Aspen Insurance Holdings Limited (AHL-PE) 10-Q 31 Oct 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	9 Months Ended
	Sep. 30, 2017shares
Document and Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
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	59,407,323

UNAUDITED CONDENSED CONSOLIDATE

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Investments:
Fixed income securities, available for sale at fair value (amortized cost — $5,305.7 and $5,620.1)	$ 5,365.4	$ 5,664.6
Fixed income securities, trading at fair value (amortized cost — $1,642.4 and $1,264.8)	1,665	1,265.7
Equity securities, trading at fair value (cost — $407.2 and $554.3)	468.6	584.7
Short-term investments, available for sale at fair value (amortized cost — $34.4 and $145.3)	34.4	145.3
Short-term investments, trading at fair value (amortized cost — $90.3 and $185.4)	90.3	185.4
Catastrophe bonds, trading at fair value (cost — $33.5 and $42.5)	30.3	42.5
Other investments, equity method	4	12.1
Total investments	7,658	7,900.3
Cash and cash equivalents (including $164.2 and $291.3 within consolidated variable interest entities)	1,209.3	1,273.8
Reinsurance recoverables
Unpaid losses	1,369.5	560.7
Ceded unearned premiums	446.4	255.2
Receivables
Underwriting premiums	1,529	1,399.4
Other	175.8	95.5
Funds withheld	100.1	73.1
Deferred policy acquisition costs	354.1	358.4
Derivatives at fair value	8.4	7.2
Receivables for securities sold	19	1.6
Office properties and equipment	89.8	83.8
Tax recoverable	13.1	0.5
Other assets	1	1
Intangible assets and goodwill	78.1	79.6
Total assets	13,051.6	12,090.1
Insurance reserves
Losses and loss adjustment expenses	6,490.6	5,319.9
Unearned premiums	1,926.5	1,618.6
Total insurance reserves	8,417.1	6,938.5
Payables
Reinsurance premiums	439.4	345.3
Deferred taxation	0.5	6.1
Accrued expenses and other payables	377.9	469.2
Liabilities under derivative contracts	2.9	18.4
Total payables	820.7	839
Loan notes issued by variable interest entities, at fair value	101.5	115
Long-term debt	549.4	549.3
Total liabilities	9,888.7	8,441.8
Commitments and contingent liabilities (see Note 16)	0	0
SHAREHOLDERS’ EQUITY
Ordinary shares, value	0.1	0.1
Preference shares:
Non-controlling interest	2.2	1.4
Additional paid-in capital	951.8	1,259.6
Retained earnings	2,234.1	2,392.3
Accumulated other comprehensive income, net of taxes	(25.3)	(5.1)
Total shareholders’ equity	3,162.9	3,648.3
Total liabilities and shareholders’ equity	13,051.6	12,090.1
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, 2017	Dec. 31, 2016
Fixed income securities, available for sale, amortized cost	$ 5,305.7	$ 5,620.1
Fixed income securities, trading at fair value (amortized cost )	1,642.4	1,264.8
Equity securities, trading at fair value, cost	407.2	554.3
Short-term Investments, available for sale, amortized cost	34.4	145.3
Short-term investments, trading at fair value, amortized cost	90.3	185.4
Catastrophe bonds, trading at fair value, cost	33.5	42.5
Cash and cash equivalents, within consolidated VIEs	$ 164.2	$ 291.3
Ordinary shares, par value (in usd per share)	$ 0.0015144558	$ 0.0015144558
Ordinary shares, issued (in shares)	59,407,323	59,774,464
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)	0	5,327,500
Preference shares, rate	7.401%	7.401%
Preference shares, par value (in usd per share)		$ 0.0015144558
7.250% preference shares
Preference shares, issued (in shares)	0	6,400,000
Preference shares, rate	7.25%	7.25%
Preference shares, par value (in usd per share)		$ 0.0015144558
5.625% preference shares
Preference shares, issued (in shares)	10,000,000	10,000,000
Preference shares, rate	5.625%	5.625%
Preference shares, par value (in usd per share)	$ 0.0015144558	$ 0.0015144558

UNAUDITED CONDENSED CONSOLIDAT4

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND OTHER COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016
Revenues
Net earned premium	$ 652.5		$ 681	$ 1,795.6		$ 2,024.9
Net investment income	46.4		46.4	141.5		143.9
Realized and unrealized investment gains	29.9		26.7	130.1		137.4
Other income	(2.2)		1.5	5		2.4
Total revenues	726.6		755.6	2,072.2		2,308.6
Expenses
Losses and loss adjustment expenses	776.2		389.2	1,450.5		1,188.8
Amortization of deferred policy acquisition costs	105.4		130.9	315.4		387.8
General, administrative and corporate expenses	110.9		125	352.1		361.2
Interest on long-term debt	7.4		7.3	22.2		22.1
Change in fair value of derivatives	(4.5)		(0.6)	(25.2)		7
Change in fair value of loan notes issued by variable interest entities	(9.8)		9.8	(3.6)		13.7
Realized and unrealized investment losses	12.4		5.2	24.4		34.1
Net realized and unrealized foreign exchange (gains)/losses	(8.4)		(10.8)	21.1		10.2
Other (income)/expenses	0		(0.9)	2		0.1
Total expenses	989.6		655.1	2,158.9		2,025
(Loss)/income from operations before income tax	(263)		100.5	(86.7)		283.6
Income tax credit/(expense)	9.2		(4.9)	5.2		(8.7)
Net (loss)/income	(253.8)		95.6	(81.5)		274.9
Amount attributable to non-controlling interest	(0.6)		0.2	(0.8)		0
Net (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	(254.4)		95.8	(82.3)		274.9
Available for sale investments:
Reclassification adjustment for net realized gains on investments included in net income	(1)		(4.4)	(2.8)		(9.7)
Change in net unrealized gains/losses on available for sale securities held	1.3		(18.8)	18		113.7
Net change from current period hedged transactions	(0.4)		3.1	3.3		(1.9)
Change in foreign currency translation adjustment	(4.9)		(10.2)	(49.7)		(26.8)
Other comprehensive (loss)/income, gross of tax	(5)		(30.3)	(31.2)		75.3
Tax thereon:
Reclassification adjustment for net realized gains on investments included in net income	0.2		0	0.4		0.6
Change in net unrealized gains on available for sale securities held	(0.6)		2	(1.8)		(11.7)
Net change from current period hedged transactions	0		(0.6)	(0.5)		0.6
Change in foreign currency translation adjustment	2.3		3.7	12.9		7.5
Total tax on other comprehensive income	1.9		5.1	11		(3)
Other comprehensive (loss)/income net of tax	(3.1)		(25.2)	(20.2)		72.3
Total comprehensive (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	$ (257.5)		$ 70.6	$ (102.5)		$ 347.2
Weighted average number of ordinary share and share equivalents (1)
Basic (in shares)	59,759,730	[1]	60,225,705	59,862,540	[1]	60,588,307
Diluted (in shares)	59,759,730	[1]	61,577,018	59,862,540	[1]	62,043,440
Basic (loss)/earnings per ordinary share adjusted for preference share dividends	$ (4.48)		$ 1.43	$ (1.99)		$ 4.07
Diluted (loss)/earnings per ordinary share adjusted for preference share dividends	$ (4.48)		$ 1.40	$ (1.99)		$ 3.97
[1]	The basic and diluted number of ordinary shares for the three and nine months ended September 30, 2017 is the same as the inclusion of dilutive securities in a loss making period would be anti-dilutive.

UNAUDITED CONDENSED CONSOLIDAT5

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Millions		Total	Ordinary shares	Preference shares	7.401% preference shares	7.250% preference shares	Non-controlling interest	Additional paid-in capital	Additional paid-in capitalOrdinary shares	Additional paid-in capitalPreference shares		Retained earnings		Retained earningsOrdinary shares	Retained earningsPreference shares		Cumulative foreign currency translation adjustments, net of taxes:	Loss on derivatives, net of taxes:	Unrealized appreciation on investments, net of taxes:
Shareholder's equity, beginning balance at Dec. 31, 2015			$ 0.1	$ 0			$ 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 shares issued									$ 2	$ 241.3
Ordinary shares repurchased and cancelled		(50)							(50)
Share-based compensation	[1]							11.6
Net (loss)/income for the period		274.9										274.9
Dividends on ordinary shares														$ (39.5)
Dividends on preference shares		(28.4)													$ (28.4)
Preference shares redemption		0								0	[2]				0	[2]
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]
Preference share redemption										0
Shareholder's equity, beginning balance at Dec. 31, 2015			0.1	0			1.3	1,075.3				2,283.6					0.6	(1.2)	60.2
Shareholder's equity, ending balance at Dec. 31, 2016		3,648.3	0.1	0			1.4	1,259.6				2,392.3					(27.1)	(0.5)	22.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preference shares, rate					7.401%	7.25%
Net change from current period hedged transactions																		(1.3)
Total accumulated other comprehensive (loss)/income, net of taxes		131.9
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-09	[3]											0
Total accumulated other comprehensive (loss)/income, net of taxes		(5.1)
Net change attributable to non-controlling interest for the period		(0.8)					0.8					(0.8)
New shares issued									0.4	0
Ordinary shares repurchased and cancelled		(30)							$ (30)
Share-based compensation	[1]							7
Net (loss)/income for the period		(81.5)										(81.5)
Dividends on ordinary shares														$ (42)
Dividends on preference shares		(28.7)													(28.7)
Preference shares redemption		8								(8)	[2]				$ 8	[2]
Change for the period, net of income tax																	(36.8)		13.8
Shareholder's equity, ending balance at Sep. 30, 2017		3,162.9	$ 0.1	$ 0			$ 2.2	951.8				2,234.1					$ (63.9)	2.3	$ 36.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preference shares, rate					7.401%	7.25%
Preference share redemption										$ (293.2)
Net change from current period hedged transactions																		$ 2.8
Total accumulated other comprehensive (loss)/income, net of taxes		$ (25.3)
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-09								$ 7.9				$ 2.8	[3]
[1]	The balance in 2017 includes $7.9 million reclassification from accrued expenses and other payable as a result of the classification of restricted share units as equity following the adoption of ASU 2016-09.
[2]	The $8.0 million reclassification from additional paid-in capital to retained earnings is the difference between the capital raised upon issuance of the 7.401% and the 7.250% Perpetual Non-Cumulative Preference Shares, net of issuance costs, and the total redemption costs of $293.2 million.
[3]	The $2.8 million relates to the cumulative effect-adjustment to opening retained earnings as a result of the classification of restricted share units as equity following the adoption of ASU 2016-09. The adjustment has been applied using a modified retrospective approach.

UNAUDITED CONDENSED CONSOLIDAT6

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Cash flows from/(used in) operating activities:
Net (loss)/income		$ (81.5)	$ 274.9
Proportion due to non-controlling interest		(0.8)	0
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization		39.7	35.6
Share-based compensation		7	11.6
Realized and unrealized investment (gains)		(130.1)	(137.4)
Realized and unrealized investment losses		24.4	34.1
Change in fair value of loan notes issued by variable interest entities		(3.6)	13.7
Net realized and unrealized investment foreign exchange losses		(10.7)	(3.1)
Net change from current period hedged transactions		2.8	(1.3)
Insurance reserves:
Losses and loss adjustment expenses		1,045.5	349.7
Unearned premiums		278.1	181.4
Reinsurance recoverables:
Unpaid losses		(797.9)	(73.1)
Ceded unearned premiums		(188.9)	(62)
Other receivables		(70.1)	(34)
Deferred policy acquisition costs		9.6	(26.4)
Reinsurance premiums payable		108.3	76.4
Funds withheld		(27)	(15.6)
Premiums receivable		(133.3)	(306.9)
Deferred taxes		(5.6)	9.5
Income tax payable		(1)	8
Accrued expenses and other payables		13	26.4
Fair value of derivatives and settlement of liabilities under derivatives		(16.7)	5.4
Long-term debt and loan notes issued by variable interest entities		(13.4)	9.8
Other assets		0	3.1
Net cash from operating activities		47.8	379.8
Cash flows from/(used in) investing activities:
(Purchases) of fixed income securities — Available for sale		(1,239.4)	(1,671.4)
(Purchases) of fixed income securities — Trading		(1,125.1)	(605.1)
Proceeds from sales and maturities of fixed income securities — Available for sale		1,581.9	1,668.7
Proceeds from sales and maturities of fixed income securities — Trading		763.9	481
(Purchases) of equity securities — Trading		(111.5)	(159.9)
Net proceeds from sales of catastrophe bonds — Trading		8.7	37.6
Proceeds from sales of equity securities — Trading		301.2	146.4
(Purchases) of short-term investments — Available for sale		(42.7)	(202.7)
Proceeds from sales of short-term investments — Available for sale		156.4	199.9
(Purchases) of short-term investments — Trading		(68.7)	(166.3)
Proceeds from sales of short-term investments — Trading		167.8	6.3
Net change in (payable)/receivable for securities (purchased)/sold		20.6	14.5
Net (purchases) of equipment		(26.7)	(15.7)
Sale of investment		9.3	0
Equity investment		(0.1)	(3.3)
Payments for acquisitions and investments, net of cash acquired		(2.3)	(52.7)
Net cash from/(used in) investing activities		393.3	(322.7)
Cash flows (used in)/from financing activities:
Proceeds from the issuance of ordinary shares, net of issuance costs		0.4	2
Ordinary shares repurchased		(30)	(50)
Proceeds from the issuance of preference shares, net of issuance costs		0	241.3
Preference share redemption		(293.2)	0
Repayment of long-term debt issued by Silverton		(115.9)	(89.3)
Dividends paid on ordinary shares		(42)	(39.5)
Dividends paid on preference shares		(28.7)	(28.4)
Cash paid for tax withholding purposes (1)	[1]	(9.5)	0
Net cash (used in)/from financing activities		(518.9)	36.1
Effect of exchange rate movements on cash and cash equivalents		13.3	(9.4)
Decrease in cash and cash equivalents		(64.5)	83.8
Cash and cash equivalents at beginning of period		1,273.8	1,099.5
Cash and cash equivalents at end of period		1,209.3	1,183.3
Supplemental disclosure of cash flow information:
Net cash paid (received) during the period for income tax		(0.2)	0.2
Cash paid during the period for interest		$ 22.2	$ 14.5
[1]	The cash paid to the tax authority when withholding shares from employees’ awards for tax-withholding purposes has been reclassified from operating activity to financing activity following the adoption of ASU 2016-09.

History and Organization

History and Organization	9 Months Ended
	Sep. 30, 2017
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 as a holding company headquartered in Bermuda. We underwrite specialty insurance and reinsurance on a global basis through our Operating Subsidiaries (as defined below) based in Bermuda, the United States and the United Kingdom: Aspen Insurance UK Limited (“Aspen U.K.”) and Aspen Underwriting Limited (corporate member of Lloyd’s Syndicate 4711, “AUL” and managed by Aspen Managing Agency Limited (“AMAL”)) (United Kingdom), Aspen Bermuda Limited (“Aspen Bermuda”) (Bermuda), Aspen Specialty Insurance Company (“Aspen Specialty”) and Aspen American Insurance Company (“AAIC”) (United States) (collectively, the “Operating Subsidiaries”). We also have branches in Australia, Canada, France, Germany, Ireland, Singapore, Switzerland and the United Arab Emirates. We established Aspen Capital Management, Ltd and other related entities (collectively, “ACM”) to leverage our existing underwriting franchise, increase our operational flexibility in the capital markets and provide investors direct access to our underwriting expertise. References to the “Company,” the “Group,” “we,” “us” or “our” refer to Aspen Holdings or Aspen Holdings and its subsidiaries.

Basis of Preparation

Basis of Preparation	9 Months Ended
	Sep. 30, 2017
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 considered necessary for a fair presentation have been included. Results for the three and nine months ended September 30, 2017 are not necessarily indicative of the results that may be expected for the year ended December 31, 2017 . 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, 2016 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, 2016 contained in the Company’s Annual Report on Form 10-K filed with the United States Securities and Exchange Commission (the “SEC”) on February 22, 2017 (File No. 001-31909). Except for the changes associated with ASU 2016-09, “ Compensation — Stock Compensation ” listed below, there have been no changes to significant accounting policies from those disclosed in the Company’s 2016 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 assumptions and estimates relate to losses and loss adjustment expenses, reinsurance recoverables, gross written premiums and commissions which have not been reported to the Company such as those relating to proportional treaty reinsurance contracts, unrecognized tax benefits, the fair value of derivatives and the fair value of other investments. All material assumptions and estimates are regularly reviewed and adjustments made as necessary, but actual results could turn out significantly different from those expected when the assumptions or estimates were made. Accounting Pronouncements Adopted in 2017 On March 30, 2016, the Financial Accounting Standards Board (“FASB”) issued ASU 2016-09, “ Compensation — Stock Compensation ” which provides guidance on several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. This ASU is effective for fiscal years beginning after December 15, 2016 and interim periods beginning after December 15, 2016. Following the adoption of this ASU, all excess tax benefits or expense related to stock-based compensation transactions are recognized prospectively as income tax benefits or expense in the Consolidated Income Statement and the excess tax benefits or expense from stock-based compensation transactions previously included in “Financing activities” on the Consolidated Statements of Cash Flows are prospectively included on that statement as “Operating activities.” The cash paid to the tax authority for tax withholding purposes has also been reclassified from operating to financing activity in the Consolidated Statement of Cash Flows and the comparative period has been restated. This ASU also allows share withholding up to the maximum statutory withholding requirement while still avoiding liability accounting. As a result, the Company has applied the equity accounting method for its restricted share units retrospectively and has recorded a cumulative effect adjustment of $2.8 million through opening retained earnings and $7.9 million through additional paid-in capital. Accounting Pronouncements Not Yet Adopted On March 10, 2017, FASB issued ASU 2017-7, “ Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post Retirement Benefit Cost ” which changes how employers report defined benefit pension and/or other post-retirement benefit costs in their financial statements. This ASU is effective for fiscal years beginning after December 15, 2017 and interim periods beginning after December 15, 2017. The Company has assessed the impact from the adoption of ASU 2017-7 and no material impact is expected on the consolidated financial statements. On August 28, 2017, the FASB issued ASU 2017-12, “Derivatives and Hedging (Topic 815)” enabling entities to better align their hedge accounting and risk management activities, while also simplifying the application of hedge accounting in certain situations. This ASU is effective for fiscal years beginning after 15 December, 2018 using a modified retrospective approach for cash flow and net investment hedge relationships that exist on the date of adoption. The Company is currently evaluating the provisions of ASU 2017-12 to determine how it will be affected, but no material impact is expected on the consolidated financial statements. Other accounting pronouncements were issued during the three months ended September 30, 2017 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, 2017
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, 2017 and 2016 : Amount Reclassified from AOCI Details about the AOCI Components Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 Affected Line Item in the Unaudited ($ in millions) Available for sale securities: Realized gains on sale of securities $ 2.6 $ 5.3 Realized and unrealized investment gains Realized (losses) on sale of securities (1.6 ) (0.9 ) Realized and unrealized investment losses 1.0 4.4 Income from operations before income tax Tax on net realized gains of securities (0.2 ) — Income tax expense $ 0.8 $ 4.4 Net (loss)/income Realized derivatives: Net realized gains/(losses) on settled derivatives $ 1.2 $ (3.1 ) General, administrative and corporate expenses Tax on settled derivatives (0.2 ) 1.1 Income tax expense $ 1.0 $ (2.0 ) Net (loss)/income Total reclassifications from AOCI to the statement of operations, net of income tax $ 1.8 $ 2.4 Net (loss)/income Amount Reclassified from AOCI Details about the AOCI Components Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2016 Affected Line Item in the Unaudited ($ in millions) Available for sale securities: Realized gains on sale of securities $ 8.2 $ 14.8 Realized and unrealized investment gains Realized (losses) on sale of securities (5.4 ) (5.1 ) Realized and unrealized investment losses 2.8 9.7 Income from operations before income tax Tax on net realized gains of securities (0.4 ) (0.6 ) Income tax expense $ 2.4 $ 9.1 Net (loss)/income Realized derivatives: Net realized gains/(losses) on settled derivatives $ 2.4 $ (5.6 ) General, administrative and corporate expenses Tax on settled derivatives (0.5 ) 1.1 Income tax expense $ 1.9 $ (4.5 ) Net (loss)/income Total reclassifications from AOCI to the statement of operations, net of income tax $ 4.3 $ 4.6 Net (loss)/income

Earnings per Ordinary Share

Earnings per Ordinary Share	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Earnings per Ordinary Share	Earnings per Ordinary Share Basic earnings per ordinary share are calculated by dividing net income available to holders of Aspen Holdings’ ordinary shares by the weighted average number of ordinary shares outstanding. Net income available to ordinary shareholders is calculated by deducting preference share dividends and net income/(loss) attributable to non-controlling interest from net income/ (loss) after tax for the period. Diluted earnings per ordinary share are based on the weighted average number of ordinary shares and dilutive potential ordinary shares outstanding during the period of calculation using the treasury stock method. The following table sets forth the computation of basic and diluted earnings per ordinary share for the three and nine months ended September 30, 2017 and 2016 , respectively: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 ($ in millions, except share and per share amounts) Net (loss) income $ (253.8 ) $ 95.6 $ (81.5 ) $ 274.9 Preference share dividends (7.7 ) (9.5 ) (28.7 ) (28.4 ) Preference share redemption costs (1) (5.6 ) — (8.0 ) — Net amount attributable to non-controlling interest (0.6 ) 0.2 (0.8 ) — Basic and diluted net (loss) income available to ordinary shareholders $ (267.7 ) $ 86.3 $ (119.0 ) $ 246.5 Ordinary shares: Basic weighted average ordinary shares 59,759,730 60,225,705 59,862,540 60,588,307 Weighted average effect of dilutive securities (2) (3) — 1,351,313 — 1,455,133 Total diluted weighted average ordinary shares 59,759,730 61,577,018 59,862,540 62,043,440 (Loss) earnings per ordinary share: Basic $ (4.48 ) $ 1.43 $ (1.99 ) $ 4.07 Diluted (3) $ (4.48 ) $ 1.40 $ (1.99 ) $ 3.97 (1) The $8.0 million deduction from net income in 2017 is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the 7.401% and 7.250% Perpetual Non-Cumulative Preference Shares, net of issuance costs, and the total redemption costs of $293.2 million . (2) Dilutive securities consist of employee restricted share units and performance shares associated with the Company’s long-term incentive plan, employee share purchase plans and director restricted share units as described in Note 14. (3) The basic and diluted number of ordinary shares for the three and nine months ended September 30, 2017 is the same as the inclusion of dilutive securities in a loss making period would be anti-dilutive. Dividends. On October 25, 2017, the Company’s Board of Directors (the “Board of Directors”) declared the following quarterly dividends: Dividend Payable on: Record Date: Ordinary shares $ 0.24 November 28, 2017 November 10, 2017 5.95% preference shares $ 0.3719 January 1, 2018 December 15, 2017 5.625% preference shares $ 0.3516 January 1, 2018 December 15, 2017

