Aspen Insurance Holdings Limited (AHL-PE) 10-Q 7 Nov 16 2016 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2016	Nov. 01, 2016
Document and Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2,016
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		60,217,228

UNAUDITED CONDENSED CONSOLIDATE

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Investments:
Fixed income securities, available for sale at fair value (amortized cost — $5,777.2 and $5,867.5)	$ 5,964.7	$ 5,951.1
Fixed income securities, trading at fair value (amortized cost — $960.6 and $794.2)	996.9	788
Equity securities, trading at fair value (cost — $737.1 and $722.5)	797.7	736.4
Short-term investments, available for sale at fair value (amortized cost — $169.1 and $162.9)	169.1	162.9
Short-term investments, trading at fair value (amortized cost — $169.6 and $9.5)	169.6	9.5
Catastrophe bonds, trading at fair value (cost — $17.5 and $55.2)	17.8	55.4
Other investments, equity method	13	8.9
Total investments	8,128.8	7,712.2
Cash and cash equivalents (including $160.9 and $243.3 within consolidated variable interest entities)	1,183.3	1,099.5
Reinsurance recoverables
Unpaid losses	419.8	354.8
Ceded unearned premiums	229.5	168.9
Receivables
Underwriting premiums	1,437.7	1,115.6
Other	131.6	94.3
Funds withheld	51.6	36
Deferred policy acquisition costs	388.2	361.1
Derivatives at fair value	6.3	9.2
Receivables for securities sold	10.8	0.6
Office properties and equipment	84	70.6
Deferred taxation	0	3.7
Other assets	1	4.1
Intangible assets and goodwill	73.1	18.2
Total assets	12,145.7	11,048.8
Insurance reserves
Losses and loss adjustment expenses	5,246.6	4,938.2
Unearned premiums	1,781.2	1,587.2
Total insurance reserves	7,027.8	6,525.4
Payables
Reinsurance premiums	182.8	92.7
Current taxation	12.1	10.8
Deferred taxation	6.2	0
Accrued expenses and other payables	344.2	343.8
Liabilities under derivative contracts	6.5	4
Total payables	551.8	451.3
Loan notes issued by variable interest entities, at fair value	112.7	103
Long-term debt	549.3	549.2
Total liabilities	8,241.6	7,628.9
Commitments and contingent liabilities (see Note 16)	0	0
SHAREHOLDERS’ EQUITY
Ordinary shares, value	0.1	0.1
Preference shares:
Non-controlling interest	1.3	1.3
Additional paid-in capital	1,280.2	1,075.3
Retained earnings	2,490.6	2,283.6
Accumulated other comprehensive income, net of taxes	131.9	59.6
Total shareholders’ equity	3,904.1	3,419.9
Total liabilities and shareholders’ equity	12,145.7	11,048.8
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
5.625% preference shares
Preference shares:
Preference shares, value	$ 0	$ 0

UNAUDITED CONDENSED CONSOLIDAT3

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Fixed income securities, available for sale, amortized cost	$ 5,777.2	$ 5,867.5
Fixed income securities, trading at fair value (amortized cost )	960.6	794.2
Equity securities, trading at fair value, cost	737.1	722.5
Short-term Investments, available for sale, amortized cost	169.1	162.9
Short-term investments, trading at fair value, amortized cost	169.6	9.5
Catastrophe bonds, trading at fair value, cost	17.5	55.2
Cash and cash equivalents, within consolidated VIEs	$ 160.9	$ 243.3
Ordinary shares, par value (in usd per share)	$ 0.0015144558	$ 0.0015144558
Ordinary shares, issued (in shares)	60,210,770	60,918,373
Preference shares, par value (in usd per share)	$ 0.0015144558	$ 0.0015144558
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)	$ 0.0015144558	$ 0.0015144558
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)	$ 0.0015144558	$ 0.0015144558
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)	$ 0.0015144558	$ 0.0015144558
5.625% preference shares
Preference shares, issued (in shares)	10,000,000	0
Preference shares, rate	5.625%	0.00%
Preference shares, par value (in usd per share)	$ 0.0015144558	$ 0

UNAUDITED CONDENSED CONSOLIDAT4

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND OTHER COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenues
Net earned premium	$ 681	$ 640.6	$ 2,024.9	$ 1,843.6
Net investment income	46.4	45	143.9	139.1
Realized and unrealized investment gains	26.7	10.7	137.4	81.6
Other income	1.5	(2.3)	2.4	0.4
Total revenues	755.6	694	2,308.6	2,064.7
Expenses
Losses and loss adjustment expenses	389.2	365.6	1,188.8	1,032.2
Amortization of deferred policy acquisition costs	130.9	132	387.8	365.4
General, administrative and corporate expenses	125	100.5	361.2	298.1
Interest on long-term debt	7.3	7.4	22.1	22.1
Change in fair value of derivatives	(0.6)	(10.1)	7	(4.3)
Change in fair value of loan notes issued by variable interest entities	9.8	8.3	13.7	14.5
Realized and unrealized investment losses	5.2	51.9	34.1	95.2
Net realized and unrealized foreign exchange (gains)/losses	(10.8)	8.4	10.2	26.4
Other expenses	(0.9)	0	0.1	0.8
Total expenses	655.1	664	2,025	1,850.4
Income from operations before income tax	100.5	30	283.6	214.3
Income tax expense	(4.9)	(1.8)	(8.7)	(9.1)
Net income	95.6	28.2	274.9	205.2
Amount attributable to non-controlling interest	0.2	(0.3)	0	(0.8)
Net income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	95.8	27.9	274.9	204.4
Available for sale investments:
Reclassification adjustment for net realized (gains) on investments included in net income	(4.4)	(2.1)	(9.7)	(33.8)
Change in net unrealized gains on available for sale securities held	(18.8)	24.8	113.7	(21)
Net change from current period hedged transactions	3.1	(0.2)	(1.9)	2.5
Change in foreign currency translation adjustment	(10.2)	(29)	(26.8)	(72.4)
Other comprehensive income, gross of tax	(30.3)	(6.5)	75.3	(124.7)
Tax thereon:
Reclassification adjustment for net realized (gains) on investments included in net income	0	0.3	0.6	0.7
Change in net unrealized gains on available for sale securities held	2	0.5	(11.7)	1.4
Net change from current period hedged transactions	(0.6)	(0.3)	0.6	(0.3)
Change in foreign currency translation adjustment	3.7	1.1	7.5	2.2
Total tax on other comprehensive income	5.1	1.6	(3)	4
Other comprehensive income/(loss) net of tax	(25.2)	(4.9)	72.3	(120.7)
Total comprehensive income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	$ 70.6	$ 23	$ 347.2	$ 83.7
Weighted average number of ordinary share and share equivalents
Basic (in shares)	60,225,705	60,779,295	60,588,307	61,442,033
Diluted (in shares)	61,577,018	62,155,125	62,043,440	62,878,436
Basic earnings per ordinary share adjusted for preference share dividends (in usd per share)	$ 1.43	$ 0.30	$ 4.07	$ 2.86
Diluted earnings per ordinary share adjusted for preference share dividends (in usd per share)	$ 1.40	$ 0.30	$ 3.97	$ 2.80

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	Additional paid-in capitalPreference 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, 2014				$ 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	$ 0
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 transactions												2.2
Total accumulated other comprehensive income, net of taxes	113.6
Total accumulated other comprehensive income, net of taxes	59.6
Shareholder's equity, beginning balance at Dec. 31, 2015	3,419.9			1.3	1,075.3			2,283.6			0.6	(1.2)	60.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change attributable to non-controlling interest for the period	0			0				0
New ordinary shares issued						2	$ 241.3
Ordinary shares repurchased and cancelled						$ (50)
Share-based compensation					11.6
Net income for the period	274.9							274.9
Dividends on ordinary shares									$ (39.5)
Dividends on preference shares	(28.4)									$ (28.4)
Change for the period, net of income tax											(19.3)		92.9
Shareholder's equity, ending balance at Sep. 30, 2016	3,904.1	$ 0.1	$ 0	$ 1.3	$ 1,280.2			$ 2,490.6			$ (18.7)	(2.5)	$ 153.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change from current period hedged transactions												$ (1.3)
Total accumulated other comprehensive income, net of taxes	$ 131.9

UNAUDITED CONDENSED CONSOLIDAT6

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities:
Net income	$ 274.9	$ 205.2
Proportion due to non-controlling interest	0	(0.8)
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization	35.6	21.3
Share-based compensation	11.6	13.3
Realized and unrealized investment (gains)	(137.4)	(81.6)
Realized and unrealized investment losses	34.1	95.2
Change in fair value of loan notes issued by variable interest entities	13.7	14.5
Net realized and unrealized investment foreign exchange losses	(3.1)	2.3
Net change from current period hedged transactions	(1.3)	2.2
Insurance reserves:
Losses and loss adjustment expenses	349.7	202.2
Unearned premiums	181.4	219.1
Reinsurance recoverables:
Unpaid losses	(73.1)	(1)
Ceded unearned premiums	(62)	(17.6)
Other receivables	(34)	(19.7)
Deferred policy acquisition costs	(26.4)	(47.4)
Reinsurance premiums payable	76.4	44.3
Funds withheld	(15.6)	7.9
Premiums receivable	(306.9)	(209.9)
Deferred taxes	9.5	2.8
Income tax payable	8	(7)
Accrued expenses and other payables	26.4	(78.2)
Fair value of derivatives and settlement of liabilities under derivatives	5.4	(13.6)
Long-term debt and loan notes issued by variable interest entities	9.8	0.1
Other assets	3.1	10.1
Net cash generated from operating activities	379.8	363.7
Cash flows (used in) investing activities:
(Purchases) of fixed income securities — Available for sale	(1,671.4)	(1,368.2)
(Purchases) of fixed income securities — Trading	(605.1)	(387)
Proceeds from sales and maturities of fixed income securities — Available for sale	1,668.7	1,211.9
Proceeds from sales and maturities of fixed income securities — Trading	481	354.4
(Purchases) of equity securities — Trading	(159.9)	(335.2)
Net proceeds of catastrophe bonds — Trading	37.6	(2)
Proceeds from sales of equity securities — Available for sale	0	108.6
Proceeds from sales of equity securities — Trading	146.4	214.7
(Purchases) of short-term investments — Available for sale	(202.7)	(118.6)
Proceeds from sales of short-term investments — Available for sale	199.9	205.7
(Purchases) of short-term investments — Trading	(166.3)	(38.2)
Proceeds from sales of short-term investments — Trading	6.3	31.5
Net change in (payable)/receivable for securities (purchased)/sold	14.5	14.7
Net (purchases) of equipment	(15.7)	(10.4)
Other investments	(3.3)	(0.8)
Payments for acquisitions and investments, net of cash acquired	(52.7)	0
Net cash (used in) investing activities	(322.7)	(118.9)
Cash flows from/(used in) financing activities:
Proceeds from the issuance of ordinary shares, net of issuance costs	2	4.4
Ordinary shares repurchased	(50)	(83.7)
Proceeds from the issuance of preference shares, net of issuance costs	241.3	0
Dividends paid on ordinary shares	(89.3)	(67.8)
Dividends paid on preference shares	(39.5)	(38.1)
Dividends paid to non-controlling interest	(28.4)	(28.4)
Net cash (used in) financing activities	36.1	(213.6)
Effect of exchange rate movements on cash and cash equivalents	(9.4)	(13)
Increase in cash and cash equivalents	83.8	18.2
Cash and cash equivalents at beginning of period	1,099.5	1,178.5
Cash and cash equivalents at end of period	1,183.3	1,196.7
Supplemental disclosure of cash flow information:
Net cash paid during the period for income tax	0.2	2.4
Cash paid during the period for interest	$ 14.5	$ 14.5

History and Organization

History and Organization	9 Months Ended
	Sep. 30, 2016
Organization, Consolidation and Presentation of Financial Statements [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. 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, 2016
Accounting Policies [Abstract]
Basis of Presentation	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, 2016 are not necessarily indicative of the results that may be expected for the year ended December 31, 2016 . 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, 2015 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, 2015 contained in the Company’s Annual Report on Form 10-K filed with the United States Securities and Exchange Commission (the “SEC”) on February 19, 2016 (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 Adopted in 2016 On May 21, 2015, the Financial Accounting Standards Board (the “FASB”) issued ASU 2015-09, “Financial Services - Insurance (Topic 944) Disclosures About Short-Duration Contracts ” which requires insurance entities to disclose additional information about the liability for unpaid claims and claim adjustment expenses, disclose information about significant changes in methodologies and assumptions used to calculate the liability for unpaid claims and claim adjustment expenses and disclose a roll forward of the liability for unpaid claims and claims adjustment expenses. This ASU is effective for annual periods beginning after December 15, 2015 and interim periods within annual periods beginning after December 15, 2016. The Company does not expect this ASU to have a material impact on its consolidated financial results but it will have an impact on the disclosures in the Company’s 2016 Annual Report on Form 10-K and its Quarterly Reports on Form 10-Q throughout 2017. 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. This ASU is effective for annual periods beginning after December 15, 2015 and may impact the Company’s provisional amounts booked in respect to the acquisition of AG Logic Holdings LLC (“AgriLogic”). Accounting Pronouncements Not Yet Adopted On June 16, 2016, the FASB issued ASU 2016-13, “ Financial Instruments - Credit Losses (Topic 326) ” which introduces a new impairment model, known as the current expected credit loss model, which is based on expected losses rather than incurred losses. Under the new credit loss model, the Company would recognize an allowance for its estimate of expected credit losses and this would apply to most debt instruments (other than those measured at fair value), trade receivables, lease receivables, reinsurance receivables, financial guarantee contracts and loan commitments. Available-for-sale debt securities are outside the model’s scope and the ASU made limited amendments to the impairment model for available-for-sale debt securities. There are other amendments required as a result of this ASU that are effective for fiscal years beginning after December 15, 2019. The Company is currently assessing the impact the adoption of this ASU will have on future financial statements and disclosures. Other accounting pronouncements were issued during the three months ended September 30, 2016 which were either not relevant to the Company or did not impact the Company’s consolidated financial statements.

Reclassifications from Accumula

Reclassifications from Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 2016
Equity [Abstract]
Reclassifications from Accumulated Other Comprehensive Income	Reclassifications from Accumulated Other Comprehensive Income The following tables set 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, 2016 and 2015 : Amount Reclassified from AOCI Details about the AOCI Components Three Months Ended September 30, 2016 Three Months Ended September 30, 2015 Affected Line Item in the Unaudited Condensed Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gains on sale of securities $ 5.3 $ 2.1 Realized and unrealized investment gains Realized (losses) on sale of securities (0.9 ) — Realized and unrealized investment losses 4.4 2.1 Income from operations before income tax Tax on net realized gains of securities — (0.3 ) Income tax expense $ 4.4 $ 1.8 Net income Foreign currency translation adjustments: Foreign currency translation adjustments, before income tax — $ 1.1 Net realized and unrealized foreign exchange gains/(losses) Tax on foreign currency translation adjustments — — Income tax expense $ — $ 1.1 Net income Realized derivatives: Net realized (losses) on settled derivatives $ (3.1 ) $ (0.8 ) General, administrative and corporate expenses Tax on settled derivatives 1.1 — Income tax expense $ (2.0 ) $ (0.8 ) Net income Total reclassifications from AOCI to the statement of operations, net of income tax $ 2.4 $ 2.1 Net income Amount Reclassified from AOCI Details about the AOCI Components Nine Months Ended September 30, 2016 Nine Months Ended September 30, 2015 Affected Line Item in the Unaudited Condensed Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gains on sale of securities $ 14.8 $ 37.7 Realized and unrealized investment gains Realized (losses) on sale of securities (5.1 ) (3.9 ) Realized and unrealized investment losses 9.7 33.8 Income from operations before income tax Tax on net realized gains of securities (0.6 ) (0.7 ) Income tax expense $ 9.1 $ 33.1 Net income Foreign currency translation adjustments: Foreign currency translation adjustments, before income tax — $ 2.2 Net realized and unrealized foreign exchange gains/(losses) Tax on foreign currency translation adjustments — — Income tax expense $ — $ 2.2 Net income Realized derivatives: Net realized (losses) on settled derivatives $ (5.6 ) $ (3.5 ) General, administrative and corporate expenses Tax on settled derivatives 1.1 — Income tax expense $ (4.5 ) $ (3.5 ) Net income Total reclassifications from AOCI to the statement of operations, net of income tax $ 4.6 $ 31.8 Net income

Earnings per Ordinary Share

Earnings per Ordinary Share	9 Months Ended
	Sep. 30, 2016
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 ordinary share for the three and nine months ended September 30, 2016 and 2015 , respectively: Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 ($ in millions, except share and per share amounts) Net income $ 95.6 $ 28.2 $ 274.9 $ 205.2 Preference share dividends (9.5 ) (9.5 ) (28.4 ) (28.4 ) Net amount attributable to non-controlling interest 0.2 (0.3 ) — (0.8 ) Basic and diluted net income available to ordinary shareholders $ 86.3 $ 18.4 $ 246.5 $ 176.0 Ordinary shares: Basic weighted average ordinary shares 60,225,705 60,779,295 60,588,307 61,442,033 Weighted average effect of dilutive securities (1) 1,351,313 1,375,830 1,455,133 1,436,403 Total diluted weighted average ordinary shares 61,577,018 62,155,125 62,043,440 62,878,436 Earnings per ordinary share: Basic $ 1.43 $ 0.30 $ 4.07 $ 2.86 Diluted $ 1.40 $ 0.30 $ 3.97 $ 2.80 (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 26, 2016, the Company’s Board of Directors (“the Board of Directors”) declared the following quarterly dividends: Dividend Payable on: Record Date: Ordinary shares $ 0.22 November 29, 2016 November 11, 2016 7.401% preference shares $ 0.462563 January 1, 2017 December 15, 2016 7.250% preference shares $ 0.4531 January 1, 2017 December 15, 2016 5.95% preference shares $ 0.3719 January 1, 2017 December 15, 2016 5.625% preference shares $ 0.3945 January 1, 2017 December 15, 2016

Segment Reporting

Segment Reporting	9 Months Ended
	Sep. 30, 2016
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 business segments and quantitative thresholds to determine the Company’s reportable segments. Segment profit or loss for each of the Company’s business segments is measured by underwriting profit or loss. Underwriting profit is the excess of net earned premiums over the sum of losses and loss expenses, amortization of deferred policy acquisition costs and general and administrative expenses. Underwriting profit or loss provides a basis for management to evaluate the business 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 insurance and reinsurance, marine, aviation, terrorism, engineering and other specialty lines). 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 business segment, see Part I, Item 1, “Business — Business Segments — Reinsurance” in the Company’s 2015 Annual Report on Form 10-K filed with the SEC. 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 business segment, see Part I, Item 1 “Business — Business Segments — Insurance” in the Company’s 2015 Annual Report on Form 10-K filed with the SEC. 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, none of which are allocated to the business segments. Corporate expenses are not allocated to the Company’s business segments as they typically do not fluctuate with the levels of premiums written and are not directly related to the Company’s business segment operations. The Company does not allocate its assets by business segment as it evaluates underwriting results of each business segment separately from the results of the Company’s investment portfolio. The following tables provide a summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of the Company’s business segments for the three months ended September 30, 2016 and 2015 : Three Months Ended September 30, 2016 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 365.9 $ 397.6 $ 763.5 Net written premiums 314.5 323.9 638.4 Gross earned premiums 364.3 445.5 809.8 Net earned premiums 316.3 364.7 681.0 Underwriting Expenses Losses and loss adjustment expenses 178.7 210.5 389.2 Amortization of deferred policy acquisition costs 53.0 77.9 130.9 General and administrative expenses 47.4 57.9 105.3 Underwriting income $ 37.2 $ 18.4 55.6 Corporate expenses (13.4 ) Non-operating expenses (6.3 ) Net investment income 46.4 Realized and unrealized investment gains 26.7 Realized and unrealized investment losses (5.2 ) Change in fair value of loan notes issued by variable interest entities (9.8 ) Change in fair value of derivatives 0.6 Interest expense on long term debt (7.3 ) Net realized and unrealized foreign exchange gains 10.8 Net other income 2.4 Income before tax $ 100.5 Net reserves for loss and loss adjustment expenses $ 2,495.4 $ 2,331.4 $ 4,826.8 Ratios Loss ratio 56.5 % 57.7 % 57.2 % Policy acquisition expense ratio 16.8 21.4 19.2 General and administrative expense ratio 15.0 15.9 17.4 (1) Expense ratio 31.8 37.3 36.6 Combined ratio 88.3 % 95.0 % 93.8 % (1) The general and administrative expense ratio in the total column includes corporate expenses. 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 ) Net 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. 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, 2016 and 2015 : Nine Months Ended September 30, 2016 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,216.1 $ 1,324.8 $ 2,540.9 Net written premiums 1,070.8 1,092.1 2,162.9 Gross earned premiums 1,000.9 1,345.8 2,346.7 Net earned premiums 896.0 1,128.9 2,024.9 Underwriting Expenses Losses and loss adjustment expenses 494.3 694.5 1,188.8 Amortization of deferred policy acquisition costs 163.1 224.7 387.8 General and administrative expenses 130.6 173.7 304.3 Underwriting income $ 108.0 $ 36.0 144.0 Corporate expenses (50.6 ) Non-operating expenses (6.3 ) Net investment income 143.9 Realized and unrealized investment gains 137.4 Realized and unrealized investment losses (34.1 ) Change in fair value of loan notes issued by variable interest entities (13.7 ) Change in fair value of derivatives (7.0 ) Interest expense on long term debt (22.1 ) Net realized and unrealized foreign exchange (losses) (10.2 ) Other income 2.4 Other expenses (0.1 ) Income before tax $ 283.6 Net reserves for loss and loss adjustment expenses $ 2,495.4 $ 2,331.4 $ 4,826.8 Ratios Loss ratio 55.2 % 61.5 % 58.7 % Policy acquisition expense ratio 18.2 19.9 19.2 General and administrative expense ratio 14.6 15.4 17.5 (1) Expense ratio 32.8 35.3 36.7 Combined ratio 88.0 % 96.8 % 95.4 % (1) The general and administrative expense ratio in the total column includes corporate expenses. 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. The Company uses underwriting ratios as measures of performance. The loss ratio is the ratio of losses and loss adjustment expenses to net premiums earned. The policy acquisition expense ratio is the ratio of amortization of deferred policy acquisition costs to net premiums earned. The general and administrative expense ratio is the ratio of general, administrative and corporate expenses to net premiums earned. The combined ratio is the sum of the loss ratio, the policy acquisition expense ratio and the general and administrative expense ratio.

