ 301.6 $ 346.8 $ 301.6

Reserves for Losses and Loss Ad

Reserves for Losses and Loss Adjustment Expenses	9 Months Ended
	Sep. 30, 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 nine months ended September 30, 2015 and twelve months ended December 31, 2014 : Nine Months Ended September 30, 2015 Twelve Months Ended December 31, 2014 ($ in millions) Provision for losses and LAE at the start of the year $ 4,750.8 $ 4,678.9 Less reinsurance recoverable (350.0 ) (332.7 ) Net loss and LAE at the start of the year 4,400.8 4,346.2 Net loss and LAE expenses (disposed) — (24.2 ) Provision for losses and LAE for claims incurred: Current year 1,129.8 1,411.6 Prior years (97.6 ) (104.1 ) Total incurred 1,032.2 1,307.5 Losses and LAE payments for claims incurred: Current year (93.4 ) (112.1 ) Prior years (718.7 ) (995.6 ) Total paid (812.1 ) (1,107.7 ) Foreign exchange (gains) (55.7 ) (121.0 ) Net losses and LAE reserves at period end 4,565.2 4,400.8 Plus reinsurance recoverable on unpaid losses at period end 348.7 350.0 Provision for losses and LAE at the end of the relevant period $ 4,913.9 $ 4,750.8 For the nine months ended September 30, 2015 , there was a reduction of $97.6 million in the Company’s estimate of the ultimate claims to be paid in respect of prior accident years compared to a reduction of $92.6 million for the nine months ended September 30, 2014 . The Company has not assumed any loss reserves as part of any transaction and there have been no disposals or transfers of reserves relating to commutations during the nine months ended September 30, 2015 (twelve months ended December 31, 2014 — $24.2 million ). 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	9 Months Ended
	Sep. 30, 2015
Equity [Abstract]
Capital Structure	Capital Structure The following table provides a summary of the Company’s authorized and issued share capital as at September 30, 2015 and December 31, 2014 : As at September 30, 2015 As at December 31, 2014 Number $ in Thousands Number $ in Thousands Authorized share capital: Ordinary Shares 0.15144558¢ per share 969,629,030 1,469 969,629,030 1,469 Non-Voting Shares 0.15144558¢ per share 6,787,880 10 6,787,880 10 Preference Shares 0.15144558¢ per share 100,000,000 152 100,000,000 152 Total authorized share capital 1,631 1,631 Issued share capital: Issued ordinary shares of 0.15144558¢ per share 60,781,958 92 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 127 129 Additional paid-in capital as at September 30, 2015 was $1,068.3 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 nine months ended September 30, 2015 : Number of Ordinary Shares Ordinary shares in issue at December 31, 2014 62,017,368 Ordinary share transactions in the nine months ended September 30, 2015 Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or 2008 share purchase plan 546,117 Ordinary shares issued to non-employee directors 8,806 Ordinary shares repurchased (1,790,333 ) Ordinary shares in issue at September 30, 2015 60,781,958 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 for the three and nine months ended September 30, 2015 , Nil and 1,790,333 ordinary shares, respectively. The total consideration paid for the three and nine months ended September 30, 2015 was $ Nil and $83.7 million , respectively, with an average price per ordinary share for the three and nine months ended September 30, 2015 of $ Nil and $46.74 , respectively. Under open market repurchases, the Company acquired and cancelled 2,120,625 and 2,891,130 ordinary shares, respectively, for the three and nine months ended September 30, 2014 . The total consideration paid for the three and nine months ended September 30, 2014 was $90.0 million and $120.9 million , respectively, and the average price per ordinary share was $42.46 and $41.82 , respectively. 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 September 30, 2015 , no Rights were exercisable or exercised. The Rights Agreement expired on April 16, 2015.

Share-Based Payments

Share-Based Payments	9 Months Ended
	Sep. 30, 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 ordinary 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 had 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 and nine months ended September 30, 2015 ( 2014 — Nil and Nil ) and 2,000 and 85,938 options, respectively, were exercised and ordinary shares issued in the three and nine months ended September 30, 2015 ( 2014 — Nil and 53,436 options). No charges against income were made in respect of employee options for the three and nine months ended September 30, 2015 ( 2014 — $ Nil and $ 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 and nine months ended September 30, 2015 , the Company granted to its employees 11,614 and 253,766 RSUs, respectively ( 2014 — Nil and 259,640 ). Compensation costs charged against income in respect of RSUs for the three and nine months ended September 30, 2015 were $2.1 million and $6.8 million , respectively ( 2014 — $1.9 million and $7.7 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 (February 10, 2014 — 27,180 RSUs) and 12,154 RSUs to the Chairman (February 10, 2014 — 13,590 RSUs). Compensation costs charged against income in respect of non-employee director RSUs for the three and nine months ended September 30, 2015 were $0.4 million and $1.3 million , respectively ( 2014 — $0.3 million and $1.1 million ). The total fair value adjustment for all RSUs for the three and nine months ended September 30, 2015 was $ Nil and $0.3 million , respectively ( 2014 — $0.3 million loss and $0.9 million gain). The total tax credit recognized by the Company in relation to RSUs in the three and nine months ended September 30, 2015 was $0.7 million and $1.4 million , respectively ( 2014 — $0.5 million and $1.8 million ). Performance Shares. During the three and nine months ended September 30, 2015 , the Company granted Nil and 277,585 performance shares, respectively, to employees ( 2014 — Nil and 315,389 ). The performance shares are subject to a three -year vesting period with a separate annual diluted book value per share (“BVPS”) growth test for each year, adjusted to add back ordinary dividends. One-third of the grant are eligible for vesting each year based on a formula, and are only 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 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 made in the context of such actions. The calculation of BVPS for 2015 will exclude 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 employees’ 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 ordinary 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 and nine months ended September 30, 2015 in respect of performance shares were $2.6 million and $4.8 million , respectively ( 2014 — $0.3 million and $4.5 million ). The total tax recognized by the Company in relation to performance shares in the three and nine months ended September 30, 2015 was a tax credit of $0.6 million and $1.1 million , respectively ( 2014 — $0.1 million and $1.0 million ). Phantom Shares. During the three and nine months ended September 30, 2015 , the Compensation Committee approved the grant of Nil and 135,651 phantom shares, respectively, to its employees ( 2014 — Nil and 154,512 ). 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 ordinary 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 and nine months ended September 30, 2015 in respect of phantom shares were $1.4 million and $3.4 million , respectively ( 2014 — $1.2 million and $3.6 million ) with a fair value adjustment for the three and nine months ended September 30, 2015 of $0.2 million and $1.8 million , respectively ( 2014 — $0.8 million and $1.3 million ). The total tax credit recognized by the Company in relation to phantom shares in the three and nine months ended September 30, 2015 was $0.1 million and $0.8 million , respectively ( 2014 — $0.2 million and $0.7 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, 264 and 54,940 ordinary shares, respectively, were exercised and issued during the three and nine months ended September 30, 2015 ( 2014 — 627 and 11,821 shares). Compensation costs charged against income in the three and nine months ended September 30, 2015 in respect of the ESPP were $0.1 million and $0.3 million , respectively ( 2014 — $0.1 million and $0.2 million ).

Intangible Assets

Intangible Assets	9 Months Ended
	Sep. 30, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets	Intangible Assets The following tables provide a summary of the Company’s intangible assets for the three and nine months ended September 30, 2015 and 2014 : Three Months Ended September 30, 2015 Three Months Ended September 30, 2014 Trade Mark Insurance Licenses Other Total Trade Mark Insurance Licenses Other Total ($ in millions) ($ in millions) Intangible Assets Beginning of the period $ 1.6 $ 16.6 $ — $ 18.2 $ 1.6 $ 16.6 $ — $ 18.2 Amortization — — — — — — — — End of the period $ 1.6 $ 16.6 $ — $ 18.2 $ 1.6 $ 16.6 $ — $ 18.2 Nine Months Ended September 30, 2015 Nine Months Ended September 30, 2014 Trade Mark Insurance Licenses Other Total Trade Mark Insurance Licenses Other Total ($ 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 — — — — — — (0.2 ) (0.2 ) End of the period $ 1.6 $ 16.6 $ — $ 18.2 $ 1.6 $ 16.6 $ — $ 18.2 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 September 30, 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 September 30, 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 amortized. The licenses 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 September 30, 2015 was $ Nil ( December 31, 2014 — $ Nil ).

Commitments and Contingent Liab

Commitments and Contingent Liabilities	9 Months Ended
	Sep. 30, 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 specific policyholders and by obligations to certain regulatory authorities to facilitate issue of letters of credit or maintain certain balances in deposits and trust funds for the benefit of policyholders. The following table details the forms and value of the Company’s restricted assets as at September 30, 2015 and December 31, 2014 : As at September 30, 2015 As at December 31, 2014 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 1,177.1 $ 1,086.9 Third party 2,294.8 2,183.4 Letters of credit / guarantees (1) 726.6 778.7 Total restricted assets $ 4,198.5 $ 4,049.0 Total as percent of cash and invested assets 48.7 % 47.0 % (1) As at September 30, 2015 , the Company pledged funds of $712.3 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 September 30, 2015 in the amount of $418.0 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 Syndicate 4711 declares 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 September 30, 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 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 had a $100.0 million secured letter of credit facility agreement with Barclays. All letters of credit issued under the facility were 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 September 30, 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 September 30, 2015 , cash collateral with a fair value of $13.7 million was held by the Company’s counterparties to support the current valuation of the interest rate swaps ( December 31, 2014 — $22.3 million ). For more information, refer to Note 10 of these unaudited condensed consolidated financial statements. (b) Operating leases Amounts outstanding under operating leases net of subleases as of September 30, 2015 were: 2015 2016 2017 2018 2019 Later Total ($ in millions) Operating Lease Obligations (1) $ 3.4 $ 10.1 $ 14.0 $ 13.6 $ 12.6 $ 97.9 $ 151.6 (1) In 2015, the Company entered into a new 16 year lease relating to the Company’s premises in New York. (c) Contingent liabilities In common with the rest of the insurance and reinsurance industry, the Company is 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 September 30, 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)	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Accounting Pronouncements Not Yet Adopted	Accounting Pronouncements Not Yet Adopted On July 22, 2015, the Financial Accounting Standards Board (“FASB”) issued ASU 2015-11, “Inventory (Topic 330)” which requires an entity to measure inventory at the lower of cost and net realizable value. ASU 2015-11 is effective for fiscal years beginning after December 15, 2016. As the Company does not currently possess stock or inventory it does not expect this ASU to impact its consolidated financial statements. On July 31, 2015, the FASB issued ASU 2015-12, “Plan Accounting: Defined Benefit Pension Plans (Topic 960), Defined Contribution Pension Plans (Topic 962), Health and Welfare Benefit Plans (Topic 965): (Part I) Fully Benefit-Responsive Investment Contracts, (Part II) Plan Investment Disclosures, (Part III) Measurement Date Practical Expedient” which simplifies the benefit plan disclosures and reporting requirements. ASU 2015-12 is effective for annual periods beginning after December 15, 2015. The ASU does not impact the Company's financial statements as it does not classify any of its investment contracts as fully benefit-responsive investment contracts. On August 10, 2015, the FASB issued ASU 2015-13, “Derivatives and Hedging (Topic 815)” which applies to entities that enter into contracts for the purchase or sale of electricity. ASU 2015-13 is effective upon issuance and should be applied prospectively. The ASU does not impact the Company's financial statements as it does not purchase or sell electricity contracts. On August 12, 2015, the FASB issued ASU 2015-14, “Revenue From Contracts With Customers (Topic 606)” which delays the effective date of ASU 2014-09 by one year. ASU 2015-14 is effective for annual periods beginning after December 15, 2015. The ASU does not impact the Company’s financial statements as insurance contracts accounted for within the scope of Topic 944, Financial-Services are exempt from this ASU. On August 18, 2015, the FASB issued ASU 2015-15, “Interest – Imputation of Interest (Subtopic 835-30) Presentation and Subsequent Measurement of Debt Issuance Costs Associated with Line-of-Credit Arrangements” which updates the Accounting Standards Codification to include the SEC Staff Announcement: Presentation and Subsequent Measurement of Debt Issuance Costs Associated with Line-of-Credit Arrangements as proscribed in ASU 2015-03. The ASU does not impact the Company's financial statements. On September 25, 2015, the FASB issued ASU 2015-16, “Business Combinations (Topic 805)” which requires an acquirer to adjust retrospectively to provisional amounts recognized in a business combination. ASU 2015-16 is effective for annual periods beginning after December 15, 2016. The ASU is not expected to have a material impact on the Company’s financial statements.

