Aspen Insurance Holdings Limited (AHL-PE) 10-Q 8 Aug 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	6 Months Ended
	Jun. 30, 2018shares
Document and Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
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	59,687,717

UNAUDITED CONDENSED CONSOLIDATE

UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Investments:
Fixed income securities, available for sale at fair value (amortized cost — $5,155.9 and $5,201.2)	$ 5,080	$ 5,231
Fixed income securities, trading at fair value (amortized cost — $1,644.6 and $1,634.9)	1,615.9	1,649.3
Equity securities, trading at fair value (cost — $0 and $414.8)	0	491
Short-term investments, available for sale at fair value (amortized cost — $72.2 and $90.0)	72.2	89.9
Short-term investments, trading at fair value (amortized cost — $27.4 and $73.0)	27.4	73
Catastrophe bonds, trading at fair value (cost — $35.8 and $33.5)	35.5	32.4
Investments, equity method	67.1	66.4
Other investments	86.2	0
Total investments	6,984.3	7,633
Cash and cash equivalents (including $74.5 and $166.6 within consolidated variable interest entities)	1,070.7	1,054.8
Reinsurance recoverables
Unpaid losses	1,656.4	1,515.2
Ceded unearned premiums	724.8	515.5
Receivables
Underwriting premiums	1,725.2	1,496.5
Other	175.5	151.1
Funds withheld	89.3	99.8
Deferred policy acquisition costs	298.6	294.3
Derivatives at fair value	14.1	6.4
Receivables for securities sold	2.6	5.3
Office properties and equipment	73	75.5
Tax recoverable	5.8	2.3
Deferred tax assets	26.9	28.3
Other assets	0.5	0.5
Intangible assets and goodwill	27	27.9
Total assets	12,874.7	12,906.4
Insurance reserves
Losses and loss adjustment expenses	6,532.8	6,749.5
Unearned premiums	2,087.2	1,820.8
Total insurance reserves	8,620	8,570.3
Payables
Reinsurance premiums	596	357.5
Accrued expenses and other payables	336.5	455.4
Liabilities under derivative contracts	49.3	1
Total payables	981.8	813.9
Loan notes issued by variable interest entities, at fair value	20.3	44.2
Long-term debt	424.6	549.5
Total liabilities	10,046.7	9,977.9
Commitments and contingent liabilities (see Note 16)	0	0
SHAREHOLDERS’ EQUITY
Ordinary shares, value	0.1	0.1
Preference shares:
Non-controlling interest	3	2.7
Additional paid-in capital	965.4	954.7
Retained earnings	1,998.9	2,026.9
Accumulated other comprehensive income, net of taxes	(139.4)	(55.9)
Total shareholders’ equity	2,828	2,928.5
Total liabilities and shareholders’ equity	12,874.7	12,906.4
5.95% 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	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Fixed income securities, available for sale, amortized cost	$ 5,155.9	$ 5,201.2
Fixed income securities, trading at fair value (amortized cost )	1,644.6	1,634.9
Equity securities, trading at fair value, cost	0	414.8
Short-term Investments, available for sale, amortized cost	72.2	90
Short-term investments, trading at fair value, amortized cost	27.4	73
Catastrophe bonds, trading at fair value, cost	35.8	33.5
Cash and cash equivalents, within consolidated VIEs	$ 74.5	$ 166.6
Ordinary shares, par value (in usd per share)	$ 0.0015144558	$ 0.0015144558
Ordinary shares, issued (in shares)	59,687,717	59,474,085
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
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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Net earned premium	$ 519.5	$ 562	$ 1,053	$ 1,143.1
Net investment income	50.4	47.4	97.7	95.1
Realized and unrealized investment gains	3.5	49	104.1	100.2
Other income	2.1	3.6	4.2	7.2
Total revenues	575.5	662	1,259	1,345.6
Expenses
Losses and loss adjustment expenses	310.4	346.1	620.6	674.3
Amortization of deferred policy acquisition costs	85.9	96.3	176.7	210
General, administrative and corporate expenses	110.2	119.9	231.2	241.2
Interest on long-term debt	7.6	7.4	15	14.8
Change in fair value of derivatives	46.1	(17.6)	22.6	(20.7)
Change in fair value of loan notes issued by variable interest entities	3.4	3.3	2.4	6.2
Realized and unrealized investment losses	24.2	7	162.5	12
Realized loss on the debt extinguishment	8.6	0	8.6	0
Net realized and unrealized foreign exchange (gains)/losses	(5.2)	20.6	(0.5)	29.5
Other expenses	0.5	2	1.7	2
Total expenses	591.7	585	1,240.8	1,169.3
(Loss)/income from operations before income tax	(16.2)	77	18.2	176.3
Income tax credit/(expense)	1.5	(1.2)	(2.1)	(4)
Net (loss)/income	(14.7)	75.8	16.1	172.3
Amount attributable to non-controlling interest	(0.1)	(0.1)	(0.3)	(0.2)
Net (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	(14.8)	75.7	15.8	172.1
Available for sale investments:
Reclassification adjustment for net realized gains on investments included in net income	3.3	(0.8)	3.2	(1.8)
Change in net unrealized (losses)/gains on available for sale securities held	(26.1)	13.7	(108.9)	16.7
Net change from current period hedged transactions	(2.6)	2.4	(1.7)	3.7
Change in foreign currency translation adjustment	33	(27.6)	24.9	(44.8)
Other comprehensive income/(loss), gross of tax	7.6	(12.3)	(82.5)	(26.2)
Tax thereon:
Reclassification adjustment for net realized gains on investments included in net income	(0.4)	0	(0.3)	0.2
Change in net unrealized gains/(losses) on available for sale securities held	1.6	(1.1)	7	(1.2)
Net change from current period hedged transactions	0.5	(0.4)	0.3	(0.5)
Change in foreign currency translation adjustment	(9.6)	6.3	(8)	10.6
Total tax on other comprehensive income	(7.9)	4.8	(1)	9.1
Other comprehensive (loss), net of tax	(0.3)	(7.5)	(83.5)	(17.1)
Total comprehensive (loss)/income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	$ (15.1)	$ 68.2	$ (67.7)	$ 155
Weighted average number of ordinary share and share equivalents
Basic (in shares)	59,671,684	59,966,358	59,609,271	59,914,797
Diluted (in shares)	59,671,684	61,022,981	60,528,136	61,095,817
Basic (loss)/earnings per ordinary share adjusted for preference share dividends	$ (0.38)	$ 1.09	$ 0.01	$ 2.48
Diluted (loss)/earnings per ordinary share adjusted for preference share dividends	$ (0.38)	$ 1.07	$ 0.01	$ 2.43

UNAUDITED CONDENSED CONSOLIDAT5

UNAUDITED CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Millions		Total	Ordinary shares	Preference shares	7.401% 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, 2016			$ 0.1	$ 0		$ 1.4	$ 1,259.6				$ 2,392.3				$ (27.1)	$ (0.5)	$ 22.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change attributable to non-controlling interest for the period		$ (0.2)				0.2					(0.2)
New shares issued								$ 0.4
Ordinary shares purchased and cancelled		(10)						(10)
Preference share redemption									$ (133.2)
Preference shares redemption costs		2.4							(2.4)	[1]
Share-based compensation	[2]						6
Net income for the period		172.3									172.3
Dividends on ordinary shares												$ (27.6)
Dividends on preference shares		(21)											$ (21)
Change for the period, net of income tax															(34.2)		13.9
Shareholder's equity, ending balance at Jun. 30, 2017		3,620.9	0.1	0		1.6	1,125.2				2,516.2				(61.3)	2.7	36.4
Shareholder's equity, beginning balance at Dec. 31, 2016			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 redemption costs	[1]												2.4
Shareholder's equity, ending balance at Dec. 31, 2017		2,928.5	0.1	0		2.7	954.7				2,026.9				(67.7)	2.1	9.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change from current period hedged transactions																3.2
Total accumulated other comprehensive (loss), net of taxes		(22.2)
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-09	[3]										2.8
Total accumulated other comprehensive (loss), net of taxes		(55.9)
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-09							7.9				2.8
Net change attributable to non-controlling interest for the period		(0.3)				0.3					(0.3)
New shares issued								2.5
Ordinary shares purchased and cancelled								$ 0
Preference share redemption									0
Preference shares redemption costs		0							$ 0	[1]			0	[1]
Share-based compensation	[2]						8.2
Net income for the period		16.1									16.1
Dividends on ordinary shares												$ (28.6)
Dividends on preference shares		(15.2)											$ (15.2)
Change for the period, net of income tax															16.9		(99)
Shareholder's equity, ending balance at Jun. 30, 2018		2,828	$ 0.1	$ 0		$ 3	$ 965.4				1,998.9				$ (50.8)	0.7	$ (89.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Dividend Rate, Percentage					7.401%
Net change from current period hedged transactions																$ (1.4)
Total accumulated other comprehensive (loss), net of taxes		$ (139.4)
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-09	[3]										$ 0
[1]	The $2.4 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% Perpetual Non-Cumulative Preference Shares, net of issuance costs, and the final redemption costs of $133.2 million.
[2]	The balance in 2017 includes $7.9 million reclassification from accrued expenses and other payable as a result of the classification of restricted share units as equity following the adoption of ASU 2016-09.
[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		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017
Cash flows (used in) operating activities:
Net income		$ 16.1	$ 172.3
Proportion due to non-controlling interest		(0.3)	(0.2)
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization		23	27.4
Share-based compensation		8.2	6
Realized and unrealized investment gains		(104.1)	(100.2)
Realized and unrealized investment losses		162.5	12
Deferred taxes		1.3	(1.7)
Change in fair value of loan notes issued by variable interest entities		2.4	6.2
Net realized and unrealized investment foreign exchange (gains)		(18)	(8.7)
Net change from current period hedged transactions		(1.4)	3.2
Insurance reserves:
Losses and loss adjustment expenses		(171)	166.4
Unearned premiums		283.1	339.5
Reinsurance recoverables:
Unpaid losses		(146.7)	(212.6)
Ceded unearned premiums		(210.7)	(207.5)
Other receivables		(23.5)	(9.3)
Deferred policy acquisition costs		(7.6)	(2.2)
Reinsurance premiums payable		241.7	(18.5)
Funds withheld		10.5	(16.8)
Premiums receivable		(229.5)	(213.3)
Income tax payable		(4.2)	4.3
Accrued expenses and other payables		(59.9)	(22)
Fair value of derivatives and settlement of liabilities under derivatives		40.6	(24.7)
Long-term debt and loan notes issued by variable interest entities		(23.9)	(4.1)
Net cash (used in) operating activities		(211.4)	(104.5)
Cash flows from investing activities:
(Purchases) of fixed income securities — Available for sale		(1,021.3)	(847.6)
(Purchases) of fixed income securities — Trading		(847.2)	(556)
Proceeds from sales and maturities of fixed income securities — Available for sale		1,037.5	1,039.9
Proceeds from sales and maturities of fixed income securities — Trading		825.5	497.3
(Purchases) of equity securities — Trading		(16.5)	(70.1)
Net (purchases)/sales of catastrophe bonds — Trading		(2.2)	13.8
Proceeds from sales of equity securities — Trading		505.6	58.9
(Purchases) of short-term investments — Available for sale		(32)	(33.2)
Proceeds from sales of short-term investments — Available for sale		49.1	138.4
(Purchases) of short-term investments — Trading		(7.3)	(21.3)
Proceeds from sales of short-term investments — Trading		51.9	135.2
Net change in (payable)/receivable for securities (purchased)/sold		19.5	20.9
Net payments for other investments		(86.2)	0
Net (purchases) of equipment		(12.9)	(17.6)
Sale of investment		0	9.3
Equity investment		(1.2)	0
Payments for acquisitions and investments, net of cash acquired		0	(2.3)
Net cash from investing activities		462.3	365.6
Cash flows (used in) financing activities:
Proceeds from the issuance of ordinary shares, net of issuance costs		2.5	0.4
Payments for repurchase of ordinary shares		0	(10)
Preference share redemption		0	(133.2)
Repayment of long-term debt issued by Silverton		(63.7)	(114.1)
Dividends paid on ordinary shares		(28.6)	(27.6)
Dividends paid on preference shares		(15.2)	(21)
Cash paid for tax withholding purposes (1)	[1]	(4.4)	(9.1)
Long-term debt redeemed		(125)	0
Make-whole payment		(8.6)	0
Net cash (used in) financing activities		(243)	(314.6)
Effect of exchange rate movements on cash and cash equivalents		8	8.1
Increase/(decrease) in cash and cash equivalents		15.9	(45.4)
Cash and cash equivalents at beginning of period		1,054.8	1,273.8
Cash and cash equivalents at end of period		1,070.7	1,228.4
Supplemental disclosure of cash flow information:
Net cash paid (received) during the period for income tax		5.2	(2.6)
Cash paid during the period for interest		$ 15	$ 14.8
[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	6 Months Ended
	Jun. 30, 2018
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, 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	6 Months Ended
	Jun. 30, 2018
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 six months ended June 30, 2018 are not necessarily indicative of the results that may be expected for the year ended December 31, 2018 . 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, 2017 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, 2017 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, 2018 (File No. 001-31909). Assumptions and estimates made by management have a significant effect on the amounts reported within the unaudited condensed consolidated financial statements. The most significant of these 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 differ significantly from those expected when the assumptions or estimates were made. Accounting Pronouncements Adopted in 2018 On August 12, 2015, the Financial Standards Accounting Board (“FASB”) issued ASU 2015-14, “ Revenue from Contracts with Customers (Topic 606)” which delayed the effective date of ASU 2014-09 by one year. This ASU is effective for annual periods beginning after December 15, 2017. Adoption of this ASU during the three and six months ended June 30, 2018 did not have a material impact on the Company’s consolidated financial statements because insurance contracts accounted for within the scope of Topic 944, Financial Services are exempt from this ASU and the Company has immaterial other revenue. On January 5, 2016, the FASB issued ASU 2016-1, “ Financial Instruments - Overall (Subtopic 825-10)” which enhances the reporting model for financial instruments. Included within the requirements of this ASU are the following: a) equity investments to be measured at fair value with changes in fair value recognized in net income; b) a simplification of the impairment assessment of equity investments without readily determinable fair values; c) public business entities to use the exit price concept when measuring the fair value of financial instruments for disclosure purposes; and d) clarifies that an entity should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale securities in combination with the entity’s other deferred tax assets. The amendments required as a result of this ASU are effective for fiscal years beginning after December 15, 2017. Adoption of this ASU during the three and six months ended June 30, 2018 did not have a material impact on the Company’s consolidated financial statements because the Company’s equity portfolio, prior to being sold, was classified as held for trading with changes in fair value recognized through net income and no valuation allowance was required in relation to deferred tax asset related to available-for-sale securities. 2018 Accounting Pronouncements Not Yet Adopted On February 14, 2018, the FASB issued ASU 2018-02, “ Income Statement - Reporting Comprehensive Income (Topic 220)” which allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. This ASU will be effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. The Company is currently evaluating the provisions of ASU 2018-02 to determine how it will be affected, but no material impact is expected on the consolidated financial statements. On February 28, 2018, the FASB issued ASU 2018-03, “ Technical Corrections and Improvements to Financial Instruments - Overall (Subtopic 825-10)” which amends multiple areas in Subtopic 825-10 via improvements to clarify the Codification or to correct unintended application of guidance. This ASU is effective for fiscal years beginning after December 15, 2017 and for interim periods within those fiscal years beginning after June 15, 2018. The Company is currently evaluating the provisions of ASU 2018-03 to determine how it will be affected, but no material impact is expected on the consolidated financial statements. On June 20, 2018, the FASB issued ASU 2018-07, “ Compensation - Stock Compensation (Topic 718)” which amends the scope of Topic 718 via improvements to non-employee share-based payment accounting. Amendments include allowing companies to account for share-based payment transactions with non-employees in the same way as share-based payment transactions with employees and includes elections that offer relief to non-public companies when measuring non-employee equity share options. This ASU will be effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. The Company is currently evaluating the provisions of ASU 2018-07 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 and six months ended June 30, 2018 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	6 Months Ended
	Jun. 30, 2018
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 six months ended June 30, 2018 and 2017 : Amount Reclassified from AOCI Details about the AOCI Components Three Months Ended June 30, 2018 Three Months Ended June 30, 2017 Affected Line Item in the Unaudited ($ in millions) Available for sale securities: Realized gains on sale of securities $ 2.6 $ 3.3 Realized and unrealized investment gains Realized (losses) on sale of securities (5.9 ) (2.5 ) Realized and unrealized investment losses (3.3 ) 0.8 Income from operations before income tax Tax on net realized (losses)/gains on securities 0.4 — Income tax credit/(expense) $ (2.9 ) $ 0.8 Net (loss)/income Realized derivatives: Net realized gains on settled derivatives $ 1.3 $ 0.9 General, administrative and corporate expenses Tax on settled derivatives (0.3 ) (0.2 ) Income tax credit/(expense) $ 1.0 $ 0.7 Net (loss)/income Total reclassifications from AOCI to the statement of operations, net of income tax $ (1.9 ) $ 1.5 Net (loss)/income Amount Reclassified from AOCI Details about the AOCI Components Six Months Ended June 30, 2018 Six Months Ended June 30, 2017 Affected Line Item in the Unaudited Condensed Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gains on sale of securities $ 4.6 $ 5.6 Realized and unrealized investment gains Realized (losses) on sale of securities (7.8 ) (3.8 ) Realized and unrealized investment losses (3.2 ) 1.8 Income from operations before income tax Tax on net realized (losses)/gains on securities 0.3 (0.2 ) Income tax credit/(expense) $ (2.9 ) $ 1.6 Net (loss)/income Realized derivatives: Net realized gains on settled derivatives $ 3.0 $ 1.2 General, administrative and corporate expenses Tax on settled derivatives (0.6 ) (0.2 ) Income tax credit/(expense) $ 2.4 $ 1.0 Net (loss)/income Total reclassifications from AOCI to the statement of operations, net of income tax $ (0.5 ) $ 2.6 Net (loss)/income