Investments

Investments	9 Months Ended
	Sep. 30, 2016
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, 2016 and 2015 : For the Three Months Ended For the Nine Months Ended September 30, 2016 September 30, 2015 September 30, 2016 September 30, 2015 ($ in millions) ($ in millions) Fixed income securities — Available for sale $ 34.8 $ 36.1 $ 107.9 $ 106.8 Fixed income securities — Trading 7.6 7.2 22.9 21.1 Short-term investments — Available for sale 0.2 0.3 0.5 0.9 Cash and cash equivalents 0.8 0.5 2.2 2.5 Equity securities — Available for sale — — — 0.1 Equity securities — Trading 5.5 4.6 17.9 16.5 Catastrophe bonds — Trading 0.2 0.4 1.3 1.3 Total $ 49.1 $ 49.1 $ 152.7 $ 149.2 Investment expenses (2.7 ) (4.1 ) (8.8 ) (10.1 ) Net investment income $ 46.4 $ 45.0 $ 143.9 $ 139.1 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, 2016 and 2015 : For the Three Months Ended For the Nine Months Ended September 30, 2016 September 30, 2015 September 30, 2016 September 30, 2015 ($ in millions) ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 5.5 $ 2.4 $ 14.7 $ 9.7 Fixed income securities — gross realized (losses) (0.9 ) (0.3 ) (4.6 ) (1.4 ) Equity securities — gross realized gains — — — 31.9 Equity securities — gross realized (losses) — — — (3.0 ) Short-term investments — gross realized gains 0.1 — — — Short-term investments — gross realized (losses) (0.2 ) — — — Cash and cash equivalents — gross realized gains — — 0.1 — Cash and cash equivalents — gross realized (losses) — — (0.5 ) — Trading: Fixed income securities — gross realized gains 4.3 0.5 9.5 3.8 Fixed income securities — gross realized (losses) (0.5 ) (1.2 ) (6.9 ) (3.8 ) Equity securities — gross realized gains 8.1 7.8 23.3 36.2 Equity securities — gross realized (losses) (4.2 ) (12.3 ) (22.2 ) (25.4 ) Catastrophe bonds 0.3 0.7 0.1 (0.1 ) Net change in gross unrealized gains 9.0 (38.8 ) 89.8 (61.5 ) Total net realized and unrealized investment gains/(losses) recorded in the statement of operations $ 21.5 $ (41.2 ) $ 103.3 $ (13.6 ) Change in available for sale net unrealized gains: Fixed income securities (23.2 ) 22.7 104.0 (27.4 ) Equity securities — — — (27.4 ) Total change in pre-tax available for sale unrealized gains (23.2 ) 22.7 104.0 (54.8 ) Change in taxes 2.0 0.8 (11.1 ) 2.1 Total change in net unrealized gains/(losses), net of taxes, recorded in other comprehensive income $ (21.2 ) $ 23.5 $ 92.9 $ (52.7 ) 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 2015 Annual Report on Form 10-K filed with the SEC. There was no OTTI charge recognized for the three and nine months ended September 30, 2016 ( 2015 — $ Nil and $ Nil , respectively). Balance Sheet Fixed Income Securities and Short-Term Investments — Available For Sale. The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income securities and short-term investments as at September 30, 2016 and December 31, 2015 : As at September 30, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,061.9 $ 24.7 $ (0.2 ) $ 1,086.4 U.S. agency 124.1 3.2 — 127.3 Municipal 23.1 3.2 (0.1 ) 26.2 Corporate 2,695.5 97.1 (1.8 ) 2,790.8 Non-U.S. government-backed corporate 81.2 1.2 — 82.4 Foreign government 566.0 17.2 (0.1 ) 583.1 Asset-backed 68.1 1.0 — 69.1 Non-agency commercial mortgage-backed 14.9 0.6 — 15.5 Agency mortgage-backed 1,142.4 41.8 (0.3 ) 1,183.9 Total fixed income securities — Available for sale 5,777.2 190.0 (2.5 ) 5,964.7 Total short-term investments — Available for sale 169.1 — — 169.1 Total $ 5,946.3 $ 190.0 $ (2.5 ) $ 6,133.8 There were non-cash transfers of securities from available for sale to trading of $41.7 million during the nine months ended September 30, 2016 . The transfers were made following the decision by the North Dakota regulator to reduce the held capital requirement for the recognition of ceded reinsurance recoveries from Aspen Bermuda Limited from 100% to 20% . This decision resulted in a new reduced collateral Regulation 114 trust being established and funded by non-cash transfers of securities from available for sale portfolios. As at December 31, 2015 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,113.9 $ 13.0 $ (3.8 ) $ 1,123.1 U.S. agency 154.5 4.3 (0.1 ) 158.7 Municipal 25.0 1.6 — 26.6 Corporate 2,626.2 49.5 (15.1 ) 2,660.6 Non-U.S. government-backed corporate 81.6 0.6 (0.1 ) 82.1 Foreign government 634.6 10.5 (0.9 ) 644.2 Asset-backed 75.4 0.9 (0.3 ) 76.0 Non-agency commercial mortgage-backed 25.5 1.2 — 26.7 Agency mortgage-backed 1,130.8 27.6 (5.3 ) 1,153.1 Total fixed income securities — Available for sale 5,867.5 109.2 (25.6 ) 5,951.1 Total short-term investments — Available for sale 162.9 — — 162.9 Total $ 6,030.4 $ 109.2 $ (25.6 ) $ 6,114.0 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, 2016 and December 31, 2015 : As at September 30, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 43.5 $ 0.9 $ — $ 44.4 Municipal 4.7 0.2 — 4.9 Corporate 655.0 22.3 (0.3 ) 677.0 Foreign government 200.8 12.6 (0.3 ) 213.1 Asset-backed 15.5 0.1 (0.1 ) 15.5 Agency mortgage-backed 41.1 0.9 — 42.0 Total fixed income securities — Trading 960.6 37.0 (0.7 ) 996.9 Total short-term investments — Trading 169.6 — — 169.6 Total equity securities — Trading 737.1 91.6 (31.0 ) 797.7 Total catastrophe bonds — Trading 17.5 0.3 — 17.8 Total $ 1,884.8 $ 128.9 $ (31.7 ) $ 1,982.0 As at December 31, 2015 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 27.4 $ — $ (0.1 ) $ 27.3 Municipal 0.5 — — 0.5 Corporate 561.9 5.9 (9.6 ) 558.2 Foreign government 181.5 1.7 (3.7 ) 179.5 Asset-backed 20.7 — (0.2 ) 20.5 Bank loans 2.2 — (0.2 ) 2.0 Total fixed income securities — Trading 794.2 7.6 (13.8 ) 788.0 Total short-term investments — Trading 9.5 — — 9.5 Total equity securities — Trading 722.5 57.3 (43.4 ) 736.4 Total catastrophe bonds — Trading 55.2 0.3 (0.1 ) 55.4 Total $ 1,581.4 $ 65.2 $ (57.3 ) $ 1,589.3 The Company classifies the financial instruments presented in the tables above 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 $17.8 million as at September 30, 2016 . 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 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 provides 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 of Chaspark approximates fair value. On July 26, 2016, the Company purchased through its wholly-owned subsidiary, Acorn Limited, a 20% share of Bene Assicurazioni (“Bene”), an Italian-based motor insurer for a total consideration of $3.3 million . The investment is accounted for under the equity method and adjustments to the carrying value of this investment are made based on the Company’s share of capital, including share of income and expenses. The tables below show the Company’s investments in the MVI, Chaspark and Bene for the three and nine months ended September 30, 2016 and September 30, 2015 : For the Three Months Ended September 30, 2016 MVI Chaspark Bene Total ($ in millions) Opening undistributed value of investment $ 0.6 $ 8.1 — $ 8.7 Initial investment — — 3.3 3.3 Unrealized gain for the three months to September 30, 2016 — 1.0 — 1.0 Closing undistributed value of investment $ 0.6 $ 9.1 $ 3.3 $ 13.0 For the Nine Months Ended September 30, 2016 MVI Chaspark Bene Total ($ in millions) Opening undistributed value of investment $ 0.8 $ 8.1 — $ 8.9 Initial investment — — 3.3 3.3 Unrealized gain/(loss) for the nine months to September 30, 2016 (0.2 ) 1.0 — 0.8 Closing undistributed value of investment $ 0.6 $ 9.1 $ 3.3 $ 13.0 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 $ — $ 8.7 $ 8.7 Initial investment 0.8 — $ 0.8 Closing value of investment $ 0.8 $ 8.7 $ 9.5 Fixed Income Securities. The scheduled maturity distribution of available for sale fixed income securities as at September 30, 2016 and December 31, 2015 is set forth in the tables 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, 2016 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 528.7 $ 531.8 AA Due after one year through five years 2,743.4 2,816.0 AA- Due after five years through ten years 1,174.8 1,228.5 A+ Due after ten years 104.9 119.9 A+ Subtotal 4,551.8 4,696.2 Non-agency commercial mortgage-backed 14.9 15.5 AA+ Agency mortgage-backed 1,142.4 1,183.9 AA+ Asset-backed 68.1 69.1 AAA Total fixed income securities — Available for sale $ 5,777.2 $ 5,964.7 As at December 31, 2015 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 661.8 $ 664.4 AA Due after one year through five years 2,765.2 2,806.6 AA- Due after five years through ten years 1,122.5 1,132.0 A+ Due after ten years 86.3 92.3 A+ Subtotal 4,635.8 4,695.3 Non-agency commercial mortgage-backed 25.5 26.7 AA+ Agency mortgage-backed 1,130.8 1,153.1 AA+ Asset-backed 75.4 76.0 AAA Total fixed income securities — Available for sale $ 5,867.5 $ 5,951.1 Guaranteed Investments. As at September 30, 2016 , the Company’s holding of guaranteed investments was limited to one municipal security, rated CC or higher ( December 31, 2015 — one municipal security, rated CC or higher). The standalone rating (rating without guarantee) is determined as the senior unsecured debt rating of the issuer. Where the credit ratings were split between the two main rating agencies, Standard & Poor’s Financial Services LLC (“S&P”) and Moody’s Investors Service, Inc. (“Moody’s”), the lowest rating was used. The Company’s exposure to other third-party guaranteed debt is primarily to investments backed by non-U.S. government guaranteed issuers. Gross Unrealized Loss. The following tables summarize as at September 30, 2016 and December 31, 2015 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 in the Company’s available for sale portfolio: As at September 30, 2016 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 $ 137.5 $ (0.2 ) $ — $ — $ 137.5 $ (0.2 ) 17 U.S. agency — — — — — — 0 Municipal 0.8 (0.1 ) — — 0.8 (0.1 ) 3 Corporate 240.3 (1.2 ) 25.9 (0.6 ) 266.2 (1.8 ) 87 Non-U.S. government-backed corporate 0.4 — — — 0.4 — 1 Foreign government 73.2 (0.1 ) 33.0 — 106.2 (0.1 ) 12 Asset-backed — — 2.2 — 2.2 — 5 Agency mortgage-backed 27.9 — 32.1 (0.3 ) 60.0 (0.3 ) 25 Total fixed income securities — Available for sale 480.1 (1.6 ) 93.2 (0.9 ) 573.3 (2.5 ) 150 Total short-term investments — Available for sale 8.9 — — — 8.9 — 5 Total $ 489.0 $ (1.6 ) $ 93.2 $ (0.9 ) $ 582.2 $ (2.5 ) 155 As at December 31, 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 $ 583.2 $ (3.7 ) $ 4.6 $ (0.1 ) $ 587.8 $ (3.8 ) 72 U.S. agency 17.6 (0.1 ) — — 17.6 (0.1 ) 12 Municipal 1.7 — — — 1.7 — 3 Corporate 1,179.7 (13.3 ) 81.1 (1.8 ) 1,260.8 (15.1 ) 510 Non-U.S. government-backed corporate 40.9 (0.1 ) — — 40.9 (0.1 ) 9 Foreign government 174.6 (0.8 ) 2.8 (0.1 ) 177.4 (0.9 ) 43 Asset-backed 51.4 (0.3 ) 4.2 — 55.6 (0.3 ) 39 Agency mortgage-backed 348.1 (3.6 ) 72.2 (1.7 ) 420.3 (5.3 ) 105 Total fixed income securities — Available for sale 2,397.2 (21.9 ) 164.9 (3.7 ) 2,562.1 (25.6 ) 793 Total short-term investments — Available for sale 56.7 — — — 56.7 — 12 Total $ 2,453.9 $ (21.9 ) $ 164.9 $ (3.7 ) $ 2,618.8 $ (25.6 ) 805