Reclassifications from Accumu24

Reclassifications from Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2015 and 2014 : Amount Reclassified from AOCI Details about the AOCI Components Three Months Ended September 30, 2015 Three Months Ended September 30, 2014 Affected Line Item in the Unaudited Condensed Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gains on sale of securities $ 2.1 $ 3.9 Realized and unrealized investment gains Realized (losses) on sale of securities — (0.3 ) Realized and unrealized investment losses 2.1 3.6 Income from operations before income tax Tax on net realized gains of securities (0.3 ) (0.1 ) Income tax expense $ 1.8 $ 3.5 Net income Foreign currency translation adjustments: Foreign currency translation adjustments, before tax 1.1 $ — Net realized and unrealized foreign exchange gains/(losses) $ 1.1 $ — Net income Realized derivatives: Net realized (losses) on settled derivatives (0.8 ) $ — General, administrative and corporate expenses $ (0.8 ) $ — Net income Total reclassifications from AOCI to the statement of operations, net of tax $ 2.1 $ 3.5 Net income Amount Reclassified from AOCI Details about the AOCI Components Nine Months Ended September 30, 2015 Nine Months Ended September 30, 2014 Affected Line Item in the Unaudited Condensed Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gains on sale of securities $ 37.7 $ 10.6 Realized and unrealized investment gains Realized (losses) on sale of securities (3.9 ) (4.7 ) Realized and unrealized investment losses 33.8 5.9 Income from operations before income tax Tax on net realized gains of securities (0.7 ) (0.2 ) Income tax expense Total reclassifications from AOCI to the statement of operations, net of tax $ 33.1 $ 5.7 Net income Foreign currency translation adjustments: Foreign currency translation adjustments, before tax 2.2 $ — Net realized and unrealized foreign exchange gains/(losses) $ 2.2 $ — Net income Realized derivatives: Net realized (losses) on settled derivatives (3.5 ) $ — General, administrative and corporate expenses $ (3.5 ) $ — Net income Total reclassifications from AOCI to the statement of operations, net of tax $ 31.8 $ 5.7 Net income

Earnings per Ordinary Share (Ta

Earnings per Ordinary Share (Tables)	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2015 and 2014 , respectively: Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 ($ in millions, except share and per share amounts) Net income $ 28.2 $ 37.4 $ 205.2 $ 288.6 Preference share dividends (9.5 ) (9.5 ) (28.4 ) (28.4 ) Net profit attributable to non-controlling interest (0.3 ) 0.1 (0.8 ) — Basic and diluted net income available to ordinary shareholders $ 18.4 $ 28.0 $ 176.0 $ 260.2 Ordinary shares: Basic weighted average ordinary shares 60,779,295 65,116,463 61,442,033 65,283,681 Weighted average effect of dilutive securities (1) 1,375,830 1,396,546 1,436,403 1,314,999 Total diluted weighted average ordinary shares 62,155,125 66,513,009 62,878,436 66,598,680 Earnings per ordinary share: Basic $ 0.30 $ 0.43 $ 2.86 $ 3.99 Diluted $ 0.30 $ 0.42 $ 2.80 $ 3.91 (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 October 28, 2015, the Company’s Board of Directors (“Board of Directors”) declared the following quarterly dividends: Dividend Payable on: Record Date: Ordinary shares $ 0.21 December 1, 2015 November 13, 2015 7.401% preference shares $ 0.462563 January 1, 2016 December 15, 2015 7.250% preference shares $ 0.4531 January 1, 2016 December 15, 2015 5.95% preference shares $ 0.3719 January 1, 2016 December 15, 2015

Segment Reporting (Tables)

Segment Reporting (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2015 and 2014 : Three Months Ended September 30, 2015 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 316.6 $ 403.9 $ 720.5 Net written premiums 294.7 357.1 651.8 Gross earned premiums 304.6 429.0 733.6 Net earned premiums 284.6 356.0 640.6 Underwriting Expenses Losses and loss adjustment expenses 169.9 195.7 365.6 Amortization of deferred policy acquisition costs 64.8 67.2 132.0 General and administrative expenses 34.7 51.3 86.0 Underwriting income $ 15.2 $ 41.8 57.0 Corporate expenses (14.5 ) Net investment income 45.0 Realized and unrealized investment gains 10.7 Realized and unrealized investment losses (51.9 ) Change in fair value of loan notes issued by variable interest entities (8.3 ) Change in fair value of derivatives 10.1 Interest expense on long term debt (7.4 ) Net realized and unrealized foreign exchange (losses) (8.4 ) Other income (2.3 ) Other expenses — Income before tax $ 30.0 Net reserves for loss and loss adjustment expenses $ 2,469.6 $ 2,095.6 $ 4,565.2 Ratios Loss ratio 59.7 % 55.0 % 57.1 % Policy acquisition expense ratio 22.8 18.9 20.6 General and administrative expense ratio 12.2 14.4 15.7 (1) Expense ratio 35.0 33.3 36.3 Combined ratio 94.7 % 88.3 % 93.4 % (1) The general and administrative expense ratio in the total column includes corporate expenses. Three Months Ended September 30, 2014 Reinsurance Insurance Total ( $ in millions) Underwriting Revenues Gross written premiums $ 256.9 $ 395.6 $ 652.5 Net written premiums 250.9 326.4 577.3 Gross earned premiums 291.0 403.9 694.9 Net earned premiums 279.6 330.8 610.4 Underwriting Expenses Losses and loss adjustment expenses 132.0 210.7 342.7 Amortization of deferred policy acquisition costs 52.1 63.4 115.5 General and administrative expenses 38.4 45.6 84.0 Underwriting income $ 57.1 $ 11.1 68.2 Corporate expenses (35.8 ) Net investment income 48.0 Realized and unrealized investment gains 1.1 Realized and unrealized investment losses (1.3 ) Change in fair value of loan notes issued by variable interest entities (8.5 ) Change in fair value of derivatives (5.1 ) Interest expense on long term debt (7.4 ) Net realized and unrealized foreign exchange (losses) (21.2 ) Other income 1.0 Other expenses (0.3 ) Income before tax $ 38.7 Net reserves for loss and loss adjustment expenses $ 2,554.2 $ 1,848.4 $ 4,402.6 Ratios Loss ratio 47.2 % 63.7 % 56.1 % Policy acquisition expense ratio 18.6 19.2 18.9 General and administrative expense ratio 13.7 13.8 19.6 (1) Expense ratio 32.3 33.0 38.5 Combined ratio 79.5 % 96.7 % 94.6 % (1) The general and administrative expense ratio in the total column includes corporate expenses. 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 nine months ended September 30, 2015 and 2014 : Nine Months Ended September 30, 2015 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,062.1 $ 1,300.4 $ 2,362.5 Net written premiums 975.0 1,084.4 2,059.4 Gross earned premiums 857.6 1,267.3 2,124.9 Net earned premiums 802.3 1,041.3 1,843.6 Underwriting Expenses Losses and loss adjustment expenses 391.7 640.5 1,032.2 Amortization of deferred policy acquisition costs 168.6 196.8 365.4 General and administrative expenses 102.5 151.8 254.3 Underwriting income $ 139.5 $ 52.2 191.7 Corporate expenses (43.8 ) Net investment income 139.1 Realized and unrealized investment gains 81.6 Realized and unrealized investment losses (95.2 ) Change in fair value of loan notes issued by variable interest entities (14.5 ) Change in fair value of derivatives 4.3 Interest expense on long term debt (22.1 ) Net realized and unrealized foreign exchange (losses) (26.4 ) Other income 0.4 Other expenses (0.8 ) Income before tax $ 214.3 Net reserves for loss and loss adjustment expenses $ 2,469.6 $ 2,095.6 $ 4,565.2 Ratios Loss ratio 48.8 % 61.5 % 56.0 % Policy acquisition expense ratio 21.0 18.9 19.8 General and administrative expense ratio 12.8 14.6 16.2 (1) Expense ratio 33.8 33.5 36.0 Combined ratio 82.6 % 95.0 % 92.0 % (1) The general and administrative expense ratio in the total column includes corporate expenses. Nine Months Ended September 30, 2014 Reinsurance Insurance Total ( $ in millions) Underwriting Revenues Gross written premiums $ 1,027.5 $ 1,259.8 $ 2,287.3 Net written premiums 980.4 980.8 1,961.2 Gross earned premiums 859.2 1,182.0 2,041.2 Net earned premiums 825.1 968.0 1,793.1 Underwriting Expenses Losses and loss adjustment expenses 367.4 600.5 967.9 Amortization of deferred policy acquisition costs 152.3 184.1 336.4 General and administrative expenses 107.0 142.6 249.6 Underwriting income $ 198.4 $ 40.8 239.2 Corporate expenses (74.6 ) Net investment income 143.6 Realized and unrealized investment gains 53.6 Realized and unrealized investment losses (8.9 ) Change in fair value of loan notes issued by variable interest entities (14.5 ) Change in fair value of derivatives (8.6 ) Interest expense on long term debt (22.1 ) Net realized and unrealized foreign exchange (losses) (10.4 ) Other income 4.8 Other expenses (2.2 ) Income before tax $ 299.9 Net reserves for loss and loss adjustment expenses $ 2,554.2 $ 1,848.4 $ 4,402.6 Ratios Loss ratio 44.5 % 62.0 % 54.0 % Policy acquisition expense ratio 18.5 19.0 18.8 General and administrative expense ratio 13.0 14.7 18.1 (1) Expense ratio 31.5 33.7 36.9 Combined ratio 76.0 % 95.7 % 90.9 % (1) The general and administrative expense ratio in the total column includes corporate expenses.