Segment Reporting

Segment Reporting	9 Months Ended
	Sep. 30, 2017
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 insurance and reinsurance (credit and surety, mortgage reinsurance and insurance, agriculture insurance and reinsurance, marine, aviation, terrorism, engineering, cyber and other specialty lines). ACM forms part of our property catastrophe reinsurance line of business as it focuses primarily on property catastrophe business through the use of alternative capital. For a more detailed description of this business segment, see Part I, Item 1, “Business — Business Segments — Reinsurance” in the Company’s 2016 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 2016 Annual Report on Form 10-K filed with the SEC. Non-underwriting Disclosures. The Company has provided additional disclosures for corporate and other (non-operating) income and expenses. Corporate and other income and expenses include net investment income, net realized and unrealized investment gains or losses, expenses associated with managing the Group, certain strategic and non-recurring costs, changes in fair value of derivatives and changes in fair value of the loan notes issued by variable interest entities, interest expenses, net realized and unrealized foreign exchange gains or losses, and income taxes, none of which are allocated to the business segments. Corporate expenses are not allocated to the Company’s business segments as they typically do not fluctuate with the levels of premiums written and are not directly related to the Company’s business segment operations. The Company does not allocate its assets by business segment as it evaluates underwriting results of each business segment separately from the results of the Company’s investment portfolio. The following tables provide a summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of the Company’s business segments for the three months ended September 30, 2017 and 2016 : Three Months Ended September 30, 2017 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 431.5 $ 421.0 $ 852.5 Net written premiums 363.6 243.8 607.4 Gross earned premiums 464.0 449.3 913.3 Net earned premiums 382.0 270.5 652.5 Underwriting Expenses Losses and loss adjustment expenses 502.2 274.0 776.2 Amortization of deferred policy acquisition costs 61.5 43.9 105.4 General and administrative expenses 32.8 59.4 92.2 Underwriting loss $ (214.5 ) $ (106.8 ) (321.3 ) Corporate expenses (13.5 ) Non-operating expenses (5.2 ) Net investment income 46.4 Realized and unrealized investment gains 29.9 Realized and unrealized investment losses (12.4 ) Change in fair value of loan notes issued by variable interest entities 9.8 Change in fair value of derivatives 4.5 Interest expense on long term debt (7.4 ) Net realized and unrealized foreign exchange gains 8.4 Net other expense (2.2 ) Loss before tax $ (263.0 ) Net reserves for loss and loss adjustment expenses $ 2,865.8 $ 2,255.3 $ 5,121.1 Ratios Loss ratio 131.5 % 101.3 % 119.0 % Policy acquisition expense ratio 16.1 16.2 16.2 General and administrative expense ratio 8.6 22.0 17.0 (1) Expense ratio 24.7 38.2 33.2 Combined ratio 156.2 % 139.5 % 152.2 % (1) The general and administrative expense ratio in the total column includes corporate and non-operating expenses. 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 18.4 (1) Expense ratio 31.8 37.3 37.6 Combined ratio 88.3 % 95.0 % 94.8 % (1) The general and administrative expense ratio in the total column includes corporate and non-operating 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, 2017 and 2016 : Nine Months Ended September 30, 2017 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,332.4 $ 1,340.2 $ 2,672.6 Net written premiums 1,097.3 775.0 1,872.3 Gross earned premiums 1,112.2 1,302.1 2,414.3 Net earned premiums 932.2 863.4 1,795.6 Underwriting Expenses Losses and loss adjustment expenses 797.9 652.6 1,450.5 Amortization of deferred policy acquisition costs 174.4 141.0 315.4 General and administrative expenses 117.4 186.9 304.3 Underwriting loss $ (157.5 ) $ (117.1 ) (274.6 ) Corporate expenses (38.3 ) Non-operating expenses (9.5 ) Net investment income 141.5 Realized and unrealized investment gains 130.1 Realized and unrealized investment losses (24.4 ) Change in fair value of loan notes issued by variable interest entities 3.6 Change in fair value of derivatives 25.2 Interest expense on long term debt (22.2 ) Net realized and unrealized foreign exchange losses (21.1 ) Other income 5.0 Other expense (2.0 ) Loss before tax $ (86.7 ) Net reserves for loss and loss adjustment expenses $ 2,865.8 $ 2,255.3 $ 5,121.1 Ratios Loss ratio 85.6 % 75.6 % 80.8 % Policy acquisition expense ratio 18.7 16.3 17.6 General and administrative expense ratio 12.6 21.6 19.6 (1) Expense ratio 31.3 37.9 37.2 Combined ratio 116.9 % 113.5 % 118.0 % (1) The general and administrative expense ratio in the total column includes corporate and non-operating expenses. 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 17.8 (1) Expense ratio 32.8 35.3 37.0 Combined ratio 88.0 % 96.8 % 95.7 % (1) The general and administrative expense ratio in the total column includes corporate and non-operating expenses. The Company uses underwriting ratios as measures of performance. The loss ratio is the ratio of losses and loss adjustment expenses to net earned premiums. The policy acquisition expense ratio is the ratio of amortization of deferred policy acquisition costs to net earned premiums. The general and administrative expense ratio is the ratio of general, administrative and corporate expenses to net earned premiums. The combined ratio is the sum of the loss ratio, the policy acquisition expense ratio and the general and administrative expense ratio.

Investments

Investments	9 Months Ended
	Sep. 30, 2017
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, 2017 and 2016 : For the Three Months Ended For the Nine Months Ended September 30, 2017 September 30, 2016 September 30, 2017 September 30, 2016 ($ in millions) ($ in millions) Fixed income securities — Available for sale $ 32.9 $ 34.8 $ 100.7 $ 107.9 Fixed income securities — Trading 11.1 7.6 31.6 22.9 Short-term investments — Available for sale 0.1 0.2 0.4 0.5 Short-term investments — Trading — — 0.5 — Cash and cash equivalents 2.1 0.8 4.1 2.2 Equity securities — Trading 2.8 5.5 11.7 17.9 Catastrophe bonds — Trading 0.5 0.2 1.3 1.3 Total $ 49.5 $ 49.1 $ 150.3 $ 152.7 Investment expenses (3.1 ) (2.7 ) (8.8 ) (8.8 ) Net investment income $ 46.4 $ 46.4 $ 141.5 $ 143.9 The following table summarizes the net realized and unrealized investment gains and losses recorded in the statement of operations and the change in unrealized gains and losses on investments recorded in other comprehensive income for the three and nine months ended September 30, 2017 and 2016 : For the Three Months Ended For the Nine Months Ended September 30, 2017 September 30, 2016 September 30, 2017 September 30, 2016 ($ in millions) ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 2.3 $ 5.5 $ 7.8 $ 14.7 Fixed income securities — gross realized (losses) (1.6 ) (0.9 ) (5.3 ) (4.6 ) Short-term investments — gross realized gains — 0.1 0.1 — Short-term investments — gross realized (losses) — (0.2 ) — — Cash and cash equivalents — gross realized gains 0.3 — 0.3 0.1 Cash and cash equivalents — gross realized (losses) — — (0.1 ) (0.5 ) Other-than-temporary impairments (0.1 ) — (0.5 ) — Trading: Fixed income securities — gross realized gains 3.0 4.3 8.3 9.5 Fixed income securities — gross realized (losses) (0.2 ) (0.5 ) (2.6 ) (6.9 ) Short-term investments — gross realized gains 1.9 — 2.0 — Cash and cash equivalents — gross realized gains 1.3 — 1.3 — Equity securities — gross realized gains 46.1 8.1 55.0 23.3 Equity securities — gross realized (losses) (7.4 ) (4.2 ) (12.4 ) (22.2 ) Catastrophe bonds — net unrealized (losses)/gains (3.1 ) 0.3 (3.2 ) 0.1 Net change in gross unrealized (losses)/gains (25.0 ) 8.0 54.4 88.8 Other investments: Gross realized and unrealized (loss) in MVI — — (0.1 ) — Gross realized (loss)/gain in Chaspark — 1.0 0.9 1.0 Gross realized and unrealized (loss) in Bene — — (0.2 ) — Total net realized and unrealized investment gains recorded in the statement of operations $ 17.5 $ 21.5 $ 105.7 $ 103.3 Change in available for sale net unrealized gains: Fixed income securities 0.3 (23.2 ) 15.2 104.0 Total change in pre-tax available for sale unrealized gains 0.3 (23.2 ) 15.2 104.0 Change in taxes (0.4 ) 2.0 (1.4 ) (11.1 ) Total change in net unrealized gains, net of taxes, recorded in other comprehensive income $ (0.1 ) $ (21.2 ) $ 13.8 $ 92.9 Other-Than-Temporary Impairments. A security is potentially impaired when its fair value is below its amortized cost. The Company reviews its available for sale fixed income 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. The total OTTI charge for the three and nine months ended September 30, 2017 was $0.1 million and $0.5 million , respectively ( 2016 — $ Nil and $ Nil ). For a more detailed description of accounting policies for OTTI, please refer to Note 2(c) of the “Notes to the Audited Consolidated Financial Statements” in the Company’s 2016 Annual Report on Form 10-K filed with the SEC. 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, 2017 and December 31, 2016 : As at September 30, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,171.3 $ 7.6 $ (6.2 ) $ 1,172.7 U.S. agency 61.3 0.9 (0.1 ) 62.1 Municipal 47.9 2.2 — 50.1 Corporate 2,431.5 45.4 (8.6 ) 2,468.3 Non-U.S. government-backed corporate 94.0 0.4 (0.4 ) 94.0 Non-U.S. government 525.3 7.9 (1.5 ) 531.7 Asset-backed 33.6 0.1 — 33.7 Agency mortgage-backed 940.8 17.4 (5.4 ) 952.8 Total fixed income securities — Available for sale 5,305.7 81.9 (22.2 ) 5,365.4 Total short-term investments — Available for sale 34.4 — — 34.4 Total $ 5,340.1 $ 81.9 $ (22.2 ) $ 5,399.8 As at December 31, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,207.9 $ 9.4 $ (11.2 ) $ 1,206.1 U.S. agency 117.7 1.9 — 119.6 Municipal 23.2 1.6 (0.4 ) 24.4 Corporate 2,566.9 39.6 (20.0 ) 2,586.5 Non-U.S. government-backed corporate 89.2 0.7 (0.1 ) 89.8 Non-U.S. government 477.7 11.8 (0.8 ) 488.7 Asset-backed 62.6 0.4 — 63.0 Non-agency commercial mortgage-backed 12.3 0.3 — 12.6 Agency mortgage-backed 1,062.6 19.6 (8.3 ) 1,073.9 Total fixed income securities — Available for sale 5,620.1 85.3 (40.8 ) 5,664.6 Total short-term investments — Available for sale 145.3 — — 145.3 Total $ 5,765.4 $ 85.3 $ (40.8 ) $ 5,809.9 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, 2017 and December 31, 2016 : As at September 30, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 160.6 $ 0.6 $ (0.6 ) $ 160.6 Municipal 32.4 — (0.1 ) 32.3 Corporate 1,029.8 17.9 (2.7 ) 1,045.0 Non-U.S government-backed corporate 2.0 — — 2.0 Non-U.S. government 203.5 7.8 (0.4 ) 210.9 Asset-backed 11.0 0.1 — 11.1 Agency mortgage-backed 203.1 0.7 (0.7 ) 203.1 Total fixed income securities — Trading 1,642.4 27.1 (4.5 ) 1,665.0 Total short-term investments — Trading 90.3 — — 90.3 Total equity securities — Trading 407.2 68.7 (7.3 ) 468.6 Total catastrophe bonds — Trading 33.5 — (3.2 ) 30.3 Total $ 2,173.4 $ 95.8 $ (15.0 ) $ 2,254.2 As at December 31, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 82.8 $ 0.4 $ (0.8 ) $ 82.4 Municipal 15.7 — (0.2 ) 15.5 Corporate 817.8 9.9 (7.1 ) 820.6 Non-U.S. government 203.4 3.5 (4.1 ) 202.8 Asset-backed 14.5 — — 14.5 Agency mortgage-backed 130.6 0.2 (0.9 ) 129.9 Total fixed income securities — Trading 1,264.8 14.0 (13.1 ) 1,265.7 Total short-term investments — Trading 185.4 — — 185.4 Total equity securities — Trading 554.3 55.4 (25.0 ) 584.7 Total catastrophe bonds — Trading 42.5 — — 42.5 Total $ 2,047.0 $ 69.4 $ (38.1 ) $ 2,078.3 The Company classifies the financial instruments 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 $30.3 million as at September 30, 2017 . The bonds receive quarterly interest payments based on variable interest rates with scheduled maturities ranging from 2017 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 . The Company made an additional investment of $0.1 million in the quarter ended September 30, 2017 . On October 2, 2012, the Company established a subsidiary, Aspen Recoveries Limited, to take ownership of a 58.5% shareholding in Chaspark Maritime Holdings Ltd., a Singaporean registered company (“Chaspark”), with the remaining shareholding owned by other insurers. The shareholding in Chaspark was received as a settlement for subrogation rights associated with a contract frustration claim settlement. In the three and nine months ended September 30, 2017 , the change in the value of the Company’s investment in Chaspark was a realized loss of $ Nil and a realized gain of $0.9 million , respectively ( September 30, 2016 — $1.0 million unrealized gain and $1.0 million unrealized gain). On March 10, 2017, Aspen Recoveries Limited received cash of $9.3 million as settlement of its share of subrogation assets held by Chaspark. On July 26, 2016, the Company purchased through its wholly-owned subsidiary, Acorn Limited, 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. On January 1, 2017, the Company purchased through its wholly-owned subsidiary, Aspen U.S. Holdings, Inc. (“Aspen U.S. Holdings”), a 49% share of Digital Risk Resources, LLC (“Digital Re”), a U.S.-based enterprise engaged in the business of developing, marketing and servicing turnkey information security and privacy liability insurance products, for a total consideration of $2.3 million . The investment is accounted for under the equity method and adjustments to the carrying value of this investment are made based on the Company’s share of capital, including share of income and expenses. The tables below show the Company’s investments in the MVI, Chaspark, Bene and Digital Re for the three and nine months ended September 30, 2017 and September 30, 2016 : For the Three Months Ended September 30, 2017 MVI Chaspark Bene Digital Re Total ($ in millions) Opening undistributed value of investment $ 0.4 $ — $ 3.0 $ 0.5 $ 3.9 Additional investment 0.1 — — — 0.1 Closing undistributed value of investment $ 0.5 $ — $ 3.0 $ 0.5 $ 4.0 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 Realized/unrealized gain for the three months to September 30, 2016 — 1.0 — 1.0 Closing value of investment $ 0.6 $ 9.1 $ 3.3 $ 13.0 For the Nine Months Ended September 30, 2017 MVI Chaspark Bene Digital Re Total ($ in millions) Opening undistributed value of investment $ 0.5 $ 8.4 $ 3.2 $ — $ 12.1 Initial investment — — — 2.3 2.3 Goodwill — — — (1.8 ) (1.8 ) Additional investment 0.1 — — — 0.1 Distribution received — (9.3 ) — — (9.3 ) Realized/unrealized gain/(losses) for the nine months to September 30, 2017 (0.1 ) 0.9 (0.2 ) — 0.6 Closing undistributed value of investment $ 0.5 $ — $ 3.0 $ 0.5 $ 4.0 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 Realized/unrealized gain for the nine months to September 30, 2016 (0.2 ) 1.0 — 0.8 Closing value of investment $ 0.6 $ 9.1 $ 3.3 $ 13.0 Fixed Income Securities. The scheduled maturity distribution of available for sale fixed income securities as at September 30, 2017 and December 31, 2016 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, 2017 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 577.8 $ 579.2 AA Due after one year through five years 2,597.3 2,623.2 AA- Due after five years through ten years 1,060.6 1,070.1 AA- Due after ten years 95.6 106.4 A+ Subtotal 4,331.3 4,378.9 Agency mortgage-backed 940.8 952.8 AA+ Asset-backed 33.6 33.7 AAA Total fixed income securities — Available for sale $ 5,305.7 $ 5,365.4 As at December 31, 2016 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 567.2 $ 570.0 AA Due after one year through five years 2,643.7 2,671.9 AA- Due after five years through ten years 1,172.3 1,168.1 A+ Due after ten years 99.4 105.1 A+ Subtotal 4,482.6 4,515.1 Non-agency commercial mortgage-backed 12.3 12.6 AAA Agency mortgage-backed 1,062.6 1,073.9 AA+ Asset-backed 62.6 63.0 AAA Total fixed income securities — Available for sale $ 5,620.1 $ 5,664.6 Guaranteed Investments. As at September 30, 2017 , the Company held no investments which are guaranteed by mono-line insurers, excluding those with explicit government guarantees. As at December 31, 2016 , the Company held one municipal bond security guaranteed by a mono-line insurer with fair value less than $0.1 million rated CC or higher. The standalone rating (rating without guarantee) is determined as the senior unsecured debt rating of the issuer. Where the credit ratings were split between the two main rating agencies, Standard & Poor’s Financial Services LLC (“S&P”) and Moody’s Investors Service, Inc. (“Moody’s”), the lowest rating was used. The Company’s exposure to other third-party guaranteed debt is primarily to investments backed by non-U.S. government guaranteed issuers. Gross Unrealized Loss. The following tables summarize, by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position in the Company’s available for sale portfolio as at September 30, 2017 and December 31, 2016 : As at September 30, 2017 0-12 months Over 12 months Total Fair Market Value Gross Unrealized Loss Fair Market Value Gross Unrealized Loss Fair Market Value Gross Unrealized Loss Number of Securities ($ in millions) U.S. government $ 617.5 $ (5.0 ) $ 62.5 $ (1.2 ) $ 680.0 $ (6.2 ) 80 U.S. agency 19.6 (0.1 ) — — 19.6 (0.1 ) 7 Municipal 19.2 — — — 19.2 — 6 Corporate 483.9 (4.6 ) 154.1 (4.0 ) 638.0 (8.6 ) 255 Non-U.S. government-backed corporate 63.4 (0.4 ) 0.4 — 63.8 (0.4 ) 15 Non-U.S. government 211.5 (1.2 ) 36.8 (0.3 ) 248.3 (1.5 ) 43 Asset-backed 12.2 — — — 12.2 — 9 Agency mortgage-backed 426.0 (4.5 ) 33.9 (0.9 ) 459.9 (5.4 ) 133 Total fixed income securities — Available for sale 1,853.3 (15.8 ) 287.7 (6.4 ) 2,141.0 (22.2 ) 548 Total short-term investments — Available for sale 25.7 — — — 25.7 — 10 Total $ 1,879.0 $ (15.8 ) $ 287.7 $ (6.4 ) $ 2,166.7 $ (22.2 ) 558 As at December 31, 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 $ 724.4 $ (11.2 ) $ — $ — $ 724.4 $ (11.2 ) 78 U.S. agency 14.1 — — — 14.1 — 4 Municipal 7.7 (0.2 ) 0.8 (0.2 ) 8.5 (0.4 ) 6 Corporate 1,044.4 (19.4 ) 6.6 (0.6 ) 1,051.0 (20.0 ) 386 Non-U.S. government-backed corporate 29.6 (0.1 ) — — 29.6 (0.1 ) 11 Non-U.S. government 143.5 (0.8 ) 1.0 — 144.5 (0.8 ) 29 Asset-backed 25.8 — 1.4 — 27.2 — 15 Agency mortgage-backed 527.6 (7.6 ) 27.2 (0.7 ) 554.8 (8.3 ) 148 Total fixed income securities — Available for sale 2,517.1 (39.3 ) 37.0 (1.5 ) 2,554.1 (40.8 ) 677 Total short-term investments — Available for sale 1.1 — — — 1.1 — 2 Total $ 2,518.2 $ (39.3 ) $ 37.0 $ (1.5 ) $ 2,555.2 $ (40.8 ) 679