Earnings per Ordinary Share

Earnings per Ordinary Share	6 Months Ended
	Jun. 30, 2018
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 six months ended June 30, 2018 and 2017 , respectively: Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 ($ in millions, except share and per share amounts) Net (loss)/income $ (14.7 ) $ 75.8 $ 16.1 $ 172.3 Preference share dividends (7.6 ) (10.5 ) (15.2 ) (21.0 ) Preference share redemption costs (1) — — — (2.4 ) Net amount attributable to non-controlling interest (0.1 ) (0.1 ) (0.3 ) (0.2 ) Basic and diluted net (loss)/income available to ordinary shareholders (2) $ (22.4 ) $ 65.2 $ 0.6 $ 148.7 Ordinary shares: Basic weighted average ordinary shares 59,671,684 59,966,358 59,609,271 59,914,797 Weighted average effect of dilutive securities (2) (3) — 1,056,623 918,865 1,181,020 Total diluted weighted average ordinary shares 59,671,684 61,022,981 60,528,136 61,095,817 (Loss)/earnings per ordinary share: Basic $ (0.38 ) $ 1.09 $ 0.01 $ 2.48 Diluted (2) $ (0.38 ) $ 1.07 $ 0.01 $ 2.43 (1) The $2.4 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% Perpetual Non-Cumulative Preference Shares, net of issuance costs, and the final redemption costs of $133.2 million . (2) The basic and diluted number of ordinary shares for the three months ended June 30, 2018 was the same, as the inclusion of dilutive securities in a loss-making period would be anti-dilutive. (3) 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. Dividends. On August 1, 2018, the Company’s Board of Directors (the “Board of Directors”) declared the following quarterly dividends: Dividend Payable on: Record Date: Ordinary shares $ 0.24 September 4, 2018 August 17, 2018 5.95% preference shares $ 0.3719 October 1, 2018 September 15, 2018 5.625% preference shares $ 0.3516 October 1, 2018 September 15, 2018

Segment Reporting

Segment Reporting	6 Months Ended
	Jun. 30, 2018
Segment Reporting [Abstract]
Segment Reporting	Segment Reporting The Company has two reporting business segments: Insurance and Reinsurance. The Company has determined its reportable segments, Aspen Insurance and Aspen Reinsurance, by taking into account the manner in which management makes operating decisions and assesses operating performance. Profit or loss for each of the Company’s business segments is measured by underwriting profit or loss. Underwriting profit is the excess of net earned premiums over the sum of losses and loss expenses, amortization of deferred policy acquisition costs and general and administrative expenses. Underwriting profit or loss provides a basis for management to evaluate the business segment’s underwriting performance. 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. Reinsurance Segment. The reinsurance segment consists of property catastrophe reinsurance, other property reinsurance, casualty reinsurance and specialty insurance and reinsurance. ACM forms part of our property catastrophe reinsurance line of business as it focuses primarily on property catastrophe business through the use of alternative capital. For a more detailed description of this business segment, see Part I, Item 1, “Business — Business Segments — Reinsurance” in the Company’s 2017 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 2017 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 June 30, 2018 and 2017 : Three Months Ended June 30, 2018 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 326.0 $ 527.8 $ 853.8 Net written premiums 266.9 219.1 486.0 Gross earned premiums 366.2 479.3 845.5 Net earned premiums 289.0 230.5 519.5 Underwriting Expenses Losses and loss adjustment expenses 167.0 143.4 310.4 Amortization of deferred policy acquisition costs 62.8 23.1 85.9 General and administrative expenses 27.9 57.2 85.1 Underwriting income $ 31.3 $ 6.8 38.1 Corporate expenses (16.0 ) Non-operating expenses (1) (9.1 ) Net investment income 50.4 Realized and unrealized investment gains 3.5 Realized and unrealized investment losses (24.2 ) Realized loss on the debt extinguishment (8.6 ) Change in fair value of loan notes issued by variable interest entities (3.4 ) Change in fair value of derivatives (46.1 ) Interest expense on long term debt (7.6 ) Net realized and unrealized foreign exchange gains 5.2 Other income 2.1 Other expenses (0.5 ) Loss before tax $ (16.2 ) Net reserves for loss and loss adjustment expenses $ 2,744.1 $ 2,132.3 $ 4,876.4 Ratios Loss ratio 57.8 % 62.2 % 59.7 % Policy acquisition expense ratio 21.7 10.0 16.5 General and administrative expense ratio 9.7 24.8 21.2 (2) Expense ratio 31.4 34.8 37.7 Combined ratio 89.2 % 97.0 % 97.4 % (1) Non-operating expenses includes $8.6 million of expenses related to the Company’s Effectiveness and Efficiency Program. (2) The general and administrative expense ratio in the “Total” column includes corporate and non-operating expenses. Three Months Ended June 30, 2017 Reinsurance Insurance Total ( $ in millions) Underwriting Revenues Gross written premiums $ 335.6 $ 486.5 $ 822.1 Net written premiums 285.5 293.2 578.7 Gross earned premiums 320.6 429.1 749.7 Net earned premiums 272.7 289.3 562.0 Underwriting Expenses Losses and loss adjustment expenses 152.6 193.5 346.1 Amortization of deferred policy acquisition costs 53.4 42.9 96.3 General and administrative expenses 40.7 65.7 106.4 Underwriting income/(loss) $ 26.0 $ (12.8 ) 13.2 Corporate expenses (11.4 ) Non-operating expenses (2.1 ) Net investment income 47.4 Realized and unrealized investment gains 49.0 Realized and unrealized investment losses (7.0 ) Change in fair value of loan notes issued by variable interest entities (3.3 ) Change in fair value of derivatives 17.6 Interest expense on long term debt (7.4 ) Net realized and unrealized foreign exchange (losses) (20.6 ) Other income 3.6 Other expenses (2.0 ) Income before tax $ 77.0 Net reserves for loss and loss adjustment expenses $ 2,445.0 $ 2,347.0 $ 4,792.0 Ratios Loss ratio 56.0 % 66.9 % 61.6 % Policy acquisition expense ratio 19.6 14.8 17.1 General and administrative expense ratio 14.9 22.7 21.3 (1) Expense ratio 34.5 37.5 38.4 Combined ratio 90.5 % 104.4 % 100.0 % (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 six months ended June 30, 2018 and 2017 : Six Months Ended June 30, 2018 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 949.5 $ 1,021.1 $ 1,970.6 Net written premiums 691.9 429.6 1,121.5 Gross earned premiums 741.2 946.9 1,688.1 Net earned premiums 571.5 481.5 1,053.0 Underwriting Expenses Losses and loss adjustment expenses 333.9 286.7 620.6 Amortization of deferred policy acquisition costs 118.7 58.0 176.7 General and administrative expenses 59.5 120.8 180.3 Underwriting income $ 59.4 $ 16.0 75.4 Corporate expenses (29.7 ) Non-operating expenses (1) (21.2 ) Net investment income 97.7 Realized and unrealized investment gains 104.1 Realized and unrealized investment losses (162.5 ) Realized loss on the debt extinguishment (8.6 ) Change in fair value of loan notes issued by variable interest entities (2.4 ) Change in fair value of derivatives (22.6 ) Interest expense on long term debt (15.0 ) Net realized and unrealized foreign exchange gains 0.5 Other income 4.2 Other expenses (1.7 ) Income before tax $ 18.2 Net reserves for loss and loss adjustment expenses $ 2,744.1 $ 2,132.3 $ 4,876.4 Ratios Loss ratio 58.4 % 59.5 % 58.9 % Policy acquisition expense ratio 20.8 12.0 16.8 General and administrative expense ratio 10.4 25.1 22.0 (2) Expense ratio 31.2 37.1 38.8 Combined ratio 89.6 % 96.6 % 97.7 % (1) Non-operating expenses includes $20.4 million of expenses related to the Company’s Effectiveness and Efficiency Program. (2) The general and administrative expense ratio in the “Total” column includes corporate and non-operating expenses. Six Months Ended June 30, 2017 Reinsurance Insurance Total ( $ in millions) Underwriting Revenues Gross written premiums $ 900.9 $ 919.2 $ 1,820.1 Net written premiums 733.7 531.2 1,264.9 Gross earned premiums 648.2 852.8 1,501.0 Net earned premiums 550.2 592.9 1,143.1 Underwriting Expenses Losses and loss adjustment expenses 295.7 378.6 674.3 Amortization of deferred policy acquisition costs 112.9 97.1 210.0 General and administrative expenses 84.6 127.5 212.1 Underwriting income/(loss) $ 57.0 $ (10.3 ) 46.7 Corporate expenses (24.8 ) Non-operating expenses (4.3 ) Net investment income 95.1 Realized and unrealized investment gains 100.2 Realized and unrealized investment losses (12.0 ) Change in fair value of loan notes issued by variable interest entities (6.2 ) Change in fair value of derivatives 20.7 Interest expense on long term debt (14.8 ) Net realized and unrealized foreign exchange (losses) (29.5 ) Other income 7.2 Other expenses (2.0 ) Income before tax $ 176.3 Net reserves for loss and loss adjustment expenses $ 2,445.0 $ 2,347.0 $ 4,792.0 Ratios Loss ratio 53.7 % 63.9 % 59.0 % Policy acquisition expense ratio 20.5 16.4 18.4 General and administrative expense ratio 15.4 21.5 21.1 (1) Expense ratio 35.9 37.9 39.5 Combined ratio 89.6 % 101.8 % 98.5 % (1) The general and administrative expense ratio in the “Total” column includes corporate and non-operating expenses.