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Summary of Investment Income	Investment Income. The following table summarizes investment income for the three and nine months ended September 30, 2015 and 2014 : For the Three Months Ended For the Nine Months Ended September 30, 2015 September 30, 2014 September 30, 2015 September 30, 2014 ($ in millions) ($ in millions) Fixed income securities — Available for sale $ 36.1 $ 37.6 $ 106.8 $ 113.4 Fixed income securities — Trading 7.2 6.6 21.1 19.8 Short-term investments — Available for sale 0.3 0.4 0.9 1.0 Fixed term deposits (included in cash and cash equivalents) 0.5 0.5 2.5 2.4 Equity securities — Available for sale — 1.0 0.1 3.5 Equity securities — Trading 4.6 3.5 16.5 10.5 Catastrophe bonds — Trading 0.4 0.5 1.3 0.9 Total $ 49.1 $ 50.1 $ 149.2 $ 151.5 Investment expenses (4.1 ) (2.1 ) (10.1 ) (7.9 ) Net investment income $ 45.0 $ 48.0 $ 139.1 $ 143.6
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 and nine months ended September 30, 2015 and 2014 : For the Three Months Ended For the Nine Months Ended September 30, 2015 September 30, 2014 September 30, 2015 September 30, 2014 ($ in millions) ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 2.4 $ 1.7 $ 9.7 $ 8.1 Fixed income securities — gross realized (losses) (0.3 ) (1.0 ) (1.4 ) (5.5 ) Equity securities — gross realized gains — 4.2 31.9 9.7 Equity securities — gross realized (losses) — — (3.0 ) — Other-than-temporary impairments — — — (0.7 ) Trading: Fixed income securities — gross realized gains 0.5 1.9 3.8 6.6 Fixed income securities — gross realized (losses) (1.2 ) (0.2 ) (3.8 ) (2.4 ) Equity securities — gross realized gains 7.8 2.8 36.2 4.5 Equity securities — gross realized (losses) (12.3 ) (0.1 ) (25.4 ) (0.3 ) Catastrophe bonds 0.7 0.5 (0.1 ) 0.5 Net change in gross unrealized gains (38.8 ) (10.0 ) (61.5 ) 24.2 Total net realized and unrealized investment (losses)/gains recorded in the statement of operations $ (41.2 ) $ (0.2 ) $ (13.6 ) $ 44.7 Change in available for sale net unrealized gains: Fixed income securities 22.7 (28.5 ) (27.4 ) 25.0 Equity securities — (4.5 ) (27.4 ) (2.3 ) Total change in pre-tax available for sale unrealized gains 22.7 (33.0 ) (54.8 ) 22.7 Change in taxes 0.8 8.1 2.1 (3.8 ) Total change in net unrealized gains, net of taxes, recorded in other comprehensive income $ 23.5 $ (24.9 ) $ (52.7 ) $ 18.9
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 September 30, 2015 and December 31, 2014 : As at September 30, 2015 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,114.5 $ 23.9 $ (0.3 ) $ 1,138.1 U.S. agency 156.1 5.9 — 162.0 Municipal 26.7 1.8 — 28.5 Corporate 2,443.0 62.5 (8.8 ) 2,496.7 Non-U.S. government-backed corporate 76.1 0.9 — 77.0 Foreign government 615.1 13.6 (0.3 ) 628.4 Asset-backed 130.8 1.7 — 132.5 Non-agency commercial mortgage-backed 29.9 1.7 — 31.6 Agency mortgage-backed 978.0 34.2 (2.5 ) 1,009.7 Total fixed income securities — Available for sale 5,570.2 146.2 (11.9 ) 5,704.5 Total short-term investments — Available for sale 176.3 — — 176.3 Total $ 5,746.5 $ 146.2 $ (11.9 ) $ 5,880.8 The Company no longer holds equity investments in its available for sale portfolio. All equities are held in the trading portfolio. As at December 31, 2014 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,074.2 $ 21.5 $ (1.3 ) $ 1,094.4 U.S. agency 190.0 7.5 (0.1 ) 197.4 Municipal 29.1 2.4 — 31.5 Corporate 2,244.7 79.9 (5.2 ) 2,319.4 Non-U.S. government-backed corporate 76.8 1.2 — 78.0 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 1,016.7 40.8 (2.2 ) 1,055.3 Total fixed income securities — Available for sale 5,462.9 176.3 (9.2 ) 5,630.0 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 $ 5,803.7 $ 203.7 $ (9.2 ) $ 5,998.2
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 September 30, 2015 and December 31, 2014 : As at September 30, 2015 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 13.4 $ 0.1 $ — $ 13.5 Municipal 0.5 — — 0.5 Corporate 545.4 7.9 (6.1 ) 547.2 Foreign government 129.1 2.2 (2.4 ) 128.9 Asset-backed 18.7 0.1 (0.1 ) 18.7 Bank loans 84.5 0.1 (1.5 ) 83.1 Total fixed income securities — Trading 791.6 10.4 (10.1 ) 791.9 Total short-term investments — Trading 6.9 — — 6.9 Total equity securities — Trading 717.2 39.7 (60.6 ) 696.3 Total catastrophe bonds — Trading 36.3 0.5 (0.1 ) 36.7 Total $ 1,552.0 $ 50.6 $ (70.8 ) $ 1,531.8 As at December 31, 2014 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. agency $ 0.2 $ — $ — $ 0.2 Municipal 1.1 — — 1.1 Corporate 520.9 11.7 (2.8 ) 529.8 Foreign government 137.3 4.3 (1.5 ) 140.1 Asset-backed 14.6 0.1 — 14.7 Bank loans 86.8 — (1.7 ) 85.1 Total fixed income securities — Trading 760.9 16.1 (6.0 ) 771.0 Total short-term investments — Trading 0.2 — — 0.2 Total equity securities — Trading 585.2 55.5 (24.7 ) 616.0 Total catastrophe bonds — Trading 34.4 0.4 — 34.8 Total $ 1,380.7 $ 72.0 $ (30.7 ) $ 1,422.0
Other Investments	The tables below show the Company’s investments in the MVI and Chaspark for the three and nine months ended September 30, 2015 : For the Three Months Ended September 30, 2015 MVI Chaspark Total ($ in millions) Opening and closing undistributed value of investment $ 0.8 $ 8.7 $ 9.5 For the Nine Months Ended September 30, 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 September 30, 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 September 30, 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 September 30, 2015 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 647.7 $ 651.6 AA Due after one year through five years 2,638.8 2,704.6 AA- Due after five years through ten years 1,055.3 1,077.7 A+ Due after ten years 89.7 96.8 A+ Subtotal 4,431.5 4,530.7 Non-agency commercial mortgage-backed 29.9 31.6 AA+ Agency mortgage-backed 978.0 1,009.7 AA+ Asset-backed 130.8 132.5 AAA Total fixed income securities — Available for sale $ 5,570.2 $ 5,704.5 As at December 31, 2014 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 590.2 $ 594.7 AA Due after one year through five years 2,552.0 2,620.8 AA- Due after five years through ten years 1,023.5 1,059.9 A+ Due after ten years 97.7 111.0 A+ Subtotal 4,263.4 4,386.4 Non-agency commercial mortgage-backed 41.5 44.8 AA+ Agency mortgage-backed 1,016.7 1,055.3 AA+ Asset-backed 141.3 143.5 AAA Total fixed income securities — Available for sale $ 5,462.9 $ 5,630.0
Aggregate Fair Value and Gross Unrealized Loss by Type of Security	Gross Unrealized Loss. The following tables summarize as at September 30, 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: As at September 30, 2015 0-12 months Over 12 months Total Fair Market Value Gross Unrealized Loss Fair Market Value Gross Unrealized Loss Fair Market Value Gross Unrealized Loss Number of Securities ($ in millions) U.S. government $ 23.7 $ (0.3 ) $ 4.6 $ — $ 28.3 $ (0.3 ) 13 U.S. agency 1.3 — — — 1.3 — 1 Corporate 704.9 (7.4 ) 80.2 (1.4 ) 785.1 (8.8 ) 337 Non-U.S. government-backed corporate 16.1 — — — 16.1 — 6 Foreign government 124.0 (0.2 ) 2.7 (0.1 ) 126.7 (0.3 ) 18 Asset-backed 18.3 — 13.5 — 31.8 — 27 Agency mortgage-backed 150.1 (1.4 ) 61.9 (1.1 ) 212.0 (2.5 ) 62 Total fixed income securities — Available for sale 1,038.4 (9.3 ) 162.9 (2.6 ) 1,201.3 (11.9 ) 464 Total short-term investments — Available for sale 52.7 — — — 52.7 — 12 Total $ 1,091.1 $ (9.3 ) $ 162.9 $ (2.6 ) $ 1,254.0 $ (11.9 ) 476 As at December 31, 2014 0-12 months Over 12 months Total Fair Market Value Gross Unrealized Loss Fair Market Value Gross Unrealized Loss Fair Market Value Gross Unrealized Loss Number of Securities ($ 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 (0.1 ) 7 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 ) 1,213.3 (9.2 ) 428 Total short-term investments — Available for sale 4.6 — — — 4.6 — 3 Total $ 794.7 $ (3.0 ) $ 423.2 $ (6.2 ) $ 1,217.9 $ (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 and nine months ended September 30, 2015 and 2014 : For the Three Months Ended For the Nine Months Ended September 30, 2015 September 30, 2014 September 30, 2015 September 30, 2014 ($ in millions) ($ in millions) (Purchases) of fixed income securities — Available for sale $ (428.9 ) $ (522.8 ) $ (1,368.2 ) $ (1,440.9 ) (Purchases) of fixed income securities — Trading (111.5 ) (154.7 ) (387.0 ) (553.6 ) (Purchases) of equity securities — Trading (66.3 ) (121.2 ) (335.2 ) (240.1 ) Proceeds from sales and maturities of fixed income securities — Available for sale 350.4 481.0 1,211.9 1,496.9 Proceeds from sales and maturities of fixed income securities — Trading 89.9 143.5 354.4 515.7 Proceeds from sales of equity securities — Available for sale — 12.1 108.6 27.7 Proceeds from sales of equity securities — Trading 63.6 17.8 214.7 36.0 Net change in (payable)/receivable for securities (purchased)/sold 8.9 (7.0 ) 14.7 12.8 (Purchases) of short-term investments — Available for sale (9.3 ) (195.1 ) (118.6 ) (464.0 ) Proceeds from short-term investments — Available for sale 33.3 220.1 205.7 313.0 (Purchases) of short-term investments — Trading (22.5 ) (21.2 ) (38.2 ) (101.4 ) Proceeds from short-term investments — Trading 16.7 28.2 31.5 94.4 Net proceeds/(purchases) of catastrophe bonds — Trading (3.8 ) (1.7 ) (2.0 ) (26.0 ) Proceeds from other investments — — — 37.3 Net proceeds/(purchases) for the period $ (79.5 ) $ (121.0 ) $ (107.7 ) $ (292.2 )

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2015 and December 31, 2014 , respectively: As at September 30, 2015 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,138.1 $ — $ — $ 1,138.1 U.S. agency — 162.0 — 162.0 Municipal — 28.5 — 28.5 Corporate — 2,496.7 — 2,496.7 Non-U.S. government-backed corporate — 77.0 — 77.0 Foreign government 447.2 181.2 — 628.4 Asset-backed — 132.5 — 132.5 Non-agency commercial mortgage-backed — 31.6 — 31.6 Agency mortgage-backed — 1,009.7 — 1,009.7 Total fixed income securities available for sale, at fair value 1,585.3 4,119.2 — 5,704.5 Short-term investments available for sale, at fair value 143.9 32.4 — 176.3 Held for trading financial assets, at fair value U.S. government 13.5 — — 13.5 Municipal — 0.5 — 0.5 Corporate — 547.2 — 547.2 Foreign government 41.2 87.7 — 128.9 Asset-backed — 18.7 — 18.7 Bank loans — 83.1 — 83.1 Total fixed income securities trading, at fair value 54.7 737.2 — 791.9 Short-term investments trading, at fair value — 6.9 — 6.9 Equity investments trading, at fair value 696.3 — — 696.3 Catastrophe bonds trading, at fair value — 36.7 — 36.7 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 9.2 — 9.2 Liabilities under derivative contracts — interest rate swaps — — — — Liabilities under derivative contracts — foreign exchange contracts — (1.9 ) — (1.9 ) Loan notes issued by variable interest entities, at fair value — — (84.5 ) (84.5 ) Loan notes issued by variable interest entities, at fair value (classified as a current liability) — — (0.8 ) (0.8 ) Total $ 2,480.2 $ 4,939.7 $ (85.3 ) $ 7,334.6 There were no maturities or transfers between Level 1, Level 2 and Level 3 during the three and nine months ended September 30, 2015 . The Company settled $0.8 million and $67.8 million Level 3 liabilities in respect to the loan notes issued by the VIEs for the three and nine months ended September 30, 2015 , respectively. As at September 30, 2015 , there were no assets classified as Level 3 and the Company's Level 3 liabilities consisted of the loan notes issued by the VIEs. As at December 31, 2014 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,094.4 $ — $ — $ 1,094.4 U.S. agency — 197.4 — 197.4 Municipal — 31.5 — 31.5 Corporate — 2,319.4 — 2,319.4 Non-U.S. government-backed corporate — 78.0 — 78.0 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 — 1,055.3 — 1,055.3 Total fixed income securities available for sale, at fair value 1,550.9 4,079.1 — 5,630.0 Short-term investments available for sale, at fair value 229.3 29.0 — 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 36.1 104.0 — 140.1 Asset-backed — 14.7 — 14.7 Bank loans — 85.1 — 85.1 Total fixed income securities trading, at fair value 36.1 734.9 — 771.0 Short-term investments trading, at fair value 0.1 0.1 — 0.2 Equity investments trading, at fair value 616.0 — — 616.0 Catastrophe bonds trading, at fair value — 34.8 — 34.8 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 $ 2,542.3 $ 4,871.6 $ (138.6 ) $ 7,275.3
Reconciliation of Liabilities Using Level 3 Inputs	The following tables present 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 and nine months ended September 30, 2015 and 2014 : Reconciliation of Liabilities Using Level 3 Inputs Three Months Ended September 30, 2015 Three Months Ended September 30, 2014 ($ in millions) Balance at the beginning of the period (1) $ 77.8 $ 56.0 Distributed to third party (0.8 ) — Total change in fair value included in the statement of operations 8.3 8.5 Balance at the end of the period (1) $ 85.3 $ 64.5 Reconciliation of Liabilities Using Level 3 Inputs Nine Months Ended September 30, 2015 Nine Months Ended September 30, 2014 ($ in millions) Balance at the beginning of the period (1) $ 138.6 $ 50.0 Distributed to third party (67.8 ) — Total change in fair value included in the statement of operations 14.5 14.5 Balance at the end of the period (1) $ 85.3 $ 64.5 (1) The amount classified as other payables was $0.8 million and $67.9 million as at September 30, 2015 and December 31, 2014, respectively.
Pricing Sources Used in Pricing Fixed Income Investments	Pricing sources used in pricing fixed income investments as at September 30, 2015 and December 31, 2014 were as follows: As at September 30, 2015 As at December 31, 2014 Index providers 84 % 84 % Pricing services 11 11 Broker-dealers 5 5 Total 100 % 100 %
Summary of Securities Priced Using Pricing Information from Index Providers	A summary of securities priced using pricing information from index providers as at September 30, 2015 and December 31, 2014 is provided below: As at September 30, 2015 As at December 31, 2014 Fair Market Value Determined using Prices from Index Providers % of Total Fair Value by Security Type Fair Market Value Determined using Prices from Index Providers % of Total Fair Value by Security Type ($ in millions, except for percentages) U.S. government $ 1,144.1 99 % $ 1,044.4 95 % U.S. agency 149.8 92 % 186.9 95 % Municipal 11.4 39 % 13.7 42 % Corporate 2,921.4 96 % 2,731.1 96 % Non-U.S. government-backed corporate 34.0 44 % 48.7 62 % Foreign government 432.4 57 % 504.4 63 % Asset-backed 100.3 66 % 140.5 89 % Non-agency commercial mortgage-backed 24.8 79 % 44.8 100 % Agency mortgage-backed 629.7 62 % 680.6 64 % Total fixed income securities $ 5,447.9 84 % $ 5,395.1 84 % Equities 696.3 100 % 725.9 100 % Total fixed income securities and equity investments $ 6,144.2 85 % $ 6,121.0 86 %
Long-term Debt Issued by Silverton	The observable and unobservable inputs used to determine the fair value of the 2015 Loan Notes and 2014 Loan Notes as at September 30, 2015 and December 31, 2014 are presented in the table below: At September 30, 2015 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan notes held by third parties $ 85.3 (1) Internal Valuation Model Gross premiums written (O) $ 37.7 $ 39.8 Reserve for losses (U) $ 4.9 $ 4.9 Contract period (O) N/A 365 days Initial value of issuance (O) $ 70.2 $ 120.0 At December 31, 2014 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan notes held by third parties $ 138.6 (1) Internal Valuation Model Gross premiums written (O) $ — $ 40.0 Reserve for losses (U) $ — $ 4.6 Contract period (O) N/A 365 days Initial value of issuance (O) $ 120.0 $ 120.0 (1) The amount classified as other payables was $0.8 million and $67.9 million as at September 30, 2015 and December 31, 2014, respectively.