Variable Interest Entities

Variable Interest Entities	9 Months Ended
	Sep. 30, 2017
Payables and Accruals [Abstract]
Variable Interest Entities	Variable Interest Entities As at September 30, 2017 , the Company had investments in three variable interest entities (“VIE”): Chaspark, Peregrine Reinsurance Ltd (“Peregrine”) and Silverton Re Ltd (“Silverton”). Chaspark. The Company has determined that Chaspark has the characteristics of a VIE as addressed by the guidance in ASC 810, Consolidation . For more information on Chaspark, refer to Note 6 of these unaudited condensed consolidated financial statements. Peregrine. In November 2016, Peregrine, a subsidiary of the Company, was registered as a segregated accounts company under the Segregated Accounts Companies Act 2000, as amended. As at September 30, 2017 , Peregrine had three segregated accounts which were funded by a third party investor. The segregated accounts have not been consolidated as part of the Company’s consolidated financial statements. The Company has, however, determined that Peregrine has the characteristics of a VIE as addressed by the guidance in ASC 810, Consolidation . The Company concluded that it is not the primary beneficiary of the three segregated accounts of Peregrine but is the primary beneficiary of the Peregrine general fund and, similar to prior reporting periods, the Company has included the results of the Peregrine general fund in its consolidated financial statements. The Company’s exposure to Peregrine’s general fund is not material. Silverton. On September 10, 2013, the Company established Silverton, a Bermuda domiciled special purpose insurer formed to provide additional collateralized capacity to support Aspen Re’s business through retrocession agreements which are collateralized and funded by Silverton through the issuance of one or more series of participating loan notes (collectively, the “Loan Notes”). Silverton 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, 2017 and 2016 : For the Nine Months Ended September 30, 2017 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 223.4 $ 54.5 $ 277.9 Total change in fair value for the period (3.6 ) (0.9 ) (4.5 ) Total distributed in the period (115.9 ) (28.8 ) (144.7 ) Closing balance as at September 30, 2017 $ 103.9 $ 24.8 $ 128.7 Liability Loan notes (long-term liabilities) $ 101.5 $ 24.2 $ 125.7 Accrued expenses (current liabilities) 2.4 0.6 3.0 Total aggregate unpaid balance as at September 30, 2017 $ 103.9 $ 24.8 $ 128.7 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 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, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The Company’s estimates of fair value for financial assets and liabilities are based on the framework established in the fair value accounting guidance included in ASC 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, 2017 and December 31, 2016 : As at September 30, 2017 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,172.6 $ — $ — $ 1,172.6 U.S. agency — 62.1 — 62.1 Municipal — 50.1 — 50.1 Corporate — 2,468.4 — 2,468.4 Non-U.S. government-backed corporate — 94.0 — 94.0 Non-U.S. government 375.2 156.5 — 531.7 Asset-backed — 33.7 — 33.7 Agency mortgage-backed — 952.8 — 952.8 Total fixed income securities available for sale, at fair value 1,547.8 3,817.6 — 5,365.4 Short-term investments available for sale, at fair value 27.4 7.0 — 34.4 Held for trading financial assets, at fair value U.S. government 160.6 — — 160.6 U.S. agency — — — — Municipal — 32.3 — 32.3 Corporate — 1,045.0 — 1,045.0 Non-U.S. government-backed corporate — 2.0 — 2.0 Non-U.S. government 24.5 186.4 — 210.9 Asset-backed — 11.1 — 11.1 Agency mortgage-backed — 203.1 — 203.1 Total fixed income securities trading, at fair value 185.1 1,479.9 — 1,665.0 Short-term investments trading, at fair value 90.0 0.3 — 90.3 Equity investments trading, at fair value 468.6 — — 468.6 Catastrophe bonds trading, at fair value — 30.3 — 30.3 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 8.4 — 8.4 Liabilities under derivative contracts — foreign exchange contracts — (2.9 ) — (2.9 ) Loan notes issued by variable interest entities, at fair value — — (101.5 ) (101.5 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (2.4 ) (2.4 ) Total $ 2,318.9 $ 5,340.6 $ (103.9 ) $ 7,555.6 There were no transfers between Level 1, Level 2 and Level 3 during the three and nine months ended September 30, 2017 . The Company settled $1.4 million and $115.9 million of Level 3 liabilities in respect of the Loan Notes issued by Silverton for the three and nine months ended September 30, 2017 , respectively. As at September 30, 2017 , there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted solely of the Loan Notes issued by Silverton. As at December 31, 2016 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,206.1 $ — $ — $ 1,206.1 U.S. agency — 119.6 — 119.6 Municipal — 24.4 — 24.4 Corporate — 2,586.5 — 2,586.5 Non-U.S. government-backed corporate — 89.8 — 89.8 Non-U.S. government 343.7 145.0 — 488.7 Asset-backed — 63.0 — 63.0 Non-agency commercial mortgage-backed — 12.6 — 12.6 Agency mortgage-backed — 1,073.9 — 1,073.9 Total fixed income securities available for sale, at fair value 1,549.8 4,114.8 — 5,664.6 Short-term investments available for sale, at fair value 118.6 26.7 — 145.3 Held for trading financial assets, at fair value U.S. government 82.4 — — 82.4 Municipal — 15.5 — 15.5 Corporate — 820.6 — 820.6 Non-U.S. government — 202.8 — 202.8 Asset-backed — 14.5 — 14.5 Agency mortgage-backed — 129.9 — 129.9 Total fixed income securities trading, at fair value 82.4 1,183.3 — 1,265.7 Short-term investments trading, at fair value 76.1 109.3 — 185.4 Equity investments trading, at fair value 584.7 — — 584.7 Catastrophe bonds trading, at fair value — 42.5 — 42.5 Other financial assets and liabilities, at fair value Derivatives at fair value – foreign exchange contracts — 7.2 — 7.2 Liabilities under derivative contracts – foreign exchange contracts — (18.4 ) — (18.4 ) Loan notes issued by variable interest entities, at fair value — — (115.0 ) (115.0 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (108.4 ) (108.4 ) Total $ 2,411.6 $ 5,465.4 $ (223.4 ) $ 7,653.6 Transfers of assets into or out of a particular level are recorded at their fair values as of the end of each reporting period, consistent with the date of the determination of fair value. During the twelve months ended December 31, 2016 , the Company transferred $83.9 million of non-U.S. government securities from Level 1 to Level 2. The Company settled $89.3 million Level 3 liabilities in respect of the Loan Notes issued by Silverton for the twelve months ended December 31, 2016 . As at December 31, 2016 , there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted of the Loan Notes issued by Silverton. The following table presents a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Level 3 inputs for the three and nine months ended September 30, 2017 and 2016 : Reconciliation of Liabilities Using Level 3 Inputs Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 ($ in millions) Balance at the beginning of the period $ 115.1 $ 105.5 Distributed to third party (1.4 ) (0.3 ) Total change in fair value included in the statement of operations (9.8 ) 9.8 Balance at the end of the period (1) $ 103.9 $ 115.0 Reconciliation of Liabilities Using Level 3 Inputs Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2016 ($ in millions) Balance at the beginning of the period 223.4 190.6 Distributed to third party (115.9 ) (89.3 ) Total change in fair value included in the statement of operations (3.6 ) 13.7 Balance at the end of the period (1) 103.9 115.0 (1) The amount classified within accrued expenses and other payables was $2.4 million and $2.3 million as at September 30, 2017 and September 30, 2016 , 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, 2017 and December 31, 2016 , the Company’s fixed income securities were valued by pricing services, index providers or broker-dealers using standard market conventions. The market conventions utilize market quotations, market transactions in comparable instruments and various relationships between instruments including, but not limited to, yield to maturity, dollar prices and spread prices in determining value. Independent Pricing Services and Index Providers. The underlying methodology used to determine the fair value of securities in the Company’s available for sale and trading portfolios by the pricing services and index providers the Company uses is very similar. Pricing services will gather observable pricing inputs from multiple external sources, including buy and sell-side contacts and broker-dealers, in order to develop their internal prices. Index providers are those firms which provide prices for a range of securities within one or more asset classes, typically using their own in-house market makers (traders) as the primary pricing source for the indices, although ultimate valuations may also rely on other observable data inputs to derive a dollar price for all index-eligible securities. Index providers without in-house trading desks will function similarly to a pricing service in that they will gather their observable pricing inputs from multiple external sources. All prices for the Company’s securities attributed to index providers are for an individual security within the respective indices. Pricing services and index providers provide pricing for less complex, liquid securities based on market quotations in active markets. Pricing services and index providers supply prices for a broad range of securities including those for actively traded securities, such as Treasury and other Government securities, in addition to those that trade less frequently or where valuation includes reference to credit spreads, pay down and pre-pay features and other observable inputs. These securities include Government Agency, Municipals, Corporate and Asset-Backed Securities. For securities that may trade less frequently or do not trade on a listed exchange, these pricing services and index providers may use matrix pricing consisting of observable market inputs to estimate the fair value of a security. These observable market inputs include: reported trades, benchmark yields, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, reference data, and industry and economic factors. Additionally, pricing services and index providers may use a valuation model such as an option adjusted spread model commonly used for estimating fair values of mortgage-backed and asset-backed securities. Neither the Company, nor its index providers, derives dollar prices using an index as a pricing input for any individual security. Broker-Dealers. The Company obtains quotes from broker-dealers who are active in the corresponding markets when prices are unavailable from independent pricing services or index providers. Generally, broker-dealers value securities through their trading desks based on observable market inputs. Their pricing methodologies include mapping securities based on trade data, bids or offers, observed spreads and performance of newly issued securities. They may also establish pricing through observing secondary trading of similar securities. 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, 2017 , the Company obtained an average of 2.0 quotes per fixed income investment, compared to 2.1 quotes as at December 31, 2016 . Pricing sources used in pricing fixed income investments as at September 30, 2017 and December 31, 2016 were as follows: As at September 30, 2017 As at December 31, 2016 Index providers 84 % 87 % Pricing services 10 7 Broker-dealers 6 6 Total 100 % 100 % Summary Pricing Information Table. A summary of securities priced using pricing information from index providers as at September 30, 2017 and December 31, 2016 is provided below: As at September 30, 2017 As at December 31, 2016 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 $ 1,333.2 100 % $ 1,288.2 100 % U.S. agency 53.2 86 % 110.2 92 % Municipal 31.2 38 % 28.8 72 % Corporate 3,346.0 95 % 3,275.3 96 % Non-U.S. government-backed corporate 47.8 50 % 44.8 50 % Non-U.S. government 425.6 61 % 455.6 72 % Asset-backed 14.2 32 % 32.1 41 % Non-agency commercial mortgage-backed — — % 12.5 98 % Agency mortgage-backed 618.2 54 % 691.9 58 % Total fixed income securities $ 5,869.4 84 % $ 5,939.4 87 % Equities 468.6 100 % 584.7 100 % Total fixed income securities and equity investments $ 6,338.0 85 % $ 6,524.1 88 % The Company, in conjunction with its third-party accounting service provider, obtains an understanding of the methods, models and inputs used by the third-party pricing service and index providers to assess the ongoing appropriateness of vendors’ prices. The Company and its third-party accounting service provider also have controls in place to validate that amounts provided represent fair values. Processes to validate and review pricing include, but are not limited to: • quantitative analysis (e.g., comparing the quarterly return for each managed portfolio to its target benchmark, with significant differences identified and investigated); • comparison of market values obtained from pricing services, index providers and broker-dealers against alternative price sources for each security where further investigation is completed when significant differences exist for pricing of individual securities between pricing sources; • initial and ongoing evaluation of methodologies used by outside parties to calculate fair value; and • comparison of the fair value estimates to the Company’s knowledge of the current market. Prices obtained from pricing services, index providers and broker-dealers are not adjusted by us; however, prices provided by a pricing service, index provider or broker-dealer in certain instances may be challenged based on information available from market or 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, Non-U.S. 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 certain non-U.S. government bonds, primarily sourced from international indices, are based on unadjusted market prices in active markets and are therefore classified within Level 1. The remaining non-U.S government bonds are classified within Level 2 as they are not actively traded. The fair values of the non-U.S. agency securities, again primarily sourced from international indices, are priced using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of non-U.S. agency securities are classified within Level 2. In addition, foreign government securities include a portion of the Emerging Market Debt (“EMD”) portfolio which is also classified within Level 2. Corporate. Corporate securities consist primarily of U.S. and foreign corporations covering a variety of industries and are for the most part priced by index providers and pricing vendors. Some issuers may participate in government programs which guarantee timely payment of principal and interest in the event of a default. The fair values of these securities are generally determined using the spread above the risk-free yield curve. Inputs used in the evaluation of these securities include credit data, interest rate data, market observations and sector news, broker-dealer quotes and trade volumes. In addition, corporate securities include a portion of the EMD portfolio. The Company classifies all of these securities within Level 2. Mortgage-backed Securities. 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. 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 income investments and are classified within Levels 1 and 2. Equity Securities. Equity securities include U.S. and foreign common stocks and are classified as trading and carried at fair value. 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, 2017 , the Company obtained an average of 4.0 quotes per equity investment, compared to 3.9 quotes as at December 31, 2016 . Pricing sources used in pricing equities as at September 30, 2017 and December 31, 2016 were substantially 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. Loan Notes Issued by Silverton . Silverton, a licensed special purpose insurer, is consolidated into the Company’s accounts as a VIE. 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. In the fourth quarter of 2016, Silverton issued an additional $130.0 million ( $105.0 million third-party funded) of Loan Notes with a maturity date of September 16, 2019. The Company elected to account for the Loan Notes at fair value using the guidance as prescribed under ASC 825, Financial Instruments as the Company believes it represents the most meaningful measurement basis for these liabilities. The Loan Notes are recorded at fair value at each reporting period and, as they are not quoted on an active market and contain significant unobservable inputs, they have been classified as a Level 3 instrument in the Company’s fair value hierarchy. The Loan Notes are unique because they are linked to the specific risks of the Company’s property catastrophe book. To determine the fair value of the Loan Notes the Company runs an internal model which considers the seasonality of the risk assumed under the retrocessional agreement between Aspen Bermuda or a combination of Aspen Bermuda and Aspen U.K., as ceding reinsurers, and Silverton. The seasonality used in the model is initially determined by applying the percentage of property catastrophe losses planned by the Company’s actuaries to the estimated written premium to determine earned premium for each quarter. The inputs to the internal model are based on Company specific data due to the lack of observable market inputs. Reserves for losses are the most significant unobservable input. An increase in reserves for losses would normally result in a decrease in the fair value of the Loan Notes while a decrease in reserves would normally result in an increase in the fair value of the Loan Notes. The observable and unobservable inputs used to determine the fair value of the Loan Notes as at September 30, 2017 and December 31, 2016 are presented in the tables below: As at September 30, 2017 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 103.9 (1) Internal Valuation Model Gross premiums written (O) $ 50.1 $ 57.6 Reserve for losses (U) $ 4.2 $ 37.5 Contract period (O) N/A 365 days Initial value of issuance (O) $ 325.0 $ 325.0 As at December 31, 2016 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 223.4 (1) Internal Valuation Model Gross premiums written (O) $ 38.9 $ 43.4 Reserve for losses (U) $ 2.7 $ 11.8 Contract period (O) N/A 365 days Initial value of issuance (O) $ 220.0 $ 220.0 (1) The amounts classified within accrued expenses and other payables were $2.4 million and $108.4 million as at September 30, 2017 and December 31, 2016, 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. For further information regarding Silverton, refer to Note 7 of these unaudited condensed consolidated financial statements.

Reinsurance

Reinsurance	9 Months Ended
	Sep. 30, 2017
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 un-collateralized reinsurance recoverables for unpaid losses as at September 30, 2017 were 9.4% ( December 31, 2016 — 16.8% ) with Munich Re which is rated AA- by S&P, 6.6% ( December 31, 2016 — 12.8% ) with Lloyd’s Syndicates which are rated A+ by S&P and 2.5% ( December 31, 2016 — 4.6% ) with Axis which is rated A+ by S&P.

Derivative Contracts

Derivative Contracts	9 Months Ended
	Sep. 30, 2017
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, 2017 and December 31, 2016 : As at September 30, 2017 As at December 31, 2016 Derivatives Not Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Derivatives at Fair Value $ 521.5 $ 6.0 (1) $ 240.2 $ 5.0 Foreign Exchange Contracts Liabilities under Derivative Contracts $ 238.7 $ (2.9 ) $ 425.4 $ (17.7 ) (1) Net of $0.6 million cash collateral ( December 31, 2016 — $ Nil ). As at September 30, 2017 As at December 31, 2016 Derivatives Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Derivatives at Fair Value $ 27.3 $ 2.4 (1) $ — $ 2.2 Foreign Exchange Contracts Liabilities under Derivative Contracts $ — $ — $ 108.6 $ (0.7 ) (1) Net of $ Nil cash collateral ( December 31, 2016 — $2.2 million ). The following table provides the unrealized and realized gains/(losses) recorded in the statements of operations and other comprehensive income for derivatives that are not designated or designated as hedging instruments under ASC 815 - " Derivatives and Hedging" for the three and nine months ended September 30, 2017 and 2016 . Amount of Gain/(Loss) Recognized on Derivatives Three Months Ended Nine Months Ended Location of Gain/(Loss) Recognized on Derivatives September 30, 2017 September 30, 2016 September 30, 2017 September 30, 2016 Derivatives not designated as hedges ($ in millions) ($in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives 4.5 0.6 25.2 (3.7 ) Interest Rate Swaps Change in Fair Value of Derivatives — — — (3.3 ) Derivatives designated as hedges Foreign Exchange Contracts General, administrative and corporate expenses 1.2 (3.1 ) 2.4 (5.6 ) Foreign Exchange Contracts Net change from current period hedged transactions (0.4 ) 3.1 3.3 (1.9 ) 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, 2017 , the Company held foreign exchange contracts that were not designated as hedging under ASC 815 with an aggregate notional value of $760.2 million ( December 31, 2016 — $665.6 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, 2017 , the impact of foreign exchange contracts on net income was a gain of $4.5 million ( September 30, 2016 — gain of $0.6 million ) and a gain of $25.2 million ( September 30, 2016 — loss of $3.7 million ), respectively. As at September 30, 2017 , the Company held foreign exchange contracts that were designated as hedging under ASC 815 with an aggregate nominal amount of $27.3 million ( December 31, 2016 — $108.6 million ). The foreign exchange contracts are recorded as derivatives at fair value in the balance sheet with the effective portion recorded in other comprehensive income and the ineffective portion recorded as a change in fair value of derivatives in the statement of operations. The contracts are considered to be effective and therefore the movement in other comprehensive income representing the effective portion for the three and nine months ended September 30, 2017 was a net unrealized loss of $0.4 million ( September 30, 2016 — gain of $3.1 million ) and a net unrealized gain of $3.3 million ( September 30, 2016 — loss of $1.9 million ), respectively. As the foreign exchange contracts settle, the realized gain or loss is reclassified from other comprehensive income into general, administration and corporate expenses of the statement of operations and other comprehensive income. For the three and nine months ended September 30, 2017 , the amount recognized within general, administrative and corporate expenses for settled foreign exchange contracts was a realized gain of $1.2 million ( September 30, 2016 — loss of $3.1 million ) and a net realized gain of $2.4 million ( September 30, 2016 — loss of $5.6 million ), respectively. Interest Rate Swaps. In 2014, the Company decided to let its interest rate program roll-off and not renew maturing positions. This decision was made after an extensive reassessment of the costs of maintaining an interest rate swap program in a steep yield curve environment. In addition, the continued uncertainty in the global economy and low inflation make it difficult to gauge the timing and speed of interest rate rises by the Federal Reserve. On May 9, 2016, the Company terminated all remaining outstanding interest rate swaps (notional value of $256.3 million ) under its International Swap Dealers Association agreement. As at September 30, 2017 and December 31, 2016 , the Company no longer had outstanding interest rate swaps. There was no charge in respect of the interest rate swaps for the three and nine months ended September 30, 2017 ( September 30, 2016 — no charge and a charge of $3.3 million ), respectively.

Deferred Policy Acquisition Cos

Deferred Policy Acquisition Costs	9 Months Ended
	Sep. 30, 2017
Insurance [Abstract]
Deferred Policy Acquisition Costs	Deferred Policy Acquisition Costs The following table represents a reconciliation of beginning and ending deferred policy acquisition costs for the three and nine months ended September 30, 2017 and 2016 : Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 ($ in millions) ($ in millions) Balance at the beginning of the period $ 364.6 $ 409.1 $ 358.4 $ 361.1 Acquisition costs deferred 94.9 110.0 311.1 414.9 Amortization of deferred policy acquisition costs (105.4 ) (130.9 ) (315.4 ) (387.8 ) Balance at the end of the period $ 354.1 $ 388.2 $ 354.1 $ 388.2

Reserves for Losses and Loss Ad

Reserves for Losses and Loss Adjustment Expenses	9 Months Ended
	Sep. 30, 2017
Insurance [Abstract]
Reserves For Loss And Adjustment Expenses [Text Block]	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, 2017 and 2016 and the twelve months ended December 31, 2016 : Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2016 Twelve Months Ended December 31, 2016 ($ in millions) Provision for losses and LAE at the start of the year $ 5,319.9 $ 4,938.2 $ 4,938.2 Less reinsurance recoverable (560.7 ) (354.8 ) (354.8 ) Net loss and LAE at the start of the year 4,759.2 4,583.4 4,583.4 Net loss and LAE expenses assumed (125.7 ) 5.7 (80.1 ) Provision for losses and LAE for claims incurred: Current year 1,543.3 1,267.0 1,705.4 Prior years (92.8 ) (78.2 ) (129.3 ) Total incurred 1,450.5 1,188.8 1,576.1 Losses and LAE payments for claims incurred: Current year (150.4 ) (62.1 ) (241.0 ) Prior years (947.7 ) (853.1 ) (981.8 ) Total paid (1,098.1 ) (915.2 ) (1,222.8 ) Foreign exchange losses/(gains) 135.2 (35.9 ) (97.4 ) Net losses and LAE reserves at period end 5,121.1 4,826.8 4,759.2 Plus reinsurance recoverable on unpaid losses at period end 1,369.5 419.8 560.7 Provision for losses and LAE at the end of the relevant period $ 6,490.6 $ 5,246.6 $ 5,319.9 For the nine months ended September 30, 2017 , there was a reduction of $92.8 million in the Company’s estimate of the ultimate claims to be paid in respect of prior accident years compared to a reduction of $78.2 million for the nine months ended September 30, 2016 . The Company ceded $125.7 million of reserves as part of an adverse development cover purchased in the third quarter of 2017. In the nine months ended September 30, 2016 , the Company assumed $5.7 million of additional loss reserves as a result of its acquisition of AG Logic Holdings, LLC and its subsidiaries (“AgriLogic”), a wholly owned specialist U.S. admitted crop intermediary with an integrated agricultural consultancy. The Company ceded $85.8 million of reserves as part of an adverse development cover purchased during the twelve months ended December 31, 2016. 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. The following tables show an analysis of incurred claims and allocated loss adjustment expenses, net of reinsurance and cumulative paid claims and allocated claim adjustment expenses, net of reinsurance as at September 30, 2017 , December 31, 2016, 2015, 2014, 2013 and 2012. The loss development triangles are derived from all business written by the Company. Although a limited number of contracts are written which have durations of greater than one year, the contracts do not meet the definition of a long duration contract. Insurance Incurred Claims, IBNR and Allocated Loss Adjustment Expenses, Net of Reinsurance As at September 30, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 Q3 2017 $ (in millions) 2012 606.9 628.4 657.6 671.8 656.7 637.0 47.2 15,825 2013 691.9 669.9 681.0 662.4 668.9 74.6 14,728 2014 758.1 732.7 704.8 686.2 109.7 18,848 2015 919.5 909.4 859.6 206.4 21,142 2016 912.7 876.3 316.0 20,538 2017 675.8 426.5 10,694 Total $ 4,403.8 Insurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012 2013 2014 2015 2016 Q3 2017 ($ in millions) 2012 117.6 308.6 379.5 454.1 504.3 519.2 2013 91.1 255.3 365.9 456.2 503.1 2014 99.8 248.2 390.7 448.9 2015 119.5 328.0 459.5 2016 117.7 332.1 2017 59.9 Total $ 2,322.7 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 2,081.1 All outstanding liabilities before 2012, net of reinsurance 140.3 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 2,221.4 Reinsurance Incurred Claims, IBNR and Allocated Loss Adjustment Expenses, Net of Reinsurance As at September 30, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 Q3 2017 $ (in millions) 2012 695.5 740.0 723.7 694.2 692.8 683.8 93.4 3,660 2013 583.2 575.9 554.8 528.6 519.5 92.9 3,524 2014 553.5 533.8 517.9 496.7 116.8 3,440 2015 584.8 568.6 553.5 166.5 3,450 2016 756.5 763.8 290.4 3,032 2017 887.4 636.9 1,704 Total $ 3,904.7 Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012 2013 2014 2015 2016 Q3 2017 ($ in millions) 2012 63.6 249.3 362.2 416.3 460.6 491.0 2013 63.6 188.2 280.7 322.6 354.7 2014 57.2 173.5 249.6 281.8 2015 57.7 172.8 254.1 2016 127.0 297.4 2017 90.5 Total $ 1,769.5 All outstanding liabilities for 2012 and subsequent years, net of reinsurance 2,135.2 All outstanding liabilities before 2012, net of reinsurance 718.9 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 2,854.1 Nine Months Ended September 30, 2017 ($ in millions) Net outstanding liabilities: Insurance lines $ 2,221.4 Reinsurance lines 2,854.1 Net loss and LAE 5,075.5 Reinsurance recoverable on unpaid losses: Insurance lines 1,115.3 Reinsurance lines 254.2 Total reinsurance recoverable on unpaid losses 1,369.5 Insurance lines other than short-duration — Unallocated claims incurred 49.4 Other (3.8 ) $ 45.6 Provision for losses and LAE at the end of the period $ 6,490.6 Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 Insurance 14.4 % 24.5 % 15.9 % 12.5 % 7.6 % Reinsurance 11.8 % 23.3 % 16.2 % 7.9 % 6.4 %