Investments

Investments	6 Months Ended
	Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Investments	Investments Statements of Operations and Other Comprehensive Income Investment Income. The following table summarizes investment income for the three and six months ended June 30, 2018 and 2017 : For the Three Months Ended For the Six Months Ended June 30, 2018 June 30, 2017 June 30, 2018 June 30, 2017 ($ in millions) ($ in millions) Fixed income securities — Available for sale $ 33.5 $ 33.9 $ 66.8 $ 67.8 Fixed income securities — Trading 13.2 10.4 25.2 20.5 Short-term investments — Available for sale 0.3 0.2 0.5 0.3 Short-term investments — Trading 0.1 0.3 0.3 0.5 Fixed term deposits (included in cash and cash equivalents) 4.2 1.3 6.6 2.0 Equity securities — Trading 0.7 3.7 1.9 8.9 Catastrophe bonds — Trading 0.7 0.4 1.3 0.8 Total $ 52.7 $ 50.2 $ 102.6 $ 100.8 Investment expenses (2.3 ) (2.8 ) (4.9 ) (5.7 ) Net investment income $ 50.4 $ 47.4 $ 97.7 $ 95.1 The following table summarizes the net realized and unrealized investment gains and losses recorded in the statement of operations and the change in unrealized gains and losses on investments recorded in other comprehensive income for the three and six months ended June 30, 2018 and 2017 : For the Three Months Ended For the Six Months Ended June 30, 2018 June 30, 2017 June 30, 2018 June 30, 2017 ($ in millions) ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 2.6 $ 3.3 $ 4.4 $ 5.5 Fixed income securities — gross realized (losses) (5.6 ) (2.4 ) (7.4 ) (3.7 ) Short-term investments — gross realized gains — — — 0.1 Cash and cash equivalents — gross realized gains — — 0.2 — Cash and cash equivalents — gross realized (losses) (0.3 ) (0.1 ) (0.4 ) (0.1 ) Other-than-temporary impairments — (0.1 ) — (0.4 ) Trading: Fixed income securities — gross realized gains 1.0 3.5 2.6 5.3 Fixed income securities — gross realized (losses) (3.4 ) (0.4 ) (10.4 ) (2.4 ) Short-term investments — gross realized gains — 0.1 — 0.1 Cash and cash equivalents — gross realized (losses) gains (0.1 ) — 1.5 — Equity securities — gross realized gains — 4.4 94.5 8.9 Equity securities — gross realized (losses) 0.2 (3.6 ) (20.1 ) (5.0 ) Catastrophe bonds — net unrealized (losses) gains — (0.1 ) 0.9 (0.1 ) Net change in gross unrealized (losses) gains (14.9 ) 38.5 (123.7 ) 79.4 Investments — equity method: Gross realized and unrealized (loss) in MVI (0.1 ) (0.1 ) (0.2 ) (0.1 ) Gross realized (loss)/gain in Chaspark — (0.8 ) — 0.9 Gross realized and unrealized (loss) in Bene (0.1 ) (0.2 ) (0.3 ) (0.2 ) Total net realized and unrealized investment (losses) gains recorded in the statement of operations $ (20.7 ) $ 42.0 $ (58.4 ) $ 88.2 Change in available for sale net unrealized (losses) gains: Fixed income securities (22.8 ) 12.9 (105.7 ) 14.9 Total change in pre-tax available for sale unrealized (losses) gains (22.8 ) 12.9 (105.7 ) 14.9 Change in taxes 1.2 (1.1 ) 6.7 (1.0 ) Total change in net unrealized gains, net of taxes, recorded in other comprehensive income $ (21.6 ) $ 11.8 $ (99.0 ) $ 13.9 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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,253.5 $ 1.6 $ (25.5 ) $ 1,229.6 U.S. agency 47.1 0.1 (0.6 ) 46.6 Municipal 105.1 1.7 (1.1 ) 105.7 Corporate 2,304.2 10.2 (47.3 ) 2,267.1 Non-U.S. government-backed corporate 92.2 0.1 (0.5 ) 91.8 Non-U.S. government 427.5 4.0 (1.3 ) 430.2 Asset-backed 18.7 — (0.3 ) 18.4 Agency mortgage-backed 907.6 5.8 (22.8 ) 890.6 Total fixed income securities — Available for sale 5,155.9 23.5 (99.4 ) 5,080.0 Total short-term investments — Available for sale 72.2 — — 72.2 Total $ 5,228.1 $ 23.5 $ (99.4 ) $ 5,152.2 As at December 31, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,166.5 $ 4.5 $ (11.6 ) $ 1,159.4 U.S. agency 51.8 0.5 (0.2 ) 52.1 Municipal 53.0 2.1 (0.2 ) 54.9 Corporate 2,391.4 36.1 (11.8 ) 2,415.7 Non-U.S. government-backed corporate 91.5 0.3 (0.5 ) 91.3 Non-U.S. government 479.7 6.4 (1.2 ) 484.9 Asset-backed 26.3 — (0.1 ) 26.2 Agency mortgage-backed 941.0 13.7 (8.2 ) 946.5 Total fixed income securities — Available for sale 5,201.2 63.6 (33.8 ) 5,231.0 Total short-term investments — Available for sale 90.0 — (0.1 ) 89.9 Total $ 5,291.2 $ 63.6 $ (33.9 ) $ 5,320.9 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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 155.6 $ 0.5 $ (0.8 ) $ 155.3 Municipal 78.6 — (0.8 ) 77.8 Corporate 988.2 3.1 (22.8 ) 968.5 Non-U.S government-backed corporate — — — — Non-U.S. government 222.3 2.0 (5.0 ) 219.3 Asset-backed 7.3 — (0.1 ) 7.2 Agency mortgage-backed 192.6 — (4.8 ) 187.8 Total fixed income securities — Trading 1,644.6 5.6 (34.3 ) 1,615.9 Total short-term investments — Trading 27.4 — — 27.4 Total catastrophe bonds — Trading 35.8 0.1 (0.4 ) 35.5 Total $ 1,707.8 $ 5.7 $ (34.7 ) $ 1,678.8 As at December 31, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 162.3 $ 0.4 $ (0.8 ) $ 161.9 Municipal 32.4 — (0.2 ) 32.2 Corporate 1,036.5 14.0 (4.2 ) 1,046.3 Non-U.S. government-backed corporate 1.0 — — 1.0 Non-U.S. government 196.1 6.9 (0.5 ) 202.5 Asset-backed 9.9 — — 9.9 Agency mortgage-backed 196.7 0.2 (1.4 ) 195.5 Total fixed income securities — Trading 1,634.9 21.5 (7.1 ) 1,649.3 Total short-term investments — Trading 73.0 — — 73.0 Total equity securities — Trading 414.8 83.5 (7.3 ) 491.0 Total catastrophe bonds — Trading 33.5 — (1.1 ) 32.4 Total $ 2,156.2 $ 105.0 $ (15.5 ) $ 2,245.7 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 $35.5 million as at June 30, 2018 . The bonds receive quarterly interest payments based on variable interest rates with scheduled maturities ranging from 2018 to 2021. The redemption value of the bonds will adjust based on the occurrence of a covered event, such as windstorms and earthquakes in the United States, Canada, the North Atlantic, Japan or Australia. Investments — Equity Method. In January 2015, the Company established, along with seven other insurance companies, a micro-insurance venture consortium and micro-insurance incubator (“MVI”) domiciled in Bermuda. The MVI is a social impact organization that provides micro-insurance products to assist global emerging consumers. The Company’s initial investment in the MVI was $0.8 million . The Company made an additional investment of $0.1 million in the twelve months ended December 31, 2017 . On July 26, 2016, the Company purchased through its wholly-owned subsidiary, Acorn Limited, a 20% share of Bene Assicurazioni (“Bene”), an Italian-based motor insurer for a total consideration of $3.3 million . The investment is accounted for under the equity method and adjustments to the carrying value of this investment are made based on the Company’s share of capital, including share of income and expenses. The Company made an additional investment of $1.2 million in the three months ended June 30, 2018 . On January 1, 2017, the Company purchased through its wholly-owned subsidiary, Aspen U.S. Holdings, Inc. (“Aspen U.S. Holdings”), a 49% share of Digital Risk Resources, LLC (“Digital Re”), a U.S.-based enterprise engaged in the business of developing, marketing and servicing turnkey information security and privacy liability insurance products, for a total consideration of $2.3 million . The investment is accounted for under the equity method and adjustments to the carrying value of this investment are made based on the Company’s share of capital, including share of income and expenses. On December 18, 2017, the Company acquired through its wholly-owned subsidiary, Aspen U.S. Holdings, a 23.2% share of Crop Re Services LLC (“Crop Re”), a newly formed U.S.-based subsidiary of CGB Diversified Services, Inc (“CGB DS”) in exchange for the sale of AG Logic Holdings, LLC (“AgriLogic”), the Company’s former U.S. crop insurance business. Total consideration for the sale of AgriLogic consisted of the 23.2% share of Crop Re valued at $62.5 million and cash in the amount of $5.9 million . Crop Re is responsible for directing the placement of reinsurance on behalf of CGB DS and CGB Insurance Company (“CGBIC”), an Indiana insurance company affiliate of CGB DS and an RMA licensed crop insurer. The remaining 76.8% of Crop Re is owned by CGB DS. AAIC’s primary crop insurance coverage will be run-off and AAIC, or an affiliate of AAIC, will provide quota share reinsurance to CGBIC for both federal and state regulated crop insurance as part of Aspen’s ownership in Crop Re. The investment in Crop Re represents the Company’s share of the net assets of Crop Re plus the difference between the cost of the investment and the amount of underlying equity in net assets, the basis difference. The Company has determined that this basis difference of $62.5 million represents the value attributable to the ability of Crop Re to direct the placement of reinsurance business under the reinsurance commitment contained within the operating agreement between Crop Re and the Company. The investment in Crop Re is accounted for under the equity method and adjustments to the carrying value of this investment are made based on the Company’s share of capital, including share of income and expenses. The tables below show the Company’s investments in the MVI, Bene, Digital Re and Crop Re for the three and six months ended June 30, 2018 : For the Three Months Ended June 30, 2018 MVI Bene Digital Re Crop Re Total ($ in millions) Opening undistributed value of investment $ 0.4 $ 2.7 $ 0.5 $ 62.5 $ 66.1 Investment in the period — 1.2 — — 1.2 Realized/unrealized losses for the three months to June 30, 2018 (0.1 ) (0.1 ) — — (0.2 ) Closing undistributed value of investment $ 0.3 $ 3.8 $ 0.5 $ 62.5 $ 67.1 Six Months Ended June 30, 2018 MVI Bene Digital Re Crop Re Total ($ in millions) Opening undistributed value of investment $ 0.5 $ 2.9 $ 0.5 $ 62.5 $ 66.4 Investment in the period — 1.2 — — 1.2 Realized/unrealized losses for the six months to June 30, 2018 (0.2 ) (0.3 ) — — (0.5 ) Closing undistributed value of investment $ 0.3 $ 3.8 $ 0.5 $ 62.5 $ 67.1 Other Investments. On December 20, 2017, the Company committed $100.0 million as a limited partner to a real estate fund. The investment objective of the fund is to achieve attractive risk-adjusted returns through the acquisition of income producing, high quality assets in gateway cities located in the U.S. and Canada in the office, retail, industrial and multifamily sectors of the real estate market. On May 1, 2018, the Company received a demand for an initial capital call of $86.2 million and paid the capital call on May 10, 2018. For further information, refer to Note 16 in these unaudited condensed consolidated financial statements. Fixed Income Securities. The scheduled maturity distribution of available for sale fixed income securities as at June 30, 2018 and December 31, 2017 is set forth in the tables below. Actual maturities may differ from contractual maturities because issuers of securities may have the right to call or prepay obligations with or without call or prepayment penalties. As at June 30, 2018 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 437.0 $ 437.4 AA- Due after one year through five years 2,627.0 2,593.2 AA- Due after five years through ten years 1,062.2 1,031.1 AA- Due after ten years 103.4 109.3 A+ Subtotal 4,229.6 4,171.0 Agency mortgage-backed 907.6 890.6 AA+ Asset-backed 18.7 18.4 AAA Total fixed income securities — Available for sale $ 5,155.9 $ 5,080.0 As at December 31, 2017 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 561.7 $ 562.4 AA Due after one year through five years 2,486.7 2,492.2 AA- Due after five years through ten years 1,092.2 1,097.4 A+ Due after ten years 93.3 106.3 A Subtotal 4,233.9 4,258.3 Agency mortgage-backed 941.0 946.5 AA+ Asset-backed 26.3 26.2 AAA Total fixed income securities — Available for sale $ 5,201.2 $ 5,231.0 Guaranteed Investments. The Company held no investments which were guaranteed by mono-line insurers, excluding those with explicit government guarantees as June 30, 2018 and December 31, 2017 . The Company’s exposure to other third-party guaranteed debt was 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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 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 $ 808.1 $ (15.8 ) $ 256.1 $ (9.7 ) $ 1,064.2 $ (25.5 ) 123 U.S. agency 32.9 (0.6 ) — — 32.9 (0.6 ) 13 Municipal 90.0 (1.1 ) 0.7 — 90.7 (1.1 ) 13 Corporate 1,556.6 (31.9 ) 285.3 (15.4 ) 1,841.9 (47.3 ) 682 Non-U.S. government-backed corporate 42.9 (0.2 ) 20.9 (0.3 ) 63.8 (0.5 ) 14 Non-U.S. government 188.9 (0.8 ) 59.3 (0.5 ) 248.2 (1.3 ) 57 Asset-backed 13.4 (0.2 ) 5.0 (0.1 ) 18.4 (0.3 ) 7 Agency mortgage-backed 407.0 (9.7 ) 289.4 (13.1 ) 696.4 (22.8 ) 240 Total fixed income securities — Available for sale 3,139.8 (60.3 ) 916.7 (39.1 ) 4,056.5 (99.4 ) 1,149 Total short-term investments — Available for sale 41.5 — — — 41.5 — 8 Total $ 3,181.3 $ (60.3 ) $ 916.7 $ (39.1 ) $ 4,098.0 $ (99.4 ) 1,157 As at December 31, 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 $ 652.1 $ (5.1 ) $ 259.8 $ (6.5 ) $ 911.9 $ (11.6 ) 101 U.S. agency 20.1 (0.2 ) 6.1 — 26.2 (0.2 ) 10 Municipal 28.5 (0.2 ) — — 28.5 (0.2 ) 9 Corporate 699.3 (3.4 ) 360.7 (8.4 ) 1,060.0 (11.8 ) 412 Non-U.S. government-backed corporate 43.5 (0.3 ) 13.3 (0.2 ) 56.8 (0.5 ) 15 Non-U.S. government 206.2 (0.8 ) 32.0 (0.4 ) 238.2 (1.2 ) 47 Asset-backed 11.1 — 10.5 (0.1 ) 21.6 (0.1 ) 11 Agency mortgage-backed 257.6 (1.9 ) 301.9 (6.3 ) 559.5 (8.2 ) 156 Total fixed income securities — Available for sale 1,918.4 (11.9 ) 984.3 (21.9 ) 2,902.7 (33.8 ) 761 Total short-term investments — Available for sale 46.9 (0.1 ) — — 46.9 (0.1 ) 8 Total $ 1,965.3 $ (12.0 ) $ 984.3 $ (21.9 ) $ 2,949.6 $ (33.9 ) 769 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 six months ended June 30, 2018 was $ Nil ( 2017 — $0.1 million and $0.4 million ). 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 2017 Annual Report on Form 10-K filed with the SEC.

Variable Interest Entities

Variable Interest Entities	6 Months Ended
	Jun. 30, 2018
Payables and Accruals [Abstract]
Variable Interest Entities	Variable Interest Entities As at June 30, 2018 , the Company had investments in two variable interest entities (“VIE”): Peregrine Reinsurance Ltd (“Peregrine”) and Silverton Re Ltd (“Silverton”). Peregrine. In November 2016, the Company, registered Peregrine as a segregated accounts company under the Segregated Accounts Companies Act 2000, as amended. As at June 30, 2018 , Peregrine had four 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 four segregated accounts of Peregrine but is the primary beneficiary of the Peregrine general fund and, similar to prior reporting periods, the Company has included the results of the Peregrine general fund in its consolidated financial statements. The Company’s exposure to Peregrine’s general fund is not material. Silverton. On September 10, 2013, the Company established Silverton, a Bermuda domiciled special purpose insurer formed to provide additional collateralized capacity to support Aspen Re’s business through retrocession agreements which are collateralized and funded by Silverton through the issuance of one or more series of participating loan notes (collectively, the “Loan Notes”). Silverton 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. Silverton was not renewed in 2017 and has not issued any Loan Notes since, in the future, any such quota share support for Aspen Re will be provided by a separate cell of Peregrine. 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 six months ended June 30, 2018 and 2017 : For the Six Months Ended June 30, 2018 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 86.6 $ 20.6 $ 107.2 Total change in fair value for the period 2.4 0.6 3.0 Total distributed in the period (63.7 ) (14.7 ) (78.4 ) Closing balance as at June 30, 2018 $ 25.3 $ 6.5 $ 31.8 Liability Loan notes (long-term liabilities) $ 20.3 $ 5.2 $ 25.5 Accrued expenses (current liabilities) 5.0 1.3 6.3 Total aggregate unpaid balance as at June 30, 2018 $ 25.3 $ 6.5 $ 31.8 For the Six Months Ended June 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 6.2 1.6 7.8 Total distributed in the period (114.1 ) (28.6 ) (142.7 ) Closing balance as at June 30, 2017 $ 115.5 $ 27.5 $ 143.0 Liability Loan notes (long-term liabilities) $ 110.8 $ 26.4 $ 137.2 Accrued expenses (current liabilities) 4.7 1.1 5.8 Total aggregate unpaid balance as at June 30, 2017 $ 115.5 $ 27.5 $ 143.0 The Company has determined that Silverton has the characteristics of a VIE that are addressed by the guidance in ASC 810, Consolidation . The Company concluded that it is the primary beneficiary of Silverton as it owns all of Silverton’s voting shares and issued share capital, and has a significant financial interest and the power to control Silverton. As a result, the Company consolidated Silverton upon its formation. The Company has no other obligation to provide financial support to Silverton and neither the creditors nor beneficial interest holders of Silverton have recourse to the Company’s general credit. In the event of an extreme catastrophic property reinsurance event or severe credit-related event, there is a risk that Aspen Bermuda and Aspen U.K. would be unable to recover losses from Silverton. These two risks are mitigated as follows: i. Silverton has collateralized the aggregate limit provided to Aspen Bermuda and Aspen U.K. by way of a trust in favor of Aspen Bermuda and Aspen U.K. as beneficiaries; ii. the trustee is a large, well-established regulated entity; and iii. all funds within the trust account are bound by investment guidelines restricting investments to one of the institutional class money market funds run by large international investment managers. For further information regarding the Loan Notes attributable to the third-party investments in Silverton, refer to Note 8 of these unaudited condensed consolidated financial statements.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,229.6 $ — $ — $ 1,229.6 U.S. agency — 46.6 — 46.6 Municipal — 105.7 — 105.7 Corporate — 2,267.1 — 2,267.1 Non-U.S. government-backed corporate — 91.8 — 91.8 Non-U.S. government 291.6 138.6 — 430.2 Asset-backed — 18.4 — 18.4 Agency mortgage-backed — 890.6 — 890.6 Total fixed income securities available for sale, at fair value 1,521.2 3,558.8 — 5,080.0 Short-term investments available for sale, at fair value 69.4 2.8 — 72.2 Held for trading financial assets, at fair value U.S. government 155.3 — — 155.3 Municipal — 77.8 — 77.8 Corporate — 968.5 — 968.5 Non-U.S. government-backed corporate — — — — Non-U.S. government 47.2 172.1 — 219.3 Asset-backed — 7.2 — 7.2 Agency mortgage-backed — 187.8 — 187.8 Total fixed income securities trading, at fair value 202.5 1,413.4 — 1,615.9 Short-term investments trading, at fair value 23.3 4.1 — 27.4 Equity investments trading, at fair value — — — — Catastrophe bonds trading, at fair value — 35.5 — 35.5 Other investments (1) — — — 86.2 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 14.1 — 14.1 Liabilities under derivative contracts — foreign exchange contracts — (49.3 ) — (49.3 ) Loan notes issued by variable interest entities, at fair value — — (20.3 ) (20.3 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (5.0 ) (5.0 ) Total $ 1,816.4 $ 4,979.4 $ (25.3 ) $ 6,770.5 (1) Other investments represents our investment in a real estate fund and is measured at fair value using the net asset value per share practical expedient. As a result this has not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the condensed consolidated balance sheets. 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. There were no significant transfers between Level 1, Level 2 and Level 3 during the six months ended June 30, 2018 . The Company settled $63.7 million of Level 3 liabilities in respect of the Loan Notes issued by Silverton for the six months ended June 30, 2018 . As at June 30, 2018 , there were no the assets classified as Level 3 and the Company’s Level 3 liabilities consisted solely of the Loan Notes issued by Silverton. As at December 31, 2017 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,159.4 $ — $ — $ 1,159.4 U.S. agency — 52.1 — 52.1 Municipal — 54.9 — 54.9 Corporate — 2,415.7 — 2,415.7 Non-U.S. government-backed corporate — 91.3 — 91.3 Non-U.S. government 341.2 143.7 — 484.9 Asset-backed — 26.2 — 26.2 Agency mortgage-backed — 946.5 — 946.5 Total fixed income securities available for sale, at fair value 1,500.6 3,730.4 — 5,231.0 Short-term investments available for sale, at fair value 87.3 2.6 — 89.9 Held for trading financial assets, at fair value U.S. government 161.9 — — 161.9 Municipal — 32.2 — 32.2 Corporate — 1,046.3 — 1,046.3 Non-U.S. government-backed corporate — 1.0 — 1.0 Non-U.S. government 24.5 178.0 — 202.5 Asset-backed — 9.9 — 9.9 Agency mortgage-backed — 195.5 — 195.5 Total fixed income securities trading, at fair value 186.4 1,462.9 — 1,649.3 Short-term investments trading, at fair value 73.0 — — 73.0 Equity investments trading, at fair value 491.0 — — 491.0 Catastrophe bonds trading, at fair value — 32.4 — 32.4 Other financial assets and liabilities, at fair value Derivatives at fair value – foreign exchange contracts — 6.4 — 6.4 Liabilities under derivative contracts – foreign exchange contracts — (1.0 ) — (1.0 ) Loan notes issued by variable interest entities, at fair value — — (44.2 ) (44.2 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (42.4 ) (42.4 ) Total $ 2,338.3 $ 5,233.7 $ (86.6 ) $ 7,485.4 Transfers of assets into or out of a particular level are recorded at their fair values as of the end of each reporting period, consistent with the date of the determination of fair value. During the twelve months ended December 31, 2017 , there were no transfers between Level 1, 2 and 3. The Company settled $115.6 million of Level 3 liabilities in respect of the Loan Notes issued by Silverton for the twelve months ended December 31, 2017 . As at December 31, 2017 , there were no assets classified as Level 3 and the Company’s Level 3 liabilities consisted solely of the Loan Notes issued by Silverton. The following table presents a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Level 3 inputs for the three and six months ended June 30, 2018 and 2017 : Reconciliation of Liabilities Using Level 3 Inputs Three Months Ended June 30, 2018 Three Months Ended June 30, 2017 ($ in millions) Balance at the beginning of the period $ 39.8 $ 115.1 Distributed to third party (17.9 ) (2.9 ) Total change in fair value included in the statement of operations 3.4 3.3 Balance at the end of the period (1) $ 25.3 $ 115.5 Reconciliation of Liabilities Using Level 3 Inputs Six Months Ended June 30, 2018 Six Months Ended June 30, 2017 ($ in millions) Balance at the beginning of the period $ 86.6 $ 223.4 Distributed to third party (63.7 ) (114.1 ) Total change in fair value included in the statement of operations 2.4 6.2 Balance at the end of the period (1) $ 25.3 $ 115.5 (1) The amount classified within accrued expenses and other payables was $5.0 million and $4.7 million as at June 30, 2018 and June 30, 2017 , 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 June 30, 2018 and December 31, 2017 , 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 June 30, 2018 the Company obtained an average of 2.1 quotes per fixed income investment compared to 2.0 quotes as at December 31, 2017 . Pricing sources used in pricing fixed income investments as at June 30, 2018 and December 31, 2017 were as follows: As at June 30, 2018 As at December 31, 2017 Index providers 84 % 84 % Pricing services 14 11 Broker-dealers 2 5 Total 100 % 100 % Summary Pricing Information Table. A summary of securities priced using pricing information from index providers as at June 30, 2018 and December 31, 2017 is provided below: As at June 30, 2018 As at December 31, 2017 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 $ 1,384.9 100 % $ 1,321.3 100 % U.S. agency 44.8 96 % 43.4 83 % Municipal 133.6 73 % 37.4 43 % Corporate 3,075.1 95 % 3,299.6 83 % Non-U.S. government-backed corporate 40.4 44 % 44.0 48 % Non-U.S. government 342.9 53 % 399.4 58 % Asset-backed 12.5 49 % 13.5 37 % Agency mortgage-backed 570.3 53 % 605.0 53 % Total fixed income securities $ 5,604.5 84 % $ 5,763.6 84 % Equities — — % 491.0 100 % Total fixed income securities and equity investments $ 5,604.5 84 % $ 6,254.6 85 % 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 as 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. 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 globally in a deep, liquid market, providing substantial price transparency and accordingly are classified as Level 2. Other investments. The Company’s other investments represent our investment in a real estate fund. Adjustments to the fair value are made based on the net asset value of the investment. The net valuation criteria established by the manager of such investments are established in accordance with the governing documents and the asset manager's valuation guidelines, which consider a two part approach: the discounted cash flows approach; and income relative to benchmarks (the performance multiple approach). Alternative valuation methodologies may be employed for investments with unusual characteristics. 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. Silverton was not renewed in 2017 and has not issued any Loan Notes since, in the future, any such quota share support for Aspen Re will be provided by a separate cell of Peregrine. 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 June 30, 2018 and December 31, 2017 are presented in the tables below: As at June 30, 2018 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 25.3 (1) Internal Valuation Model Gross premiums written (O) $ 50.1 $ 61.1 Reserve for losses (U) $ 4.2 $ 61.9 Contract period (O) N/A 365 days Initial value of issuance (O) $ 325.0 $ 325.0 As at December 31, 2017 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 86.6 (1) Internal Valuation Model Gross premiums written (O) $ 50.1 $ 61.1 Reserve for losses (U) $ 4.2 $ 61.9 Contract period (O) N/A 365 days Initial value of issuance (O) $ 325.0 $ 325.0 (1) The amounts classified within accrued expenses and other payables were $5.0 million and $42.4 million as at June 30, 2018 and December 31, 2017 , 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	6 Months Ended
	Jun. 30, 2018
Insurance [Abstract]
Reinsurance	Reinsurance The Company purchases retrocession and reinsurance to limit and diversify the Company’s risk exposure and increase its own reinsurance and insurance 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 total amount recoverable by the Company from reinsurers as at June 30, 2018 was $1,656.4 million ( December 31, 2017 — $1,515.2 million ) of which $1,126.5 million was uncollateralized ( December 31, 2017 — $1,001.9 million ). As at June 30, 2018 16.7% ( December 31, 2017 — 17.0% ) of the Company’s uncollateralized reinsurance recoverables were with Munich Re which is rated A+ by A.M Best and AA- by S&P, 12.1% ( December 31, 2017 — 13.8% ) with Lloyd’s Syndicates which are rated A by A.M Best and A+ by S&P and 8.6% ( December 31, 2017 — 7.6% ) with Aioi Nissay Dowa Insurance Co which is rated A+ by A.M best and S&P. These are the Company’s largest exposures to individual reinsurers. The Company has made no provision for doubtful debts from any of its reinsurers as at June 30, 2018 .