Derivative Contracts (Tables)

Derivative Contracts (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2015 and December 31, 2014 : As at September 30, 2015 As at December 31, 2014 Derivatives Not Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Interest Rate Swaps Liabilities under Derivative Contracts $ 951.3 $ — (1) $ 951.3 $ 0.1 (1) Foreign Exchange Contracts Derivatives at Fair Value $ 363.3 $ 9.2 $ 165.8 $ 7.9 Foreign Exchange Contracts Liabilities under Derivative Contracts $ 34.2 $ (1.1 ) $ 237.6 $ (10.5 ) (1) Net of $13.7 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 — $22.3 million ). As at September 30, 2015 As at December 31, 2014 Derivatives Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Liabilities under Derivative Contracts $ 28.2 $ (0.8 ) (1) $ 135.8 $ (3.8 ) (1) (1) Net of $0.8 million 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 and nine months ended September 30, 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 ASC 815 Location of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income September 30, 2015 September 30, 2014 ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives $ 12.9 $ (6.4 ) Interest Rate Swaps Change in Fair Value of Derivatives $ (2.8 ) $ 1.3 Amount of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income for the Nine Months Ended Derivatives Not Designated as Hedging Instruments Under ASC 815 Location of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income September 30, 2015 September 30, 2014 ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives $ 10.5 $ (4.5 ) Interest Rate Swaps Change in Fair Value of Derivatives $ (6.2 ) $ (4.1 ) 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 ASC 815 Location of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income September 30, 2015 September 30, 2014 ($ in millions) Foreign Exchange Contracts General, administrative and corporate expenses $ (0.8 ) $ — Foreign Exchange Contracts Net change from current period hedged transactions $ (0.5 ) $ — Amount of Income/(Loss) Nine Months Ended Derivatives Designated as Hedging Instruments Under ASC 815 Location of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income September 30, 2015 September 30, 2014 ($ in millions) Foreign Exchange Contracts General, administrative and corporate expenses $ (3.5 ) $ — Foreign Exchange Contracts Net change from current period hedged transactions $ 2.2 $ —

Deferred Policy Acquisition C30

Deferred Policy Acquisition Costs (Tables)	9 Months Ended
	Sep. 30, 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 and nine months ended September 30, 2015 and 2014 : Three Months Ended September 30, Nine Months Ended September 30, 2015 2014 2015 2014 ($ in millions) ($ in millions) Balance at the beginning of the period $ 349.0 $ 306.4 $ 299.0 $ 262.2 Acquisition costs deferred 129.8 110.7 413.2 375.8 Amortization of deferred policy acquisition costs (132.0 ) (115.5 ) (365.4 ) (336.4 ) Balance at the end of the period $ 346.8 $ 301.6 $ 346.8 $ 301.6

Reserves for Losses and Loss 31

Reserves for Losses and Loss Adjustment Expenses (Tables)	9 Months Ended
	Sep. 30, 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 nine months ended September 30, 2015 and twelve months ended December 31, 2014 : Nine Months Ended September 30, 2015 Twelve Months Ended December 31, 2014 ($ in millions) Provision for losses and LAE at the start of the year $ 4,750.8 $ 4,678.9 Less reinsurance recoverable (350.0 ) (332.7 ) Net loss and LAE at the start of the year 4,400.8 4,346.2 Net loss and LAE expenses (disposed) — (24.2 ) Provision for losses and LAE for claims incurred: Current year 1,129.8 1,411.6 Prior years (97.6 ) (104.1 ) Total incurred 1,032.2 1,307.5 Losses and LAE payments for claims incurred: Current year (93.4 ) (112.1 ) Prior years (718.7 ) (995.6 ) Total paid (812.1 ) (1,107.7 ) Foreign exchange (gains) (55.7 ) (121.0 ) Net losses and LAE reserves at period end 4,565.2 4,400.8 Plus reinsurance recoverable on unpaid losses at period end 348.7 350.0 Provision for losses and LAE at the end of the relevant period $ 4,913.9 $ 4,750.8

Capital Structure (Tables)

Capital Structure (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2015 and December 31, 2014 : As at September 30, 2015 As at December 31, 2014 Number $ in Thousands Number $ in Thousands Authorized share capital: Ordinary Shares 0.15144558¢ per share 969,629,030 1,469 969,629,030 1,469 Non-Voting Shares 0.15144558¢ per share 6,787,880 10 6,787,880 10 Preference Shares 0.15144558¢ per share 100,000,000 152 100,000,000 152 Total authorized share capital 1,631 1,631 Issued share capital: Issued ordinary shares of 0.15144558¢ per share 60,781,958 92 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 127 129
Summary of Ordinary Shares	The following table summarizes transactions in the Company’s ordinary shares during the nine months ended September 30, 2015 : Number of Ordinary Shares Ordinary shares in issue at December 31, 2014 62,017,368 Ordinary share transactions in the nine months ended September 30, 2015 Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or 2008 share purchase plan 546,117 Ordinary shares issued to non-employee directors 8,806 Ordinary shares repurchased (1,790,333 ) Ordinary shares in issue at September 30, 2015 60,781,958

Intangible Assets (Tables)

Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2015
Goodwill and Intangible Assets Disclosure [Abstract]
Summary of Changes in Intangible Assets	The following tables provide a summary of the Company’s intangible assets for the three and nine months ended September 30, 2015 and 2014 : Three Months Ended September 30, 2015 Three Months Ended September 30, 2014 Trade Mark Insurance Licenses Other Total Trade Mark Insurance Licenses Other Total ($ in millions) ($ in millions) Intangible Assets Beginning of the period $ 1.6 $ 16.6 $ — $ 18.2 $ 1.6 $ 16.6 $ — $ 18.2 Amortization — — — — — — — — End of the period $ 1.6 $ 16.6 $ — $ 18.2 $ 1.6 $ 16.6 $ — $ 18.2 Nine Months Ended September 30, 2015 Nine Months Ended September 30, 2014 Trade Mark Insurance Licenses Other Total Trade Mark Insurance Licenses Other Total ($ 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 — — — — — — (0.2 ) (0.2 ) End of the period $ 1.6 $ 16.6 $ — $ 18.2 $ 1.6 $ 16.6 $ — $ 18.2

Commitments and Contingent Li34

Commitments and Contingent Liabilities (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2015 and December 31, 2014 : As at September 30, 2015 As at December 31, 2014 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 1,177.1 $ 1,086.9 Third party 2,294.8 2,183.4 Letters of credit / guarantees (1) 726.6 778.7 Total restricted assets $ 4,198.5 $ 4,049.0 Total as percent of cash and invested assets 48.7 % 47.0 % (1) As at September 30, 2015 , the Company pledged funds of $712.3 million and £9.2 million ( December 31, 2014 — $764.4 million and £9.2 million ) as collateral for the secured letters of credit.
Amounts Outstanding under Operating Leases	Amounts outstanding under operating leases net of subleases as of September 30, 2015 were: 2015 2016 2017 2018 2019 Later Total ($ in millions) Operating Lease Obligations (1) $ 3.4 $ 10.1 $ 14.0 $ 13.6 $ 12.6 $ 97.9 $ 151.6 (1) In 2015, the Company entered into a new 16 year lease relating to the Company’s premises in New York.

Reclassifications from Accumu35

Reclassifications from Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income Reclassification (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Available for sale securities:
Realized gains on sale of securities	$ 2.1	$ 3.9	$ 37.7	$ 10.6
Realized (losses) on sale of securities	0	(0.3)	(3.9)	(4.7)
Net realized and unrealized investment gains reclassified to the statement of operations, gross of tax	2.1	3.6	33.8	5.9
Tax on net realized gains of securities	(0.3)	(0.1)	(0.7)	(0.2)
Total reclassifications from AOCI to the statement of operations, net of tax	1.8	3.5	33.1	5.7
Foreign currency translation adjustments, before tax	1.1	0	2.2	0
Foreign currency translation adjustments, Net income	1.1	0	2.2	0
Reclassification adjustment for net realized losses/(gains) on investments included in net income	(0.8)	0	(3.5)	0
Realized derivatives, Net income	(0.8)	0
Total reclassifications from AOCI to the statement of operations, net of tax, Net income	$ 2.1	$ 3.5	$ 31.8	$ 5.7

Earnings per Ordinary Share - C

Earnings per Ordinary Share - Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net income		$ 28.2	$ 37.4	$ 205.2	$ 288.6
Preference share dividends		(9.5)	(9.5)	(28.4)	(28.4)
Proportion due to non-controlling interest		(0.3)	0.1	(0.8)	0
Basic and diluted net income available to ordinary shareholders		$ 18.4	$ 28	$ 176	$ 260.2
Ordinary shares:
Basic weighted average ordinary shares		60,779,295	65,116,463	61,442,033	65,283,681
Weighted average effect of dilutive securities (in shares)	[1]	1,375,830	1,396,546	1,436,403	1,314,999
Total diluted weighted average ordinary shares		62,155,125	66,513,009	62,878,436	66,598,680
Earnings per ordinary share:
Basic (in usd per share)		$ 0.30	$ 0.43	$ 2.86	$ 3.99
Diluted (in usd per share)		$ 0.30	$ 0.42	$ 2.80	$ 3.91
[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 - S

Earnings per Ordinary Share - Summary of Declared Dividends (Details) - $ / shares	Oct. 28, 2015	Sep. 30, 2015
7.401% preference shares
Dividends Payable [Line Items]
Preference shares, rate		7.401%
7.250% preference shares
Dividends Payable [Line Items]
Preference shares, rate		7.25%
5.95% preference shares
Dividends Payable [Line Items]
Preference shares, rate		5.95%
Subsequent Event [Member] | Ordinary shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.21
Payable on:	Dec. 1, 2015
Record Date:	Nov. 13, 2015
Subsequent Event [Member] | 7.401% preference shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.462563
Payable on:	Jan. 1, 2016
Record Date:	Dec. 15, 2015
Subsequent Event [Member] | 7.250% preference shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.4531
Payable on:	Jan. 1, 2016
Record Date:	Dec. 15, 2015
Subsequent Event [Member] | 5.95% preference shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.3719
Payable on:	Jan. 1, 2016
Record Date:	Dec. 15, 2015

Segment Reporting - Narrative (

Segment Reporting - Narrative (Details)	9 Months Ended
	Sep. 30, 2015Segment
Segment Reporting [Abstract]
Number of business segments	2

Segment Reporting - Summary of

Segment Reporting - Summary of Gross and Net Written and Earned Premiums, Underwriting Results, Ratios and Reserves for Each of Company's Business Segments (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2015		Sep. 30, 2014		Sep. 30, 2015		Sep. 30, 2014		Dec. 31, 2014	Dec. 31, 2013
Underwriting Revenues
Gross written premiums	$ 720.5		$ 652.5		$ 2,362.5		$ 2,287.3
Net written premiums	651.8		577.3		2,059.4		1,961.2
Gross earned premiums	733.6		694.9		2,124.9		2,041.2
Net earned premiums	640.6		610.4		1,843.6		1,793.1
Underwriting Expenses
Losses and loss adjustment expenses	365.6		342.7		1,032.2		967.9
Amortization of deferred policy acquisition costs	132		115.5		365.4		336.4
General and administrative expenses	86		84		254.3		249.6
Underwriting income	57		68.2		191.7		239.2
Corporate expenses	(14.5)		(35.8)		(43.8)		(74.6)
Net investment income	45		48		139.1		143.6
Realized and unrealized investment gains	10.7		1.1		81.6		53.6
Realized and unrealized investment losses	(51.9)		(1.3)		(95.2)		(8.9)
Change in fair value of loan notes issued by variable interest entities	(8.3)		(8.5)		(14.5)		(14.5)
Change in fair value of derivatives	10.1		(5.1)		4.3		(8.6)
Interest expense on long term debt	(7.4)		(7.4)		(22.1)		(22.1)
Net realized and unrealized foreign exchange (losses)	(8.4)		(21.2)		(26.4)		(10.4)
Other income	(2.3)		1		0.4		4.8
Other expenses	0		(0.3)		(0.8)		(2.2)
Income from operations before income tax	30		38.7		214.3		299.9
Net reserves for loss and loss adjustment expenses	$ 4,565.2		$ 4,402.6		$ 4,565.2		$ 4,402.6		$ 4,400.8	$ 4,346.2
Ratios
Loss ratio	57.10%		56.10%		56.00%		54.00%
Policy acquisition expense ratio	20.60%		18.90%		19.80%		18.80%
General and administrative expense ratio	15.70%	[1]	19.60%	[2]	16.20%	[3]	18.10%	[4]
Expense ratio	36.30%		38.50%		36.00%		36.90%
Combined ratio	93.40%		94.60%		92.00%		90.90%
Reinsurance
Underwriting Revenues
Gross written premiums	$ 316.6		$ 256.9		$ 1,062.1		$ 1,027.5
Net written premiums	294.7		250.9		975		980.4
Gross earned premiums	304.6		291		857.6		859.2
Net earned premiums	284.6		279.6		802.3		825.1
Underwriting Expenses
Losses and loss adjustment expenses	169.9		132		391.7		367.4
Amortization of deferred policy acquisition costs	64.8		52.1		168.6		152.3
General and administrative expenses	34.7		38.4		102.5		107
Underwriting income	15.2		57.1		139.5		198.4
Net reserves for loss and loss adjustment expenses	$ 2,469.6		$ 2,554.2		$ 2,469.6		$ 2,554.2
Ratios
Loss ratio	59.70%		47.20%		48.80%		44.50%
Policy acquisition expense ratio	22.80%		18.60%		21.00%		18.50%
General and administrative expense ratio	12.20%		13.70%		12.80%		13.00%
Expense ratio	35.00%		32.30%		33.80%		31.50%
Combined ratio	94.70%		79.50%		82.60%		76.00%
Insurance
Underwriting Revenues
Gross written premiums	$ 403.9		$ 395.6		$ 1,300.4		$ 1,259.8
Net written premiums	357.1		326.4		1,084.4		980.8
Gross earned premiums	429		403.9		1,267.3		1,182
Net earned premiums	356		330.8		1,041.3		968
Underwriting Expenses
Losses and loss adjustment expenses	195.7		210.7		640.5		600.5
Amortization of deferred policy acquisition costs	67.2		63.4		196.8		184.1
General and administrative expenses	51.3		45.6		151.8		142.6
Underwriting income	41.8		11.1		52.2		40.8
Net reserves for loss and loss adjustment expenses	$ 2,095.6		$ 1,848.4		$ 2,095.6		$ 1,848.4
Ratios
Loss ratio	55.00%		63.70%		61.50%		62.00%
Policy acquisition expense ratio	18.90%		19.20%		18.90%		19.00%
General and administrative expense ratio	14.40%		13.80%		14.60%		14.70%
Expense ratio	33.30%		33.00%		33.50%		33.70%
Combined ratio	88.30%		96.70%		95.00%		95.70%
[1]	The general and administrative expense ratio in the total column includes corporate expenses.
[2]	The general and administrative expense ratio in the total column includes corporate expenses.
[3]	The general and administrative expense ratio in the total column includes corporate expenses.
[4]	The general and administrative expense ratio in the total column includes corporate expenses.