Capital Structure

Capital Structure	9 Months Ended
	Sep. 30, 2017
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, 2017 and December 31, 2016 : As at September 30, 2017 As at December 31, 2016 Number $ in Thousands Number $ in Thousands Authorized share capital: Ordinary Shares 0.15144558¢ per share 969,629,030 1,469 969,629,030 1,469 Non-Voting Shares 0.15144558¢ per share 6,787,880 10 6,787,880 10 Preference Shares 0.15144558¢ per share 100,000,000 152 100,000,000 152 Total authorized share capital 1,631 1,631 Issued share capital: Issued ordinary shares of 0.15144558¢ per share 59,407,323 90 59,774,464 91 Issued 7.401% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share — — 5,327,500 8 Issued 7.250% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share — — 6,400,000 10 Issued 5.95% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share 11,000,000 17 11,000,000 17 Issued 5.625% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share 10,000,000 15 10,000,000 15 Total issued share capital 122 140 Additional paid-in capital as at September 30, 2017 was $951.8 million ( December 31, 2016 — $1,259.6 million ). Additional paid-in capital includes the aggregate liquidation preferences of the Company’s preference shares of $525.0 million ( December 31, 2016 — $818.2 million ) less issue costs of $13.1 million ( December 31, 2016 — $21.1 million ). Ordinary Shares. The following table summarizes transactions in the Company’s ordinary shares during the nine months ended September 30, 2017 : Number of Ordinary Shares Ordinary shares in issue as at December 31, 2016 59,774,464 Ordinary share transactions in the nine months ended September 30, 2017 Ordinary shares issued to employees under the 2013 share incentive plan and/or 2008 share purchase plan 272,296 Ordinary shares issued to non-employee directors 9,504 Ordinary shares repurchased (648,941 ) Ordinary shares in issue as at September 30, 2017 59,407,323 Share Repurchase Authorization Program . On February 8, 2017, the Company’s Board of Directors approved a new share repurchase authorization program of $250.0 million . The new share repurchase authorization program, which was effective immediately and expires on February 8, 2019, permits the Company to effect the repurchases of its ordinary shares from time to time through a combination of transactions, including open market purchases, privately negotiated transactions and accelerated share repurchase transactions. The Company acquired and canceled 451,268 and 648,941 ordinary shares for the three and nine months ended September 30, 2017 , respectively. The total consideration paid for the three and nine months ended September 30, 2017 was $20.0 million and $30.0 million , respectively, and the average price per ordinary share for the three and nine months ended September 30, 2017 was $44.32 and $46.23 , respectively. The Company acquired and canceled 144,289 and 1,122,328 ordinary shares for the three and nine months ended September 30, 2016 , respectively. The total consideration paid for the three and nine months ended September 30, 2016 was $6.5 million and $50.0 million , respectively, and the average price per ordinary share for the three and nine months ended September 30, 2016 was $45.17 and $44.55 , respectively. Preference Share Issuance and Redemption. 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 used a portion of the net proceeds from the offering to redeem the Company’s outstanding 7.401% Perpetual Non-Cumulative Preference Shares on January 3, 2017 and used a portion of the net proceeds from the offering to redeem the 7.250% Perpetual Non-Cumulative Preference Shares on July 3, 2017. 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, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Payments	Share-Based Payments The Company has issued options and other equity incentives under three arrangements: employee incentive plans, a non-employee director plan and employee share purchase plans. When options are exercised or other equity awards vest, new ordinary shares are issued because 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, as amended (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, as amended (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 or exercised during the nine months ended September 30, 2017 ( 2016 — Nil ) and no ordinary shares were issued in the nine months ended September 30, 2017 ( 2016 — 29,222 ordinary shares). No charges against income were made in respect of stock options for the nine months ended September 30, 2017 ( 2016 — $ Nil ). Restricted share units (“RSUs”) granted to employees typically vest over a two or three -year period subject to the employee's 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 less a deduction for illiquidity, and is expensed through the income statement evenly over the vesting period. In the nine months ended September 30, 2017 , the Company granted 200,021 RSUs ( 2016 — 328,551 ) to its employees. Compensation costs charged against income in respect of RSUs granted to employees for the nine months ended September 30, 2017 were $7.2 million ( 2016 — $7.2 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, or on the date of departure of a director for the amount vested through such date. On February 8, 2017 (with a grant date of February 10, 2017), the Board of Directors approved a total of 22,230 RSUs for non-employee directors (February 8, 2016 — 24,456 RSUs) and 8,892 RSUs for the Chairman of the Board of Directors (February 8, 2016 — 10,952 RSUs). Compensation costs charged against income in respect of RSUs granted to non-employee directors for the nine months ended September 30, 2017 were $1.1 million ( 2016 — $1.1 million ). The total fair value adjustment for all RSUs for the nine months ended September 30, 2017 was $ Nil due to the introduction of a new accounting standard which removed the requirement to fair value the RSU grants. The total fair value adjustment for all RSUs for the nine months ended September 30, 2016 was $0.6 million . The total tax credit recognized by the Company in relation to RSUs in the nine months ended September 30, 2017 was $1.7 million ( 2016 — $1.7 million ). Performance Shares. During the nine months ended September 30, 2017 , the Company granted 216,878 performance shares to its employees ( 2016 — 278,477 ). 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 the performance shares are only issuable at the end of the three-year period. If the diluted BVPS growth achieved in 2017 is: • less than 5.00% , then the portion of the performance shares subject to the vesting conditions in such year will be forfeited (i.e., one-third of the initial grant); • between 5.00% and 10.00% , then the percentage of the performance shares eligible for vesting in such year will be between 10% and 100% on a straight-line basis; or • between 10.00% and 20.00% , 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 2017 , 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 2017 , 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 2017 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 calculation of BVPS for 2017 will likewise exclude the impact of amortization of goodwill/intangibles resulting from any corporate acquisitions. The Compensation Committee believes that it would not be appropriate for employees’ performance-related remuneration to be impacted by these costs. The Compensation Committee will determine the vesting conditions for the 2018 and 2019 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 2018 and 2019 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 less a deduction for illiquidity and expected dividends which would not accrue during the vesting period. Compensation costs charged against income in the nine months ended September 30, 2017 in respect of performance shares were $0.4 million ( 2016 — $2.5 million ). This reduction is due to a decrease in the expected vesting for 2017. The total tax recognized by the Company in relation to performance shares in the nine months ended September 30, 2017 was a tax credit of $0.1 million ( 2016 — $0.5 million ). Phantom Shares. During the nine months ended September 30, 2017 , the Compensation Committee approved the grant of 173,619 phantom shares to its employees ( 2016 — 147,513 ). 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 2017 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 less a deduction for illiquidity. 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 nine months ended September 30, 2017 in respect of phantom shares were $0.7 million ( 2016 — $0.8 million ) with a fair value reduction for the nine months ended September 30, 2017 of $1.8 million ( 2016 — increase of $3.8 million ). The total tax credit recognized by the Company in relation to phantom shares in the nine months ended September 30, 2017 was $0.2 million ( 2016 — $0.6 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. 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, 10,545 ordinary shares were exercised and issued during the nine months ended September 30, 2017 ( 2016 — 13,342 ). Compensation costs charged against income in the nine months ended September 30, 2017 in respect of the ESPP were $0.3 million ( 2016 — $0.3 million ).

Intangible Assets and Goodwill

Intangible Assets and Goodwill	9 Months Ended
	Sep. 30, 2017
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, 2017 and 2016 : Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 Beginning of the Period Additions Amortization End of the Period Beginning of the Period Additions Amortization End of the Period ($ in millions) ($ in millions) Intangible Assets Trade Mark $ 6.3 $ — $ (0.3 ) $ 6.0 $ 5.4 $ — $ (0.1 ) $ 5.3 Insurance Licenses 16.7 — — 16.7 16.6 — — 16.6 Agency Relationships 25.1 — (0.5 ) 24.6 24.2 — (0.4 ) 23.8 Non-compete Agreements 3.0 — (0.3 ) 2.7 2.6 — (0.1 ) 2.5 Value of Business Acquired — — — — 0.2 — (0.2 ) — Consulting Relationships 0.8 — (0.1 ) 0.7 0.9 — — 0.9 Goodwill 26.0 — — 26.0 22.1 — — 22.1 Renewal Rights 1.5 — (0.1 ) 1.4 — 1.9 — 1.9 Total $ 79.4 $ — $ (1.3 ) $ 78.1 $ 72.0 $ 1.9 $ (0.8 ) $ 73.1 Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2016 Beginning of the Period Additions Amortization End of the Period Beginning of the Period Additions Amortization End of the Period ($ in millions) ($ in millions) Intangible Assets Trade Mark $ 6.6 $ — $ (0.6 ) $ 6.0 $ 1.6 $ 4.0 $ (0.3 ) $ 5.3 Insurance Licenses 16.7 — — 16.7 16.6 — — 16.6 Agency Relationships 26.2 — (1.6 ) 24.6 — 25.0 (1.2 ) 23.8 Non-compete Agreements 3.3 — (0.6 ) 2.7 — 2.9 (0.4 ) 2.5 Value of Business Acquired — — — — — 1.8 (1.8 ) — Consulting Relationships 0.9 — (0.2 ) 0.7 — 1.0 (0.1 ) 0.9 Goodwill 24.2 1.8 — 26.0 — 22.1 — 22.1 Renewal Rights 1.7 — (0.3 ) 1.4 — 1.9 — 1.9 Total $ 79.6 $ 1.8 $ (3.3 ) $ 78.1 $ 18.2 $ 58.7 $ (3.8 ) $ 73.1 AgriLogic On January 19, 2016, Aspen U.S. Holdings acquired 100% of the equity voting interest of AgriLogic, a specialist U.S. crop managing general agency business with an integrated agricultural consultancy, for an initial purchase price of $53.0 million . In addition, the Company recognized $14.1 million of contingent consideration, with a total maximum payable of $22.8 million , subject to the future performance of the business and $2.0 million of ceding commission. The total consideration for the acquisition was $69.1 million . A significant proportion of the acquired business was represented by intangible assets, specifically $25.0 million for agency relationships, $4.0 million for the right to use the AgriLogic trademark, $2.9 million for non-compete agreements, $1.8 million for the value of business acquired and $1.0 million for consultancy relationships. In addition, $12.0 million of software was acquired and 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, $34.0 million of intangible assets and $21.0 million of goodwill are eligible for tax deduction over the next 15 years. License to use the “AgriLogic” Trademark. The Company acquired the right to use the AgriLogic trademark in the United States. The Company valued the trademark at $4.0 million with an estimated economic useful life of 10 years. The Company will amortize the estimated value of the trademark over its estimated useful life. Agency Relationships. The Company valued the agency relationships at $25.0 million with an estimated economic useful life of 15 years. The Company will amortize the estimated value of the agency relationships over their estimated useful life. Non-compete Agreements. The Company valued the non-compete agreements at $2.9 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the non-compete agreements over their estimated useful life. Value of Business Acquired. The Company recognized a $1.8 million asset for value of business acquired (“VOBA”) consisting of the inforce unearned premium reserve and claims reserves at fair value. The Company 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 was fully amortized in 2016. Consulting Relationships. The Company valued the consulting relationships at $1.0 million with an estimated economic useful life of 10 years. The Company will amortize the estimated value of the consulting relationships over their estimated useful life. Goodwill. 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. Blue Waters On October 31, 2016, Acorn acquired 100% of the equity voting interest of Blue Waters, a specialist marine insurance agency. The total consideration for the acquisition was $8.0 million . A significant proportion of the acquired business was represented by intangible assets, specifically $3.1 million for agency relationships, $1.5 million for the right to use the Blue Waters trademark, $1.0 million for non-compete agreements and $0.05 million for the value of trading licenses. In addition, $0.3 million of residual net assets were acquired. The total net assets acquired of $5.75 million resulted in the Company recognizing a total of $2.1 million in goodwill for the acquisition of Blue Waters. Agency Relationships . The Company valued the agency relationships at $3.1 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the agency relationships over their estimated useful life. License to use the “Blue Waters” Trademark . The Company acquired the right to use the Blue Waters trademark in the United States. The Company valued the trademark at $1.5 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the trademark over its estimated useful life. Non-compete Agreements . The Company valued the non-compete agreements at $1.0 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the non-compete agreements over their estimated useful life. Insurance Licenses . The Company valued the insurance licenses at $0.05 million . The insurance licenses are considered to have an indefinite useful life and are not amortized. The licenses are tested annually for impairment. Goodwill . The Company valued the goodwill at $2.1 million . The goodwill is deemed to have an indefinite useful life and will be assessed for impairment annually. Other Intangible Assets Renewal Rights. On September 22, 2016, the Company entered into a renewal rights agreement with Liberty Specialty Markets Limited (“LSML”). The Company valued the renewal rights at $1.9 million with an estimated economic useful life of 5 years . The Company will amortize the estimated value of the renewal rights over the estimated useful life. In addition to the intangible assets and goodwill associated with the AgriLogic and Blue Waters acquisitions and the renewal rights agreement with LSML, the Company has the following intangible assets from prior transactions: License to use the “Aspen” Trademark. On April 5, 2005, the Company entered into an agreement with Aspen (Actuaries and Pension Consultants) Plc to acquire the right to use the Aspen trademark in the United Kingdom. The consideration paid was approximately $1.6 million . As at September 30, 2017 , the value of the license to use the Aspen trademark was $1.6 million ( December 31, 2016 — $1.6 million ). The trademark has an indefinite useful life and is tested for impairment annually or when events or changes in circumstances indicate that the asset might be impaired. Insurance Licenses. The total value of the Company’s licenses as at September 30, 2017 was $16.6 million ( December 31, 2016 — $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. Goodwill. On January 1, 2017, the Company purchased through its wholly-owned subsidiary, Aspen U.S. Holdings, a 49% share of Digital Re. The Company valued the goodwill at $1.8 million . The goodwill is deemed to have an indefinite useful life and will be assessed for impairment annually under the provisions of ASC 323-10-35.

Commitments and Contingent Liab

Commitments and Contingent Liabilities	9 Months Ended
	Sep. 30, 2017
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, 2017 and December 31, 2016 : As at September 30, 2017 As at December 31, 2016 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 1,438.7 $ 1,482.8 Third party 2,404.3 2,380.8 Letters of credit / guarantees 694.6 672.1 Total restricted assets $ 4,537.6 $ 4,535.7 Total as percent of investable assets (1) 51.9 % 49.3 % (1) 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, consists of investable assets as at September 30, 2017 in the amount of $458.0 million ( December 31, 2016 — $447.3 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, 2017 was £0.4 million ( December 31, 2016 — £0.4 million ). This is not available for distribution by the Company for the payment of dividends. Credit Agreement. On March 27, 2017, Aspen Holdings and certain of its direct or indirect subsidiaries (collectively, the “Borrowers”) entered into a Second Amended and Restated Credit Agreement (the “Credit Agreement”) with various lenders and Barclays Bank plc, as administrative agent. The Credit Agreement will be used by the Borrowers to finance the working capital needs of the Company and its subsidiaries, for letters of credit in connection with the insurance and reinsurance businesses of the Company and its subsidiaries and for other general corporate purposes. Initial availability under the Credit Agreement is $200,000,000 and the Company has the option (subject to obtaining commitments from acceptable lenders) to increase the Credit Agreement by up to $100,000,000 . The Credit Agreement will expire on March 27, 2022. As at September 30, 2017 , no borrowings were outstanding under the Credit Agreement. The fees and interest rates on the loans and the fees on the letters of credit payable by the Borrowers under the Credit Agreement are based upon the credit ratings for the Company’s long-term unsecured senior debt by S&P and Moody’s. In addition, the fees for a letter of credit vary based upon whether the applicable Borrower has provided collateral (in the form of cash or qualifying debt securities) to secure its reimbursement obligations with respect to such letter of credit. Under the Credit Agreement, the Company must not permit (a) consolidated tangible net worth to be less than approximately $2,323,100,000 plus 25% of consolidated net income plus 25% of aggregate net cash proceeds from the issuance by the Company of its capital stock, in each case after January 1, 2017, (b) the ratio of its total consolidated debt to the sum of such debt plus its consolidated tangible net worth to exceed 35% or (c) any material insurance subsidiary to have a financial strength rating of less than B++ from A.M. Best. The Credit Agreement contains other customary affirmative and negative covenants, including (subject to various exceptions) restrictions on the ability of the Company and its subsidiaries to incur indebtedness, create or permit liens on their assets, engage in mergers or consolidations, dispose of assets, pay dividends or other distributions, purchase or redeem the Company’s equity securities, make investments and enter into transactions with affiliates. In addition, the Credit Agreement has customary events of default, including (subject to certain materiality thresholds and grace periods) payment default, failure to comply with covenants, material inaccuracy of representation or warranty, bankruptcy or insolvency proceedings, change of control and cross-default to other debt agreements. 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. The terms of a pledge agreement between Aspen Bermuda and Citi Europe (pursuant to an assignment agreement dated October 11, 2006) dated January 17, 2006, as amended, were also amended on June 30, 2014 to change the types of securities or other assets that are acceptable as collateral under the New LOC Facility. All other agreements relating to Aspen Bermuda’s LOC Facility, which now apply to the New LOC Facility with Citi Europe, as previously filed with the SEC, remain in full force and effect. As at September 30, 2017 , we had $444.2 million of outstanding collateralized letters of credit under the New LOC Facility ( December 31, 2016 — $449.4 million under the LOC Facility). (b) Operating leases Amounts outstanding under operating leases net of subleases as at September 30, 2017 were: 2017 2018 2019 2020 2021 Later Total ($ in millions) Operating Lease Obligations $ 9.8 $ 16.2 $ 15.6 $ 14.7 $ 10.0 $ 80.8 $ 147.1 (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, noticeably the U.S., a failure to deal with such disputes or potential disputes in an appropriate manner could result in an award of “bad faith” punitive damages against the Company’s Operating Subsidiaries. In accordance with ASC 450-20-50-4b, for (a) reasonably possible losses for which no accrual is made because any of the conditions for accrual in ASC 450-20-25-2 are not met and (b) reasonably possible losses in excess of the amounts accrued pursuant to ASC 450-20-30-1, the Company will provide an estimate of the possible loss or range of possible loss or state that such an estimate cannot be made. As at September 30, 2017 and December 31, 2016 , 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, 2017
Accounting Policies [Abstract]
New Accounting Pronouncements Adopted in 2017 and Not Yet Adopted	Accounting Pronouncements Adopted in 2017 On March 30, 2016, the Financial Accounting Standards Board (“FASB”) issued ASU 2016-09, “ Compensation — Stock Compensation ” which provides guidance on several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. This ASU is effective for fiscal years beginning after December 15, 2016 and interim periods beginning after December 15, 2016. Following the adoption of this ASU, all excess tax benefits or expense related to stock-based compensation transactions are recognized prospectively as income tax benefits or expense in the Consolidated Income Statement and the excess tax benefits or expense from stock-based compensation transactions previously included in “Financing activities” on the Consolidated Statements of Cash Flows are prospectively included on that statement as “Operating activities.” The cash paid to the tax authority for tax withholding purposes has also been reclassified from operating to financing activity in the Consolidated Statement of Cash Flows and the comparative period has been restated. This ASU also allows share withholding up to the maximum statutory withholding requirement while still avoiding liability accounting. As a result, the Company has applied the equity accounting method for its restricted share units retrospectively and has recorded a cumulative effect adjustment of $2.8 million through opening retained earnings and $7.9 million through additional paid-in capital. Accounting Pronouncements Not Yet Adopted On March 10, 2017, FASB issued ASU 2017-7, “ Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post Retirement Benefit Cost ” which changes how employers report defined benefit pension and/or other post-retirement benefit costs in their financial statements. This ASU is effective for fiscal years beginning after December 15, 2017 and interim periods beginning after December 15, 2017. The Company has assessed the impact from the adoption of ASU 2017-7 and no material impact is expected on the consolidated financial statements. On August 28, 2017, the FASB issued ASU 2017-12, “Derivatives and Hedging (Topic 815)” enabling entities to better align their hedge accounting and risk management activities, while also simplifying the application of hedge accounting in certain situations. This ASU is effective for fiscal years beginning after 15 December, 2018 using a modified retrospective approach for cash flow and net investment hedge relationships that exist on the date of adoption. The Company is currently evaluating the provisions of ASU 2017-12 to determine how it will be affected, but no material impact is expected on the consolidated financial statements. Other accounting pronouncements were issued during the three months ended September 30, 2017 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, 2017
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, 2017 and 2016 : Amount Reclassified from AOCI Details about the AOCI Components Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 Affected Line Item in the Unaudited ($ in millions) Available for sale securities: Realized gains on sale of securities $ 2.6 $ 5.3 Realized and unrealized investment gains Realized (losses) on sale of securities (1.6 ) (0.9 ) Realized and unrealized investment losses 1.0 4.4 Income from operations before income tax Tax on net realized gains of securities (0.2 ) — Income tax expense $ 0.8 $ 4.4 Net (loss)/income Realized derivatives: Net realized gains/(losses) on settled derivatives $ 1.2 $ (3.1 ) General, administrative and corporate expenses Tax on settled derivatives (0.2 ) 1.1 Income tax expense $ 1.0 $ (2.0 ) Net (loss)/income Total reclassifications from AOCI to the statement of operations, net of income tax $ 1.8 $ 2.4 Net (loss)/income Amount Reclassified from AOCI Details about the AOCI Components Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2016 Affected Line Item in the Unaudited ($ in millions) Available for sale securities: Realized gains on sale of securities $ 8.2 $ 14.8 Realized and unrealized investment gains Realized (losses) on sale of securities (5.4 ) (5.1 ) Realized and unrealized investment losses 2.8 9.7 Income from operations before income tax Tax on net realized gains of securities (0.4 ) (0.6 ) Income tax expense $ 2.4 $ 9.1 Net (loss)/income Realized derivatives: Net realized gains/(losses) on settled derivatives $ 2.4 $ (5.6 ) General, administrative and corporate expenses Tax on settled derivatives (0.5 ) 1.1 Income tax expense $ 1.9 $ (4.5 ) Net (loss)/income Total reclassifications from AOCI to the statement of operations, net of income tax $ 4.3 $ 4.6 Net (loss)/income

Earnings per Ordinary Share (Ta

Earnings per Ordinary Share (Tables)	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Computation of Basic and Diluted Earnings per Share	The following table sets forth the computation of basic and diluted earnings per ordinary share for the three and nine months ended September 30, 2017 and 2016 , respectively: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 ($ in millions, except share and per share amounts) Net (loss) income $ (253.8 ) $ 95.6 $ (81.5 ) $ 274.9 Preference share dividends (7.7 ) (9.5 ) (28.7 ) (28.4 ) Preference share redemption costs (1) (5.6 ) — (8.0 ) — Net amount attributable to non-controlling interest (0.6 ) 0.2 (0.8 ) — Basic and diluted net (loss) income available to ordinary shareholders $ (267.7 ) $ 86.3 $ (119.0 ) $ 246.5 Ordinary shares: Basic weighted average ordinary shares 59,759,730 60,225,705 59,862,540 60,588,307 Weighted average effect of dilutive securities (2) (3) — 1,351,313 — 1,455,133 Total diluted weighted average ordinary shares 59,759,730 61,577,018 59,862,540 62,043,440 (Loss) earnings per ordinary share: Basic $ (4.48 ) $ 1.43 $ (1.99 ) $ 4.07 Diluted (3) $ (4.48 ) $ 1.40 $ (1.99 ) $ 3.97 (1) The $8.0 million deduction from net income in 2017 is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the 7.401% and 7.250% Perpetual Non-Cumulative Preference Shares, net of issuance costs, and the total redemption costs of $293.2 million . (2) Dilutive securities consist of employee restricted share units and performance shares associated with the Company’s long-term incentive plan, employee share purchase plans and director restricted share units as described in Note 14. (3) The basic and diluted number of ordinary shares for the three and nine months ended September 30, 2017 is the same as the inclusion of dilutive securities in a loss making period would be anti-dilutive.
Summary of Declared Dividends	Dividends. On October 25, 2017, the Company’s Board of Directors (the “Board of Directors”) declared the following quarterly dividends: Dividend Payable on: Record Date: Ordinary shares $ 0.24 November 28, 2017 November 10, 2017 5.95% preference shares $ 0.3719 January 1, 2018 December 15, 2017 5.625% preference shares $ 0.3516 January 1, 2018 December 15, 2017