Derivative Contracts

Derivative Contracts	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 As at December 31, 2017 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 $ 340.9 $ 14.2 (1) $ 577.7 $ 5.0 Foreign Exchange Contracts Liabilities under Derivative Contracts $ 812.4 $ (49.3 ) $ 173.9 $ (1.0 ) (1) Net of $2.3 million cash collateral ( December 31, 2017 — $0.6 million ). As at June 30, 2018 As at December 31, 2017 Derivatives Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Derivatives at Fair Value $ 114.3 $ (0.1 ) $ 60.6 $ 1.4 The following table provides the unrealized and realized gains 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 six months ended June 30, 2018 and 2017 . Amount of Gain Recognized on Derivatives Three Months Ended Six Months Ended Location of Gain Recognized on Derivatives June 30, 2018 June 30, 2017 June 30, 2018 June 30, 2017 Derivatives not designated as hedges ($ in millions) ($in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives (46.1 ) 17.6 (22.6 ) 20.7 Derivatives designated as hedges Foreign Exchange Contracts General, administrative and corporate expenses 1.3 0.9 3.0 1.2 Foreign Exchange Contracts Net change from current period hedged transactions (2.6 ) 2.4 (1.7 ) 3.7 Foreign Exchange Contracts. The Company uses foreign exchange contracts to manage foreign currency risk associated with our operating expenses but also foreign exchange risk associated with net assets or liabilities in currencies other than the U.S. dollar. 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 June 30, 2018 , the Company held foreign exchange contracts that were not designated as hedging under ASC 815 with an aggregate notional value of $1,153.3 million ( December 31, 2017 — $751.6 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 six months ended June 30, 2018 , the impact of foreign exchange contracts on net income was a loss of $46.1 million ( June 30, 2017 — gain of $17.6 million ) and a loss of $22.6 million ( June 30, 2017 — gain of $20.7 million ), respectively. As at June 30, 2018 , the Company held foreign exchange contracts that were designated as hedging under ASC 815 with an aggregate nominal amount of $114.3 million ( December 31, 2017 — $60.6 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 six months ended June 30, 2018 was a net unrealized loss of $2.6 million ( June 30, 2017 — gain of $2.4 million ) and a net unrealized loss of $1.7 million ( June 30, 2017 — gain of $3.7 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 six months ended June 30, 2018 , the amount recognized within general, administrative and corporate expenses for settled foreign exchange contracts was a realized gain of $1.3 million ( June 30, 2017 — gain of $0.9 million ) and a realized gain of $3.0 million ( June 30, 2017 — gain of $1.2 million ), respectively.

Deferred Policy Acquisition Cos

Deferred Policy Acquisition Costs	6 Months Ended
	Jun. 30, 2018
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 six months ended June 30, 2018 and 2017 : Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 ($ in millions) ($ in millions) Balance at the beginning of the period $ 319.9 $ 367.5 $ 294.3 $ 358.4 Acquisition costs deferred 64.6 93.4 181.0 216.2 Amortization of deferred policy acquisition costs (85.9 ) (96.3 ) (176.7 ) (210.0 ) Balance at the end of the period $ 298.6 $ 364.6 $ 298.6 $ 364.6

Reserves for Losses and Loss Ad

Reserves for Losses and Loss Adjustment Expenses	6 Months Ended
	Jun. 30, 2018
Insurance [Abstract]
Reserves For Loss And Adjustment Expenses	Reserves for Losses and Loss Adjustment Expenses The following table represents a reconciliation of beginning and ending consolidated loss and loss adjustment expenses (“LAE”) reserves for the six months ended June 30, 2018 and 2017 and the twelve months ended December 31, 2017 : Six Months Ended June 30, 2018 Six Months Ended June 30, 2017 Twelve Months Ended December 31, 2017 ($ in millions) Provision for losses and LAE at the start of the year $ 6,749.5 $ 5,319.9 $ 5,319.9 Less reinsurance recoverable (1,515.2 ) (560.7 ) (560.7 ) Net loss and LAE at the start of the year 5,234.3 4,759.2 4,759.2 Net loss and LAE expenses (disposed) — — (125.5 ) Provision for losses and LAE for claims incurred: Current year 700.8 749.2 2,100.1 Prior years (80.2 ) (74.9 ) (105.4 ) Total incurred 620.6 674.3 1,994.7 Losses and LAE payments for claims incurred: Current year (49.7 ) (54.4 ) (397.5 ) Prior years (874.7 ) (686.7 ) (1,157.6 ) Total paid (924.4 ) (741.1 ) (1,555.1 ) Foreign exchange losses/(gains) (54.1 ) 99.6 161.0 Net losses and LAE reserves at period end 4,876.4 4,792.0 5,234.3 Plus reinsurance recoverable on unpaid losses at period end 1,656.4 779.4 1,515.2 Provision for losses and LAE at the end of the relevant period $ 6,532.8 $ 5,571.4 $ 6,749.5 For the six months ended June 30, 2018 , there was a reduction of $80.2 million in the Company’s estimate of the ultimate claims to be paid in respect of prior accident years compared to a reduction of $74.9 million for the six months ended June 30, 2017 . The Company ceded $125.5 million of reserves as part of an adverse development cover purchased during the twelve months ended December 31, 2017 . 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. Insurance Incurred Claims, IBNR and Allocated Loss Adjustment Expenses, Net of Reinsurance As at June 30, 2018 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 Q2 2018 $ (in millions) 2012 605.0 626.8 655.9 669.9 654.7 638.2 647.2 45.9 15,682 2013 689.6 667.9 679.0 660.4 650.6 651.1 32.4 14,631 2014 755.6 730.1 702.4 695.1 699.5 68.0 18,836 2015 916.0 905.9 855.1 858.1 154.4 21,132 2016 909.3 881.6 873.6 284.6 21,052 2017 896.1 847.7 175.9 20,622 2018 307.0 192.9 6,929 Total $ 4,884.2 Insurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 Q2 2018 ($ in millions) 2012 117.3 307.9 378.5 452.9 502.8 524.7 531.7 2013 90.7 254.5 364.9 455.0 502.8 532.7 2014 99.5 247.5 389.8 465.9 481.0 2015 119.2 326.9 471.9 523.4 2016 117.3 346.1 415.4 2017 167.8 369.5 2018 32.2 Total $ 2,885.9 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 1,998.3 All outstanding liabilities before 2012, net of reinsurance 114.0 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 2,112.3 Reinsurance Incurred Claims, IBNR and Allocated Loss Adjustment Expenses, Net of Reinsurance As at June 30, 2018 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 Q2 2018 $ (in millions) 2012 692.8 737.3 720.9 690.6 689.4 687.9 686.7 71.2 3,838 2013 579.6 572.2 550.2 524.3 504.9 499.7 66.2 3,689 2014 550.6 529.9 514.2 487.1 477.5 82.8 3,696 2015 580.3 564.5 558.2 536.9 117.6 3,899 2016 749.1 762.8 768.9 218.1 3,709 2017 1,190.2 1,189.0 420.5 3,746 2018 362.5 289.9 1,415 Total $ 4,521.2 Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 Q2 2018 ($ in millions) 2012 63.2 248.2 360.9 414.8 458.7 496.5 511.4 2013 63.2 186.8 278.4 320.1 359.9 371.6 2014 57.0 172.6 248.5 289.2 308.1 2015 57.5 171.4 272.1 306.2 2016 125.5 350.7 400.0 2017 227.9 508.1 2018 17.5 Total $ 2,422.9 All outstanding liabilities for 2012 and subsequent years, net of reinsurance 2,098.3 All outstanding liabilities before 2012, net of reinsurance 619.9 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 2,718.2 Six Months Ended June 30, 2018 ($ in millions) Net outstanding liabilities: Insurance lines 2,112.3 Reinsurance lines 2,718.2 Net loss and LAE 4,830.5 Reinsurance recoverable on unpaid losses: Insurance lines 1,382.8 Reinsurance lines 273.6 Total reinsurance recoverable on unpaid losses 1,656.4 Insurance lines other than short-duration — Unallocated claims incurred 49.7 Other (3.8 ) 45.9 Provision for losses and LAE at the end of the period 6,532.8

Capital Structure

Capital Structure	6 Months Ended
	Jun. 30, 2018
Equity [Abstract]
Capital Structure	Capital Structure The following table provides a summary of the Company’s authorized and issued share capital as at June 30, 2018 and December 31, 2017 : As at June 30, 2018 As at December 31, 2017 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 59,687,717 90 59,474,085 90 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 122 122 Additional paid-in capital as at June 30, 2018 was $965.4 million ( December 31, 2017 — $954.7 million ). Additional paid-in capital includes the aggregate liquidation preferences of the Company’s preference shares of $525.0 million ( December 31, 2017 — $525.0 million ) less issue costs of $13.1 million ( December 31, 2017 — $13.1 million ). Ordinary Shares. The following table summarizes transactions in the Company’s ordinary shares during the six months ended June 30, 2018 : Number of Ordinary Shares Ordinary shares in issue as at December 31, 2017 59,474,085 Ordinary share transactions in the six months ended June 30, 2018 Ordinary shares issued to employees under the 2013 share incentive plan and/or 2008 share purchase plan 208,954 Ordinary shares issued to non-employee directors 4,678 Ordinary shares in issue as at June 30, 2018 59,687,717 Share Repurchase Authorization Program . On February 8, 2017, the Company’s Board of Directors approved a share repurchase authorization program of $250.0 million . The share repurchase authorization program, which 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 did not acquire or cancel any ordinary shares for the three and six months ended June 30, 2018 . For the three and six months ended June 30, 2017, the Company acquired and canceled 197,673 ordinary shares for a total consideration of $10.0 million . Preference Share Issuance. The Company did not issue any preference shares for the three and six months ended June 30, 2018 .

Share-Based Payments

Share-Based Payments	6 Months Ended
	Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Payments	Share-Based Payments The Company has issued options and other equity incentives under three arrangements: the employee share incentive plan, the employee share purchase plans and the non-employee director stock incentive plan. When options are exercised or other equity awards (excluding phantom shares) vest, new ordinary shares are issued as the Company does not hold treasury shares. Phantom shares are settled in cash in lieu of ordinary shares upon vesting. (a) Employee Equity Incentives The total number of ordinary shares that may be issued under the Company’s 2013 Share Incentive Plan is 2,845,683 ordinary shares. Restricted Share Units. Restricted share units (“RSUs”) granted to employees typically vest over a three -year period subject to the employee’s continued service. RSUs vest granted to employees vest on the anniversary of the date of grant or when the Compensation Committee of the Board of Directors agrees to deliver them. Holders of RSUs will be paid one ordinary share for each RSU that vests as soon as practicable following the vesting date. Holders of RSUs generally will not be entitled to any rights of a holder of ordinary shares, including the right to vote, unless and until their RSUs vest and ordinary shares are issued but they are entitled to receive dividend equivalents. Dividend equivalents are denominated in cash and paid in cash if and when the underlying RSUs vest. The 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 six months ended June 30, 2018 , the Company granted 230,766 RSUs ( 2017 — 174,489 ) to its employees. Compensation costs charged against income in respect of RSUs granted to employees for the six months ended June 30, 2018 were $4.6 million ( 2017 — $5.0 million ). The total tax credit recognized by the Company in relation to RSUs in the six months ended June 30, 2018 was $1.1 million ( 2017 — $1.2 million ). Performance Shares. Performance share awards are subject to a three -year service vesting period with a separate annual growth in diluted book value per share (“BVPS”) test for each year, adjusted to add back ordinary shares and movements in AOCI to shareholders' equity. Accordingly, one-third of the award may be earned in each calendar year. Performance share awards are not entitled to dividends before they vest and are subject to the employee’s continued employment. If performance goals are achieved, the performance shares will vest up to a maximum of 200% of target. Notwithstanding the vesting criteria for each given year, if in any given year the shares eligible for vesting are greater than 100% or the portion of such year’s grant and the average diluted BVPS growth over such year and the preceding year is less than the average of the minimum vesting thresholds for such year and the preceding year, then only 100% (and no more) of the shares that are eligible for vesting in such year shall vest. Notwithstanding the foregoing, if in the judgment of the Compensation Committee the main reason for the BVPS metric in the earlier year falling below the minimum threshold is due to the impact of rising interest rates and bond yields, then the Compensation Committee may, in its discretion, disapply the limitation on 100% vesting. During the six months ended June 30, 2018 , the Company granted 215,273 performance shares to its employees ( 2017 — 206,076 ). 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. Net compensation costs charged against income in the six months ended June 30, 2018 in respect of performance shares were $2.7 million ( 2017 — $1.2 million ). The total tax recognized by the Company in relation to performance shares in the six months ended June 30, 2018 was a tax credit of $0.6 million ( 2017 — $0.3 million credit), excluding excess tax benefits. Phantom Shares. Phantom share awards are subject to a three -year service vesting period with a separate annual growth in diluted BVPS test for each year, adjusted to add back ordinary shares and movements in AOCI to shareholders' equity. One-third of the award may be earned in each calendar year with the vested amount being paid in cash in lieu of ordinary shares. As ordinary shares are not issued, phantom shares have no dilutive effect. During the six months ended June 30, 2018 , the Company granted 150,185 phantom shares to its employees ( 2017 — 173,619 ). The fair value of the phantom shares is based on the closing share price on the date of the grant less a deduction for illiquidity. The fair value is expensed through the consolidated income statement evenly over the vesting period. As the payment to beneficiaries will ultimately be in cash rather than ordinary shares, an adjustment is required each quarter to revalue the accumulated liability to the balance sheet date fair value. Compensation costs charged against income in the six months ended June 30, 2018 in respect of phantom shares were $1.6 million ( 2017 — $2.1 million ). The total tax recognized by the Company in relation to phantom shares in the six months ended June 30, 2018 was $0.6 million ( 2017 — $0.1 million ), excluding excess tax benefits. (b) Employee Share Purchase Plans Employee Share Purchase Plans. On April 30, 2008, the shareholders of the Company approved the Employee Share Purchase Plan, the 2008 Sharesave Scheme and the International Employee Share Purchase Plan (collectively, the “ESPP”), which are implemented by a series of consecutive offering periods as determined by the Board of Directors. In respect of the ESPP, employees can save up to $500 per month over a two -year period, at the end of which they 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, 31,684 ordinary shares were exercised and issued during the six months ended June 30, 2018 ( 2017 — 9,979 ). No ESPP options were granted in 2018. Compensation costs charged against income in the six months ended June 30, 2018 in respect of the ESPP were $0.1 million ( 2017 — $0.2 million ). The total tax credit recognized by the Company in relation to the ESPP in the six months ended June 30, 2018 was $ Nil ( 2017 — $ Nil ). (c) Non-Employee Director Plan On April 21, 2016, the shareholders of the Company approved the 2016 Stock Incentive Plan for Non-Employee Directors which provides for the granting of options, restricted share units or other share-based awards. The total number of ordinary shares that may be issued under the 2016 Non-Employee Director Plan is 263,695 . Restricted Share Units. RSUs granted to non-employee directors, including the Chairman, vest one-twelfth on each one month anniversary of the date of grant with 100% of the restricted share units becoming vested and issued on the first anniversary of the grant date or on the date of departure of a director for the amount vested through such date. The shares that are eligible to vest following final vesting date in the calendar year of the date of grant is delivered as soon as practical thereafter and the remaining shares under the RSUs are delivered on the first anniversary of the grant date. If a director leaves the Board of Directors for any reason other than “cause” (as defined in the award agreement), then the director would receive shares under the restricted share units that had vested through the date the director leaves the Board. RSUs entitle the holder to receive one ordinary share unit for each unit that vests. Holders of RSUs are not entitled to any of the rights of a holder of ordinary shares, including the right to vote, unless and until their units vest and ordinary shares are issued but they are entitled to receive dividend equivalents with respect to their units. Dividend equivalents will be denominated in cash and paid in cash if and when the underlying units vest. The following table summarizes information about RSUs issued to non-employee directors as at June 30, 2018 : Restricted Share Units As at June 30, 2018 As at June 30, 2017 RSU Holder Amount Granted Amount Granted Non-Employee Directors 29,025 22,230 Chairman 12,900 8,892 Compensation costs charged against income in respect of RSUs granted to non-employee directors for the six months ended June 30, 2018 were $0.8 million ( 2017 — $0.7 million ). The total tax charge recognized by the Company in relation to non-employee RSUs in the six months ended June 30, 2018 was $ Nil ( 2017 — $ Nil ).