Variable Interest Entities

Variable Interest Entities	9 Months Ended
	Sep. 30, 2016
Payables and Accruals [Abstract]
Variable Interest Entities	Variable Interest Entities As at September 30, 2016 , the Company had investments in two variable interest entities (“VIE”), Chaspark and Silverton. Chaspark. The Company has determined that Chaspark has the characteristics of a VIE as addressed by the guidance in ASC 810, Consolidation . As discussed 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, 2016 , there was $1.0 million and $1.0 million unrealized gain, respectively, in the value of the Company’s investment in Chaspark ( September 30, 2015 — $ Nil and $ Nil ). The adjusted carrying value approximates fair value. For more information on Chaspark, refer to Note 6 of these unaudited condensed consolidated financial statements. Silverton. On September 10, 2013, the Company established Silverton, a Bermuda domiciled special purpose insurer formed to provide additional collateralized capacity to support Aspen Re’s business through retrocession agreements which are collateralized and funded by Silverton through the issuance of one or more series of participating loan notes (collectively, the “Loan Notes”). Silverton is a non-rated insurer and the risks are fully collateralized by way of funds held in trust for the benefit of Aspen Bermuda and Aspen U.K., the ceding reinsurers. All proceeds from the issuance of the Loan Notes were deposited into separate collateral accounts for each series of Loan Notes to fund Silverton’s obligations under a retrocession property quota share agreement entered into with Aspen Bermuda or Aspen Bermuda and Aspen U.K, as the case may be. The holders of the Loan Notes participate in any profit or loss generated by Silverton attributable to the operations of the respective Silverton segregated account. Any existing value of the Loan Notes will be returned to the noteholders in installments after the expiration of the risk period of the retrocession agreement issued by Silverton for the related series of Loan Notes with the final payment being contractually due on the respective maturity dates. The following tables show the total liability balance of the Loan Notes for the nine months ended September 30, 2016 and 2015 : For the Nine Months Ended September 30, 2016 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 190.6 $ 44.4 $ 235.0 Total change in fair value for the period 13.7 3.4 17.1 Total distributed in the period (89.3 ) (19.4 ) (108.7 ) Closing balance as at September 30, 2016 $ 115.0 $ 28.4 $ 143.4 Liability Loan notes (long-term liabilities) $ 112.7 $ 27.9 $ 140.6 Accrued expenses (current liabilities) 2.3 0.5 2.8 Total aggregate unpaid balance as at September 30, 2016 $ 115.0 $ 28.4 $ 143.4 For the Nine Months Ended September 30, 2015 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 138.6 $ 35.6 $ 174.2 Total change in fair value for the period 14.5 3.4 17.9 Total distributed in the period (67.8 ) (20.3 ) (88.1 ) Closing balance as at September 30, 2015 $ 85.3 $ 18.7 $ 104.0 Liability Loan notes (long-term liabilities) $ 84.5 $ 18.5 $ 103.0 Accrued expenses (current liabilities) 0.8 0.2 1.0 Total aggregate unpaid balance as at September 30, 2015 $ 85.3 $ 18.7 $ 104.0 The Company has determined that Silverton has the characteristics of a VIE that are addressed by the guidance in ASC 810, Consolidation . The Company concluded that it is the primary beneficiary of Silverton as it owns all of Silverton’s voting shares and issued share capital, and has a significant financial interest and the power to control Silverton. As a result, the Company consolidated Silverton upon its formation. The Company has no other obligation to provide financial support to Silverton and neither the creditors nor beneficial interest holders of Silverton have recourse to the Company’s general credit. In the event of an extreme catastrophic property reinsurance event or severe credit-related event, there is a risk that Aspen Bermuda and Aspen U.K. would be unable to recover losses from Silverton. These two risks are mitigated as follows: i. Silverton has collateralized the aggregate limit provided to Aspen Bermuda and Aspen U.K. by way of a trust in favor of Aspen Bermuda and Aspen U.K. as beneficiaries; ii. the trustee is a large, well-established regulated entity; and iii. all funds within the trust account are bound by investment guidelines restricting investments to one of the institutional class money market funds run by large international investment managers. For further information regarding the Loan Notes attributable to the third-party investments in Silverton, refer to Note 8 of these unaudited condensed consolidated financial statements.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2016
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, 2016 and December 31, 2015 , respectively: As at September 30, 2016 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,086.4 $ — $ — $ 1,086.4 U.S. agency — 127.3 — 127.3 Municipal — 26.2 — 26.2 Corporate — 2,790.8 — 2,790.8 Non-U.S. government-backed corporate — 82.4 — 82.4 Foreign government 404.1 179.0 — 583.1 Asset-backed — 69.1 — 69.1 Non-agency commercial mortgage-backed — 15.5 — 15.5 Agency mortgage-backed — 1,183.9 — 1,183.9 Total fixed income securities available for sale, at fair value 1,490.5 4,474.2 — 5,964.7 Short-term investments available for sale, at fair value 141.7 27.4 — 169.1 Held for trading financial assets, at fair value U.S. government 44.4 — — 44.4 Municipal — 4.9 — 4.9 Corporate — 677.0 — 677.0 Foreign government 93.9 119.2 — 213.1 Asset-backed — 15.5 — 15.5 Agency mortgage-backed — 42.0 — 42.0 Total fixed income securities trading, at fair value 138.3 858.6 — 996.9 Short-term investments trading, at fair value 169.6 — — 169.6 Equity investments trading, at fair value 797.7 — — 797.7 Catastrophe bonds trading, at fair value — 17.8 — 17.8 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 6.3 — 6.3 Liabilities under derivative contracts — interest rate swaps — — — — Liabilities under derivative contracts — foreign exchange contracts — (6.5 ) — (6.5 ) Loan notes issued by variable interest entities, at fair value — — (112.7 ) (112.7 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (2.3 ) (2.3 ) Total $ 2,737.8 $ 5,377.8 $ (115.0 ) $ 8,000.6 There were no maturities or transfers between Level 1, Level 2 and Level 3 during the three and nine months ended September 30, 2016 . The Company settled $0.3 million and $89.3 million of Level 3 liabilities in respect of the Loan Notes issued by Silverton for the three and nine months respectively, ended September 30, 2016 . As at September 30, 2016 , there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted solely of the Loan Notes issued by Silverton. As at December 31, 2015 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,123.1 $ — $ — $ 1,123.1 U.S. agency — 158.7 — 158.7 Municipal — 26.6 — 26.6 Corporate — 2,660.6 — 2,660.6 Non-U.S. government-backed corporate — 82.1 — 82.1 Foreign government 449.5 194.7 — 644.2 Asset-backed — 76.0 — 76.0 Non-agency commercial mortgage-backed — 26.7 — 26.7 Agency mortgage-backed — 1,153.1 — 1,153.1 Total fixed income securities available for sale, at fair value 1,572.6 4,378.5 — 5,951.1 Short-term investments available for sale, at fair value 130.5 32.4 — 162.9 Held for trading financial assets, at fair value U.S. government 27.3 — — 27.3 Municipal — 0.5 — 0.5 Corporate — 558.2 — 558.2 Foreign government 73.8 105.7 — 179.5 Asset-backed — 20.5 — 20.5 Bank loans — 2.0 — 2.0 Total fixed income securities trading, at fair value 101.1 686.9 — 788.0 Short-term investments trading, at fair value 7.4 2.1 — 9.5 Equity investments trading, at fair value 736.4 — — 736.4 Catastrophe bonds trading, at fair value — 55.4 — 55.4 Other financial assets and liabilities, at fair value Derivatives at fair value – foreign exchange contracts — 8.8 — 8.8 Derivatives at fair value – interest rate swaps — 0.4 — 0.4 Liabilities under derivative contracts – foreign exchange contracts — (4.0 ) — (4.0 ) Loan notes issued by variable interest entities, at fair value — — (103.0 ) (103.0 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (87.6 ) (87.6 ) Total $ 2,548.0 $ 5,160.5 $ (190.6 ) $ 7,517.9 There were no maturities, settlements or transfers between Level 1, Level 2 and Level 3 during the twelve months ended December 31, 2015 . The Company settled $67.8 million of Level 3 liabilities in respect of the Loan Notes issued by Silverton for the twelve months ended December 31, 2015 . As at December 31, 2015 , there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted solely of the Loan Notes issued by Silverton. The following tables present a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Level 3 inputs for the three and nine months ended September 30, 2016 and 2015 : Reconciliation of Liabilities Using Level 3 Inputs Three Months Ended September 30, 2016 Three Months Ended September 30, 2015 ($ in millions) Balance at the beginning of the period (1) $ 105.5 $ 77.8 Distributed to third party (0.3 ) (0.8 ) Total change in fair value included in the statement of operations 9.8 8.3 Balance at the end of the period (1) $ 115.0 $ 85.3 Reconciliation of Liabilities Using Level 3 Inputs Nine Months Ended September 30, 2016 Nine Months Ended September 30, 2015 ($ in millions) Balance at the beginning of the period (1) $ 190.6 $ 138.6 Distributed to third party (89.3 ) (67.8 ) Total change in fair value included in the statement of operations 13.7 14.5 Balance at the end of the period (1) $ 115.0 $ 85.3 (1) The amount classified within accrued expenses and other payables was $2.3 million and $87.6 million as at September 30, 2016 and December 31, 2015, 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, 2016 and December 31, 2015 , 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, 2016 , the Company obtained an average of 2.3 quotes per fixed income investment, consistent with 2.0 quotes as at December 31, 2015 . Pricing sources used in pricing fixed income investments as at September 30, 2016 and December 31, 2015 were as follows: As at September 30, 2016 As at December 31, 2015 Index providers 86 % 85 % Pricing services 8 10 Broker-dealers 6 5 Total 100 % 100 % A summary of securities priced using pricing information from index providers as at September 30, 2016 and December 31, 2015 is provided below: As at September 30, 2016 As at December 31, 2015 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 $ 1,130.8 100.0 % $ 1,095.4 95.0 % U.S. agency 117.7 92.5 % 148.5 94.0 % Municipal 20.4 65.6 % 10.5 39.0 % Corporate 3,327.7 96.0 % 3,083.5 96.0 % Non-U.S. government-backed corporate 38.5 46.8 % 41.7 51.0 % Foreign government 511.9 66.0 % 517.6 63.0 % Asset-backed 35.2 41.5 % 55.3 57.0 % Non-agency commercial mortgage-backed 13.7 88.4 % 22.7 85.0 % Agency mortgage-backed 763.6 62.3 % 742.9 64.0 % Total fixed income securities $ 5,959.5 85.9 % $ 5,718.1 85.0 % Equities 796.2 99.8 % 736.4 100.0 % Total fixed income securities and equity investments $ 6,755.7 87.0 % $ 6,454.5 86.0 % 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, 2016 , the Company obtained an average of 3.9 quotes per equity investment, compared to 4.0 quotes as at December 31, 2015 . Pricing sources used in pricing equities as at September 30, 2016 and December 31, 2015 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 previously used 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) of 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) of Loan Notes with a maturity date of September 18, 2017. In the fourth quarter of 2015, Silverton issued an additional $125.0 million ( $100.0 million third-party funded) of Loan Notes with a maturity date of September 17, 2018. The Company elected to account for the Loan Notes at fair value using the guidance as prescribed under ASC 825, Financial Instruments as the Company believes it represents the most meaningful measurement basis for these liabilities. The Loan Notes are recorded at fair value at each reporting period and, as they are not quoted on an active market and contain significant unobservable inputs, they have been classified as a Level 3 instrument in the Company’s fair value hierarchy. The Loan Notes are unique because they are linked to the specific risks of the Company’s property catastrophe book. To determine the fair value of the Loan Notes the Company runs an internal model which considers the seasonality of the risk assumed under the retrocessional agreement between Aspen Bermuda and Aspen U.K., as ceding reinsurers, and Silverton. The seasonality used in the model is initially 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 initial assumptions of premiums and claims are then reviewed quarterly and updated based on actual premiums written and the latest estimate of claims experience. The inputs to the internal model are based on Company specific data due to the lack of observable market inputs. Reserves for losses are the most significant unobservable input. An increase in reserves for losses would normally result in a decrease in the fair value of the Loan Notes while a decrease in reserves would normally result in an increase in the fair value of the Loan Notes. The observable and unobservable inputs used to determine the fair value of the Loan Notes as at September 30, 2016 and December 31, 2015 are presented in the tables below: As at September 30, 2016 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 115.0 (1) Internal Valuation Model Gross premiums written (O) $ 43.4 $ 57.2 Reserve for losses (U) $ 2.8 $ 8.2 Contract period (O) N/A 365 days Initial value of issuance (O) $ 220.0 $ 220.0 As at December 31, 2015 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 190.6 (1) Internal Valuation Model Gross premiums written (O) $ — $ 38.9 Reserve for losses (U) $ — $ 4.2 Contract period (O) N/A 365 days Initial value of issuance (O) $ 220.0 $ 220.0 (1) The amount classified within accrued expenses and other payables was $2.3 million and $87.6 million as at September 30, 2016 and December 31, 2015, respectively. The observable and unobservable inputs represent the potential variation around the inputs used in the internal model. The contract period is defined in the respective Loan Notes agreements and the initial value represents the funds received from third parties.

Reinsurance

Reinsurance	9 Months Ended
	Sep. 30, 2016
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, 2016 were 19.1% ( December 31, 2015 — 20.4% ) with Munich Re which is rated AA- by S&P, 18.1% ( December 31, 2015 — 20.0% ) with Lloyd’s syndicates which are rated A+ by S&P and 7.8% ( December 31, 2015 — 9.2% ) with Arch Re which is rated A+ by S&P.

Derivative Contracts

Derivative Contracts	9 Months Ended
	Sep. 30, 2016
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, 2016 and December 31, 2015 : As at September 30, 2016 As at December 31, 2015 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 Derivatives at Fair Value $ — $ — $ 756.3 $ 0.4 (1) Foreign Exchange Contracts Derivatives at Fair Value $ 309.1 $ 5.9 $ 217.7 $ 8.8 Foreign Exchange Contracts Liabilities under Derivative Contracts $ 286.5 $ (6.5 ) $ 162.2 $ (2.8 ) (1) Net of $10.1 million of cash collateral provided to the counterparty, Goldman Sachs International ( $256.3 million notional) under an International Swap Dealers Association agreement, which was terminated on May 9, 2016, as security for the Company’s net liability position. As at September 30, 2016 As at December 31, 2015 Derivatives Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Liabilities under Derivative Contracts $ — $ — $ 113.6 $ (1.2 ) (1) Foreign Exchange Contracts Derivatives at Fair Value $ 28.6 $ 0.4 (1) $ — $ — (1) Net of $3.5 million cash collateral ( December 31, 2015 — $ 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, 2016 and 2015 : 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, 2016 September 30, 2015 ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives $ 0.6 $ 12.9 Interest Rate Swaps Change in Fair Value of Derivatives $ — $ (2.8 ) 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, 2016 September 30, 2015 ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives $ (3.7 ) $ 10.5 Interest Rate Swaps Change in Fair Value of Derivatives $ (3.3 ) $ (6.2 ) 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, 2016 September 30, 2015 ($ in millions) Foreign Exchange Contracts General, administrative and corporate expenses $ (3.1 ) $ (0.8 ) Foreign Exchange Contracts Net change from current period hedged transactions $ 3.1 $ (0.2 ) Amount of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income for the 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, 2016 September 30, 2015 ($ in millions) Foreign Exchange Contracts General, administrative and corporate expenses $ (5.6 ) $ (3.5 ) Foreign Exchange Contracts Net change from current period hedged transactions $ (1.9 ) $ 2.5 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, 2016 , the Company held foreign exchange contracts that were not designated as hedging under ASC 815 with an aggregate notional value of $595.6 million ( December 31, 2015 — $379.9 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, 2016 , the impact of foreign exchange contracts on net income was a gain of $0.6 million ( September 30, 2015 — gain of $12.9 million ) and a loss of $3.7 million ( September 30, 2015 — gain of $10.5 million ), respectively. The Company previously held foreign exchange contracts that were designated as hedging under ASC 815. The foreign exchange contracts were 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. On July 1, 2016 the Company removed the designation of the hedge and crystallized in other comprehensive income the foreign exchange loss recognized as at June 30, 2016. As the foreign exchange contracts settle, this unrealized 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 months ended September 30, 2016 , the amount recognized within general, administrative and corporate expenses for settled foreign exchange contracts was a realized loss of $3.1 million ( September 30, 2015 — loss of $0.8 million ) For the nine months ended September 30, 2016 , the amount recognized within general, administrative and corporate expenses for settled foreign exchange contracts was a realized loss of $5.6 million ( September 30, 2015 — loss of $3.5 million ). As at September 30, 2016 , the aggregate notional value of the contracts was $28.6 million ( December 31, 2015 — $113.6 million ). For the three months ended September 30, 2016 , the movement in other comprehensive income representing the effective portion was a net unrealized gain of $3.1 million ( September 30, 2015 — loss of $0.2 million ). For the nine months ended September 30, 2016 , the movement in other comprehensive income representing the effective portion was a net unrealized loss of $1.9 million ( September 30, 2015 — gain of $2.5 million ). Interest Rate Swaps. In 2014, the Company decided to let its interest rate program roll-off and not renew maturing positions. This decision was made after an extensive reassessment of the costs of maintaining an interest rate swap program in a steep yield curve environment. In addition, the continued uncertainty in the global economy, weak oil prices and low inflation make it difficult to gauge the timing and speed of interest rate rises by the Federal Reserve. On May 9, 2016, the Company terminated all remaining outstanding interest rate swaps (notional value of $256.3 million ) under its International Swap Dealers Association agreement. As at September 30, 2016 , the total notional amount of the swaps was $ Nil ( December 31, 2015 — $756.3 million ). For the three months ended September 30, 2016 , there was no charge in respect of the interest rate swaps ( September 30, 2015 — charge of $2.8 million ) and for the nine months ended September 30, 2016 a charge of $3.3 million ( September 30, 2015 — charge of $6.2 million ). As at September 30, 2016 , no cash collateral was held by the Company’s counterparty ( December 31, 2015 — $10.1 million ). As at September 30, 2016 , no non-cash collateral was transferred to the Company by its counterparty ( December 31, 2015 — $ 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, 2016 , 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, 2016
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, 2016 and 2015 : Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 ($ in millions) ($ in millions) Balance at the beginning of the period $ 409.1 $ 349.0 $ 361.1 $ 299.0 Acquisition costs deferred 110.0 129.8 414.9 413.2 Amortization of deferred policy acquisition costs (130.9 ) (132.0 ) (387.8 ) (365.4 ) Balance at the end of the period $ 388.2 $ 346.8 $ 388.2 $ 346.8

Reserves for Losses and Loss Ad

Reserves for Losses and Loss Adjustment Expenses	9 Months Ended
	Sep. 30, 2016
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, 2016 and 2015 and the twelve months ended December 31, 2015 : Nine Months Ended September 30, 2016 Nine Months Ended September 30, 2015 Twelve Months Ended December 31, 2015 ($ in millions) Provision for losses and LAE at the start of the year $ 4,938.2 $ 4,750.8 $ 4,750.8 Less reinsurance recoverable (354.8 ) (350.0 ) (350.0 ) Net loss and LAE at the start of the year 4,583.4 4,400.8 4,400.8 Net loss and LAE expenses assumed 5.7 — — Provision for losses and LAE for claims incurred: Current year 1,267.0 1,129.8 1,522.7 Prior years (78.2 ) (97.6 ) (156.5 ) Total incurred 1,188.8 1,032.2 1,366.2 Losses and LAE payments for claims incurred: Current year (62.1 ) (93.4 ) (141.9 ) Prior years (853.1 ) (718.7 ) (966.6 ) Total paid (915.2 ) (812.1 ) (1,108.5 ) Foreign exchange (gains) (35.9 ) (55.7 ) (75.1 ) Net losses and LAE reserves at period end 4,826.8 4,565.2 4,583.4 Plus reinsurance recoverable on unpaid losses at period end 419.8 348.7 354.8 Provision for losses and LAE at the end of the relevant period $ 5,246.6 $ 4,913.9 $ 4,938.2 For the nine months ended September 30, 2016 , there was a reduction of $78.2 million in the Company’s estimate of the ultimate claims to be paid in respect of prior accident years compared to a reduction of $97.6 million for the nine months ended September 30, 2015 . The Company assumed $5.7 million of additional loss reserves as a result of its recent acquisition of AgriLogic. 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, 2016
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, 2016 and December 31, 2015 : As at September 30, 2016 As at December 31, 2015 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 60,210,770 91 60,918,373 92 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 Issued 5.625% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share 10,000,000 15 — — Total issued share capital 141 127 Additional paid-in capital as at September 30, 2016 was $1,280.2 million ( December 31, 2015 — $1,075.3 million ). Additional paid-in capital includes the aggregate liquidation preferences of the Company’s preference shares of $818.2 million ( December 31, 2015 — $568.2 million ) less issue costs of $21.1 million ( December 31, 2015 — $12.4 million ). Ordinary Shares. The following table summarizes transactions in the Company’s ordinary shares during the nine months ended September 30, 2016 : Number of Ordinary Shares Ordinary shares in issue as at December 31, 2015 60,918,373 Ordinary share transactions in the nine months ended September 30, 2016 Ordinary shares issued to employees under the 2013 share incentive plan and/or 2008 share purchase plan 405,290 Ordinary shares issued to non-employee directors 9,435 Ordinary shares repurchased (1,122,328 ) Ordinary shares in issue as at September 30, 2016 60,210,770 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 and pursuant to a 10b5-1 plan, the Company acquired and cancelled for the three and nine months ended September 30, 2016 144,289 and 1,122,328 ordinary shares, respectively. The total consideration paid for the three and nine months ended September 30, 2016 was $6.5 million and $50.0 million , respectively, with an average price per ordinary share for the three and nine months ended September 30, 2016 of $45.17 and $44.55 , respectively. Under open market repurchases, the Company acquired and cancelled $ Nil and 1,790,333 ordinary shares, respectively, for the three and nine months ended September 30, 2015 . The total consideration paid for the three and nine months ended September 30, 2015 was $ Nil and $83.7 million , respectively, and the average price per ordinary share was $ Nil and $46.74 , respectively. Preference Share Issuance. On September 20, 2016, the Company issued 10,000,000 shares of 5.625% Perpetual Non-Cumulative Preference Shares (the “ 5.625% Preference Shares”). The 5.625% Preference Shares have a liquidation preference of $25 per share. Net proceeds were $241.3 million , comprising $250.0 million of total liquidation preference less $8.7 million of issuance expenses. The Company intends to use the net proceeds from the offering to redeem, in whole or in part, the Company’s outstanding 7.401% Perpetual Non-Cumulative Preference Shares and 7.250% Perpetual Non-Cumulative Preference Shares on January 1, 2017 and July 1, 2017, respectively. The 5.625% Preference Shares rank equally with preference shares previously issued by the Company and have no fixed maturity date. The Company may redeem all or a portion of the 5.625% Preference Shares at a redemption price of $25 per share on or after January 1, 2027. The 5.625% Preference Shares are listed on the NYSE under the symbol “AHLPRD”.

Share-Based Payments

Share-Based Payments	9 Months Ended
	Sep. 30, 2016
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: employee incentive plans, a non-employee director plan and employee share purchase plans. When options are exercised or other equity awards have vested, new ordinary shares are issued as the Company does not currently hold treasury shares. Employee and Non-Employee Director Awards. Employee options and other stock 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 were granted under the 2006 Stock Option Plan for Non-Employee Directors prior to April 21, 2016 and thereafter under the 2016 Stock Option Plan for Non-Employee Directors (the “2016 Non-Employee Director Plan”). The total number of ordinary shares that may be issued under the 2016 Non-Employee Director Plan is 263,695 . Employee stock options are 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 with vesting dependent on time and performance conditions established at the time of grant. No stock options were granted during the three and nine months ended September 30, 2016 ( 2015 — Nil and Nil ) and Nil and 29,222 options, respectively, were exercised and ordinary shares issued in the three and nine months ended September 30, 2016 ( 2015 — 2,000 and 85,938 stock options). No charges against income were made in respect of stock options for the three and nine months ended September 30, 2016 ( 2015 — $ Nil and $ Nil ). Restricted share units (“RSUs”) granted to employees typically vest over a three -year period based on continued service. Some of the RSUs 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, 2016 , the Company granted to its employees 35,852 and 328,551 RSUs, respectively ( 2015 — 11,614 and 253,766 ). Compensation costs charged against income in respect of employee RSUs for the three and nine months ended September 30, 2016 were $2.6 million and $7.2 million , respectively ( 2015 — $2.1 million and $6.8 million ). In the case of non-employee directors, generally one-twelfth of the RSUs vest on each one month anniversary of the date of grant, with 100% of the RSUs vesting on the first anniversary of the date of grant. On February 4, 2016 (with a grant date of February 8, 2016), the Board of Directors approved a total of 24,456 RSUs for non-employee directors (February 9, 2015 — 27,620 RSUs) and 10,952 RSUs to the Chairman (February 9, 2015 — 12,154 RSUs). Compensation costs charged against income in respect of non-employee director RSUs for the three and nine months ended September 30, 2016 were $0.4 million and $1.1 million , respectively ( 2015 — $0.4 million and $1.3 million ). The total fair value adjustment for all RSUs for the three and nine months ended September 30, 2016 was $0.3 million and $0.6 million , respectively ( 2015 — $ Nil and $0.3 million ). The total tax credit recognized by the Company in relation to RSUs in the three and nine months ended September 30, 2016 was $0.6 million and $1.7 million , respectively ( 2015 — $0.7 million and $1.4 million ). Performance Shares. During the three and nine months ended September 30, 2016 , the Company granted Nil and 278,477 performance shares, respectively, to its employees ( 2015 — Nil and 277,585 ). 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 is eligible for vesting each year based on a formula and are the performance shares are only issuable at the end of the three-year period. If the diluted BVPS growth achieved in 2016 is: • less than 4.65% , 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 4.65% and 9.30% , 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 9.30% and 18.60% , 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 2016 , 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 2016 , 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 2016 will likewise 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 2017 and 2018 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 2017 and 2018 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, 2016 in respect of performance shares were $0.3 million and $2.5 million , respectively ( 2015 — $2.6 million and $4.8 million ). The total tax recognized by the Company in relation to performance shares in the three and nine months ended September 30, 2016 was a tax credit of $0.1 million and $0.5 million , respectively ( 2015 — $0.6 million and $1.1 million ). Phantom Shares. During the three and nine months ended September 30, 2016 , the Compensation Committee approved the grant of 1,156 and 147,513 phantom shares, respectively, to its employees ( 2015 — Nil and 135,651 ). 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 2016 performance shares, with the difference being that any vested amount is 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, 2016 in respect of phantom shares were $0.2 million and $0.8 million , respectively ( 2015 — $1.4 million and $3.4 million ) with a fair value adjustment for the three and nine months ended September 30, 2016 of $2.5 million and $3.8 million , respectively ( 2015 — $0.2 million and $1.8 million ). The total tax credit recognized by the Company in relation to phantom shares in the three and nine months ended September 30, 2016 was $0.3 million and $0.6 million , respectively ( 2015 — $0.1 million and $0.8 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 and the International Employee Share Purchase Plan, employees can save up to $500 per month over a two -year period, at the end of which they are 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 £500 per month over a three -year period, at the end of which they are eligible to purchase the Company’s ordinary shares at a discounted price. Employees can purchase the Company’s ordinary shares at a discounted price equivalent to eighty-five percent ( 85% ) of the fair market value of the ordinary shares on the offering date which may be adjusted upon changes in capitalization of the Company. Under the ESPP, 189 and 13,342 ordinary shares, respectively, were exercised and issued during the three and nine months ended September 30, 2016 ( 2015 — 264 and 54,940 shares). Compensation costs charged against income in the three and nine months ended September 30, 2016 in respect of the ESPP were $0.1 million and $0.3 million , respectively ( 2015 — $0.1 million and $0.3 million ).