Investments - Summary of Invest

Investments - Summary of Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	$ 49.1	$ 50.1	$ 149.2	$ 151.5
Investment expenses	(4.1)	(2.1)	(10.1)	(7.9)
Net investment income	45	48	139.1	143.6
Fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	36.1	37.6	106.8	113.4
Fixed income securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	7.2	6.6	21.1	19.8
Short-term investments — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	0.3	0.4	0.9	1
Fixed term deposits (included in cash and cash equivalents)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	0.5	0.5	2.5	2.4
Equity securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	0	1	0.1	3.5
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	4.6	3.5	16.5	10.5
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	$ 0.4	$ 0.5	$ 1.3	$ 0.9

Investments - Net Realized and

Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	$ 2,100,000	$ 3,900,000	$ 37,700,000	$ 10,600,000
Fixed income maturities — gross realized (losses)	0	(300,000)	(3,900,000)	(4,700,000)
Other than Temporary Impairment Losses, Investments	0	0	0	(700,000)
Catastrophe bonds	700,000	500,000	(100,000)	500,000
Net change in gross unrealized gains	(38,800,000)	(10,000,000)	(61,500,000)	24,200,000
Total net realized and unrealized investment (losses)/gains recorded in the statement of operations	(41,200,000)	(200,000)	(13,600,000)	44,700,000
Change in available for sale net unrealized gains:
Fixed income securities	(22,700,000)	28,500,000	27,400,000	(25,000,000)
Equity securities	0	(4,500,000)	(27,400,000)	(2,300,000)
Total change in pre-tax available for sale unrealized gains	22,700,000	(33,000,000)	(54,800,000)	22,700,000
Change in taxes	800,000	8,100,000	2,100,000	(3,800,000)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	23,500,000	(24,900,000)	(52,700,000)	18,900,000
Fixed income maturities - Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	2,400,000	1,700,000	9,700,000	8,100,000
Fixed income maturities — gross realized (losses)	(300,000)	(1,000,000)	(1,400,000)	(5,500,000)
Equity Securities - Available for Sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities — gross realized gains	0	4,200,000	31,900,000	9,700,000
Equity securities — gross realized (losses)	0	0	(3,000,000)	0
Fixed Income Maturities - Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	500,000	1,900,000	3,800,000	6,600,000
Fixed income maturities — gross realized (losses)	(1,200,000)	(200,000)	(3,800,000)	(2,400,000)
Equity Securities - Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities — gross realized gains	7,800,000	2,800,000	36,200,000	4,500,000
Equity securities — gross realized (losses)	$ (12,300,000)	$ (100,000)	$ (25,400,000)	$ (300,000)

Investments - Cost, Gross Unrea

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities, Short-Term Investments and Equities (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	$ 5,746.5	$ 5,803.7
Gross Unrealized Gains	146.2	203.7
Gross Unrealized Losses	(11.9)	(9.2)
Fair Market Value	5,880.8	5,998.2
Total fixed income securities — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	5,570.2	5,462.9
Gross Unrealized Gains	146.2	176.3
Gross Unrealized Losses	(11.9)	(9.2)
Fair Market Value	5,704.5	5,630
Total short-term investments — Available for sale
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	176.3	258.2
Gross Unrealized Gains	0	0.1
Gross Unrealized Losses	0	0
Fair Market Value	176.3	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	1,114.5	1,074.2
Gross Unrealized Gains	23.9	21.5
Gross Unrealized Losses	(0.3)	(1.3)
Fair Market Value	1,138.1	1,094.4
U.S. agency
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	156.1	190
Gross Unrealized Gains	5.9	7.5
Gross Unrealized Losses	0	(0.1)
Fair Market Value	162	197.4
Municipal
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	26.7	29.1
Gross Unrealized Gains	1.8	2.4
Gross Unrealized Losses	0	0
Fair Market Value	28.5	31.5
Corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	2,443	2,244.7
Gross Unrealized Gains	62.5	79.9
Gross Unrealized Losses	(8.8)	(5.2)
Fair Market Value	2,496.7	2,319.4
Non-U.S. government-backed corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	76.1	76.8
Gross Unrealized Gains	0.9	1.2
Gross Unrealized Losses	0	0
Fair Market Value	77	78
Foreign government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	615.1	648.6
Gross Unrealized Gains	13.6	17.3
Gross Unrealized Losses	(0.3)	(0.2)
Fair Market Value	628.4	665.7
Asset-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	130.8	141.3
Gross Unrealized Gains	1.7	2.4
Gross Unrealized Losses	0	(0.2)
Fair Market Value	132.5	143.5
Non-agency commercial mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	29.9	41.5
Gross Unrealized Gains	1.7	3.3
Gross Unrealized Losses	0	0
Fair Market Value	31.6	44.8
Agency mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	978	1,016.7
Gross Unrealized Gains	34.2	40.8
Gross Unrealized Losses	(2.5)	(2.2)
Fair Market Value	$ 1,009.7	$ 1,055.3

Investments - Cost, Gross Unr43

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	$ 791.6	$ 760.9
Fair Market Value	791.9	771
Short-term investments, trading at fair value, amortized cost	6.9	0.2
U.S. government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	13.4
Gross Unrealized Gains	0.1
Gross Unrealized Losses	0
Fair Market Value	13.5
U.S. agency
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost		0.2
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Market Value		0.2
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	545.4	520.9
Gross Unrealized Gains	7.9	11.7
Gross Unrealized Losses	(6.1)	(2.8)
Fair Market Value	547.2	529.8
Foreign government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	129.1	137.3
Gross Unrealized Gains	2.2	4.3
Gross Unrealized Losses	(2.4)	(1.5)
Fair Market Value	128.9	140.1
Asset-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	18.7	14.6
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	(0.1)	0
Fair Market Value	18.7	14.7
Bank loans
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	84.5	86.8
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	(1.5)	(1.7)
Fair Market Value	83.1	85.1
Total fixed income securities — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains	10.4	16.1
Gross Unrealized Losses	(10.1)	(6)
Fair Market Value	791.9	771
Cost or Amortized Cost, Total fixed income securities, Trading	791.6	760.9
Total short-term investments — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	6.9	0.2
Short-term investments, trading at fair value, amortized cost	6.9	0.2
Total equity securities — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	717.2	585.2
Gross Unrealized Gains	39.7	55.5
Gross Unrealized Losses	(60.6)	(24.7)
Fair Market Value	696.3	616
Total catastrophe bonds — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	36.3	34.4
Gross Unrealized Gains	0.5	0.4
Gross Unrealized Losses	(0.1)	0
Fair Market Value	36.7	34.8
Total
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,552	1,380.7
Gross Unrealized Gains	50.6	72
Gross Unrealized Losses	(70.8)	(30.7)
Fair Market Value	$ 1,531.8	$ 1,422

Investments - Other Investments

Investments - Other Investments (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening and closing undistributed value of investment	$ 8.7
Initial investment	0.8	$ 0
Closing value of investment as at June 30, 2015	9.5
MVI
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening and closing undistributed value of investment	0
Initial investment	0.8
Closing value of investment as at June 30, 2015	0.8
Chaspark
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening and closing undistributed value of investment	8.7
Initial investment	0
Closing value of investment as at June 30, 2015	$ 8.7

Investments - Summary of Fixed

Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 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	$ 5,570.2	$ 5,462.9
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,746.5	5,803.7
Available for sale investments in fixed income maturities, Fair Market Value	5,704.5	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	5,570.2	5,462.9
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	29.9	41.5
Available for sale investments in fixed income maturities, Fair Market Value	$ 31.6	$ 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	$ 978	$ 1,016.7
Available for sale investments in fixed income maturities, Fair Market Value	$ 1,009.7	$ 1,055.3
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	$ 130.8	$ 141.3
Available for sale investments in fixed income maturities, Fair Market Value	$ 132.5	$ 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	$ 647.7	$ 590.2
Due after one year through five years, Cost or Amortized Cost	2,638.8	2,552
Due after five years through ten years, Cost or Amortized Cost	1,055.3	1,023.5
Due after ten years, Cost or Amortized Cost	89.7	97.7
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	4,431.5	4,263.4
Due one year or less, Fair Market Value	651.6	594.7
Due after one year through five years, Fair Market Value	2,704.6	2,620.8
Due after five years through ten years, Fair Market Value	1,077.7	1,059.9
Due after ten years, Fair Market Value	96.8	111
Available for sale investments in fixed income maturities, Fair Market Value	$ 4,530.7	$ 4,386.4
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 Va

Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Details) $ in Millions	Sep. 30, 2015USD ($)Security	Dec. 31, 2014USD ($)Security
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1,091.1	$ 794.7
0-12 months, Gross Unrealized Loss	(9.3)	(3)
Over 12 months, Fair Market Value	162.9	423.2
Over 12 months, Gross Unrealized Loss	(2.6)	(6.2)
Total, Fair Market Value	1,254	1,217.9
Total, Gross Unrealized Loss	$ (11.9)	$ (9.2)
Number of Securities | Security	476	431
Fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1,038.4	$ 790.1
0-12 months, Gross Unrealized Loss	(9.3)	(3)
Over 12 months, Fair Market Value	162.9	423.2
Over 12 months, Gross Unrealized Loss	(2.6)	(6.2)
Total, Fair Market Value	1,201.3	1,213.3
Total, Gross Unrealized Loss	$ (11.9)	$ (9.2)
Number of Securities | Security	464	428
Total short-term investments — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 52.7	$ 4.6
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	52.7	4.6
Total, Gross Unrealized Loss	$ 0	$ 0
Number of Securities | Security	12	3
U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 23.7	$ 166.1
0-12 months, Gross Unrealized Loss	(0.3)	(0.5)
Over 12 months, Fair Market Value	4.6	79.4
Over 12 months, Gross Unrealized Loss	0	(0.8)
Total, Fair Market Value	28.3	245.5
Total, Gross Unrealized Loss	$ (0.3)	$ (1.3)
Number of Securities | Security	13	39
U.S. agency
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1.3	$ 25.1
0-12 months, Gross Unrealized Loss	0	0
Over 12 months, Fair Market Value	0	4.9
Over 12 months, Gross Unrealized Loss	0	(0.1)
Total, Fair Market Value	1.3	30
Total, Gross Unrealized Loss	$ 0	$ (0.1)
Number of Securities | Security	1	7
Corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 704.9	$ 459.4
0-12 months, Gross Unrealized Loss	(7.4)	(2.1)
Over 12 months, Fair Market Value	80.2	171.3
Over 12 months, Gross Unrealized Loss	(1.4)	(3.1)
Total, Fair Market Value	785.1	630.7
Total, Gross Unrealized Loss	$ (8.8)	$ (5.2)
Number of Securities | Security	337	274
Non-U.S. government-backed corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 16.1	$ 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	16.1	0.7
Total, Gross Unrealized Loss	$ 0	$ 0
Number of Securities | Security	6	1
Foreign government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 124	$ 30.4
0-12 months, Gross Unrealized Loss	(0.2)	0
Over 12 months, Fair Market Value	2.7	44.2
Over 12 months, Gross Unrealized Loss	(0.1)	(0.2)
Total, Fair Market Value	126.7	74.6
Total, Gross Unrealized Loss	$ (0.3)	$ (0.2)
Number of Securities | Security	18	16
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 18.3	$ 43.7
0-12 months, Gross Unrealized Loss	0	(0.1)
Over 12 months, Fair Market Value	13.5	11.7
Over 12 months, Gross Unrealized Loss	0	(0.1)
Total, Fair Market Value	31.8	55.4
Total, Gross Unrealized Loss	$ 0	$ (0.2)
Number of Securities | Security	27	43
Agency mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 150.1	$ 64.7
0-12 months, Gross Unrealized Loss	(1.4)	(0.3)
Over 12 months, Fair Market Value	61.9	111.7
Over 12 months, Gross Unrealized Loss	(1.1)	(1.9)
Total, Fair Market Value	212	176.4
Total, Gross Unrealized Loss	$ (2.5)	$ (2.2)
Number of Securities | Security	62	48