Intangible Assets and Goodwill

Intangible Assets and Goodwill	6 Months Ended
	Jun. 30, 2018
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 six months ended June 30, 2018 and 2017 : Three Months Ended June 30, 2018 Three Months Ended June 30, 2017 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 Trademarks $ 2.8 $ — $ (0.1 ) $ 2.7 $ 6.4 $ — $ (0.1 ) $ 6.3 Insurance Licenses 16.7 — — 16.7 16.7 — — 16.7 Agency Relationships 2.2 — (0.2 ) 2.0 25.8 — (0.7 ) 25.1 Non-compete Agreements 0.6 — (0.1 ) 0.5 3.1 — (0.1 ) 3.0 Consulting Relationships — — — — 0.9 — (0.1 ) 0.8 Goodwill 3.9 — — 3.9 26.0 — — 26.0 Renewal Rights 1.3 — (0.1 ) 1.2 1.6 — (0.1 ) 1.5 Total $ 27.5 $ — $ (0.5 ) $ 27.0 $ 80.5 $ — $ (1.1 ) $ 79.4 Six Months Ended June 30, 2018 Six Months Ended June 30, 2017 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 Trademarks $ 2.9 $ — $ (0.2 ) $ 2.7 $ 6.6 $ — $ (0.3 ) $ 6.3 Insurance Licenses 16.7 — — 16.7 16.7 — — 16.7 Agency Relationships 2.3 — (0.3 ) 2.0 26.2 — (1.1 ) 25.1 Non-compete Agreements 0.7 — (0.2 ) 0.5 3.3 — (0.3 ) 3.0 Consulting Relationships — — — — 0.9 — (0.1 ) 0.8 Goodwill 3.9 — — 3.9 24.2 1.8 — 26.0 Renewal Rights 1.4 — (0.2 ) 1.2 1.7 — (0.2 ) 1.5 Total $ 27.9 $ — $ (0.9 ) $ 27.0 $ 79.6 $ 1.8 $ (2.0 ) $ 79.4 Crop Re and 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 was 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. In total, $34.0 million of intangible assets and $21.0 million of goodwill was eligible for tax deduction over the following 15 years. License to use the “AgriLogic” Trademark. The Company acquired the right to use the AgriLogic trademark in the United States. The Company valued the trademark at $4.0 million with an estimated economic useful life of 10 years. The Company planned to amortize the estimated value of the trademark over its estimated useful life. Agency Relationships. The Company valued the agency relationships at $25.0 million with an estimated economic useful life of 15 years. The Company planned to amortize the estimated value of the agency relationships over its 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 planned to amortize the estimated value of the non-compete agreements over its estimated useful life. Value of Business Acquired. The Company recognized a $1.8 million asset for value of business acquired (“VOBA”) consisting of the inforce unearned premium reserve and claims reserves at fair value. The Company amortized the VOBA in line with the unwinding of the acquired unearned premium balances and loss reserves. Given the short tail nature of 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 planned to amortize the estimated value of the consulting relationships over its estimated useful life. Goodwill. The Company valued the goodwill at $22.1 million . The goodwill was deemed to have an indefinite useful life and was assessed for impairment annually. On December 18, 2017, Aspen U.S. Holdings sold its interest in AgriLogic to CGB Diversified Service, Inc (“CGB DS”) in exchange for a 23.2% equity interest in Crop Re Services LLC. Aspen U.S. Holdings retained the agricultural consulting business previously integrated within AgriLogic. Intangible assets disposed of as part of the AgriLogic sale included $21.8 million of agency relationships, $3.1 million for the right to use the AgriLogic trademark, $0.9 million of non-compete agreements and $20.6 million of goodwill. Following the sale the Company performed its annual qualitative assessment on the residual intangible assets of the agricultural consulting business and determined that it was more likely than not that the intangible assets were impaired. The Company therefore recognized an impairment charge of $3.4 million in the year ended December 31, 2017 . Blue Waters On October 31, 2016, Acorn acquired 100% of the equity voting interest of Blue Waters, a specialist marine insurance agency. The total consideration for the acquisition was $8.0 million . A significant proportion of the acquired business was represented by intangible assets, specifically $3.1 million for agency relationships, $1.5 million for the right to use the Blue Waters trademark, $1.0 million for non-compete agreements and $0.05 million for the value of trading licenses. In addition, $0.3 million of residual net assets were acquired. The total net assets acquired of $5.75 million resulted in the Company recognizing a total of $2.1 million in goodwill for the acquisition of Blue Waters. Agency Relationships . The Company valued the agency relationships at $3.1 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the agency relationships over their estimated useful life. License to use the “Blue Waters” Trademark . The Company acquired the right to use the Blue Waters trademark in the United States. The Company valued the trademark at $1.5 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the trademark over its estimated useful life. Non-compete Agreements . The Company valued the non-compete agreements at $1.0 million with an estimated economic useful life of 5 years. The Company will amortize the estimated value of the non-compete agreements over their estimated useful life. Insurance Licenses . The Company valued the insurance licenses at $0.05 million . The insurance licenses are considered to have an indefinite useful life and are not amortized. The licenses are tested annually for impairment. Goodwill . The Company valued the goodwill at $2.1 million . The goodwill is deemed to have an indefinite useful life and will be assessed for impairment annually. Other Intangible Assets Renewal Rights. On September 22, 2016, the Company entered into a renewal rights agreement with Liberty Specialty Markets Limited (“LSML”). The Company valued the renewal rights at $1.9 million with an estimated economic useful life of 5 years . The Company will amortize the estimated value of the renewal rights over the estimated useful life. In addition to the intangible assets and goodwill associated with the AgriLogic and Blue Waters acquisitions and the renewal rights agreement with LSML, the Company has the following intangible assets from prior transactions: License to use the “Aspen” Trademark. On April 5, 2005, the Company entered into an agreement with Aspen (Actuaries and Pension Consultants) Plc to acquire the right to use the Aspen trademark in the United Kingdom. The consideration paid was approximately $1.6 million . As at June 30, 2018 , the value of the license to use the Aspen trademark was $1.6 million ( December 31, 2017 — $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 June 30, 2018 was $16.6 million ( December 31, 2017 — $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.

Commitments and Contingent Liab

Commitments and Contingent Liabilities	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 As at December 31, 2017 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 1,154.5 $ 1,455.0 Third party 2,431.1 2,425.3 Letters of credit / guarantees (1) 840.3 658.5 Investment commitment — real estate fund 13.8 100.0 Other investments — real estate fund 86.2 — Total restricted assets $ 4,525.9 $ 4,638.8 Total as percent of investable assets (2) 56.0 % 53.4 % (1) As at June 30, 2018 , the Company had pledged funds in the amount of $840.3 million ( December 31, 2017 — $658.5 million ) as collateral for the secured letters of credit. (2) Investable assets comprise total investments, cash and cash equivalents, accrued interest, receivables for securities sold and payables for securities purchased. Real Estate Fund. On December 20, 2017, the Company committed $100.0 million as a limited partner to a real estate fund. The investment objective of the fund is to achieve attractive risk-adjusted returns through the acquisition of income producing, high quality assets in gateway cities located in the U.S. and Canada in the office, retail, industrial and multifamily sectors of the real estate market. On May 1, 2018, the Company received a demand for an initial capital call of $86.2 million and paid the capital call on May 10, 2018. As of June 30, 2018 , the Company had a remaining capital call of $13.8 million outstanding. Investments in this real estate fund may be redeemed on a quarterly basis with 90 days’ notice subject to available cash in the fund once the lock-up period ends two years after the capital call. If sufficient cash is not available then all requested redemptions will be made on a pro rata basis. If a redemption request has not been met in full, as of such calendar quarter, the remaining portion of the request will be redeemed in subsequent quarters. There are no assurances as to when the Company may be able to withdraw, in whole or in part, its redemption request from the fund. A lock-up period is the initial amount of time an investor is contractually required to remain invested before having the ability to redeem. Funds at Lloyd’s. AUL operates at Lloyd’s as the corporate member for Syndicate 4711. Lloyd’s determines Syndicate 4711’s required regulatory capital principally through the syndicate’s annual business plan. Such capital, called Funds at Lloyd’s, consists of investable assets as at June 30, 2018 in the amount of $494.2 million ( December 31, 2017 — $458.7 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 June 30, 2018 was £0.4 million ( December 31, 2017 — £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 million and the Company has the option (subject to obtaining commitments from acceptable lenders) to increase the Credit Agreement by up to $100 million . The Credit Agreement will expire on March 27, 2022. As at June 30, 2018 , 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 29, 2018, 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 and June 30, 2016, (the “LOC Facility”). The amendment to the LOC facility extends the term of the LOC Facility to June 30, 2020 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 LOC Facility with Citi Europe, as previously filed with the SEC, remain in full force and effect. As at June 30, 2018 , we had $444.2 million of outstanding collateralized letters of credit under the LOC Facility ( December 31, 2017 — $449.4 million under the LOC Facility). (b) Operating leases Amounts outstanding under operating leases net of subleases as at June 30, 2018 were: 2018 2019 2020 2021 2022 Later Total ($ in millions) Operating Lease Obligations $ 11.0 $ 15.9 $ 15.4 $ 10.9 $ 8.9 $ 74.6 $ 136.7 (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 June 30, 2018 and December 31, 2017 , 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.

Subsequent Events

Subsequent Events	6 Months Ended
	Jun. 30, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On August 2, 2018 we announced our decision to cease underwriting international professional indemnity, marine hull and aviation on the Lloyd’s platform.

Basis of Preparation (Policies)

Basis of Preparation (Policies)	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
New Accounting Pronouncements Adopted in 2018 and Not Yet Adopted	Accounting Pronouncements Adopted in 2018 On August 12, 2015, the Financial Standards Accounting Board (“FASB”) issued ASU 2015-14, “ Revenue from Contracts with Customers (Topic 606)” which delayed the effective date of ASU 2014-09 by one year. This ASU is effective for annual periods beginning after December 15, 2017. Adoption of this ASU during the three and six months ended June 30, 2018 did not have a material impact on the Company’s consolidated financial statements because insurance contracts accounted for within the scope of Topic 944, Financial Services are exempt from this ASU and the Company has immaterial other revenue. On January 5, 2016, the FASB issued ASU 2016-1, “ Financial Instruments - Overall (Subtopic 825-10)” which enhances the reporting model for financial instruments. Included within the requirements of this ASU are the following: a) equity investments to be measured at fair value with changes in fair value recognized in net income; b) a simplification of the impairment assessment of equity investments without readily determinable fair values; c) public business entities to use the exit price concept when measuring the fair value of financial instruments for disclosure purposes; and d) clarifies that an entity should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale securities in combination with the entity’s other deferred tax assets. The amendments required as a result of this ASU are effective for fiscal years beginning after December 15, 2017. Adoption of this ASU during the three and six months ended June 30, 2018 did not have a material impact on the Company’s consolidated financial statements because the Company’s equity portfolio, prior to being sold, was classified as held for trading with changes in fair value recognized through net income and no valuation allowance was required in relation to deferred tax asset related to available-for-sale securities. 2018 Accounting Pronouncements Not Yet Adopted On February 14, 2018, the FASB issued ASU 2018-02, “ Income Statement - Reporting Comprehensive Income (Topic 220)” which allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. This ASU will be effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. The Company is currently evaluating the provisions of ASU 2018-02 to determine how it will be affected, but no material impact is expected on the consolidated financial statements. On February 28, 2018, the FASB issued ASU 2018-03, “ Technical Corrections and Improvements to Financial Instruments - Overall (Subtopic 825-10)” which amends multiple areas in Subtopic 825-10 via improvements to clarify the Codification or to correct unintended application of guidance. This ASU is effective for fiscal years beginning after December 15, 2017 and for interim periods within those fiscal years beginning after June 15, 2018. The Company is currently evaluating the provisions of ASU 2018-03 to determine how it will be affected, but no material impact is expected on the consolidated financial statements. On June 20, 2018, the FASB issued ASU 2018-07, “ Compensation - Stock Compensation (Topic 718)” which amends the scope of Topic 718 via improvements to non-employee share-based payment accounting. Amendments include allowing companies to account for share-based payment transactions with non-employees in the same way as share-based payment transactions with employees and includes elections that offer relief to non-public companies when measuring non-employee equity share options. This ASU will be effective for fiscal years beginning after December 15, 2018 and interim periods within those fiscal years. The Company is currently evaluating the provisions of ASU 2018-07 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 and six months ended June 30, 2018 which were either not relevant to the Company or did not impact the Company’s consolidated financial statements.