Intangible Assets and Goodwill

Intangible Assets and Goodwill	9 Months Ended
	Sep. 30, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets	Intangible Assets and Goodwill The following table provides a summary of the Company’s intangible assets for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended September 30, 2016 Three Months Ended September 30, 2015 Beginning of the Period Additions Amortization End of the Period Beginning of the Period Amortization End of the Period ($ in millions) ($ in millions) Intangible Assets Trade Mark $ 5.4 — $ (0.1 ) $ 5.3 $ 1.6 $ — $ 1.6 Insurance Licenses 16.6 — — 16.6 16.6 — 16.6 Agency Relationships 24.2 — (0.4 ) 23.8 — — — Non-compete Agreements 2.6 — (0.1 ) 2.5 — — — Value of Business Acquired 0.2 — (0.2 ) — — — — Consulting Relationships 0.9 — — 0.9 — — — Goodwill 22.1 — — 22.1 — — — Renewal Rights — 1.9 — 1.9 — — — Total $ 72.0 $ 1.9 $ (0.8 ) $ 73.1 $ 18.2 $ — $ 18.2 Nine Months Ended September 30, 2016 Nine Months Ended September 30, 2015 Beginning of the Period Additions Amortization End of the Period Beginning of the Period Amortization End of the Period ($ in millions) ($ in millions) Intangible Assets Trade Mark $ 1.6 4.0 $ (0.3 ) $ 5.3 $ 1.6 $ — $ 1.6 Insurance Licenses 16.6 — — 16.6 16.6 — 16.6 Agency Relationships — 25.0 (1.2 ) 23.8 — — — Non-compete Agreements — 2.9 (0.4 ) 2.5 — — — Value of Business Acquired — 1.8 (1.8 ) — — — — Consulting Relationships — 1.0 (0.1 ) 0.9 — — — Goodwill — 22.1 — 22.1 — — — Renewal Rights — 1.9 — 1.9 — — — Total $ 18.2 $ 58.7 $ (3.8 ) $ 73.1 $ 18.2 $ — $ 18.2 On January 19, 2016, Aspen U.S. Holdings acquired 100% of the equity voting interest of AgriLogic, a specialist U.S. crop managing general agency business with an integrated agricultural consultancy, for an initial purchase price of $53.0 million . In addition, the Company recognized $14.1 million of contingent consideration, with a total maximum payable of $22.8 million , subject to the future performance of the business and $2.0 million of ceding commission. The total consideration for the acquisition was $69.1 million . A significant proportion of the acquired business was represented by intangible assets, specifically $25.0 million for agency relationships, $4.0 million for the right to use the AgriLogic trademark, $2.9 million for non-compete agreements, $1.8 million for the value of business acquired and $1.0 million for consultancy relationships. In addition, $12.0 million of software was acquired and is recognized in the balance sheet under office properties and equipment along with $0.3 million of residual net assets. The total net assets acquired of $47.0 million resulted in the Company recognizing a total of $22.1 million in goodwill for the acquisition of AgriLogic. License to use the “AgriLogic” Trademark. As part of the acquisition of AgriLogic, the Company acquired the right to use the AgriLogic trademark in the United States. The Company valued the trademark at $4.0 million with an estimated economic useful life of 10 years. The Company will amortize the estimated value of the trademark over its estimated useful life. Agency Relationships. As part of the acquisition of AgriLogic, the Company valued the agency relationships at $25.0 million with an estimated economic useful life of 15 years. The Company will amortize the estimated value of the agency relationships over their estimated useful life. Non-compete Agreements. As part of the acquisition of AgriLogic, the Company valued the non-compete agreements at $2.9 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the non-compete agreements over their estimated useful life. Value of Business Acquired. As part of the acquisition of AgriLogic, the Company recognized a $1.8 million asset for value of business acquired (“VOBA”) consisting of the inforce unearned premium reserve and claims reserves at fair value. The Company will amortize the VOBA in line with the unwinding of the acquired unearned premium balances and loss reserves. Given the short tail nature of AgriLogic’s business, the VOBA has been fully amortized in 2016. Consulting Relationships. As part of the acquisition of AgriLogic, the Company valued the consulting relationships at $1.0 million with an estimated economic useful life of 10 years. The Company will amortize the estimated value of the consulting relationships over their estimated useful life. Goodwill. As part of the acquisition of AgriLogic, the Company valued the goodwill at $22.1 million . The goodwill is deemed to have an indefinite useful life and will be assessed for impairment annually. Renewal Rights. On September 22, 2016 the Company entered into a renewal rights agreement with Liberty Specialty Markets Limited ("LSML"). The Company valued the renewal rights at $1.9 million with an estimated economic useful life of 5 years . The Company will amortize the estimated value of the renewal rights over the estimated useful life. In addition to the intangible assets and goodwill associated with the AgriLogic acquisition and the renewal rights agreement with LSML, the Company has the following intangible assets from prior transactions. License to use the “Aspen” Trademark. On April 5, 2005, the Company entered into an agreement with Aspen (Actuaries and Pension Consultants) Plc to acquire the right to use the Aspen trademark in the United Kingdom. The consideration paid was approximately $1.6 million . As at September 30, 2016 , the value of the license to use the Aspen trademark was $1.6 million ( December 31, 2015 — $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 Company’s licenses as at September 30, 2016 was $16.6 million ( December 31, 2015 — $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.

Commitments and Contingent Liab

Commitments and Contingent Liabilities	9 Months Ended
	Sep. 30, 2016
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, 2016 and December 31, 2015 : As at September 30, 2016 As at December 31, 2015 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 1,432.2 $ 1,421.0 Third party 2,449.6 2,265.6 Letters of credit / guarantees (1) 698.2 708.5 Total restricted assets $ 4,580.0 $ 4,395.1 Total as percent of investable assets (2) 49.0 % 49.6 % (1) As at September 30, 2016 , the Company pledged funds of $698.2 million and £0.0 million ( December 31, 2015 — $697.6 million and £7.1 million ) as collateral for its secured letters of credit. (2) The comparative balance has been re-presented to reflect total restricted investable assets as a percent of investable assets. Investable assets comprise total investments, cash and cash equivalents, accrued interest, receivables for securities sold and payables for securities purchased. 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, 2016 in the amount of $456.1 million ( December 31, 2015 — $436.8 million ). The amounts provided as Funds at Lloyd’s are 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, 2016 was £0.4 million ( December 31, 2015 — £0.4 million ). This is not available for distribution by the Company for the payment of dividends. Credit Facility. On June 12, 2013, Aspen Holdings and certain of its direct and indirect subsidiaries (collectively, the “Borrowers”) entered into an amended and restated credit agreement (the “Credit Agreement”) with various lenders and Barclays Bank PLC, as administrative agent (“Barclays”), which amends and restates the credit agreement dated as of July 30, 2010 among Aspen Holdings, the Borrowers, various lenders and Barclays. The credit facility may be used primarily for letters of credit in connection with the Company’s insurance and reinsurance businesses to finance its working capital needs and those of our subsidiaries and for other general corporate purposes. Initial availability under the credit facility is $200.0 million and the Company has the option (subject to obtaining commitments from acceptable lenders) to increase the facility by up to $100.0 million . The facility will expire on June 12, 2017. As at September 30, 2016 , no borrowings were outstanding under the credit facility. The fees and interest rates on the loans and the fees on the letters of credit payable by the Borrowers under the Credit Agreement are based upon the credit ratings for the Company’s long-term unsecured senior debt by S&P and Moody’s. In addition, the fees for a letter of credit vary based upon whether the applicable Borrower has provided collateral (in the form of cash or qualifying debt securities) to secure its reimbursement obligations with respect to such letter of credit. Under the credit facility, the Company must not permit (a) consolidated tangible net worth to be less than approximately $2,428.6 million plus 50% of consolidated net income and 50% of aggregate net cash proceeds from the issuance by the Company of its capital stock, in each case after January 1, 2013, (b) the ratio of our total consolidated debt to the sum of such debt plus our consolidated tangible net worth to exceed 35% or (c) any material insurance subsidiary to have a financial strength rating of less than B++ from A.M. Best. In addition, the credit facility contains other customary affirmative and negative covenants as well as certain customary events of default, including with respect to a change in control. The various affirmative and negative covenants include, among others, covenants that, subject to various exceptions, restrict the ability of the Company and its subsidiaries to: incur indebtedness; create or permit liens on assets; engage in mergers or consolidations; dispose of assets; pay dividends or other distributions; purchase or redeem the Company’s equity securities or those of its subsidiaries and make other restricted payments; make certain investments; agree with others to limit the ability of the Company’s subsidiaries to pay dividends or other restricted payments or to make loans or transfer assets to the Company or another of its subsidiaries. In addition, the credit facility has customary events of default, including (subject to certain materiality thresholds and grace periods) payment default, failure to comply with covenants, material inaccuracy of representation or warranty, bankruptcy or insolvency proceedings, change of control and cross-default to other debt agreements. The credit facility also contains customary provisions in respect of successor companies resulting from mergers and acquisitions assuming obligations thereunder. On December 12, 2014, Aspen Holdings and the Borrowers entered into a first amendment to an amended and restated credit agreement, dated June 12, 2013, with various lenders and Barclays. Aspen Holdings has 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. Other Credit Facilities. On June 30, 2016, Aspen Bermuda and Citibank Europe plc (“Citi Europe”) amended the committed letter of credit facility, dated June 30, 2012, as amended on June 30, 2014 (the “LOC Facility”). The amendment to the LOC facility extends the term of the LOC Facility to June 30, 2018 and provides a maximum aggregate amount of up to $550.0 million . Under the LOC Facility, Aspen Bermuda will pay to Citi Europe (a) a letter of credit fee based on the available amounts of each letter of credit and (b) a commitment fee, which varies based upon usage, on the unutilized portion of the LOC Facility. Aspen Bermuda will also pay interest on the amount drawn by any beneficiary under the LOC Facility at a rate per annum of LIBOR plus 1% (plus reserve asset costs, if any) from the date of drawing until the date of reimbursement by Aspen Bermuda. In addition, Aspen Bermuda and Citi Europe entered into an uncommitted letter of credit facility whereby Aspen Bermuda has the ability to request letters of credit under this facility subject to the prior approval of Citi Europe. The fee associated with the uncommitted facility is a letter of credit fee based on the available amounts of each letter of credit issued under the uncommitted facility. Both the LOC Facility and the uncommitted facility are used to secure obligations of Aspen Bermuda to its policyholders. In addition to these facilities, we also use regulatory trusts to secure our obligations to policyholders. In addition, 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, 2016 and December 31, 2015, the Company had no outstanding collateralized letters of credit under this facility. 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 2015 Annual Report on Form 10-K filed with the SEC. Interest Rate Swaps. As at September 30, 2016 , no cash collateral was held by the Company’s counterparties to support the current valuation of the interest rate swaps ( December 31, 2015 — $10.1 million ). For more information on the Company’s terminated interest rate swaps, refer to Note 10 of these unaudited condensed consolidated financial statements. (b) Operating leases Amounts outstanding under operating leases net of subleases as at September 30, 2016 were: 2016 2017 2018 2019 2020 Later Total ($ in millions) Operating Lease Obligations $ 3.3 $ 16.1 $ 15.3 $ 14.0 $ 9.9 $ 88.4 $ 147.0 (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 at September 30, 2016 and December 31, 2015 , based on available information, it was the opinion of the Company’s management that the probability of the ultimate resolution of pending or threatened litigation or arbitrations having a material effect on the Company’s financial condition, results of operations or liquidity would be remote.

Basis of Preparation (Policies)

Basis of Preparation (Policies)	9 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
New Accounting Pronouncements Policy	Accounting Pronouncements Adopted in 2016 On May 21, 2015, the Financial Accounting Standards Board (the “FASB”) issued ASU 2015-09, “Financial Services - Insurance (Topic 944) Disclosures About Short-Duration Contracts ” which requires insurance entities to disclose additional information about the liability for unpaid claims and claim adjustment expenses, disclose information about significant changes in methodologies and assumptions used to calculate the liability for unpaid claims and claim adjustment expenses and disclose a roll forward of the liability for unpaid claims and claims adjustment expenses. This ASU is effective for annual periods beginning after December 15, 2015 and interim periods within annual periods beginning after December 15, 2016. The Company does not expect this ASU to have a material impact on its consolidated financial results but it will have an impact on the disclosures in the Company’s 2016 Annual Report on Form 10-K and its Quarterly Reports on Form 10-Q throughout 2017. 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. This ASU is effective for annual periods beginning after December 15, 2015 and may impact the Company’s provisional amounts booked in respect to the acquisition of AG Logic Holdings LLC (“AgriLogic”). Accounting Pronouncements Not Yet Adopted On June 16, 2016, the FASB issued ASU 2016-13, “ Financial Instruments - Credit Losses (Topic 326) ” which introduces a new impairment model, known as the current expected credit loss model, which is based on expected losses rather than incurred losses. Under the new credit loss model, the Company would recognize an allowance for its estimate of expected credit losses and this would apply to most debt instruments (other than those measured at fair value), trade receivables, lease receivables, reinsurance receivables, financial guarantee contracts and loan commitments. Available-for-sale debt securities are outside the model’s scope and the ASU made limited amendments to the impairment model for available-for-sale debt securities. There are other amendments required as a result of this ASU that are effective for fiscal years beginning after December 15, 2019. The Company is currently assessing the impact the adoption of this ASU will have on future financial statements and disclosures. Other accounting pronouncements were issued during the three months ended September 30, 2016 which were either not relevant to the Company or did not impact the Company’s consolidated financial statements.

Reclassifications from Accumu24

Reclassifications from Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2016
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income Reclassification	The following tables set 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, 2016 and 2015 : Amount Reclassified from AOCI Details about the AOCI Components Three Months Ended September 30, 2016 Three Months Ended September 30, 2015 Affected Line Item in the Unaudited Condensed Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gains on sale of securities $ 5.3 $ 2.1 Realized and unrealized investment gains Realized (losses) on sale of securities (0.9 ) — Realized and unrealized investment losses 4.4 2.1 Income from operations before income tax Tax on net realized gains of securities — (0.3 ) Income tax expense $ 4.4 $ 1.8 Net income Foreign currency translation adjustments: Foreign currency translation adjustments, before income tax — $ 1.1 Net realized and unrealized foreign exchange gains/(losses) Tax on foreign currency translation adjustments — — Income tax expense $ — $ 1.1 Net income Realized derivatives: Net realized (losses) on settled derivatives $ (3.1 ) $ (0.8 ) General, administrative and corporate expenses Tax on settled derivatives 1.1 — Income tax expense $ (2.0 ) $ (0.8 ) Net income Total reclassifications from AOCI to the statement of operations, net of income tax $ 2.4 $ 2.1 Net income Amount Reclassified from AOCI Details about the AOCI Components Nine Months Ended September 30, 2016 Nine Months Ended September 30, 2015 Affected Line Item in the Unaudited Condensed Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gains on sale of securities $ 14.8 $ 37.7 Realized and unrealized investment gains Realized (losses) on sale of securities (5.1 ) (3.9 ) Realized and unrealized investment losses 9.7 33.8 Income from operations before income tax Tax on net realized gains of securities (0.6 ) (0.7 ) Income tax expense $ 9.1 $ 33.1 Net income Foreign currency translation adjustments: Foreign currency translation adjustments, before income tax — $ 2.2 Net realized and unrealized foreign exchange gains/(losses) Tax on foreign currency translation adjustments — — Income tax expense $ — $ 2.2 Net income Realized derivatives: Net realized (losses) on settled derivatives $ (5.6 ) $ (3.5 ) General, administrative and corporate expenses Tax on settled derivatives 1.1 — Income tax expense $ (4.5 ) $ (3.5 ) Net income Total reclassifications from AOCI to the statement of operations, net of income tax $ 4.6 $ 31.8 Net income

Earnings per Ordinary Share (Ta

Earnings per Ordinary Share (Tables)	9 Months Ended
	Sep. 30, 2016
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 ordinary share for the three and nine months ended September 30, 2016 and 2015 , respectively: Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 ($ in millions, except share and per share amounts) Net income $ 95.6 $ 28.2 $ 274.9 $ 205.2 Preference share dividends (9.5 ) (9.5 ) (28.4 ) (28.4 ) Net amount attributable to non-controlling interest 0.2 (0.3 ) — (0.8 ) Basic and diluted net income available to ordinary shareholders $ 86.3 $ 18.4 $ 246.5 $ 176.0 Ordinary shares: Basic weighted average ordinary shares 60,225,705 60,779,295 60,588,307 61,442,033 Weighted average effect of dilutive securities (1) 1,351,313 1,375,830 1,455,133 1,436,403 Total diluted weighted average ordinary shares 61,577,018 62,155,125 62,043,440 62,878,436 Earnings per ordinary share: Basic $ 1.43 $ 0.30 $ 4.07 $ 2.86 Diluted $ 1.40 $ 0.30 $ 3.97 $ 2.80 (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 26, 2016, the Company’s Board of Directors (“the Board of Directors”) declared the following quarterly dividends: Dividend Payable on: Record Date: Ordinary shares $ 0.22 November 29, 2016 November 11, 2016 7.401% preference shares $ 0.462563 January 1, 2017 December 15, 2016 7.250% preference shares $ 0.4531 January 1, 2017 December 15, 2016 5.95% preference shares $ 0.3719 January 1, 2017 December 15, 2016 5.625% preference shares $ 0.3945 January 1, 2017 December 15, 2016

Segment Reporting (Tables)

Segment Reporting (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 and 2015 : Three Months Ended September 30, 2016 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 365.9 $ 397.6 $ 763.5 Net written premiums 314.5 323.9 638.4 Gross earned premiums 364.3 445.5 809.8 Net earned premiums 316.3 364.7 681.0 Underwriting Expenses Losses and loss adjustment expenses 178.7 210.5 389.2 Amortization of deferred policy acquisition costs 53.0 77.9 130.9 General and administrative expenses 47.4 57.9 105.3 Underwriting income $ 37.2 $ 18.4 55.6 Corporate expenses (13.4 ) Non-operating expenses (6.3 ) Net investment income 46.4 Realized and unrealized investment gains 26.7 Realized and unrealized investment losses (5.2 ) Change in fair value of loan notes issued by variable interest entities (9.8 ) Change in fair value of derivatives 0.6 Interest expense on long term debt (7.3 ) Net realized and unrealized foreign exchange gains 10.8 Net other income 2.4 Income before tax $ 100.5 Net reserves for loss and loss adjustment expenses $ 2,495.4 $ 2,331.4 $ 4,826.8 Ratios Loss ratio 56.5 % 57.7 % 57.2 % Policy acquisition expense ratio 16.8 21.4 19.2 General and administrative expense ratio 15.0 15.9 17.4 (1) Expense ratio 31.8 37.3 36.6 Combined ratio 88.3 % 95.0 % 93.8 % (1) The general and administrative expense ratio in the total column includes corporate expenses. 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 ) Net 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. 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, 2016 and 2015 : Nine Months Ended September 30, 2016 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,216.1 $ 1,324.8 $ 2,540.9 Net written premiums 1,070.8 1,092.1 2,162.9 Gross earned premiums 1,000.9 1,345.8 2,346.7 Net earned premiums 896.0 1,128.9 2,024.9 Underwriting Expenses Losses and loss adjustment expenses 494.3 694.5 1,188.8 Amortization of deferred policy acquisition costs 163.1 224.7 387.8 General and administrative expenses 130.6 173.7 304.3 Underwriting income $ 108.0 $ 36.0 144.0 Corporate expenses (50.6 ) Non-operating expenses (6.3 ) Net investment income 143.9 Realized and unrealized investment gains 137.4 Realized and unrealized investment losses (34.1 ) Change in fair value of loan notes issued by variable interest entities (13.7 ) Change in fair value of derivatives (7.0 ) Interest expense on long term debt (22.1 ) Net realized and unrealized foreign exchange (losses) (10.2 ) Other income 2.4 Other expenses (0.1 ) Income before tax $ 283.6 Net reserves for loss and loss adjustment expenses $ 2,495.4 $ 2,331.4 $ 4,826.8 Ratios Loss ratio 55.2 % 61.5 % 58.7 % Policy acquisition expense ratio 18.2 19.9 19.2 General and administrative expense ratio 14.6 15.4 17.5 (1) Expense ratio 32.8 35.3 36.7 Combined ratio 88.0 % 96.8 % 95.4 % (1) The general and administrative expense ratio in the total column includes corporate expenses. 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.