Segment Reporting (Tables)

Segment Reporting (Tables)	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Summary of Gross and Net Written and Earned Premiums, Underwriting Results, Ratios and Reserves for Each of Company's Business Segments	The following tables provide a summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of the Company’s business segments for the three months ended September 30, 2017 and 2016 : Three Months Ended September 30, 2017 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 431.5 $ 421.0 $ 852.5 Net written premiums 363.6 243.8 607.4 Gross earned premiums 464.0 449.3 913.3 Net earned premiums 382.0 270.5 652.5 Underwriting Expenses Losses and loss adjustment expenses 502.2 274.0 776.2 Amortization of deferred policy acquisition costs 61.5 43.9 105.4 General and administrative expenses 32.8 59.4 92.2 Underwriting loss $ (214.5 ) $ (106.8 ) (321.3 ) Corporate expenses (13.5 ) Non-operating expenses (5.2 ) Net investment income 46.4 Realized and unrealized investment gains 29.9 Realized and unrealized investment losses (12.4 ) Change in fair value of loan notes issued by variable interest entities 9.8 Change in fair value of derivatives 4.5 Interest expense on long term debt (7.4 ) Net realized and unrealized foreign exchange gains 8.4 Net other expense (2.2 ) Loss before tax $ (263.0 ) Net reserves for loss and loss adjustment expenses $ 2,865.8 $ 2,255.3 $ 5,121.1 Ratios Loss ratio 131.5 % 101.3 % 119.0 % Policy acquisition expense ratio 16.1 16.2 16.2 General and administrative expense ratio 8.6 22.0 17.0 (1) Expense ratio 24.7 38.2 33.2 Combined ratio 156.2 % 139.5 % 152.2 % (1) The general and administrative expense ratio in the total column includes corporate and non-operating expenses. 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 18.4 (1) Expense ratio 31.8 37.3 37.6 Combined ratio 88.3 % 95.0 % 94.8 % (1) The general and administrative expense ratio in the total column includes corporate and non-operating 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, 2017 and 2016 : Nine Months Ended September 30, 2017 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 1,332.4 $ 1,340.2 $ 2,672.6 Net written premiums 1,097.3 775.0 1,872.3 Gross earned premiums 1,112.2 1,302.1 2,414.3 Net earned premiums 932.2 863.4 1,795.6 Underwriting Expenses Losses and loss adjustment expenses 797.9 652.6 1,450.5 Amortization of deferred policy acquisition costs 174.4 141.0 315.4 General and administrative expenses 117.4 186.9 304.3 Underwriting loss $ (157.5 ) $ (117.1 ) (274.6 ) Corporate expenses (38.3 ) Non-operating expenses (9.5 ) Net investment income 141.5 Realized and unrealized investment gains 130.1 Realized and unrealized investment losses (24.4 ) Change in fair value of loan notes issued by variable interest entities 3.6 Change in fair value of derivatives 25.2 Interest expense on long term debt (22.2 ) Net realized and unrealized foreign exchange losses (21.1 ) Other income 5.0 Other expense (2.0 ) Loss before tax $ (86.7 ) Net reserves for loss and loss adjustment expenses $ 2,865.8 $ 2,255.3 $ 5,121.1 Ratios Loss ratio 85.6 % 75.6 % 80.8 % Policy acquisition expense ratio 18.7 16.3 17.6 General and administrative expense ratio 12.6 21.6 19.6 (1) Expense ratio 31.3 37.9 37.2 Combined ratio 116.9 % 113.5 % 118.0 % (1) The general and administrative expense ratio in the total column includes corporate and non-operating expenses. 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 17.8 (1) Expense ratio 32.8 35.3 37.0 Combined ratio 88.0 % 96.8 % 95.7 % (1) The general and administrative expense ratio in the total column includes corporate and non-operating expenses.

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2017
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, 2017 and 2016 : For the Three Months Ended For the Nine Months Ended September 30, 2017 September 30, 2016 September 30, 2017 September 30, 2016 ($ in millions) ($ in millions) Fixed income securities — Available for sale $ 32.9 $ 34.8 $ 100.7 $ 107.9 Fixed income securities — Trading 11.1 7.6 31.6 22.9 Short-term investments — Available for sale 0.1 0.2 0.4 0.5 Short-term investments — Trading — — 0.5 — Cash and cash equivalents 2.1 0.8 4.1 2.2 Equity securities — Trading 2.8 5.5 11.7 17.9 Catastrophe bonds — Trading 0.5 0.2 1.3 1.3 Total $ 49.5 $ 49.1 $ 150.3 $ 152.7 Investment expenses (3.1 ) (2.7 ) (8.8 ) (8.8 ) Net investment income $ 46.4 $ 46.4 $ 141.5 $ 143.9
Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments	The following table summarizes the net realized and unrealized investment gains and losses recorded in the statement of operations and the change in unrealized gains and losses on investments recorded in other comprehensive income for the three and nine months ended September 30, 2017 and 2016 : For the Three Months Ended For the Nine Months Ended September 30, 2017 September 30, 2016 September 30, 2017 September 30, 2016 ($ in millions) ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 2.3 $ 5.5 $ 7.8 $ 14.7 Fixed income securities — gross realized (losses) (1.6 ) (0.9 ) (5.3 ) (4.6 ) Short-term investments — gross realized gains — 0.1 0.1 — Short-term investments — gross realized (losses) — (0.2 ) — — Cash and cash equivalents — gross realized gains 0.3 — 0.3 0.1 Cash and cash equivalents — gross realized (losses) — — (0.1 ) (0.5 ) Other-than-temporary impairments (0.1 ) — (0.5 ) — Trading: Fixed income securities — gross realized gains 3.0 4.3 8.3 9.5 Fixed income securities — gross realized (losses) (0.2 ) (0.5 ) (2.6 ) (6.9 ) Short-term investments — gross realized gains 1.9 — 2.0 — Cash and cash equivalents — gross realized gains 1.3 — 1.3 — Equity securities — gross realized gains 46.1 8.1 55.0 23.3 Equity securities — gross realized (losses) (7.4 ) (4.2 ) (12.4 ) (22.2 ) Catastrophe bonds — net unrealized (losses)/gains (3.1 ) 0.3 (3.2 ) 0.1 Net change in gross unrealized (losses)/gains (25.0 ) 8.0 54.4 88.8 Other investments: Gross realized and unrealized (loss) in MVI — — (0.1 ) — Gross realized (loss)/gain in Chaspark — 1.0 0.9 1.0 Gross realized and unrealized (loss) in Bene — — (0.2 ) — Total net realized and unrealized investment gains recorded in the statement of operations $ 17.5 $ 21.5 $ 105.7 $ 103.3 Change in available for sale net unrealized gains: Fixed income securities 0.3 (23.2 ) 15.2 104.0 Total change in pre-tax available for sale unrealized gains 0.3 (23.2 ) 15.2 104.0 Change in taxes (0.4 ) 2.0 (1.4 ) (11.1 ) Total change in net unrealized gains, net of taxes, recorded in other comprehensive income $ (0.1 ) $ (21.2 ) $ 13.8 $ 92.9
Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities, Short-Term Investments and Equities	Fixed Income Securities 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, 2017 and December 31, 2016 : As at September 30, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,171.3 $ 7.6 $ (6.2 ) $ 1,172.7 U.S. agency 61.3 0.9 (0.1 ) 62.1 Municipal 47.9 2.2 — 50.1 Corporate 2,431.5 45.4 (8.6 ) 2,468.3 Non-U.S. government-backed corporate 94.0 0.4 (0.4 ) 94.0 Non-U.S. government 525.3 7.9 (1.5 ) 531.7 Asset-backed 33.6 0.1 — 33.7 Agency mortgage-backed 940.8 17.4 (5.4 ) 952.8 Total fixed income securities — Available for sale 5,305.7 81.9 (22.2 ) 5,365.4 Total short-term investments — Available for sale 34.4 — — 34.4 Total $ 5,340.1 $ 81.9 $ (22.2 ) $ 5,399.8 As at December 31, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,207.9 $ 9.4 $ (11.2 ) $ 1,206.1 U.S. agency 117.7 1.9 — 119.6 Municipal 23.2 1.6 (0.4 ) 24.4 Corporate 2,566.9 39.6 (20.0 ) 2,586.5 Non-U.S. government-backed corporate 89.2 0.7 (0.1 ) 89.8 Non-U.S. government 477.7 11.8 (0.8 ) 488.7 Asset-backed 62.6 0.4 — 63.0 Non-agency commercial mortgage-backed 12.3 0.3 — 12.6 Agency mortgage-backed 1,062.6 19.6 (8.3 ) 1,073.9 Total fixed income securities — Available for sale 5,620.1 85.3 (40.8 ) 5,664.6 Total short-term investments — Available for sale 145.3 — — 145.3 Total $ 5,765.4 $ 85.3 $ (40.8 ) $ 5,809.9
Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities	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, 2017 and December 31, 2016 : As at September 30, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 160.6 $ 0.6 $ (0.6 ) $ 160.6 Municipal 32.4 — (0.1 ) 32.3 Corporate 1,029.8 17.9 (2.7 ) 1,045.0 Non-U.S government-backed corporate 2.0 — — 2.0 Non-U.S. government 203.5 7.8 (0.4 ) 210.9 Asset-backed 11.0 0.1 — 11.1 Agency mortgage-backed 203.1 0.7 (0.7 ) 203.1 Total fixed income securities — Trading 1,642.4 27.1 (4.5 ) 1,665.0 Total short-term investments — Trading 90.3 — — 90.3 Total equity securities — Trading 407.2 68.7 (7.3 ) 468.6 Total catastrophe bonds — Trading 33.5 — (3.2 ) 30.3 Total $ 2,173.4 $ 95.8 $ (15.0 ) $ 2,254.2 As at December 31, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 82.8 $ 0.4 $ (0.8 ) $ 82.4 Municipal 15.7 — (0.2 ) 15.5 Corporate 817.8 9.9 (7.1 ) 820.6 Non-U.S. government 203.4 3.5 (4.1 ) 202.8 Asset-backed 14.5 — — 14.5 Agency mortgage-backed 130.6 0.2 (0.9 ) 129.9 Total fixed income securities — Trading 1,264.8 14.0 (13.1 ) 1,265.7 Total short-term investments — Trading 185.4 — — 185.4 Total equity securities — Trading 554.3 55.4 (25.0 ) 584.7 Total catastrophe bonds — Trading 42.5 — — 42.5 Total $ 2,047.0 $ 69.4 $ (38.1 ) $ 2,078.3
Other Investments	The tables below show the Company’s investments in the MVI, Chaspark, Bene and Digital Re for the three and nine months ended September 30, 2017 and September 30, 2016 : For the Three Months Ended September 30, 2017 MVI Chaspark Bene Digital Re Total ($ in millions) Opening undistributed value of investment $ 0.4 $ — $ 3.0 $ 0.5 $ 3.9 Additional investment 0.1 — — — 0.1 Closing undistributed value of investment $ 0.5 $ — $ 3.0 $ 0.5 $ 4.0 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 Realized/unrealized gain for the three months to September 30, 2016 — 1.0 — 1.0 Closing value of investment $ 0.6 $ 9.1 $ 3.3 $ 13.0 For the Nine Months Ended September 30, 2017 MVI Chaspark Bene Digital Re Total ($ in millions) Opening undistributed value of investment $ 0.5 $ 8.4 $ 3.2 $ — $ 12.1 Initial investment — — — 2.3 2.3 Goodwill — — — (1.8 ) (1.8 ) Additional investment 0.1 — — — 0.1 Distribution received — (9.3 ) — — (9.3 ) Realized/unrealized gain/(losses) for the nine months to September 30, 2017 (0.1 ) 0.9 (0.2 ) — 0.6 Closing undistributed value of investment $ 0.5 $ — $ 3.0 $ 0.5 $ 4.0 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 Realized/unrealized gain for the nine months to September 30, 2016 (0.2 ) 1.0 — 0.8 Closing value of investment $ 0.6 $ 9.1 $ 3.3 $ 13.0
Summary of Fixed Maturities	Fixed Income Securities. The scheduled maturity distribution of available for sale fixed income securities as at September 30, 2017 and December 31, 2016 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, 2017 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 577.8 $ 579.2 AA Due after one year through five years 2,597.3 2,623.2 AA- Due after five years through ten years 1,060.6 1,070.1 AA- Due after ten years 95.6 106.4 A+ Subtotal 4,331.3 4,378.9 Agency mortgage-backed 940.8 952.8 AA+ Asset-backed 33.6 33.7 AAA Total fixed income securities — Available for sale $ 5,305.7 $ 5,365.4 As at December 31, 2016 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 567.2 $ 570.0 AA Due after one year through five years 2,643.7 2,671.9 AA- Due after five years through ten years 1,172.3 1,168.1 A+ Due after ten years 99.4 105.1 A+ Subtotal 4,482.6 4,515.1 Non-agency commercial mortgage-backed 12.3 12.6 AAA Agency mortgage-backed 1,062.6 1,073.9 AA+ Asset-backed 62.6 63.0 AAA Total fixed income securities — Available for sale $ 5,620.1 $ 5,664.6
Aggregate Fair Value and Gross Unrealized Loss by Type of Security	Gross Unrealized Loss. The following tables summarize, by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position in the Company’s available for sale portfolio as at September 30, 2017 and December 31, 2016 : As at September 30, 2017 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 $ 617.5 $ (5.0 ) $ 62.5 $ (1.2 ) $ 680.0 $ (6.2 ) 80 U.S. agency 19.6 (0.1 ) — — 19.6 (0.1 ) 7 Municipal 19.2 — — — 19.2 — 6 Corporate 483.9 (4.6 ) 154.1 (4.0 ) 638.0 (8.6 ) 255 Non-U.S. government-backed corporate 63.4 (0.4 ) 0.4 — 63.8 (0.4 ) 15 Non-U.S. government 211.5 (1.2 ) 36.8 (0.3 ) 248.3 (1.5 ) 43 Asset-backed 12.2 — — — 12.2 — 9 Agency mortgage-backed 426.0 (4.5 ) 33.9 (0.9 ) 459.9 (5.4 ) 133 Total fixed income securities — Available for sale 1,853.3 (15.8 ) 287.7 (6.4 ) 2,141.0 (22.2 ) 548 Total short-term investments — Available for sale 25.7 — — — 25.7 — 10 Total $ 1,879.0 $ (15.8 ) $ 287.7 $ (6.4 ) $ 2,166.7 $ (22.2 ) 558 As at December 31, 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 $ 724.4 $ (11.2 ) $ — $ — $ 724.4 $ (11.2 ) 78 U.S. agency 14.1 — — — 14.1 — 4 Municipal 7.7 (0.2 ) 0.8 (0.2 ) 8.5 (0.4 ) 6 Corporate 1,044.4 (19.4 ) 6.6 (0.6 ) 1,051.0 (20.0 ) 386 Non-U.S. government-backed corporate 29.6 (0.1 ) — — 29.6 (0.1 ) 11 Non-U.S. government 143.5 (0.8 ) 1.0 — 144.5 (0.8 ) 29 Asset-backed 25.8 — 1.4 — 27.2 — 15 Agency mortgage-backed 527.6 (7.6 ) 27.2 (0.7 ) 554.8 (8.3 ) 148 Total fixed income securities — Available for sale 2,517.1 (39.3 ) 37.0 (1.5 ) 2,554.1 (40.8 ) 677 Total short-term investments — Available for sale 1.1 — — — 1.1 — 2 Total $ 2,518.2 $ (39.3 ) $ 37.0 $ (1.5 ) $ 2,555.2 $ (40.8 ) 679

Variable Interest Entities (Tab

Variable Interest Entities (Tables)	9 Months Ended
	Sep. 30, 2017
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, 2017 and 2016 : For the Nine Months Ended September 30, 2017 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 223.4 $ 54.5 $ 277.9 Total change in fair value for the period (3.6 ) (0.9 ) (4.5 ) Total distributed in the period (115.9 ) (28.8 ) (144.7 ) Closing balance as at September 30, 2017 $ 103.9 $ 24.8 $ 128.7 Liability Loan notes (long-term liabilities) $ 101.5 $ 24.2 $ 125.7 Accrued expenses (current liabilities) 2.4 0.6 3.0 Total aggregate unpaid balance as at September 30, 2017 $ 103.9 $ 24.8 $ 128.7 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

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Financial Assets Measured on Recurring Basis	The following tables present the level within the fair value hierarchy at which the Company’s financial assets and liabilities are measured on a recurring basis as at September 30, 2017 and December 31, 2016 : As at September 30, 2017 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,172.6 $ — $ — $ 1,172.6 U.S. agency — 62.1 — 62.1 Municipal — 50.1 — 50.1 Corporate — 2,468.4 — 2,468.4 Non-U.S. government-backed corporate — 94.0 — 94.0 Non-U.S. government 375.2 156.5 — 531.7 Asset-backed — 33.7 — 33.7 Agency mortgage-backed — 952.8 — 952.8 Total fixed income securities available for sale, at fair value 1,547.8 3,817.6 — 5,365.4 Short-term investments available for sale, at fair value 27.4 7.0 — 34.4 Held for trading financial assets, at fair value U.S. government 160.6 — — 160.6 U.S. agency — — — — Municipal — 32.3 — 32.3 Corporate — 1,045.0 — 1,045.0 Non-U.S. government-backed corporate — 2.0 — 2.0 Non-U.S. government 24.5 186.4 — 210.9 Asset-backed — 11.1 — 11.1 Agency mortgage-backed — 203.1 — 203.1 Total fixed income securities trading, at fair value 185.1 1,479.9 — 1,665.0 Short-term investments trading, at fair value 90.0 0.3 — 90.3 Equity investments trading, at fair value 468.6 — — 468.6 Catastrophe bonds trading, at fair value — 30.3 — 30.3 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 8.4 — 8.4 Liabilities under derivative contracts — foreign exchange contracts — (2.9 ) — (2.9 ) Loan notes issued by variable interest entities, at fair value — — (101.5 ) (101.5 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (2.4 ) (2.4 ) Total $ 2,318.9 $ 5,340.6 $ (103.9 ) $ 7,555.6 There were no transfers between Level 1, Level 2 and Level 3 during the three and nine months ended September 30, 2017 . The Company settled $1.4 million and $115.9 million of Level 3 liabilities in respect of the Loan Notes issued by Silverton for the three and nine months ended September 30, 2017 , respectively. As at September 30, 2017 , there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted solely of the Loan Notes issued by Silverton. As at December 31, 2016 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,206.1 $ — $ — $ 1,206.1 U.S. agency — 119.6 — 119.6 Municipal — 24.4 — 24.4 Corporate — 2,586.5 — 2,586.5 Non-U.S. government-backed corporate — 89.8 — 89.8 Non-U.S. government 343.7 145.0 — 488.7 Asset-backed — 63.0 — 63.0 Non-agency commercial mortgage-backed — 12.6 — 12.6 Agency mortgage-backed — 1,073.9 — 1,073.9 Total fixed income securities available for sale, at fair value 1,549.8 4,114.8 — 5,664.6 Short-term investments available for sale, at fair value 118.6 26.7 — 145.3 Held for trading financial assets, at fair value U.S. government 82.4 — — 82.4 Municipal — 15.5 — 15.5 Corporate — 820.6 — 820.6 Non-U.S. government — 202.8 — 202.8 Asset-backed — 14.5 — 14.5 Agency mortgage-backed — 129.9 — 129.9 Total fixed income securities trading, at fair value 82.4 1,183.3 — 1,265.7 Short-term investments trading, at fair value 76.1 109.3 — 185.4 Equity investments trading, at fair value 584.7 — — 584.7 Catastrophe bonds trading, at fair value — 42.5 — 42.5 Other financial assets and liabilities, at fair value Derivatives at fair value – foreign exchange contracts — 7.2 — 7.2 Liabilities under derivative contracts – foreign exchange contracts — (18.4 ) — (18.4 ) Loan notes issued by variable interest entities, at fair value — — (115.0 ) (115.0 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (108.4 ) (108.4 ) Total $ 2,411.6 $ 5,465.4 $ (223.4 ) $ 7,653.6
Reconciliation of Liabilities Using Level 3 Inputs	The following table presents a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Level 3 inputs for the three and nine months ended September 30, 2017 and 2016 : Reconciliation of Liabilities Using Level 3 Inputs Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 ($ in millions) Balance at the beginning of the period $ 115.1 $ 105.5 Distributed to third party (1.4 ) (0.3 ) Total change in fair value included in the statement of operations (9.8 ) 9.8 Balance at the end of the period (1) $ 103.9 $ 115.0 Reconciliation of Liabilities Using Level 3 Inputs Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2016 ($ in millions) Balance at the beginning of the period 223.4 190.6 Distributed to third party (115.9 ) (89.3 ) Total change in fair value included in the statement of operations (3.6 ) 13.7 Balance at the end of the period (1) 103.9 115.0 (1) The amount classified within accrued expenses and other payables was $2.4 million and $2.3 million as at September 30, 2017 and September 30, 2016 , respectively.
Pricing Sources Used in Pricing Fixed Income Investments	Pricing sources used in pricing fixed income investments as at September 30, 2017 and December 31, 2016 were as follows: As at September 30, 2017 As at December 31, 2016 Index providers 84 % 87 % Pricing services 10 7 Broker-dealers 6 6 Total 100 % 100 %
Summary of Securities Priced Using Pricing Information from Index Providers	A summary of securities priced using pricing information from index providers as at September 30, 2017 and December 31, 2016 is provided below: As at September 30, 2017 As at December 31, 2016 Fair Market Value Determined using Prices from Index Providers % of Total Fair Value by Security Type Fair Market Value Determined using Prices from Index Providers % of Total Fair Value by Security Type ($ in millions, except for percentages) U.S. government $ 1,333.2 100 % $ 1,288.2 100 % U.S. agency 53.2 86 % 110.2 92 % Municipal 31.2 38 % 28.8 72 % Corporate 3,346.0 95 % 3,275.3 96 % Non-U.S. government-backed corporate 47.8 50 % 44.8 50 % Non-U.S. government 425.6 61 % 455.6 72 % Asset-backed 14.2 32 % 32.1 41 % Non-agency commercial mortgage-backed — — % 12.5 98 % Agency mortgage-backed 618.2 54 % 691.9 58 % Total fixed income securities $ 5,869.4 84 % $ 5,939.4 87 % Equities 468.6 100 % 584.7 100 % Total fixed income securities and equity investments $ 6,338.0 85 % $ 6,524.1 88 %
Fair Value Inputs, Assets, Quantitative Information	The observable and unobservable inputs used to determine the fair value of the Loan Notes as at September 30, 2017 and December 31, 2016 are presented in the tables below: As at September 30, 2017 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 103.9 (1) Internal Valuation Model Gross premiums written (O) $ 50.1 $ 57.6 Reserve for losses (U) $ 4.2 $ 37.5 Contract period (O) N/A 365 days Initial value of issuance (O) $ 325.0 $ 325.0 As at December 31, 2016 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 223.4 (1) Internal Valuation Model Gross premiums written (O) $ 38.9 $ 43.4 Reserve for losses (U) $ 2.7 $ 11.8 Contract period (O) N/A 365 days Initial value of issuance (O) $ 220.0 $ 220.0 (1) The amounts classified within accrued expenses and other payables were $2.4 million and $108.4 million as at September 30, 2017 and December 31, 2016, respectively.