Reclassifications from Accumu25

Reclassifications from Accumulated Other Comprehensive Income (Tables)	6 Months Ended
	Jun. 30, 2018
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 six months ended June 30, 2018 and 2017 : Amount Reclassified from AOCI Details about the AOCI Components Three Months Ended June 30, 2018 Three Months Ended June 30, 2017 Affected Line Item in the Unaudited ($ in millions) Available for sale securities: Realized gains on sale of securities $ 2.6 $ 3.3 Realized and unrealized investment gains Realized (losses) on sale of securities (5.9 ) (2.5 ) Realized and unrealized investment losses (3.3 ) 0.8 Income from operations before income tax Tax on net realized (losses)/gains on securities 0.4 — Income tax credit/(expense) $ (2.9 ) $ 0.8 Net (loss)/income Realized derivatives: Net realized gains on settled derivatives $ 1.3 $ 0.9 General, administrative and corporate expenses Tax on settled derivatives (0.3 ) (0.2 ) Income tax credit/(expense) $ 1.0 $ 0.7 Net (loss)/income Total reclassifications from AOCI to the statement of operations, net of income tax $ (1.9 ) $ 1.5 Net (loss)/income Amount Reclassified from AOCI Details about the AOCI Components Six Months Ended June 30, 2018 Six Months Ended June 30, 2017 Affected Line Item in the Unaudited Condensed Consolidated Statement of Operations ($ in millions) Available for sale securities: Realized gains on sale of securities $ 4.6 $ 5.6 Realized and unrealized investment gains Realized (losses) on sale of securities (7.8 ) (3.8 ) Realized and unrealized investment losses (3.2 ) 1.8 Income from operations before income tax Tax on net realized (losses)/gains on securities 0.3 (0.2 ) Income tax credit/(expense) $ (2.9 ) $ 1.6 Net (loss)/income Realized derivatives: Net realized gains on settled derivatives $ 3.0 $ 1.2 General, administrative and corporate expenses Tax on settled derivatives (0.6 ) (0.2 ) Income tax credit/(expense) $ 2.4 $ 1.0 Net (loss)/income Total reclassifications from AOCI to the statement of operations, net of income tax $ (0.5 ) $ 2.6 Net (loss)/income

Earnings per Ordinary Share (Ta

Earnings per Ordinary Share (Tables)	6 Months Ended
	Jun. 30, 2018
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 six months ended June 30, 2018 and 2017 , respectively: Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 ($ in millions, except share and per share amounts) Net (loss)/income $ (14.7 ) $ 75.8 $ 16.1 $ 172.3 Preference share dividends (7.6 ) (10.5 ) (15.2 ) (21.0 ) Preference share redemption costs (1) — — — (2.4 ) Net amount attributable to non-controlling interest (0.1 ) (0.1 ) (0.3 ) (0.2 ) Basic and diluted net (loss)/income available to ordinary shareholders (2) $ (22.4 ) $ 65.2 $ 0.6 $ 148.7 Ordinary shares: Basic weighted average ordinary shares 59,671,684 59,966,358 59,609,271 59,914,797 Weighted average effect of dilutive securities (2) (3) — 1,056,623 918,865 1,181,020 Total diluted weighted average ordinary shares 59,671,684 61,022,981 60,528,136 61,095,817 (Loss)/earnings per ordinary share: Basic $ (0.38 ) $ 1.09 $ 0.01 $ 2.48 Diluted (2) $ (0.38 ) $ 1.07 $ 0.01 $ 2.43 (1) The $2.4 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% Perpetual Non-Cumulative Preference Shares, net of issuance costs, and the final redemption costs of $133.2 million . (2) The basic and diluted number of ordinary shares for the three months ended June 30, 2018 was the same, as the inclusion of dilutive securities in a loss-making period would be anti-dilutive. (3) 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.
Summary of Declared Dividends	Dividends. On August 1, 2018, the Company’s Board of Directors (the “Board of Directors”) declared the following quarterly dividends: Dividend Payable on: Record Date: Ordinary shares $ 0.24 September 4, 2018 August 17, 2018 5.95% preference shares $ 0.3719 October 1, 2018 September 15, 2018 5.625% preference shares $ 0.3516 October 1, 2018 September 15, 2018

Segment Reporting (Tables)

Segment Reporting (Tables)	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and 2017 : Three Months Ended June 30, 2018 Reinsurance Insurance Total ($ in millions) Underwriting Revenues Gross written premiums $ 326.0 $ 527.8 $ 853.8 Net written premiums 266.9 219.1 486.0 Gross earned premiums 366.2 479.3 845.5 Net earned premiums 289.0 230.5 519.5 Underwriting Expenses Losses and loss adjustment expenses 167.0 143.4 310.4 Amortization of deferred policy acquisition costs 62.8 23.1 85.9 General and administrative expenses 27.9 57.2 85.1 Underwriting income $ 31.3 $ 6.8 38.1 Corporate expenses (16.0 ) Non-operating expenses (1) (9.1 ) Net investment income 50.4 Realized and unrealized investment gains 3.5 Realized and unrealized investment losses (24.2 ) Realized loss on the debt extinguishment (8.6 ) Change in fair value of loan notes issued by variable interest entities (3.4 ) Change in fair value of derivatives (46.1 ) Interest expense on long term debt (7.6 ) Net realized and unrealized foreign exchange gains 5.2 Other income 2.1 Other expenses (0.5 ) Loss before tax $ (16.2 ) Net reserves for loss and loss adjustment expenses $ 2,744.1 $ 2,132.3 $ 4,876.4 Ratios Loss ratio 57.8 % 62.2 % 59.7 % Policy acquisition expense ratio 21.7 10.0 16.5 General and administrative expense ratio 9.7 24.8 21.2 (2) Expense ratio 31.4 34.8 37.7 Combined ratio 89.2 % 97.0 % 97.4 % (1) Non-operating expenses includes $8.6 million of expenses related to the Company’s Effectiveness and Efficiency Program. (2) The general and administrative expense ratio in the “Total” column includes corporate and non-operating expenses. Three Months Ended June 30, 2017 Reinsurance Insurance Total ( $ in millions) Underwriting Revenues Gross written premiums $ 335.6 $ 486.5 $ 822.1 Net written premiums 285.5 293.2 578.7 Gross earned premiums 320.6 429.1 749.7 Net earned premiums 272.7 289.3 562.0 Underwriting Expenses Losses and loss adjustment expenses 152.6 193.5 346.1 Amortization of deferred policy acquisition costs 53.4 42.9 96.3 General and administrative expenses 40.7 65.7 106.4 Underwriting income/(loss) $ 26.0 $ (12.8 ) 13.2 Corporate expenses (11.4 ) Non-operating expenses (2.1 ) Net investment income 47.4 Realized and unrealized investment gains 49.0 Realized and unrealized investment losses (7.0 ) Change in fair value of loan notes issued by variable interest entities (3.3 ) Change in fair value of derivatives 17.6 Interest expense on long term debt (7.4 ) Net realized and unrealized foreign exchange (losses) (20.6 ) Other income 3.6 Other expenses (2.0 ) Income before tax $ 77.0 Net reserves for loss and loss adjustment expenses $ 2,445.0 $ 2,347.0 $ 4,792.0 Ratios Loss ratio 56.0 % 66.9 % 61.6 % Policy acquisition expense ratio 19.6 14.8 17.1 General and administrative expense ratio 14.9 22.7 21.3 (1) Expense ratio 34.5 37.5 38.4 Combined ratio 90.5 % 104.4 % 100.0 % (1) The general and administrative expense ratio in the “Total” column includes corporate and non-operating expenses.

Investments (Tables)

Investments (Tables)	6 Months Ended
	Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Summary of Investment Income	Investment Income. The following table summarizes investment income for the three and six months ended June 30, 2018 and 2017 : For the Three Months Ended For the Six Months Ended June 30, 2018 June 30, 2017 June 30, 2018 June 30, 2017 ($ in millions) ($ in millions) Fixed income securities — Available for sale $ 33.5 $ 33.9 $ 66.8 $ 67.8 Fixed income securities — Trading 13.2 10.4 25.2 20.5 Short-term investments — Available for sale 0.3 0.2 0.5 0.3 Short-term investments — Trading 0.1 0.3 0.3 0.5 Fixed term deposits (included in cash and cash equivalents) 4.2 1.3 6.6 2.0 Equity securities — Trading 0.7 3.7 1.9 8.9 Catastrophe bonds — Trading 0.7 0.4 1.3 0.8 Total $ 52.7 $ 50.2 $ 102.6 $ 100.8 Investment expenses (2.3 ) (2.8 ) (4.9 ) (5.7 ) Net investment income $ 50.4 $ 47.4 $ 97.7 $ 95.1
Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments	The following table summarizes the net realized and unrealized investment gains and losses recorded in the statement of operations and the change in unrealized gains and losses on investments recorded in other comprehensive income for the three and six months ended June 30, 2018 and 2017 : For the Three Months Ended For the Six Months Ended June 30, 2018 June 30, 2017 June 30, 2018 June 30, 2017 ($ in millions) ($ in millions) Available for sale: Fixed income securities — gross realized gains $ 2.6 $ 3.3 $ 4.4 $ 5.5 Fixed income securities — gross realized (losses) (5.6 ) (2.4 ) (7.4 ) (3.7 ) Short-term investments — gross realized gains — — — 0.1 Cash and cash equivalents — gross realized gains — — 0.2 — Cash and cash equivalents — gross realized (losses) (0.3 ) (0.1 ) (0.4 ) (0.1 ) Other-than-temporary impairments — (0.1 ) — (0.4 ) Trading: Fixed income securities — gross realized gains 1.0 3.5 2.6 5.3 Fixed income securities — gross realized (losses) (3.4 ) (0.4 ) (10.4 ) (2.4 ) Short-term investments — gross realized gains — 0.1 — 0.1 Cash and cash equivalents — gross realized (losses) gains (0.1 ) — 1.5 — Equity securities — gross realized gains — 4.4 94.5 8.9 Equity securities — gross realized (losses) 0.2 (3.6 ) (20.1 ) (5.0 ) Catastrophe bonds — net unrealized (losses) gains — (0.1 ) 0.9 (0.1 ) Net change in gross unrealized (losses) gains (14.9 ) 38.5 (123.7 ) 79.4 Investments — equity method: Gross realized and unrealized (loss) in MVI (0.1 ) (0.1 ) (0.2 ) (0.1 ) Gross realized (loss)/gain in Chaspark — (0.8 ) — 0.9 Gross realized and unrealized (loss) in Bene (0.1 ) (0.2 ) (0.3 ) (0.2 ) Total net realized and unrealized investment (losses) gains recorded in the statement of operations $ (20.7 ) $ 42.0 $ (58.4 ) $ 88.2 Change in available for sale net unrealized (losses) gains: Fixed income securities (22.8 ) 12.9 (105.7 ) 14.9 Total change in pre-tax available for sale unrealized (losses) gains (22.8 ) 12.9 (105.7 ) 14.9 Change in taxes 1.2 (1.1 ) 6.7 (1.0 ) Total change in net unrealized gains, net of taxes, recorded in other comprehensive income $ (21.6 ) $ 11.8 $ (99.0 ) $ 13.9
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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,253.5 $ 1.6 $ (25.5 ) $ 1,229.6 U.S. agency 47.1 0.1 (0.6 ) 46.6 Municipal 105.1 1.7 (1.1 ) 105.7 Corporate 2,304.2 10.2 (47.3 ) 2,267.1 Non-U.S. government-backed corporate 92.2 0.1 (0.5 ) 91.8 Non-U.S. government 427.5 4.0 (1.3 ) 430.2 Asset-backed 18.7 — (0.3 ) 18.4 Agency mortgage-backed 907.6 5.8 (22.8 ) 890.6 Total fixed income securities — Available for sale 5,155.9 23.5 (99.4 ) 5,080.0 Total short-term investments — Available for sale 72.2 — — 72.2 Total $ 5,228.1 $ 23.5 $ (99.4 ) $ 5,152.2 As at December 31, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 1,166.5 $ 4.5 $ (11.6 ) $ 1,159.4 U.S. agency 51.8 0.5 (0.2 ) 52.1 Municipal 53.0 2.1 (0.2 ) 54.9 Corporate 2,391.4 36.1 (11.8 ) 2,415.7 Non-U.S. government-backed corporate 91.5 0.3 (0.5 ) 91.3 Non-U.S. government 479.7 6.4 (1.2 ) 484.9 Asset-backed 26.3 — (0.1 ) 26.2 Agency mortgage-backed 941.0 13.7 (8.2 ) 946.5 Total fixed income securities — Available for sale 5,201.2 63.6 (33.8 ) 5,231.0 Total short-term investments — Available for sale 90.0 — (0.1 ) 89.9 Total $ 5,291.2 $ 63.6 $ (33.9 ) $ 5,320.9
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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 155.6 $ 0.5 $ (0.8 ) $ 155.3 Municipal 78.6 — (0.8 ) 77.8 Corporate 988.2 3.1 (22.8 ) 968.5 Non-U.S government-backed corporate — — — — Non-U.S. government 222.3 2.0 (5.0 ) 219.3 Asset-backed 7.3 — (0.1 ) 7.2 Agency mortgage-backed 192.6 — (4.8 ) 187.8 Total fixed income securities — Trading 1,644.6 5.6 (34.3 ) 1,615.9 Total short-term investments — Trading 27.4 — — 27.4 Total catastrophe bonds — Trading 35.8 0.1 (0.4 ) 35.5 Total $ 1,707.8 $ 5.7 $ (34.7 ) $ 1,678.8 As at December 31, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Market Value ($ in millions) U.S. government $ 162.3 $ 0.4 $ (0.8 ) $ 161.9 Municipal 32.4 — (0.2 ) 32.2 Corporate 1,036.5 14.0 (4.2 ) 1,046.3 Non-U.S. government-backed corporate 1.0 — — 1.0 Non-U.S. government 196.1 6.9 (0.5 ) 202.5 Asset-backed 9.9 — — 9.9 Agency mortgage-backed 196.7 0.2 (1.4 ) 195.5 Total fixed income securities — Trading 1,634.9 21.5 (7.1 ) 1,649.3 Total short-term investments — Trading 73.0 — — 73.0 Total equity securities — Trading 414.8 83.5 (7.3 ) 491.0 Total catastrophe bonds — Trading 33.5 — (1.1 ) 32.4 Total $ 2,156.2 $ 105.0 $ (15.5 ) $ 2,245.7
Other Investments	The tables below show the Company’s investments in the MVI, Bene, Digital Re and Crop Re for the three and six months ended June 30, 2018 : For the Three Months Ended June 30, 2018 MVI Bene Digital Re Crop Re Total ($ in millions) Opening undistributed value of investment $ 0.4 $ 2.7 $ 0.5 $ 62.5 $ 66.1 Investment in the period — 1.2 — — 1.2 Realized/unrealized losses for the three months to June 30, 2018 (0.1 ) (0.1 ) — — (0.2 ) Closing undistributed value of investment $ 0.3 $ 3.8 $ 0.5 $ 62.5 $ 67.1
Summary of Fixed Maturities	Fixed Income Securities. The scheduled maturity distribution of available for sale fixed income securities as at June 30, 2018 and December 31, 2017 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 June 30, 2018 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 437.0 $ 437.4 AA- Due after one year through five years 2,627.0 2,593.2 AA- Due after five years through ten years 1,062.2 1,031.1 AA- Due after ten years 103.4 109.3 A+ Subtotal 4,229.6 4,171.0 Agency mortgage-backed 907.6 890.6 AA+ Asset-backed 18.7 18.4 AAA Total fixed income securities — Available for sale $ 5,155.9 $ 5,080.0 As at December 31, 2017 Amortized Cost or Cost Fair Market Value Average S&P Ratings by Maturity ($ in millions) Due one year or less $ 561.7 $ 562.4 AA Due after one year through five years 2,486.7 2,492.2 AA- Due after five years through ten years 1,092.2 1,097.4 A+ Due after ten years 93.3 106.3 A Subtotal 4,233.9 4,258.3 Agency mortgage-backed 941.0 946.5 AA+ Asset-backed 26.3 26.2 AAA Total fixed income securities — Available for sale $ 5,201.2 $ 5,231.0
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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 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 $ 808.1 $ (15.8 ) $ 256.1 $ (9.7 ) $ 1,064.2 $ (25.5 ) 123 U.S. agency 32.9 (0.6 ) — — 32.9 (0.6 ) 13 Municipal 90.0 (1.1 ) 0.7 — 90.7 (1.1 ) 13 Corporate 1,556.6 (31.9 ) 285.3 (15.4 ) 1,841.9 (47.3 ) 682 Non-U.S. government-backed corporate 42.9 (0.2 ) 20.9 (0.3 ) 63.8 (0.5 ) 14 Non-U.S. government 188.9 (0.8 ) 59.3 (0.5 ) 248.2 (1.3 ) 57 Asset-backed 13.4 (0.2 ) 5.0 (0.1 ) 18.4 (0.3 ) 7 Agency mortgage-backed 407.0 (9.7 ) 289.4 (13.1 ) 696.4 (22.8 ) 240 Total fixed income securities — Available for sale 3,139.8 (60.3 ) 916.7 (39.1 ) 4,056.5 (99.4 ) 1,149 Total short-term investments — Available for sale 41.5 — — — 41.5 — 8 Total $ 3,181.3 $ (60.3 ) $ 916.7 $ (39.1 ) $ 4,098.0 $ (99.4 ) 1,157 As at December 31, 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 $ 652.1 $ (5.1 ) $ 259.8 $ (6.5 ) $ 911.9 $ (11.6 ) 101 U.S. agency 20.1 (0.2 ) 6.1 — 26.2 (0.2 ) 10 Municipal 28.5 (0.2 ) — — 28.5 (0.2 ) 9 Corporate 699.3 (3.4 ) 360.7 (8.4 ) 1,060.0 (11.8 ) 412 Non-U.S. government-backed corporate 43.5 (0.3 ) 13.3 (0.2 ) 56.8 (0.5 ) 15 Non-U.S. government 206.2 (0.8 ) 32.0 (0.4 ) 238.2 (1.2 ) 47 Asset-backed 11.1 — 10.5 (0.1 ) 21.6 (0.1 ) 11 Agency mortgage-backed 257.6 (1.9 ) 301.9 (6.3 ) 559.5 (8.2 ) 156 Total fixed income securities — Available for sale 1,918.4 (11.9 ) 984.3 (21.9 ) 2,902.7 (33.8 ) 761 Total short-term investments — Available for sale 46.9 (0.1 ) — — 46.9 (0.1 ) 8 Total $ 1,965.3 $ (12.0 ) $ 984.3 $ (21.9 ) $ 2,949.6 $ (33.9 ) 769