Investments - Analysis of Inves

Investments - Analysis of Investment Purchases/Sales and Maturities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Gain (Loss) on Investments [Line Items]
(Purchases) of fixed income securities — Available for sale			$ (1,368.2)	$ (1,440.9)
(Purchases) of fixed income securities — Trading			(387)	(553.6)
(Purchases) of equity securities — Trading			(335.2)	(240.1)
Proceeds from sales and maturities of fixed income securities — Available for sale	$ 350.4	$ 481	1,211.9	1,496.9
Proceeds from sales and maturities of fixed income securities — Trading	89.9	143.5	354.4	515.7
Proceeds from sales of equity securities — Available for sale			108.6	27.7
Proceeds from sales of equity securities — Trading			214.7	36
Net change in (payable)/receivable for securities (purchased)/sold	8.9	(7)	14.7	12.8
(Purchases) of short-term investments — Available for sale			(118.6)	(464)
Proceeds from Available-for-sale Securities, Short-term			205.7	313
(Purchases) of short-term investments — Trading			(38.2)	(101.4)
Proceeds from Sale of Short-term Investments			31.5	94.4
Net (purchases) of catastrophe bonds — Trading			(2)	(26)
Proceeds from other investments	0	0	0	37.3
Net proceeds/(purchases) for the period	(79.5)	(121)	(107.7)	(292.2)
Available for sale
Gain (Loss) on Investments [Line Items]
(Purchases) of fixed income securities — Available for sale	(428.9)	(522.8)	(1,368.2)	(1,440.9)
Proceeds from sales of equity securities — Available for sale	0	12.1	108.6	27.7
(Purchases) of short-term investments — Available for sale	(9.3)	(195.1)	(118.6)	(464)
Proceeds from Available-for-sale Securities, Short-term	33.3	220.1	205.7	313
Trading
Gain (Loss) on Investments [Line Items]
(Purchases) of fixed income securities — Trading	(111.5)	(154.7)	(387)	(553.6)
(Purchases) of equity securities — Trading	(66.3)	(121.2)	(335.2)	(240.1)
Proceeds from sales of equity securities — Trading	63.6	17.8	214.7	36
(Purchases) of short-term investments — Trading	(22.5)	(21.2)	(38.2)	(101.4)
Proceeds from Sale of Short-term Investments	16.7	28.2	31.5	94.4
Net (purchases) of catastrophe bonds — Trading	$ (3.8)	$ (1.7)	$ (2)	$ (26)

Investments - Narrative (Detail

Investments - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015USD ($)	Sep. 30, 2014USD ($)	Sep. 30, 2015USD ($)security	Sep. 30, 2014USD ($)	Dec. 31, 2014USD ($)security	Oct. 02, 2012
Investment Securities [Line Items]
Other than Temporary Impairment Losses, Investments	$ 0	$ 0	$ 0	$ (700,000)
Fair Market Value	791,900,000		791,900,000		$ 771,000,000
Catastrophe bonds	36,700,000		36,700,000		34,800,000
Investment funded	1,800,000		$ 1,800,000		$ 2,500,000
Number of municipal holdings | security			2		2
Bank loans
Investment Securities [Line Items]
Fair Market Value	$ 83,100,000		$ 83,100,000		$ 85,100,000
Chaspark
Investment Securities [Line Items]
Percentage of ownership acquired						58.50%

Variable Interest Entities - Na

Variable Interest Entities - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2015USD ($)Investmentvariable_interest_entity	Sep. 30, 2014USD ($)	Sep. 30, 2015USD ($)Investmentvariable_interest_entity	Sep. 30, 2014USD ($)	Dec. 31, 2014USD ($)	Dec. 27, 2013USD ($)	Oct. 02, 2012
Variable Interest Entity [Line Items]
Number of investments in variable interest entities | Investment	2		2
Number of variable interest entities | variable_interest_entity	2		2
Total aggregate unpaid balance	$ 549,200,000		$ 549,200,000		$ 549,100,000
Silverton 2,015
Variable Interest Entity [Line Items]
Loans issued					85,000,000
Loan notes issued by variable interest entities, at fair value	84,500,000		84,500,000		$ 70,000,000
Chaspark
Variable Interest Entity [Line Items]
Percentage of ownership acquired							58.50%
Change in fair value of investment	$ 0	$ 0	$ 0	$ 0
Silverton 2,014
Variable Interest Entity [Line Items]
Loans issued						$ 65,000,000
Percentage of interests acquired	100.00%		100.00%
Silverton 2014 | Third party
Variable Interest Entity [Line Items]
Loans issued						$ 50,000,000
Silverton 2014 | Loan Notes 2014
Variable Interest Entity [Line Items]
Fair value of 2014 Loan Notes	$ 1,000,000		$ 1,000,000
Silverton 2014 | Silverton Loan Notes
Variable Interest Entity [Line Items]
Repayments of debt			88,100,000
Silverton 2014 | Silverton Loan Notes | Aspen Holdings and Third Parties
Variable Interest Entity [Line Items]
Total aggregate unpaid balance	1,000,000		1,000,000
Silverton 2014 | Silverton Loan Notes | Aspen Holdings
Variable Interest Entity [Line Items]
Maximum loss exposure, amount	233,040.372441354993498		233,040.372441354993498
Silverton 2014 | Investor | Loan Notes 2014
Variable Interest Entity [Line Items]
Fair value of 2014 Loan Notes	800,000		800,000
Repayments of debt			67,800,000
Total aggregate unpaid balance	800,000		800,000
Long-term debt, current maturities	800,000		800,000
Silverton 2015 | Silverton Loan Notes | Aspen Holdings
Variable Interest Entity [Line Items]
Maximum loss exposure, amount	18,500,000		18,500,000
Silverton 2015 | Loan Notes 2015
Variable Interest Entity [Line Items]
Fair value of 2014 Loan Notes	103,000,000		103,000,000
Silverton 2015 | Loan Notes 2015 | Aspen Holdings
Variable Interest Entity [Line Items]
Fair value of 2014 Loan Notes	$ 103,000,000		$ 103,000,000

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended
		Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value				$ 36.3	$ 34.4
Derivatives at fair value				9.2	8
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value				36.7
Loan notes issued by variable interest entities, at fair value				(84.5)	(70.7)
Loan notes issued by variable interest entities, at fair value (classified as a current liability)				(0.8)	(67.9)
Total				7,334.6	7,275.3
Recurring | Short-term investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure				6.9	0.2
Recurring | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure				696.3	616
Recurring | Derivatives at fair value, assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value				9.2	7.9
Recurring | Derivatives at fair value, assets | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value				0	0.1
Recurring | Liabilities under derivative contracts — foreign exchange contracts | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value				(1.9)	(14.3)
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 Disclosure				1,138.1	1,094.4
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 Disclosure				162	197.4
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 Disclosure				28.5	31.5
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 Disclosure				2,496.7	2,319.4
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 Disclosure				77	78
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 Disclosure				628.4	665.7
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 Disclosure				132.5	143.5
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 Disclosure				31.6	44.8
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 Disclosure				1,009.7	1,055.3
Fixed income securities — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure				5,704.5	5,630
Short-term investments — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure				176.3	258.3
Equity investments available for sale, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure					109.9
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 Disclosure				791.9	771
Catastrophe bonds trading, at fair value					34.8
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 Disclosure				13.5	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 Disclosure					0.2
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 Disclosure				0.5	1.1
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 Disclosure				547.2	529.8
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 Disclosure				128.9	140.1
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 Disclosure				18.7	14.7
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 Disclosure				83.1	85.1
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				2,480.2	2,542.3
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 Disclosure				0	0.1
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 Disclosure				696.3	616
Level 1 | Recurring | Derivatives at fair value, assets | 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, assets | 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 | 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 Disclosure				1,138.1	1,094.4
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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				447.2	456.5
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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				1,585.3	1,550.9
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 Disclosure				143.9	229.3
Level 1 | Equity investments available for sale, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure					109.9
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 Disclosure				54.7	36.1
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 Disclosure				13.5	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 Disclosure					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 Disclosure				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 Disclosure				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 Disclosure				41.2	36.1
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 Disclosure				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 Disclosure				0	0
Level 2 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value				36.7
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				4,939.7	4,871.6
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 Disclosure				6.9	0.1
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 Disclosure				0	0
Level 2 | Recurring | Derivatives at fair value, assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value				9.2	7.9
Level 2 | Recurring | Derivatives at fair value, assets | Interest Rate Swaps
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value				0	0.1
Level 2 | Recurring | Liabilities under derivative contracts — foreign exchange contracts | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value				(1.9)	(14.3)
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 Disclosure				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 Disclosure				162	197.4
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 Disclosure				28.5	31.5
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 Disclosure				2,496.7	2,319.4
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 Disclosure				77	78
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 Disclosure				181.2	209.2
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 Disclosure				132.5	143.5
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 Disclosure				31.6	44.8
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 Disclosure				1,009.7	1,055.3
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 Disclosure				4,119.2	4,079.1
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 Disclosure				32.4	29
Level 2 | Equity investments available for sale, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure					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 Disclosure				737.2	734.9
Catastrophe bonds trading, at fair value					34.8
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 Disclosure				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 Disclosure					0.2
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 Disclosure				0.5	1.1
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 Disclosure				547.2	529.8
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 Disclosure				87.7	104
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 Disclosure				18.7	14.7
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 Disclosure				83.1	85.1
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				(84.5)	(70.7)
Loan notes issued by variable interest entities, at fair value (classified as a current liability)				(0.8)	(67.9)
Total				(85.3)	(138.6)
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 Disclosure				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 Disclosure				0	0
Level 3 | Recurring | Derivatives at fair value, assets | 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, assets | 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 | 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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				0	0
Level 3 | Equity investments available for sale, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure					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 Disclosure				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 Disclosure				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 Disclosure					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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				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 Disclosure				$ 0	0
Silverton 2014 | Level 3 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proceeds from Equity Method Investment, Dividends or Distributions		$ 0	$ 0
Total	[1]				$ 138.6
[1]	The amount classified as other payables was $0.8 million and $67.9 million as at September 30, 2015 and December 31, 2014, respectively.

Fair Value Measurements - Recon

Fair Value Measurements - Reconciliation of Liabilities Using Level 3 Inputs (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Loan notes issued by variable interest entities, at fair value (classified as a current liability)		$ (0.8)		$ (0.8)		$ (67.9)
Level 3
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Loan notes issued by variable interest entities, at fair value (classified as a current liability)		(0.8)		(0.8)		$ (67.9)
Level 3 | Silverton 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements		0.8		67.8
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	[1]	77.8
Total change in fair value included in the statement of operations		8.3		14.5
Ending balance	[1]	$ 85.3		85.3
Level 3 | Silverton 2104
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements			$ 0		$ 0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	[1]		56	$ 138.6	50
Total change in fair value included in the statement of operations			8.5		14.5
Ending balance	[1]		$ 64.5		$ 64.5
[1]	The amount classified as other payables was $0.8 million and $67.9 million as at September 30, 2015 and December 31, 2014, respectively.