Derivative Contracts (Tables)

Derivative Contracts (Tables)	9 Months Ended
	Sep. 30, 2017
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, 2017 and December 31, 2016 : As at September 30, 2017 As at December 31, 2016 Derivatives Not Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Derivatives at Fair Value $ 521.5 $ 6.0 (1) $ 240.2 $ 5.0 Foreign Exchange Contracts Liabilities under Derivative Contracts $ 238.7 $ (2.9 ) $ 425.4 $ (17.7 ) (1) Net of $0.6 million cash collateral ( December 31, 2016 — $ Nil ). As at September 30, 2017 As at December 31, 2016 Derivatives Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Derivatives at Fair Value $ 27.3 $ 2.4 (1) $ — $ 2.2 Foreign Exchange Contracts Liabilities under Derivative Contracts $ — $ — $ 108.6 $ (0.7 ) (1) Net of $ Nil cash collateral ( December 31, 2016 — $2.2 million ).
Gain/(Loss) Recognized in Income on Derivative	The following table provides the unrealized and realized gains/(losses) recorded in the statements of operations and other comprehensive income for derivatives that are not designated or designated as hedging instruments under ASC 815 - " Derivatives and Hedging" for the three and nine months ended September 30, 2017 and 2016 . Amount of Gain/(Loss) Recognized on Derivatives Three Months Ended Nine Months Ended Location of Gain/(Loss) Recognized on Derivatives September 30, 2017 September 30, 2016 September 30, 2017 September 30, 2016 Derivatives not designated as hedges ($ in millions) ($in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives 4.5 0.6 25.2 (3.7 ) Interest Rate Swaps Change in Fair Value of Derivatives — — — (3.3 ) Derivatives designated as hedges Foreign Exchange Contracts General, administrative and corporate expenses 1.2 (3.1 ) 2.4 (5.6 ) Foreign Exchange Contracts Net change from current period hedged transactions (0.4 ) 3.1 3.3 (1.9 )

Deferred Policy Acquisition C31

Deferred Policy Acquisition Costs (Tables)	9 Months Ended
	Sep. 30, 2017
Insurance [Abstract]
Reconciliation of Beginning and Ending Deferred Policy Acquisition Costs	The following table represents a reconciliation of beginning and ending deferred policy acquisition costs for the three and nine months ended September 30, 2017 and 2016 : Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 ($ in millions) ($ in millions) Balance at the beginning of the period $ 364.6 $ 409.1 $ 358.4 $ 361.1 Acquisition costs deferred 94.9 110.0 311.1 414.9 Amortization of deferred policy acquisition costs (105.4 ) (130.9 ) (315.4 ) (387.8 ) Balance at the end of the period $ 354.1 $ 388.2 $ 354.1 $ 388.2

Reserves for Losses and Loss 32

Reserves for Losses and Loss Adjustment Expenses (Tables)	9 Months Ended
	Sep. 30, 2017
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, 2017 and 2016 and the twelve months ended December 31, 2016 : Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2016 Twelve Months Ended December 31, 2016 ($ in millions) Provision for losses and LAE at the start of the year $ 5,319.9 $ 4,938.2 $ 4,938.2 Less reinsurance recoverable (560.7 ) (354.8 ) (354.8 ) Net loss and LAE at the start of the year 4,759.2 4,583.4 4,583.4 Net loss and LAE expenses assumed (125.7 ) 5.7 (80.1 ) Provision for losses and LAE for claims incurred: Current year 1,543.3 1,267.0 1,705.4 Prior years (92.8 ) (78.2 ) (129.3 ) Total incurred 1,450.5 1,188.8 1,576.1 Losses and LAE payments for claims incurred: Current year (150.4 ) (62.1 ) (241.0 ) Prior years (947.7 ) (853.1 ) (981.8 ) Total paid (1,098.1 ) (915.2 ) (1,222.8 ) Foreign exchange losses/(gains) 135.2 (35.9 ) (97.4 ) Net losses and LAE reserves at period end 5,121.1 4,826.8 4,759.2 Plus reinsurance recoverable on unpaid losses at period end 1,369.5 419.8 560.7 Provision for losses and LAE at the end of the relevant period $ 6,490.6 $ 5,246.6 $ 5,319.9
Short-duration Insurance Contracts, Claims Development	The following tables show an analysis of incurred claims and allocated loss adjustment expenses, net of reinsurance and cumulative paid claims and allocated claim adjustment expenses, net of reinsurance as at September 30, 2017 , December 31, 2016, 2015, 2014, 2013 and 2012. The loss development triangles are derived from all business written by the Company. Although a limited number of contracts are written which have durations of greater than one year, the contracts do not meet the definition of a long duration contract. Insurance Incurred Claims, IBNR and Allocated Loss Adjustment Expenses, Net of Reinsurance As at September 30, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 Q3 2017 $ (in millions) 2012 606.9 628.4 657.6 671.8 656.7 637.0 47.2 15,825 2013 691.9 669.9 681.0 662.4 668.9 74.6 14,728 2014 758.1 732.7 704.8 686.2 109.7 18,848 2015 919.5 909.4 859.6 206.4 21,142 2016 912.7 876.3 316.0 20,538 2017 675.8 426.5 10,694 Total $ 4,403.8 Insurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012 2013 2014 2015 2016 Q3 2017 ($ in millions) 2012 117.6 308.6 379.5 454.1 504.3 519.2 2013 91.1 255.3 365.9 456.2 503.1 2014 99.8 248.2 390.7 448.9 2015 119.5 328.0 459.5 2016 117.7 332.1 2017 59.9 Total $ 2,322.7 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 2,081.1 All outstanding liabilities before 2012, net of reinsurance 140.3 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 2,221.4 Reinsurance Incurred Claims, IBNR and Allocated Loss Adjustment Expenses, Net of Reinsurance As at September 30, 2017 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 Q3 2017 $ (in millions) 2012 695.5 740.0 723.7 694.2 692.8 683.8 93.4 3,660 2013 583.2 575.9 554.8 528.6 519.5 92.9 3,524 2014 553.5 533.8 517.9 496.7 116.8 3,440 2015 584.8 568.6 553.5 166.5 3,450 2016 756.5 763.8 290.4 3,032 2017 887.4 636.9 1,704 Total $ 3,904.7 Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012 2013 2014 2015 2016 Q3 2017 ($ in millions) 2012 63.6 249.3 362.2 416.3 460.6 491.0 2013 63.6 188.2 280.7 322.6 354.7 2014 57.2 173.5 249.6 281.8 2015 57.7 172.8 254.1 2016 127.0 297.4 2017 90.5 Total $ 1,769.5 All outstanding liabilities for 2012 and subsequent years, net of reinsurance 2,135.2 All outstanding liabilities before 2012, net of reinsurance 718.9 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 2,854.1
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability	Nine Months Ended September 30, 2017 ($ in millions) Net outstanding liabilities: Insurance lines $ 2,221.4 Reinsurance lines 2,854.1 Net loss and LAE 5,075.5 Reinsurance recoverable on unpaid losses: Insurance lines 1,115.3 Reinsurance lines 254.2 Total reinsurance recoverable on unpaid losses 1,369.5 Insurance lines other than short-duration — Unallocated claims incurred 49.4 Other (3.8 ) $ 45.6 Provision for losses and LAE at the end of the period $ 6,490.6
Short-duration Insurance Contracts, Schedule of Historical Claims Duration	Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 Insurance 14.4 % 24.5 % 15.9 % 12.5 % 7.6 % Reinsurance 11.8 % 23.3 % 16.2 % 7.9 % 6.4 %

Capital Structure (Tables)

Capital Structure (Tables)	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Summary of Authorized and Issued Share Capital	The following table provides a summary of the Company’s authorized and issued share capital as at September 30, 2017 and December 31, 2016 : As at September 30, 2017 As at December 31, 2016 Number $ in Thousands Number $ in Thousands Authorized share capital: Ordinary Shares 0.15144558¢ per share 969,629,030 1,469 969,629,030 1,469 Non-Voting Shares 0.15144558¢ per share 6,787,880 10 6,787,880 10 Preference Shares 0.15144558¢ per share 100,000,000 152 100,000,000 152 Total authorized share capital 1,631 1,631 Issued share capital: Issued ordinary shares of 0.15144558¢ per share 59,407,323 90 59,774,464 91 Issued 7.401% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share — — 5,327,500 8 Issued 7.250% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share — — 6,400,000 10 Issued 5.95% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share 11,000,000 17 11,000,000 17 Issued 5.625% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share 10,000,000 15 10,000,000 15 Total issued share capital 122 140
Summary of Ordinary Shares	The following table summarizes transactions in the Company’s ordinary shares during the nine months ended September 30, 2017 : Number of Ordinary Shares Ordinary shares in issue as at December 31, 2016 59,774,464 Ordinary share transactions in the nine months ended September 30, 2017 Ordinary shares issued to employees under the 2013 share incentive plan and/or 2008 share purchase plan 272,296 Ordinary shares issued to non-employee directors 9,504 Ordinary shares repurchased (648,941 ) Ordinary shares in issue as at September 30, 2017 59,407,323

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	9 Months Ended
	Sep. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Summary of Changes in Intangible Assets	The following table provides a summary of the Company’s intangible assets for the three and nine months ended September 30, 2017 and 2016 : Three Months Ended September 30, 2017 Three Months Ended September 30, 2016 Beginning of the Period Additions Amortization End of the Period Beginning of the Period Additions Amortization End of the Period ($ in millions) ($ in millions) Intangible Assets Trade Mark $ 6.3 $ — $ (0.3 ) $ 6.0 $ 5.4 $ — $ (0.1 ) $ 5.3 Insurance Licenses 16.7 — — 16.7 16.6 — — 16.6 Agency Relationships 25.1 — (0.5 ) 24.6 24.2 — (0.4 ) 23.8 Non-compete Agreements 3.0 — (0.3 ) 2.7 2.6 — (0.1 ) 2.5 Value of Business Acquired — — — — 0.2 — (0.2 ) — Consulting Relationships 0.8 — (0.1 ) 0.7 0.9 — — 0.9 Goodwill 26.0 — — 26.0 22.1 — — 22.1 Renewal Rights 1.5 — (0.1 ) 1.4 — 1.9 — 1.9 Total $ 79.4 $ — $ (1.3 ) $ 78.1 $ 72.0 $ 1.9 $ (0.8 ) $ 73.1 Nine Months Ended September 30, 2017 Nine Months Ended September 30, 2016 Beginning of the Period Additions Amortization End of the Period Beginning of the Period Additions Amortization End of the Period ($ in millions) ($ in millions) Intangible Assets Trade Mark $ 6.6 $ — $ (0.6 ) $ 6.0 $ 1.6 $ 4.0 $ (0.3 ) $ 5.3 Insurance Licenses 16.7 — — 16.7 16.6 — — 16.6 Agency Relationships 26.2 — (1.6 ) 24.6 — 25.0 (1.2 ) 23.8 Non-compete Agreements 3.3 — (0.6 ) 2.7 — 2.9 (0.4 ) 2.5 Value of Business Acquired — — — — — 1.8 (1.8 ) — Consulting Relationships 0.9 — (0.2 ) 0.7 — 1.0 (0.1 ) 0.9 Goodwill 24.2 1.8 — 26.0 — 22.1 — 22.1 Renewal Rights 1.7 — (0.3 ) 1.4 — 1.9 — 1.9 Total $ 79.6 $ 1.8 $ (3.3 ) $ 78.1 $ 18.2 $ 58.7 $ (3.8 ) $ 73.1

Commitments and Contingent Li35

Commitments and Contingent Liabilities (Tables)	9 Months Ended
	Sep. 30, 2017
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, 2017 and December 31, 2016 : As at September 30, 2017 As at December 31, 2016 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 1,438.7 $ 1,482.8 Third party 2,404.3 2,380.8 Letters of credit / guarantees 694.6 672.1 Total restricted assets $ 4,537.6 $ 4,535.7 Total as percent of investable assets (1) 51.9 % 49.3 % (1) 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, 2017 were: 2017 2018 2019 2020 2021 Later Total ($ in millions) Operating Lease Obligations $ 9.8 $ 16.2 $ 15.6 $ 14.7 $ 10.0 $ 80.8 $ 147.1

Basis of Preparation - Narrativ

Basis of Preparation - Narrative (Details) - Accounting Standards Update 2016-09 - USD ($) $ in Millions		Sep. 30, 2017	Sep. 30, 2016
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption	[1]	$ 2.8	$ 0
Additional paid-in capital
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ 7.9
[1]	The $2.8 million relates to the cumulative effect-adjustment to opening retained earnings as a result of the classification of restricted share units as equity following the adoption of ASU 2016-09. The adjustment has been applied using a modified retrospective approach.

Reclassifications from Accumu37

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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Available for sale securities:
(Loss)/income from operations before income tax	$ (263)	$ 100.5	$ (86.7)	$ 283.6
General, administrative and corporate expenses	110.9	125	352.1	361.2
Income tax expense	9.2	(4.9)	5.2	(8.7)
Net (loss)/income	(253.8)	95.6	(81.5)	274.9
Reclassification out of AOCI
Available for sale securities:
Net (loss)/income	1.8	2.4	4.3	4.6
Reclassification out of AOCI | Available for sale securities
Available for sale securities:
Realized gains on sale of securities	2.6	5.3	8.2	14.8
Realized (losses) on sale of securities	(1.6)	(0.9)	(5.4)	(5.1)
(Loss)/income from operations before income tax	1	4.4	2.8	9.7
Income tax expense	(0.2)	0	(0.4)	(0.6)
Net (loss)/income	0.8	4.4	2.4	9.1
Reclassification out of AOCI | Realized derivatives
Available for sale securities:
General, administrative and corporate expenses	1.2	(3.1)	2.4	(5.6)
Income tax expense	(0.2)	1.1	(0.5)	1.1
Net (loss)/income	$ 1	$ (2)	$ 1.9	$ (4.5)

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			12 Months Ended
	Sep. 30, 2017		Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016	Dec. 31, 2016
Class of Stock [Line Items]
Net (loss)/income	$ (253.8)		$ 95.6	$ (81.5)		$ 274.9
Preference share dividends	(7.7)		(9.5)	(28.7)		(28.4)
Change in redemption value	(5.6)		0	(8)		0
Proportion due to non-controlling interest	(0.6)		0.2	(0.8)		0
Basic and diluted net (loss) income available to ordinary shareholders	(267.7)		86.3	(119)		246.5
Diluted net income available to ordinary shareholders	$ (267.7)		$ 86.3	$ (119)		$ 246.5
Ordinary shares:
Basic weighted average ordinary shares	59,759,730	[1]	60,225,705	59,862,540	[1]	60,588,307
Weighted average effect of dilutive securities	0		1,351,313	0		1,455,133
Total diluted weighted average ordinary shares	59,759,730	[1]	61,577,018	59,862,540	[1]	62,043,440
(Loss) earnings per ordinary share:
Basic (in usd per share)	$ (4.48)		$ 1.43	$ (1.99)		$ 4.07
Diluted (in usd per share)	$ (4.48)		$ 1.40	$ (1.99)		$ 3.97
Payment for preferred shares redemption				$ 293.2		$ 0
7.401% preference shares
(Loss) earnings per ordinary share:
Preference shares, rate				7.401%			7.401%
7.250% preference shares
(Loss) earnings per ordinary share:
Preference shares, rate				7.25%			7.25%
[1]	The basic and diluted number of ordinary shares for the three and nine months ended September 30, 2017 is the same as the inclusion of dilutive securities in a loss making period would be anti-dilutive.

Earnings per Ordinary Share - S

Earnings per Ordinary Share - Summary of Declared Dividends (Details) - $ / shares	Oct. 25, 2017	Sep. 30, 2017	Dec. 31, 2016
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%	5.625%
Subsequent Event | Ordinary shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.24
Payable Date	Nov. 28, 2017
Record Date	Nov. 10, 2017
Subsequent Event | 5.95% preference shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.3719
Payable Date	Jan. 1, 2018
Record Date	Dec. 15, 2017
Subsequent Event | 5.625% preference shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.35156
Payable Date	Jan. 1, 2018
Record Date	Dec. 15, 2017

Segment Reporting - Narrative (

Segment Reporting - Narrative (Details)	9 Months Ended
	Sep. 30, 2017Segment
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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Underwriting Revenues
Gross written premiums	$ 852.5	$ 763.5	$ 2,672.6	$ 2,540.9
Net written premiums	607.4	638.4	1,872.3	2,162.9
Gross earned premiums	913.3	809.8	2,414.3	2,346.7
Net earned premiums	652.5	681	1,795.6	2,024.9
Underwriting Expenses
Losses and loss adjustment expenses	776.2	389.2	1,450.5	1,188.8
Amortization of deferred policy acquisition costs	105.4	130.9	315.4	387.8
General and administrative expenses	92.2	105.3	304.3	304.3
Underwriting loss	(321.3)	55.6	(274.6)	144
Corporate expenses	(13.5)	(13.4)	(38.3)	(50.6)
Non-operating expenses	(5.2)	(6.3)	(9.5)	(6.3)
Net investment income	46.4	46.4	141.5	143.9
Realized and unrealized investment gains	29.9	26.7	130.1	137.4
Realized and unrealized investment losses	(12.4)	(5.2)	(24.4)	(34.1)
Change in fair value of loan notes issued by variable interest entities	9.8	(9.8)	3.6	(13.7)
Change in fair value of derivatives	4.5	0.6	25.2	(7)
Interest expense on long term debt	(7.4)	(7.3)	(22.2)	(22.1)
Net realized and unrealized foreign exchange gains	8.4	10.8	(21.1)	(10.2)
Other income	(2.2)	1.5	5	2.4
Net other income/(expense)	(2.2)	2.4
Other expenses			(2)	(0.1)
(Loss)/income from operations before income tax	(263)	100.5	(86.7)	283.6
Net reserves for loss and loss adjustment expenses	$ 5,121.1	$ 4,826.8	$ 5,121.1	$ 4,826.8	$ 4,759.2	$ 4,583.4
Ratios
Loss ratio	119.00%	57.20%	80.80%	58.70%
Policy acquisition expense ratio	16.20%	19.20%	17.60%	19.20%
General and administrative expense ratio	17.00%	18.40%	19.60%	17.80%
Expense ratio	33.20%	37.60%	37.20%	37.00%
Combined ratio	152.20%	94.80%	118.00%	95.70%
Reinsurance
Underwriting Revenues
Gross written premiums	$ 431.5	$ 365.9	$ 1,332.4	$ 1,216.1
Net written premiums	363.6	314.5	1,097.3	1,070.8
Gross earned premiums	464	364.3	1,112.2	1,000.9
Net earned premiums	382	316.3	932.2	896
Underwriting Expenses
Losses and loss adjustment expenses	502.2	178.7	797.9	494.3
Amortization of deferred policy acquisition costs	61.5	53	174.4	163.1
General and administrative expenses	32.8	47.4	117.4	130.6
Underwriting loss	(214.5)	37.2	(157.5)	108
Net reserves for loss and loss adjustment expenses	$ 2,865.8	$ 2,495.4	$ 2,865.8	$ 2,495.4
Ratios
Loss ratio	131.50%	56.50%	85.60%	55.20%
Policy acquisition expense ratio	16.10%	16.80%	18.70%	18.20%
General and administrative expense ratio	8.60%	15.00%	12.60%	14.60%
Expense ratio	24.70%	31.80%	31.30%	32.80%
Combined ratio	156.20%	88.30%	116.90%	88.00%
Insurance
Underwriting Revenues
Gross written premiums	$ 421	$ 397.6	$ 1,340.2	$ 1,324.8
Net written premiums	243.8	323.9	775	1,092.1
Gross earned premiums	449.3	445.5	1,302.1	1,345.8
Net earned premiums	270.5	364.7	863.4	1,128.9
Underwriting Expenses
Losses and loss adjustment expenses	274	210.5	652.6	694.5
Amortization of deferred policy acquisition costs	43.9	77.9	141	224.7
General and administrative expenses	59.4	57.9	186.9	173.7
Underwriting loss	(106.8)	18.4	(117.1)	36
Net reserves for loss and loss adjustment expenses	$ 2,255.3	$ 2,331.4	$ 2,255.3	$ 2,331.4
Ratios
Loss ratio	101.30%	57.70%	75.60%	61.50%
Policy acquisition expense ratio	16.20%	21.40%	16.30%	19.90%
General and administrative expense ratio	22.00%	15.90%	21.60%	15.40%
Expense ratio	38.20%	37.30%	37.90%	35.30%
Combined ratio	139.50%	95.00%	113.50%	96.80%

Investments - Summary of Invest

Investments - Summary of Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 49.5	$ 49.1	$ 150.3	$ 152.7
Investment expenses	(3.1)	(2.7)	(8.8)	(8.8)
Net investment income	46.4	46.4	141.5	143.9
Total fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	32.9	34.8	100.7	107.9
Fixed Income Maturities - Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	11.1	7.6	31.6	22.9
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.1	0.2	0.4	0.5
Short-term Investment, Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0	0	0.5	0
Cash and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	2.1	0.8	4.1	2.2
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	2.8	5.5	11.7	17.9
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 0.5	$ 0.2	$ 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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Other-than-temporary impairments	$ (0.1)	$ 0	$ (0.5)	$ 0
Catastrophe bonds	(3.1)	0.3	(3.2)	0.1
Net change in gross unrealized (losses)/gains	(25)	8	54.4	88.8
Total net realized and unrealized investment gains recorded in the statement of operations	17.5	21.5	105.7	103.3
Change in available for sale net unrealized gains:
Fixed income securities	0.3	(23.2)	15.2	104
Total change in pre-tax available for sale unrealized gains	0.3	(23.2)	15.2	104
Change in taxes	(0.4)	2	(1.4)	(11.1)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	(0.1)	(21.2)	13.8	92.9
Total fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	2.3	5.5	7.8	14.7
Fixed income maturities — gross realized (losses)	(1.6)	(0.9)	(5.3)	(4.6)
Cash and cash equivalents - gross realized gains	0.3	0	0.3	0.1
Cash and cash equivalents — gross realized gains (losses)	0	0	(0.1)	(0.5)
Fixed Income Maturities - Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Cash and cash equivalents - gross realized gains	1.3	0	1.3	0
Fixed income securities - gross realized gains	3	4.3	8.3	9.5
Fixed income securities - gross realized (losses)	(0.2)	(0.5)	(2.6)	(6.9)
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Trading securities, gross realized gain	46.1	8.1	55	23.3
Equity securities — gross realized (losses)	(7.4)	(4.2)	(12.4)	(22.2)
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	0	0.1	0.1	0
Fixed income maturities — gross realized (losses)	0	(0.2)	0	0
Trading securities, gross realized gain	1.9	0	2	0
MVI
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	0	0	(0.1)	0
Chaspark
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	0	1	0.9	1
Bene
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	$ 0	$ 0	$ (0.2)	$ 0