Variable Interest Entities (Tab

Variable Interest Entities (Tables)	6 Months Ended
	Jun. 30, 2018
Payables and Accruals [Abstract]
Summary of Silverton Loan Notes	The following tables show the total liability balance of the Loan Notes for the six months ended June 30, 2018 and 2017 : For the Six Months Ended June 30, 2018 Third Party Aspen Holdings Total ($ in millions) Opening balance $ 86.6 $ 20.6 $ 107.2 Total change in fair value for the period 2.4 0.6 3.0 Total distributed in the period (63.7 ) (14.7 ) (78.4 ) Closing balance as at June 30, 2018 $ 25.3 $ 6.5 $ 31.8 Liability Loan notes (long-term liabilities) $ 20.3 $ 5.2 $ 25.5 Accrued expenses (current liabilities) 5.0 1.3 6.3 Total aggregate unpaid balance as at June 30, 2018 $ 25.3 $ 6.5 $ 31.8 For the Six Months Ended June 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 6.2 1.6 7.8 Total distributed in the period (114.1 ) (28.6 ) (142.7 ) Closing balance as at June 30, 2017 $ 115.5 $ 27.5 $ 143.0 Liability Loan notes (long-term liabilities) $ 110.8 $ 26.4 $ 137.2 Accrued expenses (current liabilities) 4.7 1.1 5.8 Total aggregate unpaid balance as at June 30, 2017 $ 115.5 $ 27.5 $ 143.0

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,229.6 $ — $ — $ 1,229.6 U.S. agency — 46.6 — 46.6 Municipal — 105.7 — 105.7 Corporate — 2,267.1 — 2,267.1 Non-U.S. government-backed corporate — 91.8 — 91.8 Non-U.S. government 291.6 138.6 — 430.2 Asset-backed — 18.4 — 18.4 Agency mortgage-backed — 890.6 — 890.6 Total fixed income securities available for sale, at fair value 1,521.2 3,558.8 — 5,080.0 Short-term investments available for sale, at fair value 69.4 2.8 — 72.2 Held for trading financial assets, at fair value U.S. government 155.3 — — 155.3 Municipal — 77.8 — 77.8 Corporate — 968.5 — 968.5 Non-U.S. government-backed corporate — — — — Non-U.S. government 47.2 172.1 — 219.3 Asset-backed — 7.2 — 7.2 Agency mortgage-backed — 187.8 — 187.8 Total fixed income securities trading, at fair value 202.5 1,413.4 — 1,615.9 Short-term investments trading, at fair value 23.3 4.1 — 27.4 Equity investments trading, at fair value — — — — Catastrophe bonds trading, at fair value — 35.5 — 35.5 Other investments (1) — — — 86.2 Other financial assets and liabilities, at fair value Derivatives at fair value — foreign exchange contracts — 14.1 — 14.1 Liabilities under derivative contracts — foreign exchange contracts — (49.3 ) — (49.3 ) Loan notes issued by variable interest entities, at fair value — — (20.3 ) (20.3 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (5.0 ) (5.0 ) Total $ 1,816.4 $ 4,979.4 $ (25.3 ) $ 6,770.5 (1) Other investments represents our investment in a real estate fund and is measured at fair value using the net asset value per share practical expedient. As a result this has not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the condensed consolidated balance sheets. 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. There were no significant transfers between Level 1, Level 2 and Level 3 during the six months ended June 30, 2018 . The Company settled $63.7 million of Level 3 liabilities in respect of the Loan Notes issued by Silverton for the six months ended June 30, 2018 . As at June 30, 2018 , there were no the assets classified as Level 3 and the Company’s Level 3 liabilities consisted solely of the Loan Notes issued by Silverton. As at December 31, 2017 Level 1 Level 2 Level 3 Total ($ in millions) Available for sale financial assets, at fair value U.S. government $ 1,159.4 $ — $ — $ 1,159.4 U.S. agency — 52.1 — 52.1 Municipal — 54.9 — 54.9 Corporate — 2,415.7 — 2,415.7 Non-U.S. government-backed corporate — 91.3 — 91.3 Non-U.S. government 341.2 143.7 — 484.9 Asset-backed — 26.2 — 26.2 Agency mortgage-backed — 946.5 — 946.5 Total fixed income securities available for sale, at fair value 1,500.6 3,730.4 — 5,231.0 Short-term investments available for sale, at fair value 87.3 2.6 — 89.9 Held for trading financial assets, at fair value U.S. government 161.9 — — 161.9 Municipal — 32.2 — 32.2 Corporate — 1,046.3 — 1,046.3 Non-U.S. government-backed corporate — 1.0 — 1.0 Non-U.S. government 24.5 178.0 — 202.5 Asset-backed — 9.9 — 9.9 Agency mortgage-backed — 195.5 — 195.5 Total fixed income securities trading, at fair value 186.4 1,462.9 — 1,649.3 Short-term investments trading, at fair value 73.0 — — 73.0 Equity investments trading, at fair value 491.0 — — 491.0 Catastrophe bonds trading, at fair value — 32.4 — 32.4 Other financial assets and liabilities, at fair value Derivatives at fair value – foreign exchange contracts — 6.4 — 6.4 Liabilities under derivative contracts – foreign exchange contracts — (1.0 ) — (1.0 ) Loan notes issued by variable interest entities, at fair value — — (44.2 ) (44.2 ) Loan notes issued by variable interest entities, at fair value (included within accrued expenses and other payables) — — (42.4 ) (42.4 ) Total $ 2,338.3 $ 5,233.7 $ (86.6 ) $ 7,485.4
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 six months ended June 30, 2018 and 2017 : Reconciliation of Liabilities Using Level 3 Inputs Three Months Ended June 30, 2018 Three Months Ended June 30, 2017 ($ in millions) Balance at the beginning of the period $ 39.8 $ 115.1 Distributed to third party (17.9 ) (2.9 ) Total change in fair value included in the statement of operations 3.4 3.3 Balance at the end of the period (1) $ 25.3 $ 115.5
Pricing Sources Used in Pricing Fixed Income Investments	Pricing sources used in pricing fixed income investments as at June 30, 2018 and December 31, 2017 were as follows: As at June 30, 2018 As at December 31, 2017 Index providers 84 % 84 % Pricing services 14 11 Broker-dealers 2 5 Total 100 % 100 %
Summary of Securities Priced Using Pricing Information from Index Providers	A summary of securities priced using pricing information from index providers as at June 30, 2018 and December 31, 2017 is provided below: As at June 30, 2018 As at December 31, 2017 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 $ 1,384.9 100 % $ 1,321.3 100 % U.S. agency 44.8 96 % 43.4 83 % Municipal 133.6 73 % 37.4 43 % Corporate 3,075.1 95 % 3,299.6 83 % Non-U.S. government-backed corporate 40.4 44 % 44.0 48 % Non-U.S. government 342.9 53 % 399.4 58 % Asset-backed 12.5 49 % 13.5 37 % Agency mortgage-backed 570.3 53 % 605.0 53 % Total fixed income securities $ 5,604.5 84 % $ 5,763.6 84 % Equities — — % 491.0 100 % Total fixed income securities and equity investments $ 5,604.5 84 % $ 6,254.6 85 %
Fair Value Inputs, Assets, Quantitative Information	The observable and unobservable inputs used to determine the fair value of the Loan Notes as at June 30, 2018 and December 31, 2017 are presented in the tables below: As at June 30, 2018 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 25.3 (1) Internal Valuation Model Gross premiums written (O) $ 50.1 $ 61.1 Reserve for losses (U) $ 4.2 $ 61.9 Contract period (O) N/A 365 days Initial value of issuance (O) $ 325.0 $ 325.0 As at December 31, 2017 Fair Value Level 3 Valuation Method Observable (O) and Unobservable (U) inputs Low High ($ in millions) ($ in millions) Loan Notes $ 86.6 (1) Internal Valuation Model Gross premiums written (O) $ 50.1 $ 61.1 Reserve for losses (U) $ 4.2 $ 61.9 Contract period (O) N/A 365 days Initial value of issuance (O) $ 325.0 $ 325.0 (1) The amounts classified within accrued expenses and other payables were $5.0 million and $42.4 million as at June 30, 2018 and December 31, 2017 , respectively.

Derivative Contracts (Tables)

Derivative Contracts (Tables)	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 As at December 31, 2017 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 $ 340.9 $ 14.2 (1) $ 577.7 $ 5.0 Foreign Exchange Contracts Liabilities under Derivative Contracts $ 812.4 $ (49.3 ) $ 173.9 $ (1.0 ) (1) Net of $2.3 million cash collateral ( December 31, 2017 — $0.6 million ). As at June 30, 2018 As at December 31, 2017 Derivatives Designated as Hedging Instruments Under ASC 815 Balance Sheet Location Notional Amount Fair Value Notional Amount Fair Value ($ in millions) ($ in millions) Foreign Exchange Contracts Derivatives at Fair Value $ 114.3 $ (0.1 ) $ 60.6 $ 1.4
Gain/(Loss) Recognized in Income on Derivative	The following table provides the unrealized and realized gains 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 six months ended June 30, 2018 and 2017 . Amount of Gain Recognized on Derivatives Three Months Ended Six Months Ended Location of Gain Recognized on Derivatives June 30, 2018 June 30, 2017 June 30, 2018 June 30, 2017 Derivatives not designated as hedges ($ in millions) ($in millions) Foreign Exchange Contracts Change in Fair Value of Derivatives (46.1 ) 17.6 (22.6 ) 20.7 Derivatives designated as hedges Foreign Exchange Contracts General, administrative and corporate expenses 1.3 0.9 3.0 1.2 Foreign Exchange Contracts Net change from current period hedged transactions (2.6 ) 2.4 (1.7 ) 3.7

Deferred Policy Acquisition C32

Deferred Policy Acquisition Costs (Tables)	6 Months Ended
	Jun. 30, 2018
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 six months ended June 30, 2018 and 2017 : Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 ($ in millions) ($ in millions) Balance at the beginning of the period $ 319.9 $ 367.5 $ 294.3 $ 358.4 Acquisition costs deferred 64.6 93.4 181.0 216.2 Amortization of deferred policy acquisition costs (85.9 ) (96.3 ) (176.7 ) (210.0 ) Balance at the end of the period $ 298.6 $ 364.6 $ 298.6 $ 364.6

Reserves for Losses and Loss 33

Reserves for Losses and Loss Adjustment Expenses (Tables)	6 Months Ended
	Jun. 30, 2018
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 six months ended June 30, 2018 and 2017 and the twelve months ended December 31, 2017 : Six Months Ended June 30, 2018 Six Months Ended June 30, 2017 Twelve Months Ended December 31, 2017 ($ in millions) Provision for losses and LAE at the start of the year $ 6,749.5 $ 5,319.9 $ 5,319.9 Less reinsurance recoverable (1,515.2 ) (560.7 ) (560.7 ) Net loss and LAE at the start of the year 5,234.3 4,759.2 4,759.2 Net loss and LAE expenses (disposed) — — (125.5 ) Provision for losses and LAE for claims incurred: Current year 700.8 749.2 2,100.1 Prior years (80.2 ) (74.9 ) (105.4 ) Total incurred 620.6 674.3 1,994.7 Losses and LAE payments for claims incurred: Current year (49.7 ) (54.4 ) (397.5 ) Prior years (874.7 ) (686.7 ) (1,157.6 ) Total paid (924.4 ) (741.1 ) (1,555.1 ) Foreign exchange losses/(gains) (54.1 ) 99.6 161.0 Net losses and LAE reserves at period end 4,876.4 4,792.0 5,234.3 Plus reinsurance recoverable on unpaid losses at period end 1,656.4 779.4 1,515.2 Provision for losses and LAE at the end of the relevant period $ 6,532.8 $ 5,571.4 $ 6,749.5
Short-duration Insurance Contracts, Claims Development	Insurance Incurred Claims, IBNR and Allocated Loss Adjustment Expenses, Net of Reinsurance As at June 30, 2018 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 Q2 2018 $ (in millions) 2012 605.0 626.8 655.9 669.9 654.7 638.2 647.2 45.9 15,682 2013 689.6 667.9 679.0 660.4 650.6 651.1 32.4 14,631 2014 755.6 730.1 702.4 695.1 699.5 68.0 18,836 2015 916.0 905.9 855.1 858.1 154.4 21,132 2016 909.3 881.6 873.6 284.6 21,052 2017 896.1 847.7 175.9 20,622 2018 307.0 192.9 6,929 Total $ 4,884.2 Insurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 Q2 2018 ($ in millions) 2012 117.3 307.9 378.5 452.9 502.8 524.7 531.7 2013 90.7 254.5 364.9 455.0 502.8 532.7 2014 99.5 247.5 389.8 465.9 481.0 2015 119.2 326.9 471.9 523.4 2016 117.3 346.1 415.4 2017 167.8 369.5 2018 32.2 Total $ 2,885.9 All outstanding liabilities for 2012 and subsequent years, net of reinsurance $ 1,998.3 All outstanding liabilities before 2012, net of reinsurance 114.0 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 2,112.3 Reinsurance Incurred Claims, IBNR and Allocated Loss Adjustment Expenses, Net of Reinsurance As at June 30, 2018 Total of IBNR Plus Expected Development on Reported Claims Cumulative Number of Reported Claims For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 Q2 2018 $ (in millions) 2012 692.8 737.3 720.9 690.6 689.4 687.9 686.7 71.2 3,838 2013 579.6 572.2 550.2 524.3 504.9 499.7 66.2 3,689 2014 550.6 529.9 514.2 487.1 477.5 82.8 3,696 2015 580.3 564.5 558.2 536.9 117.6 3,899 2016 749.1 762.8 768.9 218.1 3,709 2017 1,190.2 1,189.0 420.5 3,746 2018 362.5 289.9 1,415 Total $ 4,521.2 Reinsurance Cumulative Paid Claims and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Unaudited Prior Years Accident Year 2012 2013 2014 2015 2016 2017 Q2 2018 ($ in millions) 2012 63.2 248.2 360.9 414.8 458.7 496.5 511.4 2013 63.2 186.8 278.4 320.1 359.9 371.6 2014 57.0 172.6 248.5 289.2 308.1 2015 57.5 171.4 272.1 306.2 2016 125.5 350.7 400.0 2017 227.9 508.1 2018 17.5 Total $ 2,422.9 All outstanding liabilities for 2012 and subsequent years, net of reinsurance 2,098.3 All outstanding liabilities before 2012, net of reinsurance 619.9 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 2,718.2
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability	Six Months Ended June 30, 2018 ($ in millions) Net outstanding liabilities: Insurance lines 2,112.3 Reinsurance lines 2,718.2 Net loss and LAE 4,830.5 Reinsurance recoverable on unpaid losses: Insurance lines 1,382.8 Reinsurance lines 273.6 Total reinsurance recoverable on unpaid losses 1,656.4 Insurance lines other than short-duration — Unallocated claims incurred 49.7 Other (3.8 ) 45.9 Provision for losses and LAE at the end of the period 6,532.8

Capital Structure (Tables)

Capital Structure (Tables)	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 As at December 31, 2017 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 59,687,717 90 59,474,085 90 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 122 122
Summary of Ordinary Shares	The following table summarizes transactions in the Company’s ordinary shares during the six months ended June 30, 2018 : Number of Ordinary Shares Ordinary shares in issue as at December 31, 2017 59,474,085 Ordinary share transactions in the six months ended June 30, 2018 Ordinary shares issued to employees under the 2013 share incentive plan and/or 2008 share purchase plan 208,954 Ordinary shares issued to non-employee directors 4,678 Ordinary shares in issue as at June 30, 2018 59,687,717

Share-Based Payments (Tables)

Share-Based Payments (Tables)	6 Months Ended
	Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of Non-Employee Director Restricted Stock Units Award	The following table summarizes information about RSUs issued to non-employee directors as at June 30, 2018 : Restricted Share Units As at June 30, 2018 As at June 30, 2017 RSU Holder Amount Granted Amount Granted Non-Employee Directors 29,025 22,230 Chairman 12,900 8,892

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	6 Months Ended
	Jun. 30, 2018
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 six months ended June 30, 2018 and 2017 : Three Months Ended June 30, 2018 Three Months Ended June 30, 2017 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 Trademarks $ 2.8 $ — $ (0.1 ) $ 2.7 $ 6.4 $ — $ (0.1 ) $ 6.3 Insurance Licenses 16.7 — — 16.7 16.7 — — 16.7 Agency Relationships 2.2 — (0.2 ) 2.0 25.8 — (0.7 ) 25.1 Non-compete Agreements 0.6 — (0.1 ) 0.5 3.1 — (0.1 ) 3.0 Consulting Relationships — — — — 0.9 — (0.1 ) 0.8 Goodwill 3.9 — — 3.9 26.0 — — 26.0 Renewal Rights 1.3 — (0.1 ) 1.2 1.6 — (0.1 ) 1.5 Total $ 27.5 $ — $ (0.5 ) $ 27.0 $ 80.5 $ — $ (1.1 ) $ 79.4 Six Months Ended June 30, 2018 Six Months Ended June 30, 2017 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 Trademarks $ 2.9 $ — $ (0.2 ) $ 2.7 $ 6.6 $ — $ (0.3 ) $ 6.3 Insurance Licenses 16.7 — — 16.7 16.7 — — 16.7 Agency Relationships 2.3 — (0.3 ) 2.0 26.2 — (1.1 ) 25.1 Non-compete Agreements 0.7 — (0.2 ) 0.5 3.3 — (0.3 ) 3.0 Consulting Relationships — — — — 0.9 — (0.1 ) 0.8 Goodwill 3.9 — — 3.9 24.2 1.8 — 26.0 Renewal Rights 1.4 — (0.2 ) 1.2 1.7 — (0.2 ) 1.5 Total $ 27.9 $ — $ (0.9 ) $ 27.0 $ 79.6 $ 1.8 $ (2.0 ) $ 79.4