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 and 2015 : For the Three Months Ended For the Nine Months Ended September 30, 2016 September 30, 2015 September 30, 2016 September 30, 2015 ($ in millions) ($ in millions) Fixed income securities — Available for sale $ 34.8 $ 36.1 $ 107.9 $ 106.8 Fixed income securities — Trading 7.6 7.2 22.9 21.1 Short-term investments — Available for sale 0.2 0.3 0.5 0.9 Cash and cash equivalents 0.8 0.5 2.2 2.5 Equity securities — Available for sale — — — 0.1 Equity securities — Trading 5.5 4.6 17.9 16.5 Catastrophe bonds — Trading 0.2 0.4 1.3 1.3 Total $ 49.1 $ 49.1 $ 152.7 $ 149.2 Investment expenses (2.7 ) (4.1 ) (8.8 ) (10.1 ) Net investment income $ 46.4 $ 45.0 $ 143.9 $ 139.1
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, 2016 and 2015 : For the Three Months Ended For the Nine Months Ended September 30, 2016 September 30, 2015 September 30, 2016 September 30, 2015 ($ in millions) ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 5.5 $ 2.4 $ 14.7 $ 9.7 Fixed income securities — gross realized (losses) (0.9 ) (0.3 ) (4.6 ) (1.4 ) Equity securities — gross realized gains — — — 31.9 Equity securities — gross realized (losses) — — — (3.0 ) Short-term investments — gross realized gains 0.1 — — — Short-term investments — gross realized (losses) (0.2 ) — — — Cash and cash equivalents — gross realized gains — — 0.1 — Cash and cash equivalents — gross realized (losses) — — (0.5 ) — Trading: Fixed income securities — gross realized gains 4.3 0.5 9.5 3.8 Fixed income securities — gross realized (losses) (0.5 ) (1.2 ) (6.9 ) (3.8 ) Equity securities — gross realized gains 8.1 7.8 23.3 36.2 Equity securities — gross realized (losses) (4.2 ) (12.3 ) (22.2 ) (25.4 ) Catastrophe bonds 0.3 0.7 0.1 (0.1 ) Net change in gross unrealized gains 9.0 (38.8 ) 89.8 (61.5 ) Total net realized and unrealized investment gains/(losses) recorded in the statement of operations $ 21.5 $ (41.2 ) $ 103.3 $ (13.6 ) Change in available for sale net unrealized gains: Fixed income securities (23.2 ) 22.7 104.0 (27.4 ) Equity securities — — — (27.4 ) Total change in pre-tax available for sale unrealized gains (23.2 ) 22.7 104.0 (54.8 ) Change in taxes 2.0 0.8 (11.1 ) 2.1 Total change in net unrealized gains/(losses), net of taxes, recorded in other comprehensive income $ (21.2 ) $ 23.5 $ 92.9 $ (52.7 )
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 and Short-Term Investments — Available For Sale. The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income securities and short-term investments as at September 30, 2016 and December 31, 2015 : As at September 30, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,061.9 $ 24.7 $ (0.2 ) $ 1,086.4 U.S. agency 124.1 3.2 — 127.3 Municipal 23.1 3.2 (0.1 ) 26.2 Corporate 2,695.5 97.1 (1.8 ) 2,790.8 Non-U.S. government-backed corporate 81.2 1.2 — 82.4 Foreign government 566.0 17.2 (0.1 ) 583.1 Asset-backed 68.1 1.0 — 69.1 Non-agency commercial mortgage-backed 14.9 0.6 — 15.5 Agency mortgage-backed 1,142.4 41.8 (0.3 ) 1,183.9 Total fixed income securities — Available for sale 5,777.2 190.0 (2.5 ) 5,964.7 Total short-term investments — Available for sale 169.1 — — 169.1 Total $ 5,946.3 $ 190.0 $ (2.5 ) $ 6,133.8 There were non-cash transfers of securities from available for sale to trading of $41.7 million during the nine months ended September 30, 2016 . The transfers were made following the decision by the North Dakota regulator to reduce the held capital requirement for the recognition of ceded reinsurance recoveries from Aspen Bermuda Limited from 100% to 20% . This decision resulted in a new reduced collateral Regulation 114 trust being established and funded by non-cash transfers of securities from available for sale portfolios. As at December 31, 2015 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,113.9 $ 13.0 $ (3.8 ) $ 1,123.1 U.S. agency 154.5 4.3 (0.1 ) 158.7 Municipal 25.0 1.6 — 26.6 Corporate 2,626.2 49.5 (15.1 ) 2,660.6 Non-U.S. government-backed corporate 81.6 0.6 (0.1 ) 82.1 Foreign government 634.6 10.5 (0.9 ) 644.2 Asset-backed 75.4 0.9 (0.3 ) 76.0 Non-agency commercial mortgage-backed 25.5 1.2 — 26.7 Agency mortgage-backed 1,130.8 27.6 (5.3 ) 1,153.1 Total fixed income securities — Available for sale 5,867.5 109.2 (25.6 ) 5,951.1 Total short-term investments — Available for sale 162.9 — — 162.9 Total $ 6,030.4 $ 109.2 $ (25.6 ) $ 6,114.0
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, 2016 and December 31, 2015 : As at September 30, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 43.5 $ 0.9 $ — $ 44.4 Municipal 4.7 0.2 — 4.9 Corporate 655.0 22.3 (0.3 ) 677.0 Foreign government 200.8 12.6 (0.3 ) 213.1 Asset-backed 15.5 0.1 (0.1 ) 15.5 Agency mortgage-backed 41.1 0.9 — 42.0 Total fixed income securities — Trading 960.6 37.0 (0.7 ) 996.9 Total short-term investments — Trading 169.6 — — 169.6 Total equity securities — Trading 737.1 91.6 (31.0 ) 797.7 Total catastrophe bonds — Trading 17.5 0.3 — 17.8 Total $ 1,884.8 $ 128.9 $ (31.7 ) $ 1,982.0 As at December 31, 2015 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 27.4 $ — $ (0.1 ) $ 27.3 Municipal 0.5 — — 0.5 Corporate 561.9 5.9 (9.6 ) 558.2 Foreign government 181.5 1.7 (3.7 ) 179.5 Asset-backed 20.7 — (0.2 ) 20.5 Bank loans 2.2 — (0.2 ) 2.0 Total fixed income securities — Trading 794.2 7.6 (13.8 ) 788.0 Total short-term investments — Trading 9.5 — — 9.5 Total equity securities — Trading 722.5 57.3 (43.4 ) 736.4 Total catastrophe bonds — Trading 55.2 0.3 (0.1 ) 55.4 Total $ 1,581.4 $ 65.2 $ (57.3 ) $ 1,589.3
Other Investments	The tables below show the Company’s investments in the MVI, Chaspark and Bene for the three and nine months ended September 30, 2016 and September 30, 2015 : For the Three Months Ended September 30, 2016 MVI Chaspark Bene Total ($ in millions) Opening undistributed value of investment $ 0.6 $ 8.1 — $ 8.7 Initial investment — — 3.3 3.3 Unrealized gain for the three months to September 30, 2016 — 1.0 — 1.0 Closing undistributed value of investment $ 0.6 $ 9.1 $ 3.3 $ 13.0 For the Nine Months Ended September 30, 2016 MVI Chaspark Bene Total ($ in millions) Opening undistributed value of investment $ 0.8 $ 8.1 — $ 8.9 Initial investment — — 3.3 3.3 Unrealized gain/(loss) for the nine months to September 30, 2016 (0.2 ) 1.0 — 0.8 Closing undistributed value of investment $ 0.6 $ 9.1 $ 3.3 $ 13.0 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 $ — $ 8.7 $ 8.7 Initial investment 0.8 — $ 0.8 Closing value of investment $ 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, 2016 and December 31, 2015 is set forth in the tables 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, 2016 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 528.7 $ 531.8 AA Due after one year through five years 2,743.4 2,816.0 AA- Due after five years through ten years 1,174.8 1,228.5 A+ Due after ten years 104.9 119.9 A+ Subtotal 4,551.8 4,696.2 Non-agency commercial mortgage-backed 14.9 15.5 AA+ Agency mortgage-backed 1,142.4 1,183.9 AA+ Asset-backed 68.1 69.1 AAA Total fixed income securities — Available for sale $ 5,777.2 $ 5,964.7 As at December 31, 2015 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 661.8 $ 664.4 AA Due after one year through five years 2,765.2 2,806.6 AA- Due after five years through ten years 1,122.5 1,132.0 A+ Due after ten years 86.3 92.3 A+ Subtotal 4,635.8 4,695.3 Non-agency commercial mortgage-backed 25.5 26.7 AA+ Agency mortgage-backed 1,130.8 1,153.1 AA+ Asset-backed 75.4 76.0 AAA Total fixed income securities — Available for sale $ 5,867.5 $ 5,951.1
Aggregate Fair Value and Gross Unrealized Loss by Type of Security	Gross Unrealized Loss. The following tables summarize as at September 30, 2016 and December 31, 2015 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 in the Company’s available for sale portfolio: As at September 30, 2016 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 $ 137.5 $ (0.2 ) $ — $ — $ 137.5 $ (0.2 ) 17 U.S. agency — — — — — — 0 Municipal 0.8 (0.1 ) — — 0.8 (0.1 ) 3 Corporate 240.3 (1.2 ) 25.9 (0.6 ) 266.2 (1.8 ) 87 Non-U.S. government-backed corporate 0.4 — — — 0.4 — 1 Foreign government 73.2 (0.1 ) 33.0 — 106.2 (0.1 ) 12 Asset-backed — — 2.2 — 2.2 — 5 Agency mortgage-backed 27.9 — 32.1 (0.3 ) 60.0 (0.3 ) 25 Total fixed income securities — Available for sale 480.1 (1.6 ) 93.2 (0.9 ) 573.3 (2.5 ) 150 Total short-term investments — Available for sale 8.9 — — — 8.9 — 5 Total $ 489.0 $ (1.6 ) $ 93.2 $ (0.9 ) $ 582.2 $ (2.5 ) 155 As at December 31, 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 $ 583.2 $ (3.7 ) $ 4.6 $ (0.1 ) $ 587.8 $ (3.8 ) 72 U.S. agency 17.6 (0.1 ) — — 17.6 (0.1 ) 12 Municipal 1.7 — — — 1.7 — 3 Corporate 1,179.7 (13.3 ) 81.1 (1.8 ) 1,260.8 (15.1 ) 510 Non-U.S. government-backed corporate 40.9 (0.1 ) — — 40.9 (0.1 ) 9 Foreign government 174.6 (0.8 ) 2.8 (0.1 ) 177.4 (0.9 ) 43 Asset-backed 51.4 (0.3 ) 4.2 — 55.6 (0.3 ) 39 Agency mortgage-backed 348.1 (3.6 ) 72.2 (1.7 ) 420.3 (5.3 ) 105 Total fixed income securities — Available for sale 2,397.2 (21.9 ) 164.9 (3.7 ) 2,562.1 (25.6 ) 793 Total short-term investments — Available for sale 56.7 — — — 56.7 — 12 Total $ 2,453.9 $ (21.9 ) $ 164.9 $ (3.7 ) $ 2,618.8 $ (25.6 ) 805

Variable Interest Entities (Tab

Variable Interest Entities (Tables)	9 Months Ended
	Sep. 30, 2016
Payables and Accruals [Abstract]
Summary of Silverton Loan Notes	The following tables show the total liability balance of the Loan Notes for the nine months ended September 30, 2016 and 2015 : For the Nine Months Ended September 30, 2016 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 190.6 $ 44.4 $ 235.0 Total change in fair value for the period 13.7 3.4 17.1 Total distributed in the period (89.3 ) (19.4 ) (108.7 ) Closing balance as at September 30, 2016 $ 115.0 $ 28.4 $ 143.4 Liability Loan notes (long-term liabilities) $ 112.7 $ 27.9 $ 140.6 Accrued expenses (current liabilities) 2.3 0.5 2.8 Total aggregate unpaid balance as at September 30, 2016 $ 115.0 $ 28.4 $ 143.4 For the Nine Months Ended September 30, 2015 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 138.6 $ 35.6 $ 174.2 Total change in fair value for the period 14.5 3.4 17.9 Total distributed in the period (67.8 ) (20.3 ) (88.1 ) Closing balance as at September 30, 2015 $ 85.3 $ 18.7 $ 104.0 Liability Loan notes (long-term liabilities) $ 84.5 $ 18.5 $ 103.0 Accrued expenses (current liabilities) 0.8 0.2 1.0 Total aggregate unpaid balance as at September 30, 2015 $ 85.3 $ 18.7 $ 104.0

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 and December 31, 2015 , respectively: As at September 30, 2016 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,086.4 $ — $ — $ 1,086.4 U.S. agency — 127.3 — 127.3 Municipal — 26.2 — 26.2 Corporate — 2,790.8 — 2,790.8 Non-U.S. government-backed corporate — 82.4 — 82.4 Foreign government 404.1 179.0 — 583.1 Asset-backed — 69.1 — 69.1 Non-agency commercial mortgage-backed — 15.5 — 15.5 Agency mortgage-backed — 1,183.9 — 1,183.9 Total fixed income securities available for sale, at fair value 1,490.5 4,474.2 — 5,964.7 Short-term investments available for sale, at fair value 141.7 27.4 — 169.1 Held for trading financial assets, at fair value U.S. government 44.4 — — 44.4 Municipal — 4.9 — 4.9 Corporate — 677.0 — 677.0 Foreign government 93.9 119.2 — 213.1 Asset-backed — 15.5 — 15.5 Agency mortgage-backed — 42.0 — 42.0 Total fixed income securities trading, at fair value 138.3 858.6 — 996.9 Short-term investments trading, at fair value 169.6 — — 169.6 Equity investments trading, at fair value 797.7 — — 797.7 Catastrophe bonds trading, at fair value — 17.8 — 17.8 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 6.3 — 6.3 Liabilities under derivative contracts — interest rate swaps — — — — Liabilities under derivative contracts — foreign exchange contracts — (6.5 ) — (6.5 ) Loan notes issued by variable interest entities, at fair value — — (112.7 ) (112.7 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (2.3 ) (2.3 ) Total $ 2,737.8 $ 5,377.8 $ (115.0 ) $ 8,000.6 There were no maturities or transfers between Level 1, Level 2 and Level 3 during the three and nine months ended September 30, 2016 . The Company settled $0.3 million and $89.3 million of Level 3 liabilities in respect of the Loan Notes issued by Silverton for the three and nine months respectively, ended September 30, 2016 . As at September 30, 2016 , there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted solely of the Loan Notes issued by Silverton. As at December 31, 2015 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,123.1 $ — $ — $ 1,123.1 U.S. agency — 158.7 — 158.7 Municipal — 26.6 — 26.6 Corporate — 2,660.6 — 2,660.6 Non-U.S. government-backed corporate — 82.1 — 82.1 Foreign government 449.5 194.7 — 644.2 Asset-backed — 76.0 — 76.0 Non-agency commercial mortgage-backed — 26.7 — 26.7 Agency mortgage-backed — 1,153.1 — 1,153.1 Total fixed income securities available for sale, at fair value 1,572.6 4,378.5 — 5,951.1 Short-term investments available for sale, at fair value 130.5 32.4 — 162.9 Held for trading financial assets, at fair value U.S. government 27.3 — — 27.3 Municipal — 0.5 — 0.5 Corporate — 558.2 — 558.2 Foreign government 73.8 105.7 — 179.5 Asset-backed — 20.5 — 20.5 Bank loans — 2.0 — 2.0 Total fixed income securities trading, at fair value 101.1 686.9 — 788.0 Short-term investments trading, at fair value 7.4 2.1 — 9.5 Equity investments trading, at fair value 736.4 — — 736.4 Catastrophe bonds trading, at fair value — 55.4 — 55.4 Other financial assets and liabilities, at fair value Derivatives at fair value – foreign exchange contracts — 8.8 — 8.8 Derivatives at fair value – interest rate swaps — 0.4 — 0.4 Liabilities under derivative contracts – foreign exchange contracts — (4.0 ) — (4.0 ) Loan notes issued by variable interest entities, at fair value — — (103.0 ) (103.0 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (87.6 ) (87.6 ) Total $ 2,548.0 $ 5,160.5 $ (190.6 ) $ 7,517.9 There were no maturities, settlements or transfers between Level 1, Level 2 and Level 3 during the twelve months ended December 31, 2015 . The Company settled $67.8 million of Level 3 liabilities in respect of the Loan Notes issued by Silverton for the twelve months ended December 31, 2015 . As at December 31, 2015 , there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted solely of the Loan Notes issued by Silverton.
Reconciliation of Liabilities Using Level 3 Inputs	The following tables present a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Level 3 inputs for the three and nine months ended September 30, 2016 and 2015 : Reconciliation of Liabilities Using Level 3 Inputs Three Months Ended September 30, 2016 Three Months Ended September 30, 2015 ($ in millions) Balance at the beginning of the period (1) $ 105.5 $ 77.8 Distributed to third party (0.3 ) (0.8 ) Total change in fair value included in the statement of operations 9.8 8.3 Balance at the end of the period (1) $ 115.0 $ 85.3 Reconciliation of Liabilities Using Level 3 Inputs Nine Months Ended September 30, 2016 Nine Months Ended September 30, 2015 ($ in millions) Balance at the beginning of the period (1) $ 190.6 $ 138.6 Distributed to third party (89.3 ) (67.8 ) Total change in fair value included in the statement of operations 13.7 14.5 Balance at the end of the period (1) $ 115.0 $ 85.3 (1) The amount classified within accrued expenses and other payables was $2.3 million and $87.6 million as at September 30, 2016 and December 31, 2015, respectively.
Pricing Sources Used in Pricing Fixed Income Investments	Pricing sources used in pricing fixed income investments as at September 30, 2016 and December 31, 2015 were as follows: As at September 30, 2016 As at December 31, 2015 Index providers 86 % 85 % Pricing services 8 10 Broker-dealers 6 5 Total 100 % 100 %
Summary of Securities Priced Using Pricing Information from Index Providers	A summary of securities priced using pricing information from index providers as at September 30, 2016 and December 31, 2015 is provided below: As at September 30, 2016 As at December 31, 2015 Fair Market Value Determined using Prices from Index Providers % of Total Fair Value by Security Type Fair Market Value Determined using Prices from Index Providers % of Total Fair Value by Security Type ($ in millions, except for percentages) U.S. government $ 1,130.8 100.0 % $ 1,095.4 95.0 % U.S. agency 117.7 92.5 % 148.5 94.0 % Municipal 20.4 65.6 % 10.5 39.0 % Corporate 3,327.7 96.0 % 3,083.5 96.0 % Non-U.S. government-backed corporate 38.5 46.8 % 41.7 51.0 % Foreign government 511.9 66.0 % 517.6 63.0 % Asset-backed 35.2 41.5 % 55.3 57.0 % Non-agency commercial mortgage-backed 13.7 88.4 % 22.7 85.0 % Agency mortgage-backed 763.6 62.3 % 742.9 64.0 % Total fixed income securities $ 5,959.5 85.9 % $ 5,718.1 85.0 % Equities 796.2 99.8 % 736.4 100.0 % Total fixed income securities and equity investments $ 6,755.7 87.0 % $ 6,454.5 86.0 %
Fair Value Inputs, Assets, Quantitative Information	The observable and unobservable inputs used to determine the fair value of the Loan Notes as at September 30, 2016 and December 31, 2015 are presented in the tables below: As at September 30, 2016 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 115.0 (1) Internal Valuation Model Gross premiums written (O) $ 43.4 $ 57.2 Reserve for losses (U) $ 2.8 $ 8.2 Contract period (O) N/A 365 days Initial value of issuance (O) $ 220.0 $ 220.0 As at December 31, 2015 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 190.6 (1) Internal Valuation Model Gross premiums written (O) $ — $ 38.9 Reserve for losses (U) $ — $ 4.2 Contract period (O) N/A 365 days Initial value of issuance (O) $ 220.0 $ 220.0 (1) The amount classified within accrued expenses and other payables was $2.3 million and $87.6 million as at September 30, 2016 and December 31, 2015, respectively.