Investments - Cost, Gross Unrea

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities, Short-Term Investments and Equities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Statement [Line Items]
Cost or Amortized Cost	$ 5,340.1	$ 5,765.4
Gross Unrealized Gains	81.9	85.3
Gross Unrealized Losses	(22.2)	(40.8)
Fair Market Value	5,399.8	5,809.9
Total fixed income securities — Available for sale
Statement [Line Items]
Cost or Amortized Cost	5,305.7	5,620.1
Gross Unrealized Gains	81.9	85.3
Gross Unrealized Losses	(22.2)	(40.8)
Fair Market Value	5,365.4	5,664.6
Total short-term investments — Available for sale
Statement [Line Items]
Cost or Amortized Cost	34.4	145.3
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	34.4	145.3
U.S. government
Statement [Line Items]
Cost or Amortized Cost	1,171.3	1,207.9
Gross Unrealized Gains	7.6	9.4
Gross Unrealized Losses	(6.2)	(11.2)
Fair Market Value	1,172.7	1,206.1
U.S. agency
Statement [Line Items]
Cost or Amortized Cost	61.3	117.7
Gross Unrealized Gains	0.9	1.9
Gross Unrealized Losses	(0.1)	0
Fair Market Value	62.1	119.6
Municipal
Statement [Line Items]
Cost or Amortized Cost	47.9	23.2
Gross Unrealized Gains	2.2	1.6
Gross Unrealized Losses	0	(0.4)
Fair Market Value	50.1	24.4
Corporate
Statement [Line Items]
Cost or Amortized Cost	2,431.5	2,566.9
Gross Unrealized Gains	45.4	39.6
Gross Unrealized Losses	(8.6)	(20)
Fair Market Value	2,468.3	2,586.5
Non-U.S. government-backed corporate
Statement [Line Items]
Cost or Amortized Cost	94	89.2
Gross Unrealized Gains	0.4	0.7
Gross Unrealized Losses	(0.4)	(0.1)
Fair Market Value	94	89.8
Non-U.S. government
Statement [Line Items]
Cost or Amortized Cost	525.3	477.7
Gross Unrealized Gains	7.9	11.8
Gross Unrealized Losses	(1.5)	(0.8)
Fair Market Value	531.7	488.7
Asset-backed
Statement [Line Items]
Cost or Amortized Cost	33.6	62.6
Gross Unrealized Gains	0.1	0.4
Gross Unrealized Losses	0	0
Fair Market Value	33.7	63
Non-agency commercial mortgage-backed
Statement [Line Items]
Cost or Amortized Cost		12.3
Gross Unrealized Gains		0.3
Gross Unrealized Losses		0
Fair Market Value		12.6
Agency mortgage-backed
Statement [Line Items]
Cost or Amortized Cost	940.8	1,062.6
Gross Unrealized Gains	17.4	19.6
Gross Unrealized Losses	(5.4)	(8.3)
Fair Market Value	$ 952.8	$ 1,073.9

Investments - Cost, Gross Unr45

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Statement [Line Items]
Fair Market Value	$ 1,665	$ 1,265.7
U.S. government
Statement [Line Items]
Cost or Amortized Cost	160.6	82.8
Gross Unrealized Gains	0.6	0.4
Gross Unrealized Losses	(0.6)	(0.8)
Fair Market Value	160.6	82.4
Municipal
Statement [Line Items]
Cost or Amortized Cost	32.4	15.7
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.1)	(0.2)
Fair Market Value	32.3	15.5
Corporate
Statement [Line Items]
Cost or Amortized Cost	1,029.8	817.8
Gross Unrealized Gains	17.9	9.9
Gross Unrealized Losses	(2.7)	(7.1)
Fair Market Value	1,045	820.6
Non-U.S. government-backed corporate
Statement [Line Items]
Cost or Amortized Cost	2
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Market Value	2
Non-U.S. government
Statement [Line Items]
Cost or Amortized Cost	203.5	203.4
Gross Unrealized Gains	7.8	3.5
Gross Unrealized Losses	(0.4)	(4.1)
Fair Market Value	210.9	202.8
Asset-backed
Statement [Line Items]
Cost or Amortized Cost	11	14.5
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	0	0
Fair Market Value	11.1	14.5
Agency mortgage-backed
Statement [Line Items]
Cost or Amortized Cost	203.1	130.6
Gross Unrealized Gains	0.7	0.2
Gross Unrealized Losses	(0.7)	(0.9)
Fair Market Value	203.1	129.9
Total fixed income securities — Trading
Statement [Line Items]
Cost or Amortized Cost	1,642.4	1,264.8
Gross Unrealized Gains	27.1	14
Gross Unrealized Losses	(4.5)	(13.1)
Fair Market Value	1,665	1,265.7
Total short-term investments — Trading
Statement [Line Items]
Cost or Amortized Cost	90.3	185.4
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	90.3	185.4
Total equity securities — Trading
Statement [Line Items]
Cost or Amortized Cost	407.2	554.3
Gross Unrealized Gains	68.7	55.4
Gross Unrealized Losses	(7.3)	(25)
Fair Market Value	468.6	584.7
Total catastrophe bonds — Trading
Statement [Line Items]
Cost or Amortized Cost	33.5	42.5
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3.2)	0
Fair Market Value	30.3	42.5
Total
Statement [Line Items]
Cost or Amortized Cost	2,173.4	2,047
Gross Unrealized Gains	95.8	69.4
Gross Unrealized Losses	(15)	(38.1)
Fair Market Value	$ 2,254.2	$ 2,078.3

Investments - Other Investments

Investments - Other Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	$ 3.9	$ 8.7	$ 12.1	$ 8.9
Initial investment		3.3	2.3	3.3
Goodwill	0	0	(1.8)	(22.1)
Additional investment	0.1		0.1
Distribution received			(9.3)	0
Unrealized/realized gain/(loss)		1	0.6	0.8
Closing value of investment	4	13	4	13
MVI
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0.4	0.6	0.5	0.8
Initial investment		0	0	0
Goodwill			0
Additional investment	0.1		0.1
Distribution received			0
Unrealized/realized gain/(loss)		0	(0.1)	(0.2)
Closing value of investment	0.5	0.6	0.5	0.6
Chaspark
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0	8.1	8.4	8.1
Initial investment		0	0	0
Goodwill			0
Additional investment	0		0
Distribution received			(9.3)
Unrealized/realized gain/(loss)		1	0.9	1
Closing value of investment	0	9.1	0	9.1
Bene
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	3	0	3.2
Initial investment		3.3	0
Goodwill			0
Additional investment	0		0
Distribution received			0
Unrealized/realized gain/(loss)		0	(0.2)
Closing value of investment	3	$ 3.3	3	$ 3.3
Digital Re
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0.5		0
Initial investment			2.3
Goodwill			(1.8)
Additional investment	0		0
Distribution received			0
Unrealized/realized gain/(loss)			0
Closing value of investment	$ 0.5		$ 0.5

Investments - Summary of Fixed

Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
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	$ 5,305.7		$ 5,620.1
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,340.1		5,765.4
Available for sale investments in fixed income maturities, Fair Market Value	5,365.4		5,664.6
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	5,305.7		5,620.1
Available for sale investments in fixed income maturities, Fair Market Value	5,365.4		5,664.6
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			12.3
Available for sale investments in fixed income maturities, Fair Market Value			12.6
Available For Sale Securities Average Ratings By Maturity		AAA
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	940.8		1,062.6
Available for sale investments in fixed income maturities, Fair Market Value	$ 952.8		1,073.9
Available For Sale Securities 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	$ 33.6		62.6
Available for sale investments in fixed income maturities, Fair Market Value	$ 33.7		63
Available For Sale Securities 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	$ 577.8		567.2
Due after one year through five years, Cost or Amortized Cost	2,597.3		2,643.7
Due after five years through ten years, Cost or Amortized Cost	1,060.6		1,172.3
Due after ten years, Cost or Amortized Cost	95.6		99.4
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	4,331.3		4,482.6
Due one year or less, Fair Market Value	579.2		570
Due after one year through five years, Fair Market Value	2,623.2		2,671.9
Due after five years through ten years, Fair Market Value	1,070.1		1,168.1
Due after ten years, Fair Market Value	106.4		105.1
Available for sale investments in fixed income maturities, Fair Market Value	$ 4,378.9		$ 4,515.1
Available For Sale Securities Average for one year or less	AA	AA
Available For Sale Securities Average after one year through five years	AA-	AA-
Available For Sale Securities Average after five years through ten years	AA-	A+
Available For Sale Securities Debt Maturities 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, 2017USD ($)Security	Dec. 31, 2016USD ($)Security
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1,879	$ 2,518.2
0-12 months, Gross Unrealized Loss	(15.8)	(39.3)
Over 12 months, Fair Market Value	287.7	37
Over 12 months, Gross Unrealized Loss	(6.4)	(1.5)
Total, Fair Market Value	2,166.7	2,555.2
Total, Gross Unrealized Loss	$ (22.2)	$ (40.8)
Number of Securities | Security	558	679
Total fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1,853.3	$ 2,517.1
0-12 months, Gross Unrealized Loss	(15.8)	(39.3)
Over 12 months, Fair Market Value	287.7	37
Over 12 months, Gross Unrealized Loss	(6.4)	(1.5)
Total, Fair Market Value	2,141	2,554.1
Total, Gross Unrealized Loss	$ (22.2)	$ (40.8)
Number of Securities | Security	548	677
Total short-term investments — Trading
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 25.7	$ 1.1
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	25.7	1.1
Total, Gross Unrealized Loss	$ 0	$ 0
Number of Securities | Security	10	2
U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 617.5	$ 724.4
0-12 months, Gross Unrealized Loss	(5)	(11.2)
Over 12 months, Fair Market Value	62.5	0
Over 12 months, Gross Unrealized Loss	(1.2)	0
Total, Fair Market Value	680	724.4
Total, Gross Unrealized Loss	$ (6.2)	$ (11.2)
Number of Securities | Security	80	78
U.S. agency
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 19.6	$ 14.1
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	19.6	14.1
Total, Gross Unrealized Loss	$ (0.1)	$ 0
Number of Securities | Security	7	4
Municipal
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 19.2	$ 7.7
0-12 months, Gross Unrealized Loss	0	(0.2)
Over 12 months, Fair Market Value	0	0.8
Over 12 months, Gross Unrealized Loss	0	(0.2)
Total, Fair Market Value	19.2	8.5
Total, Gross Unrealized Loss	$ 0	$ (0.4)
Number of Securities | Security	6	6
Corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 483.9	$ 1,044.4
0-12 months, Gross Unrealized Loss	(4.6)	(19.4)
Over 12 months, Fair Market Value	154.1	6.6
Over 12 months, Gross Unrealized Loss	(4)	(0.6)
Total, Fair Market Value	638	1,051
Total, Gross Unrealized Loss	$ (8.6)	$ (20)
Number of Securities | Security	255	386
Non-U.S. government-backed corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 63.4	$ 29.6
0-12 months, Gross Unrealized Loss	(0.4)	(0.1)
Over 12 months, Fair Market Value	0.4	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	63.8	29.6
Total, Gross Unrealized Loss	$ (0.4)	$ (0.1)
Number of Securities | Security	15	11
Non-U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 211.5	$ 143.5
0-12 months, Gross Unrealized Loss	(1.2)	(0.8)
Over 12 months, Fair Market Value	36.8	1
Over 12 months, Gross Unrealized Loss	(0.3)	0
Total, Fair Market Value	248.3	144.5
Total, Gross Unrealized Loss	$ (1.5)	$ (0.8)
Number of Securities | Security	43	29
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 12.2	$ 25.8
0-12 months, Gross Unrealized Loss	0	0
Over 12 months, Fair Market Value	0	1.4
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	12.2	27.2
Total, Gross Unrealized Loss	$ 0	$ 0
Number of Securities | Security	9	15
Agency mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 426	$ 527.6
0-12 months, Gross Unrealized Loss	(4.5)	(7.6)
Over 12 months, Fair Market Value	33.9	27.2
Over 12 months, Gross Unrealized Loss	(0.9)	(0.7)
Total, Fair Market Value	459.9	554.8
Total, Gross Unrealized Loss	$ (5.4)	$ (8.3)
Number of Securities | Security	133	148

Investments - Narrative (Detail

Investments - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Dec. 31, 2016USD ($)security	Jun. 30, 2017USD ($)	Jan. 01, 2017USD ($)	Jul. 26, 2016USD ($)	Jun. 30, 2016USD ($)	Dec. 31, 2015USD ($)	Jan. 01, 2015USD ($)	Oct. 02, 2012
Investment Holdings [Line Items]
OTTI charge recognized	$ 100,000	$ 0	$ 500,000	$ 0
Catastrophe bonds	30,300,000		30,300,000		$ 42,500,000
Equity Method Investment, Aggregate Cost	4,000,000	13,000,000	4,000,000	13,000,000	$ 12,100,000	$ 3,900,000			$ 8,700,000	$ 8,900,000
Additional investment	100,000		100,000
Distribution received			(9,300,000)	0
Investment Funds	0		0
Number Of Municipal Holdings | security					1
MVI
Investment Holdings [Line Items]
Equity Method Investment, Aggregate Cost	500,000	600,000	500,000	600,000	$ 500,000	400,000			600,000	800,000	$ 800,000
Additional investment	100,000		100,000
Change in fair value of investment	0	0	(100,000)	0
Distribution received			0
Chaspark
Investment Holdings [Line Items]
Equity Method Investment, Aggregate Cost	0	9,100,000	0	9,100,000	8,400,000	0			8,100,000	$ 8,100,000
Additional investment	0		0
Equity Method Investment, Ownership Percentage												58.50%
Change in fair value of investment	0	1,000,000	900,000	1,000,000
Distribution received			(9,300,000)
Bene
Investment Holdings [Line Items]
Equity Method Investment, Aggregate Cost	3,000,000	3,300,000	3,000,000	3,300,000	3,200,000	3,000,000		$ 3,300,000	$ 0
Additional investment	0		0
Equity Method Investment, Ownership Percentage								20.00%
Change in fair value of investment	0	$ 0	(200,000)	$ 0
Distribution received			0
Digital Re
Investment Holdings [Line Items]
Equity Method Investment, Aggregate Cost	500,000		500,000		$ 0	$ 500,000	$ 2,300,000
Additional investment	$ 0		0
Equity Method Investment, Ownership Percentage							49.00%
Distribution received			$ 0

Variable Interest Entities - Na

Variable Interest Entities - Narrative (Details)	Sep. 30, 2017variable_interest_entity
Payables and Accruals [Abstract]
Number of Variable Interest Entities	3

Variable Interest Entities - Su

Variable Interest Entities - Summary of Silverton Loan Notes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Movement in Silverton Loan Notes [Roll Forward]
Total liability balance of Loan Notes, Beginning Balance			$ 277.9	$ 235
Total change in fair value for the period	$ 9.8	$ (9.8)	3.6	(13.7)
Total distributed in the period			(144.7)	(108.7)
Total liability balance of Loan Notes, Ending Balance	128.7	143.4	128.7	143.4
Loan notes (long-term liabilities)					$ 125.7	$ 140.6
Accrued expenses (current liabilities)					3	2.8
Total aggregate unpaid balance	128.7	143.4	277.9	235	128.7	143.4
Third party
Movement in Silverton Loan Notes [Roll Forward]
Total liability balance of Loan Notes, Beginning Balance			223.4	190.6
Total change in fair value for the period			(3.6)	13.7
Total distributed in the period			(115.9)	(89.3)
Total liability balance of Loan Notes, Ending Balance	103.9	115	103.9	115
Loan notes (long-term liabilities)					101.5	112.7
Accrued expenses (current liabilities)					2.4	2.3
Total aggregate unpaid balance	103.9	115	223.4	190.6	103.9	115
Aspen Holdings
Movement in Silverton Loan Notes [Roll Forward]
Total liability balance of Loan Notes, Beginning Balance			54.5	44.4
Total change in fair value for the period			(0.9)	3.4
Total distributed in the period			(28.8)	(19.4)
Total liability balance of Loan Notes, Ending Balance	24.8	28.4	24.8	28.4
Loan notes (long-term liabilities)					24.2	27.9
Accrued expenses (current liabilities)					0.6	0.5
Total aggregate unpaid balance	$ 24.8	$ 28.4	54.5	44.4	$ 24.8	$ 28.4
Aspen Holdings | Third party
Movement in Silverton Loan Notes [Roll Forward]
Total change in fair value for the period			$ (4.5)	$ 17.1

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value	$ 33.5	$ 42.5
Derivatives at fair value	8.4	7.2
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value	30.3
Loan notes issued by variable interest entities, at fair value	(101.5)	(115)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)	(2.4)	(108.4)
Total	7,555.6	7,653.6
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, at fair value	90.3	185.4
Fair Value, Measurements, Recurring | Equity investments trading, at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	468.6	584.7
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	8.4	7.2
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	(2.9)	(18.4)
Available for sale financial assets | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	5,365.4	5,664.6
Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	1,172.6	1,206.1
Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	62.1	119.6
Available for sale financial assets | Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	50.1	24.4
Available for sale financial assets | Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	2,468.4	2,586.5
Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	94	89.8
Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	531.7	488.7
Available for sale financial assets | Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	33.7	63
Available for sale financial assets | Fair Value, Measurements, Recurring | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value		12.6
Available for sale financial assets | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	952.8	1,073.9
Short-term investments | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	34.4	145.3
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, at fair value	1,665	1,265.7
Catastrophe bonds trading, at fair value		42.5
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, at fair value	160.6	82.4
Held for trading 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, at fair value	0
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, at fair value	32.3	15.5
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, at fair value	1,045	820.6
Held for trading 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, at fair value	2
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	210.9	202.8
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, at fair value	11.1	14.5
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, at fair value	203.1	129.9
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	2,318.9	2,411.6
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, at fair value	90	76.1
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, at fair value	468.6	584.7
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 | 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 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	1,547.8	1,549.8
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	1,172.6	1,206.1
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	375.2	343.7
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value		0
Level 1 | Available for sale financial assets | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 1 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	27.4	118.6
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, at fair value	185.1	82.4
Catastrophe bonds trading, at fair value		0
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, at fair value	160.6	82.4
Level 1 | Held for trading 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, at fair value	0
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, at fair value	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, at fair value	0	0
Level 1 | Held for trading 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, at fair value	0
Level 1 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	24.5	0
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, at fair value	0	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, at fair value	0	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	30.3
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	5,340.6	5,465.4
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, at fair value	0.3	109.3
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, at fair value	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	8.4	7.2
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	(2.9)	(18.4)
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	3,817.6	4,114.8
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	62.1	119.6
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	50.1	24.4
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	2,468.4	2,586.5
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	94	89.8
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	156.5	145
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	33.7	63
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value		12.6
Level 2 | Available for sale financial assets | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	952.8	1,073.9
Level 2 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	7	26.7
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, at fair value	1,479.9	1,183.3
Catastrophe bonds trading, at fair value		42.5
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, at fair value	0	0
Level 2 | Held for trading 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, at fair value	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, at fair value	32.3	15.5
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, at fair value	1,045	820.6
Level 2 | Held for trading 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, at fair value	2
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	186.4	202.8
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, at fair value	11.1	14.5
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, at fair value	203.1	129.9
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	(101.5)	(115)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)	(2.4)	(108.4)	$ (2.3)
Total	103.9	223.4
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, at fair value	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, at fair value	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 | 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 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | U.S. agency
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government-backed corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Non-agency commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value		0
Level 3 | Available for sale financial assets | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 3 | Short-term investments | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	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, at fair value	0	0
Catastrophe bonds trading, at fair value		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, at fair value	0	0
Level 3 | Held for trading 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, at fair value	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, at fair value	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, at fair value	0	0
Level 3 | Held for trading 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, at fair value	0
Level 3 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	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, at fair value	0	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, at fair value	$ 0	$ 0

Fair Value Measurements - Recon

Fair Value Measurements - Reconciliation of Liabilities Using Level 3 Inputs (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
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)	$ (2.4)		$ (2.4)		$ (108.4)
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)	(2.4)	$ (2.3)	(2.4)	$ (2.3)	$ (108.4)
Level 3 | Silverton 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		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		$ 115		$ 115
Level 3 | Silverton 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	115.1		223.4
Distributed to third party	(1.4)		(115.9)
Total change in fair value included in the statement of operations	(9.8)		(3.6)
Ending balance	$ 103.9		$ 103.9

Fair Value Measurements - Prici

Fair Value Measurements - Pricing Sources Used in Pricing Fixed Income Investments (Details)	Sep. 30, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Index providers	84.00%	87.00%
Pricing services	10.00%	7.00%
Broker-dealers	6.00%	6.00%
Total	100.00%	100.00%

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 6,338	$ 6,524.1
% of Total Fair Value by Security Type	85.00%	88.00%
Fixed Income Maturities
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 5,869.4	$ 5,939.4
% of Total Fair Value by Security Type	84.00%	87.00%
Fixed Income Maturities | U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 1,333.2	$ 1,288.2
% of Total Fair Value by Security Type	100.00%	100.00%
Fixed Income Maturities | U.S. agency
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 53.2	$ 110.2
% of Total Fair Value by Security Type	86.00%	92.00%
Fixed Income Maturities | Municipal
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 31.2	$ 28.8
% of Total Fair Value by Security Type	38.00%	72.00%
Fixed Income Maturities | Corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 3,346	$ 3,275.3
% of Total Fair Value by Security Type	95.00%	96.00%
Fixed Income Maturities | Non-U.S. government-backed corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 47.8	$ 44.8
% of Total Fair Value by Security Type	50.00%	50.00%
Fixed Income Maturities | Non-U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 425.6	$ 455.6
% of Total Fair Value by Security Type	61.00%	72.00%
Fixed Income Maturities | Asset-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 14.2	$ 32.1
% of Total Fair Value by Security Type	32.00%	41.00%
Fixed Income Maturities | Non-agency commercial mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 0	$ 12.5
% of Total Fair Value by Security Type	0.00%	98.00%
Fixed Income Maturities | Agency mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 618.2	$ 691.9
% of Total Fair Value by Security Type	54.00%	58.00%
Equity securities — Trading
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 468.6	$ 584.7
% of Total Fair Value by Security Type	100.00%	100.00%

Fair Value Measurements - Loan

Fair Value Measurements - Loan Notes Issued by Variable Interest Entities (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	$ 852.5	$ 763.5	$ 2,672.6	$ 2,540.9
Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums			57.6	43.4
Reserve for losses			$ 37.5	$ 11.8
Contract period			365 days	365 days
Loan notes held by third parties	(325)		$ (325)		$ (220)
Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums			50.1	$ 38.9
Reserve for losses			4.2	2.7
Loan notes held by third parties	(325)		(325)		(220)
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	7,555.6		7,555.6		7,653.6
Loan notes held by third parties	(101.5)		(101.5)		(115)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)	(2.4)		(2.4)		(108.4)
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	103.9		103.9		223.4
Loan notes held by third parties	(101.5)		(101.5)		(115)
Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables)	(2.4)	$ (2.3)	(2.4)	$ (2.3)	(108.4)
Fair Value, Measurements, Recurring | Level 3 | Silverton 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 103.9		$ 103.9
Fair Value, Measurements, Recurring | Level 3 | Silverton 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total					$ 223.4