Fair Value Measurements - Prici

Fair Value Measurements - Pricing Sources Used in Pricing Fixed Income Investments (Details)	Sep. 30, 2015	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Index providers	84.00%	84.00%
Pricing services	11.00%	11.00%
Broker-dealers	5.00%	5.00%
Total	100.00%	100.00%

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 6,144.2	$ 6,121
% of Total Fair Value by Security Type	85.00%	86.00%
Fixed Income Maturities
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 5,447.9	$ 5,395.1
% of Total Fair Value by Security Type	84.00%	84.00%
Fixed Income Maturities | U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 1,144.1	$ 1,044.4
% 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	$ 149.8	$ 186.9
% of Total Fair Value by Security Type	92.00%	95.00%
Fixed Income Maturities | Municipal
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 11.4	$ 13.7
% of Total Fair Value by Security Type	39.00%	42.00%
Fixed Income Maturities | Corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 2,921.4	$ 2,731.1
% of Total Fair Value by Security Type	96.00%	96.00%
Fixed Income Maturities | Foreign Corporate Debt Securities [Member]
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 34	$ 48.7
% of Total Fair Value by Security Type	44.00%	62.00%
Fixed Income Maturities | Foreign government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 432.4	$ 504.4
% of Total Fair Value by Security Type	57.00%	63.00%
Fixed Income Maturities | Asset-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 100.3	$ 140.5
% of Total Fair Value by Security Type	66.00%	89.00%
Fixed Income Maturities | Non-agency commercial mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 24.8	$ 44.8
% of Total Fair Value by Security Type	79.00%	100.00%
Fixed Income Maturities | Agency mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 629.7	$ 680.6
% of Total Fair Value by Security Type	62.00%	64.00%
Equities
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 696.3	$ 725.9
% of Total Fair Value by Security Type	100.00%	100.00%

Fair Value Measurements - Loan

Fair Value Measurements - Loan Notes Issued by Variable Interest Entities (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds, trading at fair value, cost		$ 36.3		$ 36.3		$ 34.4
Gross written premiums		720.5	$ 652.5	2,362.5	$ 2,287.3
Derivatives at fair value		9.2		9.2		8
Level 3 | Low
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums				37.7		0
Reserve for losses				4.9		0
Loan notes held by third parties		70.2		70.2		120
Level 3 | High
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums				39.8		40
Reserve for losses				$ 4.9		$ 4.6
Contract period				365 days		365 days
Loan notes held by third parties		120		$ 120		$ 120
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds, trading at fair value, cost		36.7		36.7
Loan notes held by third parties		84.5		84.5		70.7
Loan notes issued by variable interest entities, at fair value (classified as a current liability)		(0.8)		(0.8)		(67.9)
Net change from current period hedged transaction		(7,334.6)		(7,334.6)		(7,275.3)
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds, trading at fair value, cost		0		0
Loan notes held by third parties		0		0		0
Loan notes issued by variable interest entities, at fair value (classified as a current liability)		0		0		0
Net change from current period hedged transaction		(2,480.2)		(2,480.2)		(2,542.3)
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds, trading at fair value, cost		36.7		36.7
Loan notes held by third parties		0		0		0
Loan notes issued by variable interest entities, at fair value (classified as a current liability)		0		0		0
Net change from current period hedged transaction		(4,939.7)		(4,939.7)		(4,871.6)
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds, trading at fair value, cost		0		0
Loan notes held by third parties		84.5		84.5		70.7
Loan notes issued by variable interest entities, at fair value (classified as a current liability)		(0.8)		(0.8)		(67.9)
Net change from current period hedged transaction		85.3		85.3		138.6
Fixed income securities — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		5,704.5		5,704.5		5,630
Fixed income securities — Available for sale | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		1,585.3		1,585.3		1,550.9
Fixed income securities — Available for sale | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		4,119.2		4,119.2		4,079.1
Fixed income securities — Available for sale | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Total short-term investments — Available for sale | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		176.3		176.3		258.3
Total short-term investments — Available for sale | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		143.9		143.9		229.3
Total short-term investments — Available for sale | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		32.4		32.4		29
Total short-term investments — Available for sale | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Equity investments available for sale, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						109.9
Equity investments available for sale, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						109.9
Equity investments available for sale, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						0
Equity investments available for sale, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						0
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 Disclosure		791.9		791.9		771
Catastrophe bonds, trading at fair value, cost						34.8
Held for trading financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		54.7		54.7		36.1
Catastrophe bonds, trading at fair value, cost						0
Held for trading financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		737.2		737.2		734.9
Catastrophe bonds, trading at fair value, cost						34.8
Held for trading financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Catastrophe bonds, trading at fair value, cost						0
U.S. government | Available for sale financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		1,138.1		1,138.1		1,094.4
U.S. government | Available for sale financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		1,138.1		1,138.1		1,094.4
U.S. government | Available for sale financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
U.S. government | Available for sale financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
U.S. government | 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 Disclosure		13.5		13.5		0
U.S. government | Held for trading financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		13.5		13.5		0
U.S. government | Held for trading financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
U.S. government | Held for trading financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
U.S. agency | Available for sale financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		162		162		197.4
U.S. agency | Available for sale financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
U.S. agency | Available for sale financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		162		162		197.4
U.S. agency | Available for sale financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
U.S. agency | 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 Disclosure						0.2
U.S. agency | Held for trading financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						0
U.S. agency | Held for trading financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						0.2
U.S. agency | Held for trading financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure						0
Municipal | Available for sale financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		28.5		28.5		31.5
Municipal | Available for sale financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Municipal | Available for sale financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		28.5		28.5		31.5
Municipal | Available for sale financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Municipal | 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 Disclosure		0.5		0.5		1.1
Municipal | Held for trading financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Municipal | Held for trading financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0.5		0.5		1.1
Municipal | Held for trading financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Corporate | Available for sale financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		2,496.7		2,496.7		2,319.4
Corporate | Available for sale financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Corporate | Available for sale financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		2,496.7		2,496.7		2,319.4
Corporate | Available for sale financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Corporate | 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 Disclosure		547.2		547.2		529.8
Corporate | Held for trading financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Corporate | Held for trading financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		547.2		547.2		529.8
Corporate | Held for trading financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Foreign government | Available for sale financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		628.4		628.4		665.7
Foreign government | Available for sale financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		447.2		447.2		456.5
Foreign government | Available for sale financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		181.2		181.2		209.2
Foreign government | Available for sale financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Foreign government | 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 Disclosure		128.9		128.9		140.1
Foreign government | Held for trading financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		41.2		41.2		36.1
Foreign government | Held for trading financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		87.7		87.7		104
Foreign government | Held for trading financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Asset-backed | Available for sale financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		132.5		132.5		143.5
Asset-backed | Available for sale financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Asset-backed | Available for sale financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		132.5		132.5		143.5
Asset-backed | Available for sale financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Asset-backed | 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 Disclosure		18.7		18.7		14.7
Asset-backed | Held for trading financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Asset-backed | Held for trading financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		18.7		18.7		14.7
Asset-backed | Held for trading financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Non-agency commercial mortgage-backed | Available for sale financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		31.6		31.6		44.8
Non-agency commercial mortgage-backed | Available for sale financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Non-agency commercial mortgage-backed | Available for sale financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		31.6		31.6		44.8
Non-agency commercial mortgage-backed | Available for sale financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Agency mortgage-backed | Available for sale financial assets, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		1,009.7		1,009.7		1,055.3
Agency mortgage-backed | Available for sale financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Agency mortgage-backed | Available for sale financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		1,009.7		1,009.7		1,055.3
Agency mortgage-backed | Available for sale financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Bank loans | 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 Disclosure		83.1		83.1		85.1
Bank loans | Held for trading financial assets, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Bank loans | Held for trading financial assets, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		83.1		83.1		85.1
Bank loans | Held for trading financial assets, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Short-term investments trading, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		6.9		6.9		0.2
Short-term investments trading, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0.1
Short-term investments trading, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		6.9		6.9		0.1
Short-term investments trading, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Equity investments trading, at fair value | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		696.3		696.3		616
Equity investments trading, at fair value | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		696.3		696.3		616
Equity investments trading, at fair value | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Equity investments trading, at fair value | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0		0		0
Foreign Exchange Contracts | Derivatives at fair value, assets | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		9.2		9.2		7.9
Foreign Exchange Contracts | Derivatives at fair value, assets | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		0		0		0
Foreign Exchange Contracts | Derivatives at fair value, assets | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		9.2		9.2		7.9
Foreign Exchange Contracts | Derivatives at fair value, assets | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		0		0		0
Foreign Exchange Contracts | Liabilities under derivative contracts — foreign exchange contracts | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value		(1.9)		(1.9)		(14.3)
Foreign Exchange Contracts | Liabilities under derivative contracts — foreign exchange contracts | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value		0		0		0
Foreign Exchange Contracts | Liabilities under derivative contracts — foreign exchange contracts | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value		(1.9)		(1.9)		(14.3)
Foreign Exchange Contracts | Liabilities under derivative contracts — foreign exchange contracts | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities under derivative contracts, fair value		0		0		0
Interest Rate Swaps | Derivatives at fair value, assets | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		0		0		0.1
Interest Rate Swaps | Derivatives at fair value, assets | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		0		0		0
Interest Rate Swaps | Derivatives at fair value, assets | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		0		0		0.1
Interest Rate Swaps | Derivatives at fair value, assets | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives at fair value		0		0		$ 0
Silverton 2015 | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net change from current period hedged transaction	[1]	$ 85.3		$ 85.3
[1]	The amount classified as other payables was $0.8 million and $67.9 million as at September 30, 2015 and December 31, 2014, respectively.

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015USD ($)quote / Investment	Sep. 30, 2014USD ($)	Sep. 30, 2015USD ($)quote / Investment	Sep. 30, 2014USD ($)	Dec. 31, 2014USD ($)quote / Investment	Dec. 31, 2013USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Quotes per fixed income investment | quote / Investment	2		2		2
Quotes per equity investment | quote / Investment	2		2		4
Gross written premiums	$ 720,500,000	$ 652,500,000	$ 2,362,500,000	$ 2,287,300,000
Silverton 2,014
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 2,015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans issued					$ 85,000,000
Loan notes held by third parties	84,500,000		84,500,000		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	$ 70,200,000		70,200,000		120,000,000
Gross written premiums			37,700,000		0
Reserve for losses			$ 4,900,000		$ 0

Reinsurance - Narrative (Detail

Reinsurance - Narrative (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Munich Re
Reinsurance [Line Items]
Concentration Risk, Percentage	22.90%	18.60%
Lloyd's
Reinsurance [Line Items]
Concentration Risk, Percentage	17.10%	27.30%
Arch Re
Reinsurance [Line Items]
Concentration Risk, Percentage	8.20%	8.40%

Derivative Contracts - Fair Val

Derivative Contracts - Fair Value of Derivative Instruments (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Derivatives at fair value		$ 9,200,000		$ 9,200,000		$ 8,000,000
Cash collateral provided		13,700,000		13,700,000		22,300,000
Foreign Exchange Contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative notional amount		28,200,000		28,200,000		135,800,000
Cash collateral provided		800,000		800,000		0
Foreign Exchange Contracts | Designated as Hedging Instrument | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Fair Value				(800,000)		(3,800,000)	[1]
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Derivative notional amount		397,500,000		397,500,000		403,400,000
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Liability notional amount		34,200,000		34,200,000		237,600,000
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments | Derivatives At Fair Value [Member]
Derivatives, Fair Value [Line Items]
Derivative notional amount		363,300,000		363,300,000		165,800,000
Interest Rate Swaps | Derivatives Not Designated as Hedging Instruments | Derivatives At Fair Value [Member]
Derivatives, Fair Value [Line Items]
Derivative notional amount		951,300,000		951,300,000		951,300,000
Derivatives at fair value, assets | Fair Value, Measurements, Recurring | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Derivatives at fair value		9,200,000		9,200,000		7,900,000
Derivatives at fair value, assets | Fair Value, Measurements, Recurring | Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Derivatives at fair value		0		0		100,000
Derivatives at fair value, assets | Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Derivatives at fair value		9,200,000		9,200,000		7,900,000
Derivatives at fair value, assets | Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Derivatives at fair value		9,200,000		9,200,000		7,900,000
Derivatives at fair value, assets | Level 2 | Fair Value, Measurements, Recurring | Interest Rate Swaps
Derivatives, Fair Value [Line Items]
Derivatives at fair value		0		0		100,000
Derivatives at fair value, assets | Level 2 | Fair Value, Measurements, Recurring | Interest Rate Swaps | Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Derivatives at fair value	[2]	0		0		100,000
Liabilities under derivative contracts | Fair Value, Measurements, Recurring | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Liabilities at fair value		(1,900,000)		(1,900,000)		(14,300,000)
Liabilities under derivative contracts | Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts
Derivatives, Fair Value [Line Items]
Liabilities at fair value		(1,900,000)		(1,900,000)		(14,300,000)
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		(1,100,000)		(1,100,000)		$ (10,500,000)
Change in Fair Value of Derivatives | Foreign Exchange Contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Fair Value		$ (500,000)	$ 0	$ 2,200,000	$ 0
[1]	Net of $0.8 million cash collateral (December 31, 2014 — $Nil).
[2]	Net of $13.7 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 - Gain_(Lo

Derivative Contracts - Gain/(Loss) Recognized in Income on Derivative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contract settlement	$ 800,000	$ 0
Derivatives Not Designated as Hedging Instruments | Foreign Exchange Contracts | Change in Fair Value of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	12,900,000	(6,400,000)	$ 10,500,000	$ (4,500,000)
Derivatives Not Designated as Hedging Instruments | Interest Rate Swaps | Change in Fair Value of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	(2,800,000)	1,300,000	(6,200,000)	(4,100,000)
Designated as Hedging Instrument | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contract settlement	(800,000)	0	(3,500,000)	0
Designated as Hedging Instrument | Foreign Exchange Contracts | Change in Fair Value of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value	$ (500,000)	$ 0	$ 2,200,000	$ 0

Derivative Contracts - Narrativ

Derivative Contracts - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Cash collateral provided	$ 13,700,000		$ 13,700,000		$ 22,300,000
Pledged assets	0		0		0
Goldman Sachs International
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	451,300,000		451,300,000
Credit Agricole CIB
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	500,000,000		500,000,000
Foreign Exchange Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contract settlement	800,000	$ 0
Foreign Exchange Contracts | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	28,200,000		28,200,000		135,800,000
Foreign exchange contract settlement	(800,000)	0	(3,500,000)	$ 0
Cash collateral provided	800,000		800,000		0
Foreign Exchange Contracts | Designated as Hedging Instrument | Change in Fair Value of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value	(500,000)	0	2,200,000	0
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	397,500,000		397,500,000		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	12,900,000	(6,400,000)	10,500,000	(4,500,000)
Foreign Exchange Contracts | Derivatives at Fair Value [Member] | Derivatives Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	363,300,000		363,300,000		165,800,000
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Other assets pledged	0		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	(2,800,000)	$ 1,300,000	(6,200,000)	$ (4,100,000)
Interest Rate Swaps | Derivatives at Fair Value [Member] | Derivatives Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	$ 951,300,000		$ 951,300,000		$ 951,300,000