Derivative Contracts (Tables)

Derivative Contracts (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 and December 31, 2015 : As at September 30, 2016 As at December 31, 2015 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 Derivatives at Fair Value $ — $ — $ 756.3 $ 0.4 (1) Foreign Exchange Contracts Derivatives at Fair Value $ 309.1 $ 5.9 $ 217.7 $ 8.8 Foreign Exchange Contracts Liabilities under Derivative Contracts $ 286.5 $ (6.5 ) $ 162.2 $ (2.8 ) (1) Net of $10.1 million of cash collateral provided to the counterparty, Goldman Sachs International ( $256.3 million notional) under an International Swap Dealers Association agreement, which was terminated on May 9, 2016, as security for the Company’s net liability position. As at September 30, 2016 As at December 31, 2015 Derivatives Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Liabilities under Derivative Contracts $ — $ — $ 113.6 $ (1.2 ) (1) Foreign Exchange Contracts Derivatives at Fair Value $ 28.6 $ 0.4 (1) $ — $ — (1) Net of $3.5 million cash collateral ( December 31, 2015 — $ 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, 2016 and 2015 : 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, 2016 September 30, 2015 ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives $ 0.6 $ 12.9 Interest Rate Swaps Change in Fair Value of Derivatives $ — $ (2.8 ) 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, 2016 September 30, 2015 ($ in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives $ (3.7 ) $ 10.5 Interest Rate Swaps Change in Fair Value of Derivatives $ (3.3 ) $ (6.2 ) 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, 2016 September 30, 2015 ($ in millions) Foreign Exchange Contracts General, administrative and corporate expenses $ (3.1 ) $ (0.8 ) Foreign Exchange Contracts Net change from current period hedged transactions $ 3.1 $ (0.2 ) Amount of Income/(Loss) Recognized in the Statement of Operations and Other Comprehensive Income for the 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, 2016 September 30, 2015 ($ in millions) Foreign Exchange Contracts General, administrative and corporate expenses $ (5.6 ) $ (3.5 ) Foreign Exchange Contracts Net change from current period hedged transactions $ (1.9 ) $ 2.5

Deferred Policy Acquisition C31

Deferred Policy Acquisition Costs (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 and 2015 : Three Months Ended September 30, Nine Months Ended September 30, 2016 2015 2016 2015 ($ in millions) ($ in millions) Balance at the beginning of the period $ 409.1 $ 349.0 $ 361.1 $ 299.0 Acquisition costs deferred 110.0 129.8 414.9 413.2 Amortization of deferred policy acquisition costs (130.9 ) (132.0 ) (387.8 ) (365.4 ) Balance at the end of the period $ 388.2 $ 346.8 $ 388.2 $ 346.8

Reserves for Losses and Loss 32

Reserves for Losses and Loss Adjustment Expenses (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 and 2015 and the twelve months ended December 31, 2015 : Nine Months Ended September 30, 2016 Nine Months Ended September 30, 2015 Twelve Months Ended December 31, 2015 ($ in millions) Provision for losses and LAE at the start of the year $ 4,938.2 $ 4,750.8 $ 4,750.8 Less reinsurance recoverable (354.8 ) (350.0 ) (350.0 ) Net loss and LAE at the start of the year 4,583.4 4,400.8 4,400.8 Net loss and LAE expenses assumed 5.7 — — Provision for losses and LAE for claims incurred: Current year 1,267.0 1,129.8 1,522.7 Prior years (78.2 ) (97.6 ) (156.5 ) Total incurred 1,188.8 1,032.2 1,366.2 Losses and LAE payments for claims incurred: Current year (62.1 ) (93.4 ) (141.9 ) Prior years (853.1 ) (718.7 ) (966.6 ) Total paid (915.2 ) (812.1 ) (1,108.5 ) Foreign exchange (gains) (35.9 ) (55.7 ) (75.1 ) Net losses and LAE reserves at period end 4,826.8 4,565.2 4,583.4 Plus reinsurance recoverable on unpaid losses at period end 419.8 348.7 354.8 Provision for losses and LAE at the end of the relevant period $ 5,246.6 $ 4,913.9 $ 4,938.2

Capital Structure (Tables)

Capital Structure (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 and December 31, 2015 : As at September 30, 2016 As at December 31, 2015 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 60,210,770 91 60,918,373 92 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 Issued 5.625% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share 10,000,000 15 — — Total issued share capital 141 127
Summary of Ordinary Shares	The following table summarizes transactions in the Company’s ordinary shares during the nine months ended September 30, 2016 : Number of Ordinary Shares Ordinary shares in issue as at December 31, 2015 60,918,373 Ordinary share transactions in the nine months ended September 30, 2016 Ordinary shares issued to employees under the 2013 share incentive plan and/or 2008 share purchase plan 405,290 Ordinary shares issued to non-employee directors 9,435 Ordinary shares repurchased (1,122,328 ) Ordinary shares in issue as at September 30, 2016 60,210,770

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	9 Months Ended
	Sep. 30, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Summary of Changes in Intangible Assets	The following table provides a summary of the Company’s intangible assets for the three and nine months ended September 30, 2016 and 2015 : Three Months Ended September 30, 2016 Three Months Ended September 30, 2015 Beginning of the Period Additions Amortization End of the Period Beginning of the Period Amortization End of the Period ($ in millions) ($ in millions) Intangible Assets Trade Mark $ 5.4 — $ (0.1 ) $ 5.3 $ 1.6 $ — $ 1.6 Insurance Licenses 16.6 — — 16.6 16.6 — 16.6 Agency Relationships 24.2 — (0.4 ) 23.8 — — — Non-compete Agreements 2.6 — (0.1 ) 2.5 — — — Value of Business Acquired 0.2 — (0.2 ) — — — — Consulting Relationships 0.9 — — 0.9 — — — Goodwill 22.1 — — 22.1 — — — Renewal Rights — 1.9 — 1.9 — — — Total $ 72.0 $ 1.9 $ (0.8 ) $ 73.1 $ 18.2 $ — $ 18.2 Nine Months Ended September 30, 2016 Nine Months Ended September 30, 2015 Beginning of the Period Additions Amortization End of the Period Beginning of the Period Amortization End of the Period ($ in millions) ($ in millions) Intangible Assets Trade Mark $ 1.6 4.0 $ (0.3 ) $ 5.3 $ 1.6 $ — $ 1.6 Insurance Licenses 16.6 — — 16.6 16.6 — 16.6 Agency Relationships — 25.0 (1.2 ) 23.8 — — — Non-compete Agreements — 2.9 (0.4 ) 2.5 — — — Value of Business Acquired — 1.8 (1.8 ) — — — — Consulting Relationships — 1.0 (0.1 ) 0.9 — — — Goodwill — 22.1 — 22.1 — — — Renewal Rights — 1.9 — 1.9 — — — Total $ 18.2 $ 58.7 $ (3.8 ) $ 73.1 $ 18.2 $ — $ 18.2

Commitments and Contingent Li35

Commitments and Contingent Liabilities (Tables)	9 Months Ended
	Sep. 30, 2016
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, 2016 and December 31, 2015 : As at September 30, 2016 As at December 31, 2015 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 1,432.2 $ 1,421.0 Third party 2,449.6 2,265.6 Letters of credit / guarantees (1) 698.2 708.5 Total restricted assets $ 4,580.0 $ 4,395.1 Total as percent of investable assets (2) 49.0 % 49.6 % (1) As at September 30, 2016 , the Company pledged funds of $698.2 million and £0.0 million ( December 31, 2015 — $697.6 million and £7.1 million ) as collateral for its secured letters of credit. (2) The comparative balance has been re-presented to reflect total restricted investable assets as a percent of investable assets. Investable assets comprise total investments, cash and cash equivalents, accrued interest, receivables for securities sold and payables for securities purchased.
Amounts Outstanding under Operating Leases	Amounts outstanding under operating leases net of subleases as at September 30, 2016 were: 2016 2017 2018 2019 2020 Later Total ($ in millions) Operating Lease Obligations $ 3.3 $ 16.1 $ 15.3 $ 14.0 $ 9.9 $ 88.4 $ 147.0

Reclassifications from Accumu36

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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Available for sale securities:
Income from operations before income tax	$ 100.5	$ 30	$ 283.6	$ 214.3
Income tax expense	4.9	1.8	8.7	9.1
Net income	95.6	28.2	274.9	205.2
Net realized and unrealized foreign exchange gains/(losses)	10.8	(8.4)	(10.2)	(26.4)
General, administrative and corporate expenses	125	100.5	361.2	298.1
Reclassification out of Accumulated Other Comprehensive Income [Member]
Available for sale securities:
Net income	2.4	2.1	4.6	31.8
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Available for sale securities:
Realized gains on sale of securities	5.3	2.1	14.8	37.7
Realized (losses) on sale of securities	(0.9)	0	(5.1)	(3.9)
Income from operations before income tax	4.4	2.1	9.7	33.8
Income tax expense	0	0.3	0.6	0.7
Net income	4.4	1.8	9.1	33.1
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
Available for sale securities:
Income tax expense	0		0	0
Net income	0	1.1	0	2.2
Net realized and unrealized foreign exchange gains/(losses)	0	1.1	0	2.2
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member] | Foreign Exchange Contracts
Available for sale securities:
Income tax expense	1.1	0	1.1	0
Net income	2	0.8	4.5	3.5
General, administrative and corporate expenses	$ (3.1)	$ (0.8)	$ (5.6)	$ (3.5)

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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Net income		$ 95.6	$ 28.2	$ 274.9	$ 205.2
Preference share dividends		(9.5)	(9.5)	(28.4)	(28.4)
Proportion due to non-controlling interest		0.2	(0.3)	0	(0.8)
Basic and diluted net income available to ordinary shareholders		$ 86.3	$ 18.4	$ 246.5	$ 176
Ordinary shares:
Basic weighted average ordinary shares		60,225,705	60,779,295	60,588,307	61,442,033
Weighted average effect of dilutive securities(1)	[1]	1,351,313	1,375,830	1,455,133	1,436,403
Total diluted weighted average ordinary shares		61,577,018	62,155,125	62,043,440	62,878,436
Earnings per ordinary share:
Basic (in usd per share)		$ 1.43	$ 0.30	$ 4.07	$ 2.86
Diluted (in usd per share)		$ 1.40	$ 0.30	$ 3.97	$ 2.80
[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. 26, 2016	Sep. 30, 2016	Dec. 31, 2015
7.401% preference shares
Dividends Payable [Line Items]
Preference shares, rate		7.401%	7.401%
7.250% preference shares
Dividends Payable [Line Items]
Preference shares, rate		7.25%	7.25%
5.95% preference shares
Dividends Payable [Line Items]
Preference shares, rate		5.95%	5.95%
5.625% preference shares
Dividends Payable [Line Items]
Preference shares, rate		5.625%	0.00%
Subsequent Event | Ordinary shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.22
Payable Date	Nov. 29, 2016
Record Date	Nov. 11, 2016
Subsequent Event | 7.401% preference shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.462563
Payable Date	Jan. 1, 2017
Record Date	Dec. 15, 2016
Subsequent Event | 7.250% preference shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.4531
Payable Date	Jan. 1, 2017
Record Date	Dec. 15, 2016
Subsequent Event | 5.95% preference shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.3719
Payable Date	Jan. 1, 2017
Record Date	Dec. 15, 2016
Subsequent Event | 5.625% preference shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.3945
Payable Date	Jan. 1, 2017
Record Date	Dec. 15, 2016

Segment Reporting - Narrative (

Segment Reporting - Narrative (Details)	9 Months Ended
	Sep. 30, 2016Segment
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, 2016		Sep. 30, 2015		Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Underwriting Revenues
Gross written premiums	$ 763.5		$ 720.5		$ 2,540.9	$ 2,362.5
Net written premiums	638.4		651.8		2,162.9	2,059.4
Gross earned premiums	809.8		733.6		2,346.7	2,124.9
Net earned premiums	681		640.6		2,024.9	1,843.6
Underwriting Expenses
Losses and loss adjustment expenses	389.2		365.6		1,188.8	1,032.2
Amortization of deferred policy acquisition costs	130.9		132		387.8	365.4
General and administrative expenses	105.3		86		304.3	254.3
Underwriting income	55.6		57		144	191.7
Corporate expenses	(13.4)		(14.5)		(50.6)	(43.8)
Non-operating expenses	(6.3)				(6.3)
Net investment income	46.4		45		143.9	139.1
Realized and unrealized investment gains	26.7		10.7		137.4	81.6
Realized and unrealized investment losses	(5.2)		(51.9)		(34.1)	(95.2)
Change in fair value of loan notes issued by variable interest entities	(9.8)		(8.3)		(13.7)	(14.5)
Change in fair value of derivatives	0.6		10.1		(7)	4.3
Interest expense on long term debt	(7.3)		(7.4)		(22.1)	(22.1)
Net realized and unrealized foreign exchange gains	10.8		(8.4)		(10.2)	(26.4)
Other income	1.5		(2.3)		2.4	0.4
Other expenses	(0.9)		0		0.1	0.8
Net other income	2.4
Income from operations before income tax	100.5		30		283.6	214.3
Net reserves for loss and loss adjustment expenses	$ 4,826.8		$ 4,565.2		$ 4,826.8	$ 4,565.2	$ 4,583.4	$ 4,400.8
Ratios
Loss ratio	57.20%		57.10%		58.70%	56.00%
Policy acquisition expense ratio	19.20%		20.60%		19.20%	19.80%
General and administrative expense ratio	17.40%	[1]	15.70%	[2]	17.50%	16.20%
Expense ratio	36.60%		36.30%		36.70%	36.00%
Combined ratio	93.80%		93.40%		95.40%	92.00%
Reinsurance
Underwriting Revenues
Gross written premiums	$ 365.9		$ 316.6		$ 1,216.1	$ 1,062.1
Net written premiums	314.5		294.7		1,070.8	975
Gross earned premiums	364.3		304.6		1,000.9	857.6
Net earned premiums	316.3		284.6		896	802.3
Underwriting Expenses
Losses and loss adjustment expenses	178.7		169.9		494.3	391.7
Amortization of deferred policy acquisition costs	53		64.8		163.1	168.6
General and administrative expenses	47.4		34.7		130.6	102.5
Underwriting income	37.2		15.2		108	139.5
Net reserves for loss and loss adjustment expenses	$ 2,495.4		$ 2,469.6		$ 2,495.4	$ 2,469.6
Ratios
Loss ratio	56.50%		59.70%		55.20%	48.80%
Policy acquisition expense ratio	16.80%		22.80%		18.20%	21.00%
General and administrative expense ratio	15.00%		12.20%		14.60%	12.80%
Expense ratio	31.80%		35.00%		32.80%	33.80%
Combined ratio	88.30%		94.70%		88.00%	82.60%
Insurance
Underwriting Revenues
Gross written premiums	$ 397.6		$ 403.9		$ 1,324.8	$ 1,300.4
Net written premiums	323.9		357.1		1,092.1	1,084.4
Gross earned premiums	445.5		429		1,345.8	1,267.3
Net earned premiums	364.7		356		1,128.9	1,041.3
Underwriting Expenses
Losses and loss adjustment expenses	210.5		195.7		694.5	640.5
Amortization of deferred policy acquisition costs	77.9		67.2		224.7	196.8
General and administrative expenses	57.9		51.3		173.7	151.8
Underwriting income	18.4		41.8		36	52.2
Net reserves for loss and loss adjustment expenses	$ 2,331.4		$ 2,095.6		$ 2,331.4	$ 2,095.6
Ratios
Loss ratio	57.70%		55.00%		61.50%	61.50%
Policy acquisition expense ratio	21.40%		18.90%		19.90%	18.90%
General and administrative expense ratio	15.90%		14.40%		15.40%	14.60%
Expense ratio	37.30%		33.30%		35.30%	33.50%
Combined ratio	95.00%		88.30%		96.80%	95.00%
[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.

Investments - Summary of Invest

Investments - Summary of Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	$ 49.1	$ 49.1	$ 152.7	$ 149.2
Investment expenses	(2.7)	(4.1)	(8.8)	(10.1)
Net investment income	46.4	45	143.9	139.1
Total fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	34.8	36.1	107.9	106.8
Fixed Income Maturities - Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	7.6	7.2	22.9	21.1
Total short-term investments — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	0.2	0.3	0.5	0.9
Cash and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	0.8	0.5	2.2	2.5
Equity Securities - Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	0	0	0	0.1
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	5.5	4.6	17.9	16.5
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Total	$ 0.2	$ 0.4	$ 1.3	$ 1.3

Investments - Net Realized and

Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Catastrophe bonds	$ 0.3	$ 0.7	$ 0.1	$ (0.1)
Net change in gross unrealized gains	9	(38.8)	89.8	(61.5)
Total net realized and unrealized investment gains/(losses) recorded in the statement of operations	21.5	(41.2)	103.3	(13.6)
Change in available for sale net unrealized gains:
Fixed income securities	(23.2)	22.7	104	(27.4)
Equity securities	0	0	0	(27.4)
Total change in pre-tax available for sale unrealized gains	(23.2)	22.7	104	(54.8)
Change in taxes	2	0.8	(11.1)	2.1
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	(21.2)	23.5	92.9	(52.7)
Total fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	5.5	2.4	14.7	9.7
Fixed income maturities — gross realized (losses)	(0.9)	(0.3)	(4.6)	(1.4)
Cash and Cash Equivalents Gross Realized Gains	0	0	0.1	0
Cash and cash equivalents — gross realized gains (losses)	0	0	(0.5)	0
Equity securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Equity securities — gross realized gains	0	0	0	31.9
Equity securities — gross realized (losses)	0	0	0	(3)
Equity securities — gross realized gains	8.1	7.8	23.3	36.2
Equity securities — gross realized (losses)	(4.2)	(12.3)	(22.2)	(25.4)
Total short-term investments — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	0.1	0	0	0
Fixed income maturities — gross realized (losses)	(0.2)	0	0	0
Fixed Income Maturities - Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	4.3	0.5	9.5	3.8
Fixed income maturities — gross realized (losses)	$ (0.5)	$ (1.2)	$ (6.9)	$ (3.8)

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, 2016	Dec. 31, 2015
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 5,946.3	$ 6,030.4
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	190	109.2
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(2.5)	(25.6)
Available-for-sale Securities	6,133.8	6,114
U.S. government
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,061.9	1,113.9
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	24.7	13
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(0.2)	(3.8)
Available-for-sale Securities	1,086.4	1,123.1
U.S. agency
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	124.1	154.5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	3.2	4.3
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	(0.1)
Available-for-sale Securities	127.3	158.7
Municipal
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	23.1	25
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	3.2	1.6
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(0.1)	0
Available-for-sale Securities	26.2	26.6
Corporate
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,695.5	2,626.2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	97.1	49.5
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(1.8)	(15.1)
Available-for-sale Securities	2,790.8	2,660.6
Non-U.S. government-backed corporate
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	81.2	81.6
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	1.2	0.6
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	(0.1)
Available-for-sale Securities	82.4	82.1
Foreign government
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	566	634.6
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	17.2	10.5
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(0.1)	(0.9)
Available-for-sale Securities	583.1	644.2
Asset-backed
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	68.1	75.4
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	1	0.9
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	(0.3)
Available-for-sale Securities	69.1	76
Non-agency commercial mortgage-backed
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	14.9	25.5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0.6	1.2
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Available-for-sale Securities	15.5	26.7
Agency mortgage-backed
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,142.4	1,130.8
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	41.8	27.6
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(0.3)	(5.3)
Available-for-sale Securities	1,183.9	1,153.1
Total fixed income securities — Available for sale
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	5,777.2	5,867.5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	190	109.2
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(2.5)	(25.6)
Available-for-sale Securities	5,964.7	5,951.1
Total short-term investments — Available for sale
Statement [Line Items]
Available-for-sale Securities, Amortized Cost Basis	169.1	162.9
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Available-for-sale Securities	$ 169.1	$ 162.9