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) $ in Millions	3 Months Ended					9 Months Ended		12 Months Ended
	Sep. 30, 2017USD ($)quote / Investment	Dec. 31, 2016USD ($)quote / Investment	Sep. 30, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Sep. 30, 2017USD ($)quote / Investment	Sep. 30, 2016USD ($)	Dec. 31, 2016USD ($)quote / Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Quotes per fixed income investment | quote / Investment	2	2.1				2		2.1
Quotes per equity investment | quote / Investment	4	3.9				4		3.9
Silverton 2,015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued					$ 85
Silverton 2015, Third-party Funded
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued					$ 70
Silverton 2,016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued				$ 125
Silverton 2016, Third-party Funded
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued				$ 100
Silverton 2,017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued		$ 130
Silverton 2017, Third-party Funded
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Issued		105
Non-U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 1 to Level 2 transfers, amount		$ 83.9						$ 83.9
Fair Value, Measurements, Recurring | Level 3 | Silverton
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repayments of debt								$ 89.3
Fair Value, Measurements, Recurring | Level 3 | Silverton 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of Level 3 liabilities settled			$ 0.3				$ 89.3
Fair Value, Measurements, Recurring | Level 3 | Silverton 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of Level 3 liabilities settled	$ 1.4					$ 115.9

Reinsurance - Narrative (Detail

Reinsurance - Narrative (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Munich Re
Concentration Risk [Line Items]
Concentration Risk, Percentage	9.40%	16.80%
Lloyd's
Concentration Risk [Line Items]
Concentration Risk, Percentage	6.60%	12.80%
Axis Re
Concentration Risk [Line Items]
Concentration Risk, Percentage	2.50%	4.60%

Derivative Contracts - Fair Val

Derivative Contracts - Fair Value of Derivative Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivatives at fair value	$ 8.4	$ 7.2
Foreign Exchange Contracts | Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Derivative notional amount	760.2	665.6
Cash collateral provided	0.6	0
Foreign Exchange Contracts | Not Designated as Hedging Instruments | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivative notional amount	521.5	240.2
Foreign Exchange Contracts | Not Designated as Hedging Instruments | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Liability notional amount	238.7	425.4
Foreign Exchange Contracts | Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Cash collateral provided	0	2.2
Foreign Exchange Contracts | Designated as Hedging Instrument | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivative notional amount	27.3	0
Foreign Exchange Contracts | Designated as Hedging Instrument | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Derivative notional amount	0	108.6
Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts | Not Designated as Hedging Instruments | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivatives at fair value	6	5
Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts | Not Designated as Hedging Instruments | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Liabilities at fair value	(2.9)	(17.7)
Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts | Designated as Hedging Instrument | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivatives at fair value	2.4	2.2
Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts | Designated as Hedging Instrument | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Liabilities at fair value	$ 0	$ (0.7)

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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
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	$ 4.5	$ 0.6	$ 25.2	$ (3.7)
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	0	0	(3.3)
Designated as Hedging Instrument | Foreign Exchange Contracts | General and Administrative Expense
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contract settlement	1.2	(3.1)	2.4	(5.6)
Designated as Hedging Instrument | Foreign Exchange Contracts | Net change from current period hedged transactions
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value	$ (0.4)	$ 3.1	$ 3.3	$ (1.9)

Derivative Contracts - Narrativ

Derivative Contracts - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	May 09, 2016
Goldman Sachs International
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount						$ 256.3
Foreign Exchange Contracts | Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Cash collateral provided	$ 0.6		$ 0.6		$ 0
Derivative notional amount	760.2		760.2		665.6
Foreign Exchange Contracts | Not Designated as Hedging Instruments | Change in Fair Value of Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments not designated as hedging instruments, gain (loss), net	4.5	$ 0.6	25.2	$ (3.7)
Foreign Exchange Contracts | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Cash collateral provided	0		0		2.2
Foreign Exchange Contracts | Designated as Hedging Instrument | General and Administrative Expense
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contract settlement	1.2	(3.1)	2.4	(5.6)
Foreign Exchange Contracts | Designated as Hedging Instrument | Net change from current period hedged transactions
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain (loss)	(0.4)	3.1	3.3	(1.9)
Foreign Exchange Contracts | Derivatives at Fair Value | Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	521.5		521.5		240.2
Foreign Exchange Contracts | Derivatives at Fair Value | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	27.3		27.3		0
Foreign Exchange Contracts | Liabilities under Derivative Contracts | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	0		0		$ 108.6
Interest Rate Swaps | 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	$ 0	$ 0	$ (3.3)

Deferred Policy Acquisition C62

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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance at the beginning of the period	$ 364.6	$ 409.1	$ 358.4	$ 361.1
Acquisition costs deferred	94.9	110	311.1	414.9
Amortization of deferred policy acquisition costs	(105.4)	(130.9)	(315.4)	(387.8)
Balance at the end of the period	$ 354.1	$ 388.2	$ 354.1	$ 388.2

Reserves for Losses and Loss 63

Reserves for Losses and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses ("LAE") Reserves (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Provision for losses and LAE at the start of the year	$ 5,319.9	$ 4,938.2	$ 4,938.2
Net loss and LAE at the start of the year	4,759.2	4,583.4	4,583.4
Net loss and LAE expenses assumed	(125.7)	5.7	(80.1)
Provision for losses and LAE for claims incurred:
Current year	1,543.3	1,267	1,705.4
Prior years	(92.8)	(78.2)	(129.3)
Total incurred	1,450.5	1,188.8	1,576.1
Losses and LAE payments for claims incurred:
Current year	(150.4)	(62.1)	(241)
Prior years	(947.7)	(853.1)	(981.8)
Total paid	(1,098.1)	(915.2)	(1,222.8)
Foreign exchange losses/(gains)	135.2	(35.9)	(97.4)
Net losses and LAE reserves at period end	5,121.1	4,826.8	4,759.2
Plus reinsurance recoverable on unpaid losses at period end	1,369.5	419.8	560.7	$ 354.8
Provision for losses and LAE at the end of the relevant period	$ 6,490.6	$ 5,246.6	$ 5,319.9

Reserves for Losses and Loss 64

Reserves for Losses and Loss Adjustment Expenses - Short-duration Insurance Contracts, Claims Development by Accident Year (Details) $ in Millions	Sep. 30, 2017USD ($)Claim	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)
Claims Development [Line Items]
Liability for claims and claims adjustment expenses, net of reinsurance	$ 5,075.5
Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	4,403.8
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	2,322.7
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	2,081.1
All outstanding liabilities before 2012, net of reinsurance	140.3
Liability for claims and claims adjustment expenses, net of reinsurance	2,221.4
Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	3,904.7
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	1,769.5
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	2,135.2
All outstanding liabilities before 2012, net of reinsurance	718.9
Liability for claims and claims adjustment expenses, net of reinsurance	2,854.1
Accident Year 2012 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	637	$ 656.7	$ 671.8	$ 657.6	$ 628.4	$ 606.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 47.2
Number of Reported Claims | Claim	15,825
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 519.2	504.3	454.1	379.5	308.6	117.6
Accident Year 2012 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	683.8	692.8	694.2	723.7	740	695.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 93.4
Number of Reported Claims | Claim	3,660
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 491	460.6	416.3	362.2	249.3	$ 63.6
Accident Year 2013 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	668.9	662.4	681	669.9	691.9
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 74.6
Number of Reported Claims | Claim	14,728
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 503.1	456.2	365.9	255.3	91.1
Accident Year 2013 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	519.5	528.6	554.8	575.9	583.2
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 92.9
Number of Reported Claims | Claim	3,524
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 354.7	322.6	280.7	188.2	$ 63.6
Accident Year 2014 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	686.2	704.8	732.7	758.1
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 109.7
Number of Reported Claims | Claim	18,848
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 448.9	390.7	248.2	99.8
Accident Year 2014 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	496.7	517.9	533.8	553.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 116.8
Number of Reported Claims | Claim	3,440
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 281.8	249.6	173.5	$ 57.2
Accident Year 2015 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	859.6	909.4	919.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 206.4
Number of Reported Claims | Claim	21,142
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 459.5	328	119.5
Accident Year 2015 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	553.5	568.6	584.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 166.5
Number of Reported Claims | Claim	3,450
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 254.1	172.8	$ 57.7
Accident Year 2016 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	876.3	912.7
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 316
Number of Reported Claims | Claim	20,538
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 332.1	117.7
Accident Year 2016 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	763.8	756.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 290.4
Number of Reported Claims | Claim	3,032
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 297.4	$ 127
Accident Year 2017 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	675.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 426.5
Number of Reported Claims | Claim	10,694
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 59.9
Accident Year 2017 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	887.4
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 636.9
Number of Reported Claims | Claim	1,704
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 90.5

Reserves for Losses and Loss 65

Reserves for Losses and Loss Adjustment Expenses - Short-duration Insurance Contracts Reconciliation Of Claims Development To Liability (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claims adjustment expenses, net of reinsurance	$ 5,075.5
Reinsurance recoverable on unpaid losses	1,369.5
Unallocated claims incurred	49.4
Other	(3.8)
Liability for unpaid claims and claim adjustment expense, aggregate reconciling items	45.6
Losses and loss adjustment expenses	6,490.6	$ 5,319.9	$ 5,246.6	$ 4,938.2
Insurance lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claims adjustment expenses, net of reinsurance	2,221.4
Reinsurance recoverable on unpaid losses	1,115.3
Reinsurance lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claims adjustment expenses, net of reinsurance	2,854.1
Reinsurance recoverable on unpaid losses	254.2
Insurance lines other than short-duration
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Losses and loss adjustment expenses	$ 0

Reserves for Losses and Loss 66

Reserves for Losses and Loss Adjustment Expenses - Short-duration Insurance Contracts Schedule Of Historical Claims Duration (Details)	Sep. 30, 2017
Insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year One	14.40%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Two	24.50%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Three	15.90%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Four	12.50%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Five	7.60%
Reinsurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year One	11.80%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Two	23.30%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Three	16.20%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Four	7.90%
Average Annual Percentage Payout of Incurred Claims, Net of Reinsurance, Year Five	6.40%

Reserves for Losses and Loss 67

Reserves for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Insurance [Abstract]
Reserve releases	$ (92.8)	$ (78.2)	$ (129.3)
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss portfolio transfers (disposed)	(125.7)	5.7	(80.1)
Liability For Unpaid Claims And Claims Adjustment Expense Amount Disposed of or Ceded	$ 125.7		$ 85.8
AgriLogic
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss portfolio transfers (disposed)		$ 5.7

Capital Structure - Summary of

Capital Structure - Summary of Authorized and Issued Share Capital (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 20, 2016
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)	59,407,323	59,774,464
Issued ordinary shares of 0.15144558¢ per share	$ 90	$ 91
Total issued share capital	$ 122	$ 140
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 shares, liquidation preference (in USD per share)	$ 25	$ 25
7.401% preference shares
Issued share capital:
Preference shares, issued (in shares)	0	5,327,500
Issued preference shares of 0.15144558¢ each with a liquidation preference	$ 0	$ 8
Preference shares, par value (in usd per share)		$ 0.0015144558
Preferred shares, liquidation preference (in USD per share)		$ 25
Preferred Stock Dividends Rate Percentage		7.401%
7.250% preference shares
Issued share capital:
Preference shares, issued (in shares)	0	6,400,000
Issued preference shares of 0.15144558¢ each with a liquidation preference	$ 0	$ 10
Preference shares, par value (in usd per share)		$ 0.0015144558
Preferred shares, liquidation preference (in USD per share)		$ 25
Preferred Stock Dividends Rate Percentage		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 shares, liquidation preference (in USD 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	10,000,000	10,000,000
Issued preference shares of 0.15144558¢ each with a liquidation preference	$ 15	$ 15
Preference shares, par value (in usd per share)	$ 0.0015144558	$ 0.0015144558
Preferred shares, liquidation preference (in USD per share)	$ 25	$ 25	$ 25
Preferred Stock Dividends Rate Percentage	5.625%	5.625%

Capital Structure - Summary o69

Capital Structure - Summary of Ordinary Shares (Details)	9 Months Ended
	Sep. 30, 2017shares
Ordinary Shares [Roll Forward]
Ordinary shares in issue as at December 31, 2016	59,774,464
Ordinary shares issued to employees under the 2013 share incentive plan and/or 2008 share purchase plan	272,296
Ordinary shares issued to non-employee directors	9,504
Ordinary shares repurchased	(648,941)
Ordinary shares in issue as at September 30, 2017	59,407,323

Capital Structure - Narrative (

Capital Structure - Narrative (Details) - USD ($)	Sep. 20, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Feb. 08, 2017
Class of Stock [Line Items]
Additional paid-in capital		$ 951,800,000		$ 951,800,000		$ 1,259,600,000
Aggregate liquidation preferences		$ 525,000,000		525,000,000		818,200,000
Issuance costs				$ 13,100,000		$ 21,100,000
Shares repurchase and cancelled during period (in shares)		451,268	144,289	648,941	1,122,328
Shares repurchase and cancelled during period, value		$ 20,000,000	$ 6,500,000	$ 30,000,000	$ 50,000,000
Average price (in usd per share)		$ 44.32	$ 45.17	$ 46.23	$ 44.55
Preferred shares, liquidation preference (in USD per share)		$ 25		$ 25		$ 25
Repurchase Program 2017
Class of Stock [Line Items]
Existing share repurchase authorization amount							$ 250,000,000
5.625% preference shares
Class of Stock [Line Items]
Issuance costs	$ 8,700,000
Preference shares, issued (in shares)	10,000,000	10,000,000		10,000,000		10,000,000
Preference shares, rate				5.625%		5.625%
Preferred shares, liquidation preference (in USD per share)	$ 25	$ 25		$ 25		$ 25
Net proceeds from issuance of preference shares	$ 241,300,000
Preferred shares, liquidation preference, value	$ 250,000,000
7.401% preference shares
Class of Stock [Line Items]
Preference shares, issued (in shares)		0		0		5,327,500
Preference shares, rate				7.401%		7.401%
Preferred shares, liquidation preference (in USD per share)						$ 25
7.250% preference shares
Class of Stock [Line Items]
Preference shares, issued (in shares)		0		0		6,400,000
Preference shares, rate				7.25%		7.25%
Preferred shares, liquidation preference (in USD per share)						$ 25

Share-Based Payments - Narrativ

Share-Based Payments - Narrative (Details)	9 Months Ended
	Sep. 30, 2017USD ($)Agreementshares	Sep. 30, 2017GBP (£)Agreementshares	Sep. 30, 2016USD ($)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
Options exercised (in shares) | shares	0	0	29,222
Compensation costs	$ 0		$ 0
Award vesting rights, percentage	8.33%	8.33%
Percentage of restricted share units vested and issued	100.00%	100.00%
Achievement One
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Achieved and required performance growth, percentage	5.00%	5.00%
Percentage of initial shares granted	33.33%	33.33%
Achievement Two | Minimum
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Achieved and required performance growth, percentage	5.00%	5.00%
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	10.00%	10.00%
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	10.00%	10.00%
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	20.00%	20.00%
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
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
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
Sharesave Scheme
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Employee contribution period	3 years	3 years
Employee savings under ESPP | £		£ 500
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 contribution period	2 years	2 years
Employee savings under ESPP	$ 500
Shares issued (in shares) | shares	10,545	10,545	13,342
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	$ 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	$ (400,000)		$ (2,500,000)
Granted, number of shares (in shares) | shares	216,878	216,878	278,477
Tax benefit from compensation expense	$ (100,000)		$ (500,000)
Phantom Shares
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Compensation costs	$ (700,000)		$ (800,000)
Granted, number of shares (in shares) | shares	173,619	173,619	147,513
Tax benefit from compensation expense	$ (200,000)		$ (600,000)
Fair value adjustment capitalized amount	$ 1,800,000		3,800,000
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	$ (7,200,000)		$ (7,200,000)
Granted, number of shares (in shares) | shares	200,020.707095856	200,020.707095856	328,551
Tax benefit from compensation expense	$ (1,700,000)		$ (1,700,000)
Fair value adjustment capitalized amount	0		600,000
Restricted Share Units | Director
Non Employee Share Based Compensation Arrangement With Individual Share Based Payments By Title Of Individual [Line Items]
Compensation costs	$ (1,100,000)		$ (1,100,000)
RSUs for non-employee directors (in shares) | shares				22,230	24,456
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				8,892	10,952
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 Goodwil72

Intangible Assets and Goodwill - Summary of Changes in Intangible Assets (Details) - USD ($) $ in Millions	Jan. 19, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Amortization		$ (1.3)	$ (0.8)	$ (3.3)	$ (3.8)
Goodwill [Roll Forward]
Goodwill, Beginning of the period		26	22.1	24.2	0
Goodwill, Additions		0	0	1.8	22.1
Goodwill, End of the period		26	22.1	26	22.1
Intangible assets and goodwill, Beginning of the period		79.4	72	79.6	18.2
Intangible Assets and Goodwill, Acquired		0	1.9	1.8	58.7
Intangible assets and goodwill, End of the period		78.1	73.1	78.1	73.1
Trade Mark
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		6.3	5.4	6.6	1.6
Intangible Assets, Additions		0	0	0	4
Amortization		(0.3)	(0.1)	(0.6)	(0.3)
Intangible Assets, End of the period	$ 4	6	5.3	6	5.3
Insurance Licenses
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		16.7	16.6	16.7	16.6
Intangible Assets, Additions		0	0	0	0
Amortization		0	0	0	0
Intangible Assets, End of the period		16.7	16.6	16.7	16.6
Agency Relationships
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		25.1	24.2	26.2	0
Intangible Assets, Additions		0	0	0	25
Amortization		(0.5)	(0.4)	(1.6)	(1.2)
Intangible Assets, End of the period	25	24.6	23.8	24.6	23.8
Non-compete Agreements
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		3	2.6	3.3	0
Intangible Assets, Additions	2.9	0	0	0	2.9
Amortization		(0.3)	(0.1)	(0.6)	(0.4)
Intangible Assets, End of the period		2.7	2.5	2.7	2.5
Value of Business Acquired
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		0	0.2	0	0
Intangible Assets, Additions		0	0	0	1.8
Amortization		0	(0.2)	0	(1.8)
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		0.8	0.9	0.9	0
Intangible Assets, Additions		0	0	0	1
Amortization		(0.1)	0	(0.2)	(0.1)
Intangible Assets, End of the period	$ 1	0.7	0.9	0.7	0.9
Contractual Rights
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		1.5	0	1.7	0
Intangible Assets, Additions		0	1.9	0	1.9
Amortization		(0.1)	0	(0.3)	0
Intangible Assets, End of the period		$ 1.4	$ 1.9	$ 1.4	$ 1.9

Intangible Assets and Goodwil73

Intangible Assets and Goodwill - Narrative (Details) - USD ($) $ in Thousands	Oct. 31, 2016	Sep. 22, 2016	Jan. 19, 2016	Apr. 05, 2005	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Jan. 01, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Contingent consideration			$ 14,100
Consideration Transferred, Other			22,800
Ceding commission			2,000
Consideration transferred			69,100
Software acquired			12,000
Residual assets			300
Total assets acquired, net			$ 47,000
Goodwill					$ 0	$ 0	$ 1,800	$ 22,100
AgriLogic
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Percentage of interests acquired			100.00%
Aggregate consideration for sale and purchase agreement			$ 53,000
Goodwill			22,100
Intangible assets, expected tax deductible amount			34,000
Goodwill, expected tax deductible amount			$ 21,000
Eligible tax deduction period			15 years
Blue Waters
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Percentage of interests acquired	100.00%
Consideration transferred	$ 8,000
Residual assets	300
Total assets acquired, net	5,750
Goodwill	2,100
Agency Relationships
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset			$ 25,000		24,600	23,800	24,600	23,800	$ 25,100		$ 26,200	$ 24,200	$ 0
Intangible Assets, Additions					0	0	$ 0	25,000
Asset amortization period							15 years
Trade Mark
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset			4,000		6,000	5,300	$ 6,000	5,300	6,300		6,600	5,400	1,600
Intangible Assets, Additions					0	0	$ 0	4,000
Asset amortization period							10 years
Trade Mark | Blue Waters
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset	1,500
Asset amortization period							5 years
Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset					16,700	16,600	$ 16,700	16,600	16,700		16,700	16,600	16,600
Intangible Assets, Additions					0	0	0	0
Insurance Licenses | Blue Waters
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of trading licenses	50
Contractual Rights
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset					1,400	1,900	1,400	1,900	1,500		1,700	0	0
Intangible Assets, Additions					0	1,900	0	1,900
Asset amortization period		5 years
Consulting Relationships
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset			1,000		700	900	700	900	800		900	900	0
Intangible Assets, Additions					0	0	$ 0	1,000
Asset amortization period							10 years
Customer Lists | Blue Waters
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset	3,100
Asset amortization period							5 years
Non-compete Agreements
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset					2,700	2,500	$ 2,700	2,500	3,000		3,300	2,600	0
Intangible Assets, Additions			2,900		0	0	$ 0	$ 2,900
Asset amortization period								5 years
Non-compete Agreements | Blue Waters
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset	$ 1,000
Asset amortization period							5 years
Value of Business Acquired
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset					0	0	$ 0	$ 0	$ 0		0	$ 200	$ 0
Intangible Assets, Additions					0	$ 0	0	$ 1,800
Value of Business Acquired | AgriLogic
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Intangible Assets, Additions			$ 1,800
Digital Re
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Goodwill							1,800
Equity Method Investment, Ownership Percentage										49.00%
LSML | Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset		$ 1,900
Aspen U.K.
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Aggregate consideration for sale and purchase agreement				$ 1,600
Aspen U.K. | Trade Mark
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset					1,600		1,600				1,600
Aspen U.K. | Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses					2,100		2,100
AAIC, Aspen Specialty, AIUK | Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset					16,600		16,600				$ 16,600
AAIC | Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses					10,000		10,000
Aspen Specialty | Insurance Licenses
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Acquired licenses					$ 4,500		$ 4,500

Commitments and Contingent Li74

Commitments and Contingent Liabilities - Schedule of Company's Restricted Assets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	$ 4,537.6	$ 4,535.7
Total as percent of investable assets(1)	51.90%	49.30%
Letters of credit / guarantees
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	$ 694.6	$ 672.1
Affiliated transactions
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	1,438.7	1,482.8
Third party
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total restricted assets	$ 2,404.3	$ 2,380.8

Commitments and Contingent Li75

Commitments and Contingent Liabilities - Narrative (Details) £ in Millions	Mar. 27, 2017USD ($)	Jun. 30, 2016	Sep. 30, 2017USD ($)	Sep. 30, 2017GBP (£)	Dec. 31, 2016USD ($)	Dec. 31, 2016GBP (£)	Sep. 30, 2016USD ($)
Line of Credit Facility [Line Items]
Assets held by insurance regulators			$ 458,000,000		$ 447,300,000
Minimum capital required | £				£ 0.4		£ 0.4
Maximum borrowing capacity	$ 200,000,000
Line Of Credit Facility Increasable Capacity	$ 100,000,000
Long-term Line of Credit			0
Line Of Credit Facility Tangible Net Worth Restrictions Minimum			$ 2,323,100,000
Line Of Credit Facility Tangible Net Worth Net Income Restriction			25.00%
Line Of Credit Facility Tangible Net Worth Cash Proceeds From Issuance Of Capital Stock Restriction			25.00%
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
Outstanding letters of credit			$ 444,200,000		$ 449,400,000
LIBOR | Citi Europe | Letter of Credit
Line of Credit Facility [Line Items]
Variable rate basis spread		1.00%

Commitments and Contingent Li76

Commitments and Contingent Liabilities - Amounts Outstanding under Operating Leases (Details) $ in Millions	Sep. 30, 2017USD ($)
Leases [Abstract]
2,017	$ 9.8
2,018	16.2
2,019	15.6
2,020	14.7
2,021	10
Later Years	80.8
Total	$ 147.1