Deferred Policy Acquisition C60

Deferred Policy Acquisition Costs - Reconciliation of Beginning and Ending Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance at the beginning of the period	$ 349	$ 306.4	$ 299	$ 262.2
Acquisition costs deferred	129.8	110.7	413.2	375.8
Amortization of deferred policy acquisition costs	(132)	(115.5)	(365.4)	(336.4)
Balance at the end of the period	$ 346.8	$ 301.6	$ 346.8	$ 301.6

Reserves for Losses and Loss 61

Reserves for Losses and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses ("LAE") Reserves (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Provision for losses and LAE at the start of the year	$ 4,750,800,000	$ 4,678,900,000	$ 4,678,900,000
Less reinsurance recoverable			(350,000,000)	$ (332,700,000)
Net loss and LAE at the start of the year	4,400,800,000	4,346,200,000	4,346,200,000
Net loss and LAE expenses (disposed)	0	(24,200,000)	(24,200,000)
Provision for losses and LAE for claims incurred:
Current year	1,129,800,000		1,411,600,000
Prior years	(97,600,000)	(92,600,000)	(104,100,000)
Total incurred	1,032,200,000		1,307,500,000
Losses and LAE payments for claims incurred:
Current year	(93,400,000)		(112,100,000)
Prior years	(718,700,000)		(995,600,000)
Total paid	(812,100,000)		(1,107,700,000)
Foreign exchange (gains)	(55,700,000)		(121,000,000)
Net losses and LAE reserves at period end	4,565,200,000	$ 4,402,600,000	4,400,800,000
Plus reinsurance recoverable on unpaid losses at period end	348,700,000		350,000,000
Provision for losses and LAE at the end of the relevant period	$ 4,913,900,000		$ 4,750,800,000

Reserves for Losses and Loss 62

Reserves for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Insurance [Abstract]
Reserve releases	$ 97,600,000	$ 92,600,000	$ 104,100,000
Loss portfolio transfers (disposed)	$ 0	$ 24,200,000	$ 24,200,000

Capital Structure - Summary of

Capital Structure - Summary of Authorized and Issued Share Capital (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Authorized share capital:
Number of ordinary shares (in shares)	969,629,030	969,629,030
Number of non-voting shares (in shares)	6,787,880	6,787,880
Number of preference shares (in 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 (in shares)	60,781,958	62,017,368
Issued ordinary shares of 0.15144558¢ per share	$ 92	$ 94
Total issued share capital	$ 127	$ 129
Ordinary shares, par value (in usd per share)	$ 0.15144558	$ 0.15144558
Non-voting shares, par value (in usd per share)	0.15144558	0.15144558
Preference shares, par value (in usd per share)	0.15144558	0.15144558
Preference shares, liquidation preference (in usd per share)	$ 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 usd per share)	$ 0.15144558	$ 0.15144558
Preference shares, liquidation preference (in usd per share)	$ 25	$ 25
Preference shares, rate	7.401%	7.401%
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 usd per share)	$ 0.15144558	$ 0.15144558
Preference shares, liquidation preference (in usd per share)	$ 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 usd per share)	$ 0.15144558	$ 0.15144558
Preference shares, liquidation preference (in usd per share)	$ 25	$ 25
Preference shares, rate	5.95%	5.95%

Capital Structure - Summary o64

Capital Structure - Summary of Ordinary Shares (Details)	9 Months Ended
	Sep. 30, 2015shares
Ordinary Shares [Roll Forward]
Ordinary shares in issue at the beginning of the year (in shares)	62,017,368
Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or 2008 share purchase plan (in shares)	546,117
Ordinary shares issued to non-employee directors (in shares)	8,806
Ordinary shares repurchased (in shares)	(1,790,333)
Ordinary shares in issue at the end of the period (in shares)	60,781,958

Capital Structure - Narrative (

Capital Structure - Narrative (Details) - USD ($)	Feb. 05, 2015	Apr. 17, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Class of Stock [Line Items]
Additional paid-in capital			$ 1,068,300,000		$ 1,068,300,000		$ 1,134,300,000
Aggregate liquidation preferences			$ 568,200,000		568,200,000		568,200,000
Issuance costs					$ 12,400,000		$ 12,400,000
Convertible preferred stock, shares issued upon conversion (in shares)		0.001
Class of warrant or right, exercise price of warrants or rights (in usd per share)		$ 160
Number of days rights are not exercisable until beneficial ownership percentage is announced		10 days
Equity method investment, ownership percentage		10.00%
Minimum period before exercisable		10 days
Repurchase Program 2015
Class of Stock [Line Items]
Share Repurchase Agreement, commenced date	Feb. 5, 2015
Existing share repurchase authorization amount	$ 500,000,000
Open Markets
Class of Stock [Line Items]
Shares repurchase and cancelled during period (in shares)			0	2,120,625	1,790,333	2,891,130
Shares repurchase and cancelled during period, value			$ 0	$ 90,000,000	$ 83,700,000	$ 120,900,000
Average price (in usd per share)			$ 0	$ 42.46	$ 46.74	$ 41.82
Passive Institutional Investors
Class of Stock [Line Items]
Equity method investment, ownership percentage		15.00%
Preference shares
Class of Stock [Line Items]
Convertible preferred stock, shares issued upon conversion (in shares)		1

Share-Based Payments - Narrativ

Share-Based Payments - Narrative (Details)	Apr. 06, 2014GBP (£)	Sep. 30, 2015USD ($)shares	Sep. 30, 2014USD ($)shares	Sep. 30, 2015USD ($)Agreementshares	Sep. 30, 2015GBP (£)Agreementshares	Sep. 30, 2014USD ($)shares	Feb. 05, 2015shares	Feb. 10, 2014shares
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Vesting period				3 years	3 years
Options grants in period (in shares) | shares		0	0	0	0	0
Options exercised (in shares) | shares		2,000	0	85,938	85,938	53,436
Compensation costs		$ 0	$ 0	$ 0		$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage				8.33%	8.33%
Percentage of restricted share units vested and issued				100.00%	100.00%
Achievement One
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Achieved and required performance growth, percentage				5.60%	5.60%
Percentage of initial shares granted				33.33%	33.33%
Achievement Two | Minimum
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Achieved and required performance growth, percentage				5.60%	5.60%
Eligible for vesting, percent				10.00%	10.00%
Achievement Two | Maximum
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Achieved and required performance growth, percentage				11.10%	11.10%
Eligible for vesting, percent				100.00%	100.00%
Achievement Three | Minimum
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Achieved and required performance growth, percentage				11.10%	11.10%
Eligible for vesting, percent				100.00%	100.00%
Achievement Three | Maximum
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Achieved and required performance growth, percentage				22.20%	22.20%
Eligible for vesting, percent				200.00%	200.00%
Employee Share Purchase Plans
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Compensation costs charged in ESPP		$ 100,000	$ 100,000	$ 300,000		$ 200,000
Sharesave Scheme
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Employee savings under ESPP | £	£ 500				£ 250
Employee contribution period				3 years	3 years
Purchase price of fair market value				(85.00%)	(85.00%)
2013 Share Incentive Plan
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Shares available to grant under incentive plan (in shares) | shares		2,845,683		2,845,683	2,845,683
2003 Share Incentive Plan
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Shares available to grant under incentive plan (in shares) | shares		595,683		595,683	595,683
International Employee Share Purchase Plan
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Employee savings under ESPP				$ 500
Employee contribution period				2 years	2 years
Additional employee contribution period				1 year	1 year
Shares issued (in shares) | shares		264	627	54,940	54,940	11,821
Investor Options
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 | Agreement				3	3
Performance Shares
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Vesting period				3 years	3 years
Compensation costs		$ 2,600,000	$ 300,000	$ 4,800,000		$ 4,500,000
Granted, number of shares (in shares) | shares		0	0	277,585	277,585	315,389
Tax benefit from compensation expense		$ (600,000)	$ (100,000)	$ (1,100,000)		$ (1,000,000)
Stock Options
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Vesting period				3 years	3 years
Exercise period				10 years	10 years
Restricted Share Units
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Vesting period				3 years	3 years
Compensation costs		$ 2,100,000	$ 1,900,000	$ 6,800,000		$ 7,700,000
Granted, number of shares (in shares) | shares		11,614	0	253,766	253,766	259,640
Fair value adjustment capitalized amount		$ 0	$ 300,000	$ (300,000)		$ (900,000)
Tax benefit from compensation expense		(700,000)	(500,000)	(1,400,000)		(1,800,000)
Restricted Share Units | Director
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Compensation costs		400,000	300,000	1,300,000		1,100,000
RSUs for non-employee directors (in shares) | shares							27,620	27,180
Restricted Share Units | Board of Directors Chairman
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
RSUs for non-employee directors (in shares) | shares							12,154	13,590
Phantom Shares
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Compensation costs		$ 1,400,000	$ 1,200,000	$ 3,400,000		$ 3,600,000
Granted, number of shares (in shares) | shares		0	0	135,651	135,651	154,512
Fair value adjustment capitalized amount		$ (200,000)	$ (800,000)	$ (1,800,000)		$ (1,300,000)
Tax benefit from compensation expense		$ (100,000)	$ (200,000)	$ (800,000)		$ (700,000)
Grants in 2007 | Stock Options
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Vesting period				7 years	7 years
Grant Year 2015 and 2016 | Performance Shares
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Vesting rule, threshold percent of eligible shares				100.00%	100.00%
Vesting rule, maximum vesting percent of eligible shares				100.00%	100.00%

Intangible Assets - Summary of

Intangible Assets - Summary of Changes in Intangible Assets (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Beginning of the period	$ 18,200,000	$ 18,200,000	$ 18,200,000	$ 18,400,000
Amortization	0	0	0	(200,000)
End of the period	18,200,000	18,200,000	18,200,000	18,200,000
Trade Mark
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Beginning of the period	1,600,000	1,600,000	1,600,000	1,600,000
Amortization	0	0	0	0
End of the period	1,600,000	1,600,000	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	16,600,000	16,600,000
Amortization	0	0	0	0
End of the period	16,600,000	16,600,000	16,600,000	16,600,000
Other
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Beginning of the period	0	0	0	200,000
Amortization	0	0	0	(200,000)
End of the period	$ 0	$ 0	$ 0	$ 0

Intangible Assets - Narrative (

Intangible Assets - Narrative (Details) - USD ($)	Apr. 05, 2005	Sep. 30, 2015	Dec. 31, 2010	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 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	$ 18,200,000	$ 18,200,000	$ 18,200,000	$ 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	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	16,600,000	16,600,000
Insurance Licenses | Aspen U.K.
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses		2,100,000
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
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	$ 0	$ 0	$ 0	$ 200,000

Commitments and Contingent Li69

Commitments and Contingent Liabilities - Schedule of Company's Restricted Assets (Details) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets		$ 4,198.5	$ 4,049
Total as percent of cash and invested assets		48.70%	47.00%
Letters of credit / guarantees
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	[1]	$ 726.6	$ 778.7
Affiliated transactions
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets		1,177.1	1,086.9
Third party
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets		$ 2,294.8	$ 2,183.4
[1]	As at September 30, 2015, the Company pledged funds of $712.3 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 Li70

Commitments and Contingent Liabilities - Narrative (Details)	9 Months Ended
	Sep. 30, 2015USD ($)	Sep. 30, 2015GBP (£)	Dec. 31, 2014USD ($)	Dec. 31, 2014GBP (£)	Sep. 30, 2014USD ($)	Jul. 30, 2012USD ($)tranche
Line of Credit Facility [Line Items]
Deposit securities and cash as collateral secured letters of credit	$ 712,300,000	£ 9,200,000	$ 764,400,000	£ 9,200,000
Assets held by insurance regulators	418,000,000		411,900,000
Minimum capital required | £		£ 400,000		£ 400,000
Cash collateral provided	13,700,000		22,300,000
Citi Europe | Letter of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity					$ 575,000,000	$ 950,000,000
Number of maturity tranches | tranche						2
Citi Europe | Letter of Credit, Tranche I
Line of Credit Facility [Line Items]
Maximum borrowing capacity						$ 650,000,000
Citi Europe | Letter of Credit, Tranche II
Line of Credit Facility [Line Items]
Maximum borrowing capacity						$ 300,000,000
Barclays Bank PLC | Letter of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	100,000,000
Outstanding letters of credit	$ 0		$ 5,000,000
LIBOR | Citi Europe | Letter of Credit
Line of Credit Facility [Line Items]
Variable rate basis spread	1.00%

Commitments and Contingent Li71

Commitments and Contingent Liabilities - Amounts Outstanding under Operating Leases (Details) $ in Millions	Sep. 30, 2015USD ($)	[1]
Leases [Abstract]
2,015	$ 3.4
2,016	10.1
2,017	14
2,018	13.6
2,019	12.6
Later Years	97.9
Total	$ 151.6
[1]	In 2015, the Company entered into a new 16 year lease relating to the Company’s premises in New York.