Investments - Cost, Gross Unr44

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Statement [Line Items]
Fair Market Value	$ 996.9	$ 788
U.S. government
Statement [Line Items]
Cost or Amortized Cost	43.5	27.4
Gross Unrealized Gains	0.9	0
Gross Unrealized Losses	0	(0.1)
Fair Market Value	44.4	27.3
Municipal
Statement [Line Items]
Cost or Amortized Cost	4.7	0.5
Gross Unrealized Gains	0.2	0
Gross Unrealized Losses	0	0
Fair Market Value	4.9	0.5
Corporate
Statement [Line Items]
Cost or Amortized Cost	655	561.9
Gross Unrealized Gains	22.3	5.9
Gross Unrealized Losses	(0.3)	(9.6)
Fair Market Value	677	558.2
Foreign government
Statement [Line Items]
Cost or Amortized Cost	200.8	181.5
Gross Unrealized Gains	12.6	1.7
Gross Unrealized Losses	(0.3)	(3.7)
Fair Market Value	213.1	179.5
Asset-backed
Statement [Line Items]
Cost or Amortized Cost	15.5	20.7
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	(0.1)	(0.2)
Fair Market Value	15.5	20.5
Agency mortgage-backed
Statement [Line Items]
Cost or Amortized Cost	41.1
Gross Unrealized Gains	0.9
Gross Unrealized Losses	0
Fair Market Value	42
Bank loans
Statement [Line Items]
Cost or Amortized Cost		2.2
Gross Unrealized Gains		0
Gross Unrealized Losses		(0.2)
Fair Market Value		2
Total fixed income securities — Trading
Statement [Line Items]
Cost or Amortized Cost	960.6	794.2
Gross Unrealized Gains	37	7.6
Gross Unrealized Losses	(0.7)	(13.8)
Fair Market Value	996.9	788
Total short-term investments — Trading
Statement [Line Items]
Cost or Amortized Cost	169.6	9.5
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	169.6	9.5
Total equity securities — Trading
Statement [Line Items]
Cost or Amortized Cost	737.1	722.5
Gross Unrealized Gains	91.6	57.3
Gross Unrealized Losses	(31)	(43.4)
Fair Market Value	797.7	736.4
Total catastrophe bonds — Trading
Statement [Line Items]
Cost or Amortized Cost	17.5	55.2
Gross Unrealized Gains	0.3	0.3
Gross Unrealized Losses	0	(0.1)
Fair Market Value	17.8	55.4
Total
Statement [Line Items]
Cost or Amortized Cost	1,884.8	1,581.4
Gross Unrealized Gains	128.9	65.2
Gross Unrealized Losses	(31.7)	(57.3)
Fair Market Value	$ 1,982	$ 1,589.3

Investments - Other Investments

Investments - Other Investments (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	$ 8.7	$ 8.9	$ 8.7
Initial investment	3.3	3.3	0.8
Equity Method Investment, Unrealized Intercompany Profit (Loss) Not Eliminated, Amount	1	0.8
Closing value of investment	13	13	9.5
MVI
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0.6	0.8	0
Initial investment	0	0	0.8
Equity Method Investment, Unrealized Intercompany Profit (Loss) Not Eliminated, Amount	0	(0.2)
Closing value of investment	0.6	0.6	0.8
Chaspark
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	8.1	8.1	8.7
Initial investment	0	0	0
Equity Method Investment, Unrealized Intercompany Profit (Loss) Not Eliminated, Amount	1	1
Closing value of investment	9.1	9.1	$ 8.7
Bene
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0	0
Initial investment	3.3	3.3
Equity Method Investment, Unrealized Intercompany Profit (Loss) Not Eliminated, Amount	0	0
Closing value of investment	$ 3.3	$ 3.3

Investments - Summary of Fixed

Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
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	$ 5,777.2	$ 5,867.5
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,946.3	6,030.4
Available for sale investments in fixed income maturities, Fair Market Value	5,964.7	5,951.1
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	5,777.2	5,867.5
Available for sale investments in fixed income maturities, Fair Market Value	5,964.7	5,951.1
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	14.9	25.5
Available for sale investments in fixed income maturities, Fair Market Value	$ 15.5	$ 26.7
Available for sale investments Average Ratings by Maturity	AA+	AA+
Agency mortgage-backed
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	$ 1,142.4	$ 1,130.8
Available for sale investments in fixed income maturities, Fair Market Value	$ 1,183.9	$ 1,153.1
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	$ 68.1	$ 75.4
Available for sale investments in fixed income maturities, Fair Market Value	$ 69.1	$ 76
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	$ 528.7	$ 661.8
Due after one year through five years, Cost or Amortized Cost	2,743.4	2,765.2
Due after five years through ten years, Cost or Amortized Cost	1,174.8	1,122.5
Due after ten years, Cost or Amortized Cost	104.9	86.3
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	4,551.8	4,635.8
Due one year or less, Fair Market Value	531.8	664.4
Due after one year through five years, Fair Market Value	2,816	2,806.6
Due after five years through ten years, Fair Market Value	1,228.5	1,132
Due after ten years, Fair Market Value	119.9	92.3
Available for sale investments in fixed income maturities, Fair Market Value	$ 4,696.2	$ 4,695.3
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, 2016USD ($)Security	Dec. 31, 2015USD ($)Security
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 489	$ 2,453.9
0-12 months, Gross Unrealized Loss	(1.6)	(21.9)
Over 12 months, Fair Market Value	93.2	164.9
Over 12 months, Gross Unrealized Loss	(0.9)	(3.7)
Total, Fair Market Value	582.2	2,618.8
Total, Gross Unrealized Loss	$ (2.5)	$ (25.6)
Number of Securities | Security	155	805
Total fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 480.1	$ 2,397.2
0-12 months, Gross Unrealized Loss	(1.6)	(21.9)
Over 12 months, Fair Market Value	93.2	164.9
Over 12 months, Gross Unrealized Loss	(0.9)	(3.7)
Total, Fair Market Value	573.3	2,562.1
Total, Gross Unrealized Loss	$ (2.5)	$ (25.6)
Number of Securities | Security	150	793
Total short-term investments — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 8.9	$ 56.7
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	8.9	56.7
Total, Gross Unrealized Loss	$ 0	$ 0
Number of Securities | Security	5	12
U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 137.5	$ 583.2
0-12 months, Gross Unrealized Loss	(0.2)	(3.7)
Over 12 months, Fair Market Value	0	4.6
Over 12 months, Gross Unrealized Loss	0	(0.1)
Total, Fair Market Value	137.5	587.8
Total, Gross Unrealized Loss	$ (0.2)	$ (3.8)
Number of Securities | Security	17	72
U.S. agency
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 0	$ 17.6
0-12 months, Gross Unrealized Loss	0	(0.1)
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	0	17.6
Total, Gross Unrealized Loss	$ 0	$ (0.1)
Number of Securities | Security	0	12
Municipal
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 0.8	$ 1.7
0-12 months, Gross Unrealized Loss	(0.1)	0
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	0.8	1.7
Total, Gross Unrealized Loss	$ (0.1)	$ 0
Number of Securities | Security	3	3
Corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 240.3	$ 1,179.7
0-12 months, Gross Unrealized Loss	(1.2)	(13.3)
Over 12 months, Fair Market Value	25.9	81.1
Over 12 months, Gross Unrealized Loss	(0.6)	(1.8)
Total, Fair Market Value	266.2	1,260.8
Total, Gross Unrealized Loss	$ (1.8)	$ (15.1)
Number of Securities | Security	87	510
Non-U.S. government-backed corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 0.4	$ 40.9
0-12 months, Gross Unrealized Loss	0	(0.1)
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	0.4	40.9
Total, Gross Unrealized Loss	$ 0	$ (0.1)
Number of Securities | Security	1	9
Foreign government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 73.2	$ 174.6
0-12 months, Gross Unrealized Loss	(0.1)	(0.8)
Over 12 months, Fair Market Value	33	2.8
Over 12 months, Gross Unrealized Loss	0	(0.1)
Total, Fair Market Value	106.2	177.4
Total, Gross Unrealized Loss	$ (0.1)	$ (0.9)
Number of Securities | Security	12	43
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 0	$ 51.4
0-12 months, Gross Unrealized Loss	0	(0.3)
Over 12 months, Fair Market Value	2.2	4.2
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	2.2	55.6
Total, Gross Unrealized Loss	$ 0	$ (0.3)
Number of Securities | Security	5	39
Agency mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 27.9	$ 348.1
0-12 months, Gross Unrealized Loss	0	(3.6)
Over 12 months, Fair Market Value	32.1	72.2
Over 12 months, Gross Unrealized Loss	(0.3)	(1.7)
Total, Fair Market Value	60	420.3
Total, Gross Unrealized Loss	$ (0.3)	$ (5.3)
Number of Securities | Security	25	105

Investments - Narrative (Detail

Investments - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)security	Sep. 30, 2015USD ($)	Dec. 31, 2015USD ($)security	Jul. 26, 2016	Jun. 30, 2016USD ($)	Jun. 30, 2015USD ($)	Dec. 31, 2014USD ($)	Oct. 02, 2012
Investment Securities [Line Items]
Other than Temporary Impairment Losses, Investments	$ 0	$ 0		$ 0
Held capital requirement, percentage	20.00%		20.00%		100.00%
Catastrophe bonds	$ 17,800,000		$ 17,800,000		$ 55,400,000
Initial investment	3,300,000		3,300,000	800,000
Equity Method Investment, Aggregate Cost	13,000,000	9,500,000	$ 13,000,000	9,500,000	$ 8,900,000		$ 8,700,000	$ 9,500,000	$ 8,700,000
Number Of Municipal Holdings | security			1		1
Aspen Bermuda Limited
Investment Securities [Line Items]
Transfer from Investments			$ 41,700,000
MVI
Investment Securities [Line Items]
Initial investment	0		0	800,000
Equity Method Investment, Aggregate Cost	600,000	800,000	600,000	800,000	$ 800,000		600,000	800,000	0
Chaspark Maritime Holdings Ltd [Member]
Investment Securities [Line Items]
Initial investment	0		0	0
Percentage of ownership acquired										58.50%
Equity Method Investment, Aggregate Cost	9,100,000	$ 8,700,000	9,100,000	$ 8,700,000	8,100,000		8,100,000	$ 8,700,000	$ 8,700,000
Bene
Investment Securities [Line Items]
Initial investment	3,300,000		3,300,000
Percentage of ownership acquired						20.00%
Equity Method Investment, Aggregate Cost	$ 3,300,000		$ 3,300,000		$ 0		$ 0

Variable Interest Entities - Na

Variable Interest Entities - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2016USD ($)variable_interest_entity	Sep. 30, 2015USD ($)	Sep. 30, 2016USD ($)variable_interest_entity	Sep. 30, 2015USD ($)
Variable Interest Entity [Line Items]
Number of variable interest entities | variable_interest_entity	2		2
Chaspark Maritime Holdings Ltd [Member]
Variable Interest Entity [Line Items]
Change in fair value of investment | $	$ 1,000,000	$ 0	$ 1,000,000	$ 0

Variable Interest Entities - Su

Variable Interest Entities - Summary of Silverton Loan Notes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Movement in Silverton Loan Notes [Roll Forward]
Change in fair value of loan notes issued by variable interest entities	$ (9.8)	$ (8.3)	$ (13.7)	$ (14.5)
Third party
Movement in Silverton Loan Notes [Roll Forward]
Notes Payable, Related Parties, Beginning Balance			190.6	138.6
Change in fair value of loan notes issued by variable interest entities			13.7	14.5
Total distributed in the period			(89.3)	(67.8)
Notes Payable, Related Parties, Ending Balance	115	85.3	115	85.3
Other Accrued Liabilities, Noncurrent	112.7	84.5	112.7	84.5
Other Accrued Liabilities, Current	2.3	0.8	2.3	0.8
Aspen Holdings
Movement in Silverton Loan Notes [Roll Forward]
Notes Payable, Related Parties, Beginning Balance			44.4	35.6
Change in fair value of loan notes issued by variable interest entities			3.4	3.4
Total distributed in the period			(19.4)	(20.3)
Notes Payable, Related Parties, Ending Balance	28.4	18.7	28.4	18.7
Other Accrued Liabilities, Noncurrent	27.9	18.5	27.9	18.5
Other Accrued Liabilities, Current	0.5	0.2	0.5	0.2
Aspen Holdings and Third Parties
Movement in Silverton Loan Notes [Roll Forward]
Notes Payable, Related Parties, Beginning Balance			235	174.2
Change in fair value of loan notes issued by variable interest entities			17.1	17.9
Total distributed in the period			(108.7)	(88.1)
Notes Payable, Related Parties, Ending Balance	143.4	104	143.4	104
Other Accrued Liabilities, Noncurrent	140.6	103	140.6	103
Other Accrued Liabilities, Current	$ 2.8	$ 1	$ 2.8	$ 1

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value		$ 17.5	$ 55.2
Derivatives at fair value		6.3	9.2
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value		17.8
Loan notes issued by variable interest entities, at fair value		(112.7)	(103)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)		(2.3)	(87.6)
Total		8,000.6	7,517.9
Fair Value, Measurements, 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		169.6	9.5
Fair Value, Measurements, Recurring | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		797.7	736.4
Fair Value, Measurements, 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		6.3	8.8
Fair Value, Measurements, 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			0.4
Liabilities under derivative contracts, fair value		0
Fair Value, Measurements, 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		(6.5)	(4)
Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		1,086.4	1,123.1
Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		127.3	158.7
Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		26.2	26.6
Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		2,790.8	2,660.6
Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		82.4	82.1
Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		583.1	644.2
Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		69.1	76
Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		15.5	26.7
Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		1,183.9	1,153.1
Fixed income securities — Available for sale | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		5,964.7	5,951.1
Short-term investments — Available for sale | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		169.1	162.9
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		996.9	788
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		44.4	27.3
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		4.9	0.5
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		677	558.2
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		213.1	179.5
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		15.5	20.5
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Bank loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure			2
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		42
Catastrophe bonds trading, at fair value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value			55.4
Level 1 | Fair Value, Measurements, 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 (included within accrued expenses and other payables)		0	0
Total		2,737.8	2,548
Level 1 | Fair Value, Measurements, 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		169.6	7.4
Level 1 | Fair Value, Measurements, Recurring | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		797.7	736.4
Level 1 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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
Liabilities under derivative contracts, fair value		0
Level 1 | Fair Value, Measurements, 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 | Fair Value, Measurements, Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		1,086.4	1,123.1
Level 1 | Available for sale financial assets, at fair value | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		404.1	449.5
Level 1 | Available for sale financial assets, at fair value | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		1,490.5	1,572.6
Level 1 | Short-term investments — Available for sale | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		141.7	130.5
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		138.3	101.1
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		44.4	27.3
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		93.9	73.8
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, 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 | Fair Value, Measurements, Recurring | Bank loans
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 | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0
Level 1 | Catastrophe bonds trading, at fair value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value			0
Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value		17.8
Loan notes issued by variable interest entities, at fair value		0	0
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)		0	0
Total		5,377.8	5,160.5
Level 2 | Fair Value, Measurements, 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	2.1
Level 2 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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		6.3	8.8
Level 2 | Fair Value, Measurements, 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			0.4
Liabilities under derivative contracts, fair value		0
Level 2 | Fair Value, Measurements, 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		(6.5)	(4)
Level 2 | Available for sale financial assets, at fair value | Fair Value, Measurements, 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 | Fair Value, Measurements, Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		127.3	158.7
Level 2 | Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		26.2	26.6
Level 2 | Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		2,790.8	2,660.6
Level 2 | Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		82.4	82.1
Level 2 | Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		179	194.7
Level 2 | Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		69.1	76
Level 2 | Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		15.5	26.7
Level 2 | Available for sale financial assets, at fair value | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		1,183.9	1,153.1
Level 2 | Fixed income securities — Available for sale | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		4,474.2	4,378.5
Level 2 | Short-term investments — Available for sale | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		27.4	32.4
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		858.6	686.9
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, 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 | Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		4.9	0.5
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		677	558.2
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		119.2	105.7
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		15.5	20.5
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Bank loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure			2
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		42
Level 2 | Catastrophe bonds trading, at fair value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value			55.4
Level 3 | Fair Value, Measurements, 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		(112.7)	(103)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)		(2.3)	(87.6)
Total		(115)	(190.6)
Level 3 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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
Liabilities under derivative contracts, fair value		0
Level 3 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, Recurring
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 | Fair Value, Measurements, Recurring
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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, 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 | Fair Value, Measurements, Recurring | Bank loans
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 | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure		0
Level 3 | Catastrophe bonds trading, at fair value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value			0
Silverton 2016 | Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	[1]	$ (115)
Silverton 2015 | Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	[1]		$ 190.6
[1]	The amount classified within accrued expenses and other payables was $2.3 million and $87.6 million as at September 30, 2016 and December 31, 2015, 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		12 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)		$ (2.3)		$ (2.3)		$ (87.6)
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 (included within accrued expenses and other payables)		(2.3)		(2.3)		(87.6)
Level 3 | Silverton 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	[1]	105.5		190.6
Distributed to third party		(0.3)		(89.3)
Total change in fair value included in the statement of operations		9.8		13.7
Ending balance	[1]	$ 115		$ 115		190.6
Level 3 | Silverton 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	[1]		$ 77.8		$ 138.6	138.6
Distributed to third party			(0.8)		(67.8)	$ 67.8
Total change in fair value included in the statement of operations			8.3		14.5
Ending balance	[1]		$ 85.3		$ 85.3
[1]	The amount classified within accrued expenses and other payables was $2.3 million and $87.6 million as at September 30, 2016 and December 31, 2015, respectively.

Fair Value Measurements - Prici

Fair Value Measurements - Pricing Sources Used in Pricing Fixed Income Investments (Details)	Sep. 30, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Index providers	86.00%	85.00%
Pricing services	8.00%	10.00%
Broker-dealers	6.00%	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, 2016	Dec. 31, 2015
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 6,755.7	$ 6,454.5
% of Total Fair Value by Security Type	87.00%	86.00%
Fixed Income Maturities
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 5,959.5	$ 5,718.1
% of Total Fair Value by Security Type	85.90%	85.00%
Fixed Income Maturities | U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 1,130.8	$ 1,095.4
% of Total Fair Value by Security Type	100.00%	95.00%
Fixed Income Maturities | U.S. agency
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 117.7	$ 148.5
% of Total Fair Value by Security Type	92.50%	94.00%
Fixed Income Maturities | Municipal
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 20.4	$ 10.5
% of Total Fair Value by Security Type	65.60%	39.00%
Fixed Income Maturities | Corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 3,327.7	$ 3,083.5
% 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	$ 38.5	$ 41.7
% of Total Fair Value by Security Type	46.80%	51.00%
Fixed Income Maturities | Foreign government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 511.9	$ 517.6
% of Total Fair Value by Security Type	66.00%	63.00%
Fixed Income Maturities | Asset-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 35.2	$ 55.3
% of Total Fair Value by Security Type	41.50%	57.00%
Fixed Income Maturities | Non-agency commercial mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 13.7	$ 22.7
% of Total Fair Value by Security Type	88.40%	85.00%
Fixed Income Maturities | Agency mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 763.6	$ 742.9
% of Total Fair Value by Security Type	62.30%	64.00%
Equities
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 796.2	$ 736.4
% of Total Fair Value by Security Type	99.80%	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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums		$ 763.5	$ 720.5	$ 2,540.9	$ 2,362.5
Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums				57.2		$ 38.9
Reserve for losses				$ 8.2		$ 4.2
Contract period				365 days		365 days
Loan notes held by third parties		(220)		$ (220)		$ (220)
Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums				43.4		0
Reserve for losses				2.8		0
Loan notes held by third parties		(220)		(220)		(220)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net change from current period hedged transactions		(8,000.6)		(8,000.6)		(7,517.9)
Loan notes held by third parties		(112.7)		(112.7)		(103)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)		(2.3)		(2.3)		(87.6)
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net change from current period hedged transactions		115		115		190.6
Loan notes held by third parties		(112.7)		(112.7)		(103)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)		$ (2.3)		$ (2.3)		(87.6)
Fair Value, Measurements, Recurring | Level 3 | Silverton 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net change from current period hedged transactions	[1]					$ (190.6)
[1]	The amount classified within accrued expenses and other payables was $2.3 million and $87.6 million as at September 30, 2016 and December 31, 2015, respectively.

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details)	Sep. 30, 2016quote / Investment	Dec. 31, 2015USD ($)quote / Investment	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Quotes per fixed income investment | quote / Investment	2.3	2
Quotes per equity investment | quote / Investment	3.9	4
Silverton 2,014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans issued				$ 65,000,000
Silverton 2,015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans issued			$ 85,000,000
Silverton 2,016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans issued		$ 125,000,000
Third party | Silverton 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans issued				$ 50,000,000
Third party | Silverton 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans issued			$ 70,000,000
Third party | Silverton 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans issued		$ 100,000,000

Reinsurance - Narrative (Detail

Reinsurance - Narrative (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Munich Re
Concentration Risk [Line Items]
Concentration Risk, Percentage	19.10%	20.40%
Lloyd's
Concentration Risk [Line Items]
Concentration Risk, Percentage	18.10%	20.00%
Arch Re
Concentration Risk [Line Items]
Concentration Risk, Percentage	7.80%	9.20%

Derivative Contracts - Fair Val

Derivative Contracts - Fair Value of Derivative Instruments (Details) - USD ($)		9 Months Ended		12 Months Ended
		Sep. 30, 2016		Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Derivatives at fair value		$ 6,300,000		$ 9,200,000
Cash collateral provided		0		10,100,000
Foreign Exchange Contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Cash collateral provided		3,500,000		0
Foreign Exchange Contracts | Designated as Hedging Instrument | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivative notional amount		28,600,000		0
Fair Value		400,000	[1]	0
Foreign Exchange Contracts | Designated as Hedging Instrument | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Derivative notional amount		0		113,600,000
Fair Value	[1]	0		(1,200,000)
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Derivative notional amount		595,600,000		379,900,000
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivative notional amount		309,100,000		217,700,000
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Liability notional amount		286,500,000		162,200,000
Interest Rate Swaps | Derivatives Not Designated as Hedging Instruments | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivative notional amount		0		756,300,000
Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivatives at fair value		5,900,000		8,800,000
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		6,500,000		2,800,000
Level 2 | Fair Value, Measurements, Recurring | Interest Rate Swaps | Derivatives Not Designated as Hedging Instruments | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Liabilities at fair value	[2]	$ 0		$ 400,000
[1]	Net of $3.5 million cash collateral (December 31, 2015 — $Nil).
[2]	Net of $10.1 million of cash collateral provided to the counterparty, Goldman Sachs International ($256.3 million notional) under an International Swap Dealers Association agreement, which was terminated on May 9, 2016, as security for the Company’s net liability position

Derivative Contracts - Gain_(Lo

Derivative Contracts - Gain/(Loss) Recognized in Income on Derivative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
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	$ 0.6	$ 12.9	$ (3.7)	$ 10.5
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	0	(2.8)	(3.3)	(6.2)
Designated as Hedging Instrument | Foreign Exchange Contracts | General and Administrative Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contract settlement	(3.1)	(0.8)	(5.6)	(3.5)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net change from current period hedged transactions [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value	$ 3.1	$ (0.2)	$ (1.9)	$ 2.5

Derivative Contracts - Narrativ

Derivative Contracts - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended			12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016		Sep. 30, 2015	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Cash collateral provided	$ 0		$ 0			$ 10,100,000
Pledged assets	0		0			0
Goldman Sachs International
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	256,300,000		256,300,000
Foreign Exchange Contracts | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Cash collateral provided	3,500,000		3,500,000			0
Foreign Exchange Contracts | Designated as Hedging Instrument | General and Administrative Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contract settlement	(3,100,000)	$ (800,000)	(5,600,000)		$ (3,500,000)
Foreign Exchange Contracts | Designated as Hedging Instrument | Net change from current period hedged transactions [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value	3,100,000	(200,000)	(1,900,000)		2,500,000
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	595,600,000		595,600,000			379,900,000
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	600,000	12,900,000	(3,700,000)		10,500,000
Foreign Exchange Contracts | Derivatives at Fair Value | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	28,600,000		28,600,000			0
Fair Value			400,000	[1]		0
Foreign Exchange Contracts | Derivatives at Fair Value | Derivatives Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	309,100,000		309,100,000			217,700,000
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	0	$ (2,800,000)	(3,300,000)		$ (6,200,000)
Interest Rate Swaps | Derivatives at Fair Value | Derivatives Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	$ 0		$ 0			$ 756,300,000
[1]	Net of $3.5 million cash collateral (December 31, 2015 — $Nil).

Deferred Policy Acquisition C61

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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance at the beginning of the period	$ 409.1	$ 349	$ 361.1	$ 299
Acquisition costs deferred	110	129.8	414.9	413.2
Amortization of deferred policy acquisition costs	(130.9)	(132)	(387.8)	(365.4)
Balance at the end of the period	$ 388.2	$ 346.8	$ 388.2	$ 346.8

Reserves for Losses and Loss 62

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, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Provision for losses and LAE at the start of the year	$ 4,938,200,000	$ 4,750,800,000	$ 4,750,800,000
Less reinsurance recoverable			(354,800,000)	$ (350,000,000)
Net loss and LAE at the start of the year	4,583,400,000	4,400,800,000	4,400,800,000
Net loss and LAE expenses assumed	5,700,000	0	0
Provision for losses and LAE for claims incurred:
Current year	1,267,000,000	1,129,800,000	1,522,700,000
Prior years	(78,200,000)	(97,600,000)	(156,500,000)
Total incurred	1,188,800,000	1,032,200,000	1,366,200,000
Losses and LAE payments for claims incurred:
Current year	(62,100,000)	(93,400,000)	(141,900,000)
Prior years	(853,100,000)	(718,700,000)	(966,600,000)
Total paid	(915,200,000)	(812,100,000)	(1,108,500,000)
Foreign exchange (gains)	(35,900,000)	(55,700,000)	(75,100,000)
Net losses and LAE reserves at period end	4,826,800,000	4,565,200,000	4,583,400,000
Plus reinsurance recoverable on unpaid losses at period end	419,800,000	348,700,000	354,800,000
Provision for losses and LAE at the end of the relevant period	$ 5,246,600,000	$ 4,913,900,000	$ 4,938,200,000

Reserves for Losses and Loss 63

Reserves for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Insurance [Abstract]
Reserve releases	$ (78,200,000)	$ (97,600,000)	$ (156,500,000)
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss portfolio transfers (disposed)	5,700,000	$ 0	$ 0
AgriLogic
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss portfolio transfers (disposed)	$ 5,700,000

Capital Structure - Summary of

Capital Structure - Summary of Authorized and Issued Share Capital (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Ordinary shares, par value (in usd per share)	$ 0.0015144558	$ 0.0015144558
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)	60,210,770	60,918,373
Issued ordinary shares of 0.15144558¢ per share	$ 91	$ 92
Total issued share capital	$ 141	$ 127
Non Voting Shares Par Or Stated Value Per Share	$ 0.0015144558	$ 0.0015144558
Preference shares, par value (in usd per share)	0.0015144558	0.0015144558
Preferred Stock, Liquidation Preference Per Share	$ 25	$ 25
7.401% preference shares
Issued share capital:
Preference shares, issued (in shares)	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)	$ 0.0015144558	$ 0.0015144558
Preferred Stock, Liquidation Preference Per Share	$ 25	$ 25
Preferred Stock Dividends Rate Percentage	7.401%	7.401%
7.250% preference shares
Issued share capital:
Preference shares, issued (in shares)	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)	$ 0.0015144558	$ 0.0015144558
Preferred Stock, Liquidation Preference Per Share	$ 25	$ 25
Preferred Stock Dividends Rate Percentage	7.25%	7.25%
5.95% preference shares
Issued share capital:
Preference shares, issued (in shares)	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)	$ 0.0015144558	$ 0.0015144558
Preferred Stock, Liquidation Preference Per Share	$ 25	$ 25
Preferred Stock Dividends Rate Percentage	5.95%	5.95%
5.625% preference shares
Issued share capital:
Preference shares, issued (in shares)	10,000,000	0
Issued preference shares of 0.15144558¢ each with a liquidation preference	$ 15	$ 0
Preference shares, par value (in usd per share)	$ 0.0015144558	$ 0
Preferred Stock, Liquidation Preference Per Share	$ 25	$ 0
Preferred Stock Dividends Rate Percentage	5.625%	0.00%

Capital Structure - Summary o65

Capital Structure - Summary of Ordinary Shares (Details)	9 Months Ended
	Sep. 30, 2016shares
Ordinary Shares [Roll Forward]
Ordinary shares in issue as at December 31, 2015	60,918,373
Ordinary shares issued to employees under the 2013 share incentive plan and/or 2008 share purchase plan	405,290
Ordinary shares issued to non-employee directors	9,435
Ordinary shares repurchased	(1,122,328)
Ordinary shares in issue as at September 30, 2016	60,210,770

Capital Structure - Narrative (

Capital Structure - Narrative (Details) - USD ($)	Feb. 05, 2015	Apr. 17, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Class of Stock [Line Items]
Additional paid-in capital			$ 1,280,200,000		$ 1,280,200,000		$ 1,075,300,000
Aggregate liquidation preferences			$ 818,200,000		$ 818,200,000		$ 568,200,000
Preferred Stock, Liquidation Preference Per Share			$ 25		$ 25		$ 25
Convertible preferred stock, shares issued upon conversion (in shares)		0.001
Minimum period before exercisable		10 days
Preferred Stock, Liquidation Preference, Value			$ 250,000,000		$ 250,000,000
Issuance costs					$ 21,100,000		$ 12,400,000
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)			144,289	0	1,122,328	1,790,333
Shares repurchase and cancelled during period, value			$ 6,500,000	$ 0	$ 50,000,000	$ 83,700,000
Average price (in usd per share)			$ 45.17	$ 0	$ 44.55	$ 46.74
5.625% preference shares
Class of Stock [Line Items]
Preferred Stock, Shares Issued			10,000,000		10,000,000		0
Preferred Stock, Dividend Rate, Percentage					5.625%		0.00%
Preferred Stock, Liquidation Preference Per Share			$ 25		$ 25		$ 0
Stock Issued During Period, Value, New Issues					$ 241,300,000
Issuance costs					$ 8,700,000
7.401% preference shares
Class of Stock [Line Items]
Preferred Stock, Shares Issued			5,327,500		5,327,500		5,327,500
Preferred Stock, Dividend Rate, Percentage					7.401%		7.401%
Preferred Stock, Liquidation Preference Per Share			$ 25		$ 25		$ 25
7.250% preference shares
Class of Stock [Line Items]
Preferred Stock, Shares Issued			6,400,000		6,400,000		6,400,000
Preferred Stock, Dividend Rate, Percentage					7.25%		7.25%
Preferred Stock, Liquidation Preference Per Share			$ 25		$ 25		$ 25

Share-Based Payments - Narrativ

Share-Based Payments - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2016USD ($)shares	Sep. 30, 2015USD ($)shares	Sep. 30, 2016USD ($)Agreementshares	Sep. 30, 2016GBP (£)Agreementshares	Sep. 30, 2015USD ($)shares	Feb. 08, 2016shares	Feb. 10, 2015shares
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
Options exercised (in shares) | shares	0	2,000	29,222	29,222	85,938
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			4.65%	4.65%
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			4.65%	4.65%
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			9.30%	9.30%
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			9.30%	9.30%
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			18.60%	18.60%
Eligible for vesting, percent			200.00%	200.00%
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
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
2016 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	263,695		263,695	263,695
Sharesave Scheme
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			3 years	3 years
Purchase price of fair market value			(85.00%)	(85.00%)
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
Additional employee contribution period			1 year	1 year
Employee contribution period			2 years	2 years
Shares issued (in shares) | shares	189	264	13,342	13,342	54,940
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		$ 300,000
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
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
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	$ (300,000)	$ 2,600,000	$ 2,500,000		$ 4,800,000
Granted, number of shares (in shares) | shares	0	0	278,477	278,477	277,585
Tax benefit from compensation expense	$ 100,000	$ (600,000)	$ (500,000)		$ (1,100,000)
Phantom Shares
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Compensation costs	$ (200,000)	$ 1,400,000	$ 800,000		$ 3,400,000
Granted, number of shares (in shares) | shares	1,156	0	147,513	147,513	135,651
Fair value adjustment capitalized amount	$ 2,500,000	$ (200,000)	$ 3,800,000		$ 1,800,000
Tax benefit from compensation expense	300,000	100,000	$ 600,000		800,000
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	$ 2,600,000	$ 2,100,000	$ 7,200,000		$ 6,800,000
Granted, number of shares (in shares) | shares	35,852	11,614	328,551	328,551	253,766
Fair value adjustment capitalized amount	$ 300,000	$ 0	$ 600,000		$ 300,000
Tax benefit from compensation expense	(600,000)	(700,000)	(1,700,000)		(1,400,000)
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	$ 400,000	$ 1,100,000		$ 1,300,000
RSUs for non-employee directors (in shares) | shares						24,456	27,620
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						10,952	12,154
Grant Year 2017 and 2018 | 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 and Goodwil68

Intangible Assets and Goodwill - Summary of Changes in Intangible Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Amortization	$ (0.8)	$ 0	$ (3.8)	$ 0
Goodwill [Roll Forward]
Goodwill, Beginning of the period	22.1	0	0	0
Goodwill, Acquired During Period	0		22.1
Goodwill, End of the period	22.1	0	22.1	0
Intangible assets and goodwill, Beginning of the period	72	18.2	18.2	18.2
Intangible Assets and Goodwill, Acquired	1.9		58.7
Intangible assets and goodwill, End of the period	73.1	18.2	73.1	18.2
Trade Mark
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period			1.6
Intangible Assets, Additions	0		4
Intangible Assets, End of the period	1.6		1.6
Insurance Licenses
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period	16.6	16.6	16.6	16.6
Intangible Assets, Additions	0		0
Amortization	0	0	0	0
Intangible Assets, End of the period	16.6	16.6	16.6	16.6
Agency Relationships
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period	24.2	0	0	0
Intangible Assets, Additions	0		25
Amortization	(0.4)	0	(1.2)	0
Intangible Assets, End of the period	23.8	0	23.8	0
Non-compete Agreements
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period	2.6	0	0	0
Intangible Assets, Additions	0		2.9
Amortization	(0.1)	0	(0.4)	0
Intangible Assets, End of the period	2.5	0	2.5	0
Value of Business Acquired
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period	0.2	0	0	0
Intangible Assets, Additions	0		1.8
Amortization	0.2	0	(1.8)	0
Intangible Assets, End of the period	0	0	0	0
Consulting Relationships
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period	0.9	0	0	0
Intangible Assets, Additions	0		1
Amortization	0	0	(0.1)	0
Intangible Assets, End of the period	0.9	0	0.9	0
Customer Lists
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period	0	0	0	0
Intangible Assets, Additions	1.9		1.9
Amortization	0	0	0	0
Intangible Assets, End of the period	1.9	0	1.9	0
Trade Mark
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period	5.4	1.6	1.6	1.6
Amortization	(0.1)	0	(0.3)	0
Intangible Assets, End of the period	$ 5.3	$ 1.6	$ 5.3	$ 1.6

Intangible Assets and Goodwil69

Intangible Assets and Goodwill - Narrative (Details) - USD ($) $ in Millions	Apr. 05, 2005	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Jan. 19, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Business Combination, Contingent Consideration, Asset, Noncurrent						$ 14.1
Business Combination, Consideration Transferred, Other		$ 22.8		$ 22.8
Business Combination, Consideration Transferred, Liabilities Incurred				2
Business Combination, Consideration Transferred				69.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment						12
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Residual Assets				0.3
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net						$ 47
Goodwill, Acquired During Period		0		22.1
AgriLogic [Member]
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Percentage of interests acquired						100.00%
Aggregate consideration for sale and purchase agreement				53
Aspen U.K.
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Aggregate consideration for sale and purchase agreement	$ 1.6
Agency Relationships
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		23.8		23.8	$ 24.2	$ 25	$ 0	$ 0	$ 0	$ 0
Intangible Assets, Additions		0		$ 25
Asset amortization period				15 years
Trade Mark
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		1.6		$ 1.6		4	1.6
Intangible Assets, Additions		0		$ 4
Asset amortization period				10 years
Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		16.6		$ 16.6	16.6		16.6	16.6	16.6	16.6
Intangible Assets, Additions		0		0
Insurance Licenses | Aspen U.K.
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses		2.1		2.1
Insurance Licenses | Aspen Specialty
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses		4.5		4.5
Insurance Licenses | AAIC
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses		10		10
Consulting Relationships
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		0.9		0.9	0.9	$ 1	0	0	0	0
Intangible Assets, Additions		0		$ 1
Asset amortization period				10 years
Customer Lists
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		1.9		$ 1.9	0		0	0	0	0
Intangible Assets, Additions		1.9		$ 1.9
Asset amortization period				5 years
Non-compete Agreements
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		2.5		$ 2.5	2.6		0	0	0	0
Intangible Assets, Additions		0		2.9
Asset amortization period			5 years
Value of Business Acquired
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		0		0	$ 0.2		$ 0	$ 0	$ 0	$ 0
Intangible Assets, Additions		$ 0		$ 1.8

Commitments and Contingent Li70

Commitments and Contingent Liabilities - Schedule of Company's Restricted Assets (Details) £ in Millions, $ in Millions		Sep. 30, 2016USD ($)	Sep. 30, 2016GBP (£)	Dec. 31, 2015USD ($)	Dec. 31, 2015GBP (£)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets		$ 4,580		$ 4,395.1
Total as percent of investable assets(2)	[1]	49.00%	49.00%	49.60%	49.60%
Deposit securities and cash as collateral secured letters of credit		$ 698.2	£ 0	$ 697.6	£ 7.1
Letters of credit / guarantees
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	[2]	698.2		708.5
Affiliated transactions
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets		1,432.2		1,421
Third party
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets		$ 2,449.6		$ 2,265.6
[1]	The comparative balance has been re-presented to reflect total restricted investable assets as a percent of investable assets. Investable assets comprise total investments, cash and cash equivalents, accrued interest, receivables for securities sold and payables for securities purchased.
[2]	As at September 30, 2016, the Company pledged funds of $698.2 million and £0.0 million (December 31, 2015 — $697.6 million and £7.1 million) as collateral for its secured letters of credit.

Commitments and Contingent Li71

Commitments and Contingent Liabilities - Narrative (Details)	Jun. 12, 2013USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2016GBP (£)	Dec. 31, 2015USD ($)	Dec. 31, 2015GBP (£)
Line of Credit Facility [Line Items]
Deposit securities and cash as collateral secured letters of credit		$ 698,200,000	£ 0	$ 697,600,000	£ 7,100,000
Assets held by insurance regulators		456,100,000		436,800,000
Minimum capital required | £			£ 400,000		£ 400,000
Maximum borrowing capacity	$ 200,000,000
Cash collateral provided		0		$ 10,100,000
Line Of Credit Facility Increasable Capacity	$ 100,000,000
Long-term Line of Credit		0
Line Of Credit Facility Tangible Net Worth Restrictions Minimum		$ 2,428,600,000
Line Of Credit Facility Tangible Net Worth Net Income Restriction		50.00%
Line Of Credit Facility Tangible Net Worth Cash Proceeds From Issuance Of Capital Stock Restriction		50.00%
Percentage Of Consolidated Leverage Ratio Permitted		35.00%
Citi Europe | Letter of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 550,000,000
Barclays Bank PLC | Letter of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity		100,000,000
Outstanding letters of credit		$ 0
LIBOR | Citi Europe | Letter of Credit
Line of Credit Facility [Line Items]
Variable rate basis spread		1.00%

Commitments and Contingent Li72

Commitments and Contingent Liabilities - Amounts Outstanding under Operating Leases (Details) $ in Millions	Sep. 30, 2016USD ($)
Leases [Abstract]
2,016	$ 3.3
2,017	16.1
2,018	15.3
2,019	14
2,020	9.9
Later Years	88.4
Total	$ 147