Commitments and Contingent Li37

Commitments and Contingent Liabilities (Tables)	6 Months Ended
	Jun. 30, 2018
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 June 30, 2018 and December 31, 2017 : As at June 30, 2018 As at December 31, 2017 ($ in millions, except percentages) Regulatory trusts and deposits: Affiliated transactions $ 1,154.5 $ 1,455.0 Third party 2,431.1 2,425.3 Letters of credit / guarantees (1) 840.3 658.5 Investment commitment — real estate fund 13.8 100.0 Other investments — real estate fund 86.2 — Total restricted assets $ 4,525.9 $ 4,638.8 Total as percent of investable assets (2) 56.0 % 53.4 % (1) As at June 30, 2018 , the Company had pledged funds in the amount of $840.3 million ( December 31, 2017 — $658.5 million ) as collateral for the secured letters of credit. (2) Investable assets comprise total investments, cash and cash equivalents, accrued interest, receivables for securities sold and payables for securities purchased.
Amounts Outstanding under Operating Leases	Amounts outstanding under operating leases net of subleases as at June 30, 2018 were: 2018 2019 2020 2021 2022 Later Total ($ in millions) Operating Lease Obligations $ 11.0 $ 15.9 $ 15.4 $ 10.9 $ 8.9 $ 74.6 $ 136.7

Reclassifications from Accumu38

Reclassifications from Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income Reclassification (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Available for sale securities:
(Loss)/income from operations before income tax	$ (16.2)	$ 77	$ 18.2	$ 176.3
General, administrative and corporate expenses	110.2	119.9	231.2	241.2
Income tax credit/(expense)	1.5	(1.2)	(2.1)	(4)
Net (loss)/income	(14.7)	75.8	16.1	172.3
Reclassification out of AOCI
Available for sale securities:
Net (loss)/income	(1.9)	1.5	(0.5)	2.6
Reclassification out of AOCI | Available for sale securities
Available for sale securities:
Realized gains on sale of securities	2.6	3.3	4.6	5.6
Realized (losses) on sale of securities	(5.9)	(2.5)	(7.8)	(3.8)
(Loss)/income from operations before income tax	(3.3)	0.8	(3.2)	1.8
Income tax credit/(expense)	0.4	0	0.3	(0.2)
Net (loss)/income	(2.9)	0.8	(2.9)	1.6
Reclassification out of AOCI | Realized derivatives
Available for sale securities:
General, administrative and corporate expenses	1.3	0.9	3	1.2
Income tax credit/(expense)	(0.3)	(0.2)	(0.6)	(0.2)
Net (loss)/income	$ 1	$ 0.7	$ 2.4	$ 1

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		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Class of Stock [Line Items]
Net income		$ (14.7)	$ 75.8	$ 16.1	$ 172.3
Preference share dividends		(7.6)	(10.5)	(15.2)	(21)
Change in redemption value		0	0	0	(2.4)
Proportion due to non-controlling interest		(0.1)	(0.1)	(0.3)	(0.2)
Basic and diluted net (loss)/income available to ordinary shareholders (2)		(22.4)	65.2	0.6	148.7
Diluted net income available to ordinary shareholders		$ (22.4)	$ 65.2	$ 0.6	$ 148.7
Ordinary shares:
Basic weighted average ordinary shares		59,671,684	59,966,358	59,609,271	59,914,797
Weighted average effect of dilutive securities		0	1,056,623	918,865	1,181,020
Total diluted weighted average ordinary shares		59,671,684	61,022,981	60,528,136	61,095,817
(Loss)/earnings per ordinary share:
Basic (in usd per share)		$ (0.38)	$ 1.09	$ 0.01	$ 2.48
Diluted (in usd per share)		$ (0.38)	$ 1.07	$ 0.01	$ 2.43
7.401% preference shares
(Loss)/earnings per ordinary share:
Preference shares, rate				7.401%
Additional paid-in capital | Redemption costs of preference shares
Class of Stock [Line Items]
Change in redemption value	[1]			$ 0	$ 2.4
(Loss)/earnings per ordinary share:
Stock Redeemed or Called During Period, Value				$ 0	$ 133.2
[1]	The $2.4 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% Perpetual Non-Cumulative Preference Shares, net of issuance costs, and the final redemption costs of $133.2 million.

Earnings per Ordinary Share - S

Earnings per Ordinary Share - Summary of Declared Dividends (Details) - $ / shares	Aug. 01, 2018	Jun. 30, 2018	Dec. 31, 2017
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	Sep. 4, 2018
Record Date	Aug. 17, 2018
Subsequent Event | 5.95% preference shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.3719
Payable Date	Oct. 1, 2018
Record Date	Sep. 15, 2018
Subsequent Event | 5.625% preference shares
Dividends Payable [Line Items]
Dividend (in usd per share)	$ 0.35156
Payable Date	Oct. 1, 2018
Record Date	Sep. 15, 2018

Segment Reporting - Narrative (

Segment Reporting - Narrative (Details)	6 Months Ended
	Jun. 30, 2018Segment
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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Underwriting Revenues
Gross written premiums	$ 853.8	$ 822.1	$ 1,970.6	$ 1,820.1
Net written premiums	486	578.7	1,121.5	1,264.9
Gross earned premiums	845.5	749.7	1,688.1	1,501
Net earned premiums	519.5	562	1,053	1,143.1
Underwriting Expenses
Losses and loss adjustment expenses	310.4	346.1	620.6	674.3
Amortization of deferred policy acquisition costs	85.9	96.3	176.7	210
General and administrative expenses	85.1	106.4	180.3	212.1
Underwriting income	38.1	13.2	75.4	46.7
Corporate expenses	(16)	(11.4)	(29.7)	(24.8)
Nonoperating Income (Expense)	(9.1)	(2.1)	(21.2)	(4.3)
Net investment income	50.4	47.4	97.7	95.1
Realized and unrealized investment gains	3.5	49	104.1	100.2
Realized and unrealized investment losses	(24.2)	(7)	(162.5)	(12)
Realized loss on the debt extinguishment	(8.6)	0	(8.6)	0
Change in fair value of loan notes issued by variable interest entities	(3.4)	(3.3)	(2.4)	(6.2)
Change in fair value of derivatives	(46.1)	17.6	(22.6)	20.7
Interest expense on long term debt	(7.6)	(7.4)	(15)	(14.8)
Net realized and unrealized foreign exchange gains	5.2	(20.6)	0.5	(29.5)
Other income	2.1	3.6	4.2	7.2
Other expenses	(0.5)	(2)	(1.7)	(2)
(Loss)/income from operations before income tax	(16.2)	77	18.2	176.3
Net reserves for loss and loss adjustment expenses	$ 4,876.4	$ 4,792	$ 4,876.4	$ 4,792	$ 5,234.3	$ 4,759.2
Ratios
Loss ratio	59.70%	61.60%	58.90%	59.00%
Policy acquisition expense ratio	16.50%	17.10%	16.80%	18.40%
General and administrative expense ratio	21.20%	21.30%	22.00%	21.10%
Expense ratio	37.70%	38.40%	38.80%	39.50%
Combined ratio	97.40%	100.00%	97.70%	98.50%
Non-operating expenses related to Effectiveness and Efficiency Program	$ 8.6		$ 20.4
Reinsurance
Underwriting Revenues
Gross written premiums	326	$ 335.6	949.5	$ 900.9
Net written premiums	266.9	285.5	691.9	733.7
Gross earned premiums	366.2	320.6	741.2	648.2
Net earned premiums	289	272.7	571.5	550.2
Underwriting Expenses
Losses and loss adjustment expenses	167	152.6	333.9	295.7
Amortization of deferred policy acquisition costs	62.8	53.4	118.7	112.9
General and administrative expenses	27.9	40.7	59.5	84.6
Underwriting income	31.3	26	59.4	57
Net reserves for loss and loss adjustment expenses	$ 2,744.1	$ 2,445	$ 2,744.1	$ 2,445
Ratios
Loss ratio	57.80%	56.00%	58.40%	53.70%
Policy acquisition expense ratio	21.70%	19.60%	20.80%	20.50%
General and administrative expense ratio	9.70%	14.90%	10.40%	15.40%
Expense ratio	31.40%	34.50%	31.20%	35.90%
Combined ratio	89.20%	90.50%	89.60%	89.60%
Insurance
Underwriting Revenues
Gross written premiums	$ 527.8	$ 486.5	$ 1,021.1	$ 919.2
Net written premiums	219.1	293.2	429.6	531.2
Gross earned premiums	479.3	429.1	946.9	852.8
Net earned premiums	230.5	289.3	481.5	592.9
Underwriting Expenses
Losses and loss adjustment expenses	143.4	193.5	286.7	378.6
Amortization of deferred policy acquisition costs	23.1	42.9	58	97.1
General and administrative expenses	57.2	65.7	120.8	127.5
Underwriting income	6.8	(12.8)	16	(10.3)
Net reserves for loss and loss adjustment expenses	$ 2,132.3	$ 2,347	$ 2,132.3	$ 2,347
Ratios
Loss ratio	62.20%	66.90%	59.50%	63.90%
Policy acquisition expense ratio	10.00%	14.80%	12.00%	16.40%
General and administrative expense ratio	24.80%	22.70%	25.10%	21.50%
Expense ratio	34.80%	37.50%	37.10%	37.90%
Combined ratio	97.00%	104.40%	96.60%	101.80%

Investments - Summary of Invest

Investments - Summary of Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 52.7	$ 50.2	$ 102.6	$ 100.8
Investment expenses	(2.3)	(2.8)	(4.9)	(5.7)
Net investment income	50.4	47.4	97.7	95.1
Total fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	33.5	33.9	66.8	67.8
Fixed Income Maturities - Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	13.2	10.4	25.2	20.5
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.3	0.2	0.5	0.3
Short-term Investment, Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.1	0.3	0.3	0.5
Fixed term deposits (included in cash and cash equivalents)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	4.2	1.3	6.6	2
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	0.7	3.7	1.9	8.9
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross investment income	$ 0.7	$ 0.4	$ 1.3	$ 0.8

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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Other-than-temporary impairments	$ 0	$ (0.1)	$ 0	$ (0.4)
Catastrophe bonds	0	(0.1)	0.9	(0.1)
Net change in gross unrealized (losses) gains	(14.9)	38.5	(123.7)	79.4
Total net realized and unrealized investment (losses) gains recorded in the statement of operations	(20.7)	42	(58.4)	88.2
Change in available for sale net unrealized (losses) gains:
Fixed income securities	(22.8)	12.9	(105.7)	14.9
Total change in pre-tax available for sale unrealized (losses) gains	(22.8)	12.9	(105.7)	14.9
Change in taxes	1.2	(1.1)	6.7	(1)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	(21.6)	11.8	(99)	13.9
Total fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	2.6	3.3	4.4	5.5
Fixed income maturities — gross realized (losses)	(5.6)	(2.4)	(7.4)	(3.7)
Cash and cash equivalents - gross realized gains	0	0	0.2	0
Cash and cash equivalents — gross realized gains (losses)	(0.3)	(0.1)	(0.4)	(0.1)
Fixed Income Maturities - Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Cash and cash equivalents — gross realized gains (losses)	(0.1)	0	1.5	0
Fixed income securities - gross realized gains	1	3.5	2.6	5.3
Fixed income securities - gross realized (losses)	(3.4)	(0.4)	(10.4)	(2.4)
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Trading securities, gross realized gain	0	4.4	94.5	8.9
Equity securities — gross realized (losses)	0.2	(3.6)	(20.1)	(5)
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	0	0	0	0.1
Trading securities, gross realized gain	0	0.1	0	0.1
MVI
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	(0.1)	(0.1)	(0.2)	(0.1)
Chaspark
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	0	(0.8)	0	0.9
Bene
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	$ (0.1)	$ (0.2)	$ (0.3)	$ (0.2)

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	Jun. 30, 2018	Dec. 31, 2017
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	$ 5,228.1	$ 5,291.2
Gross Unrealized Gains	23.5	63.6
Gross Unrealized Losses	(99.4)	(33.9)
Fair Market Value	5,152.2	5,320.9
Total fixed income securities — Available for sale
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,155.9	5,201.2
Gross Unrealized Gains	23.5	63.6
Gross Unrealized Losses	(99.4)	(33.8)
Fair Market Value	5,080	5,231
Total short-term investments — Available for sale
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	72.2	90
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(0.1)
Fair Market Value	72.2	89.9
U.S. government
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	1,253.5	1,166.5
Gross Unrealized Gains	1.6	4.5
Gross Unrealized Losses	(25.5)	(11.6)
Fair Market Value	1,229.6	1,159.4
U.S. agency
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	47.1	51.8
Gross Unrealized Gains	0.1	0.5
Gross Unrealized Losses	(0.6)	(0.2)
Fair Market Value	46.6	52.1
Municipal
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	105.1	53
Gross Unrealized Gains	1.7	2.1
Gross Unrealized Losses	(1.1)	(0.2)
Fair Market Value	105.7	54.9
Corporate
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	2,304.2	2,391.4
Gross Unrealized Gains	10.2	36.1
Gross Unrealized Losses	(47.3)	(11.8)
Fair Market Value	2,267.1	2,415.7
Non-U.S. government-backed corporate
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	92.2	91.5
Gross Unrealized Gains	0.1	0.3
Gross Unrealized Losses	(0.5)	(0.5)
Fair Market Value	91.8	91.3
Non-U.S. government
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	427.5	479.7
Gross Unrealized Gains	4	6.4
Gross Unrealized Losses	(1.3)	(1.2)
Fair Market Value	430.2	484.9
Asset-backed
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	18.7	26.3
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.3)	(0.1)
Fair Market Value	18.4	26.2
Agency mortgage-backed
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	907.6	941
Gross Unrealized Gains	5.8	13.7
Gross Unrealized Losses	(22.8)	(8.2)
Fair Market Value	$ 890.6	$ 946.5

Investments - Cost, Gross Unr46

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Statement [Line Items]
Cost or Amortized Cost	$ 1,644.6	$ 1,634.9
Fair Market Value	1,615.9	1,649.3
U.S. government
Statement [Line Items]
Cost or Amortized Cost	155.6	162.3
Gross Unrealized Gains	0.5	0.4
Gross Unrealized Losses	(0.8)	(0.8)
Fair Market Value	155.3	161.9
Municipal
Statement [Line Items]
Cost or Amortized Cost	78.6	32.4
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.8)	(0.2)
Fair Market Value	77.8	32.2
Corporate
Statement [Line Items]
Cost or Amortized Cost	988.2	1,036.5
Gross Unrealized Gains	3.1	14
Gross Unrealized Losses	(22.8)	(4.2)
Fair Market Value	968.5	1,046.3
Non-U.S. government-backed corporate
Statement [Line Items]
Cost or Amortized Cost	0	1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	0	1
Non-U.S. government
Statement [Line Items]
Cost or Amortized Cost	222.3	196.1
Gross Unrealized Gains	2	6.9
Gross Unrealized Losses	(5)	(0.5)
Fair Market Value	219.3	202.5
Asset-backed
Statement [Line Items]
Cost or Amortized Cost	7.3	9.9
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.1)	0
Fair Market Value	7.2	9.9
Agency mortgage-backed
Statement [Line Items]
Cost or Amortized Cost	192.6	196.7
Gross Unrealized Gains	0	0.2
Gross Unrealized Losses	(4.8)	(1.4)
Fair Market Value	187.8	195.5
Total fixed income securities — Trading
Statement [Line Items]
Cost or Amortized Cost	1,644.6	1,634.9
Gross Unrealized Gains	5.6	21.5
Gross Unrealized Losses	(34.3)	(7.1)
Fair Market Value	1,615.9	1,649.3
Total short-term investments — Trading
Statement [Line Items]
Cost or Amortized Cost	27.4	73
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Market Value	27.4	73
Total equity securities — Trading
Statement [Line Items]
Cost or Amortized Cost		414.8
Gross Unrealized Gains		83.5
Gross Unrealized Losses		(7.3)
Fair Market Value		491
Total catastrophe bonds — Trading
Statement [Line Items]
Cost or Amortized Cost	35.8	33.5
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	(0.4)	(1.1)
Fair Market Value	35.5	32.4
Total
Statement [Line Items]
Cost or Amortized Cost	1,707.8	2,156.2
Gross Unrealized Gains	5.7	105
Gross Unrealized Losses	(34.7)	(15.5)
Fair Market Value	$ 1,678.8	$ 2,245.7

Investments - Other Investments

Investments - Other Investments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	$ 66.1	$ 66.4
Investment in the period	1.2	1.2
Unrealized/realized gain/(loss)	(0.2)	(0.5)
Closing value of investment	67.1	67.1
MVI
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0.4	0.5
Investment in the period	0	0
Unrealized/realized gain/(loss)	(0.1)	(0.2)
Closing value of investment	0.3	0.3
Bene
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	2.7	2.9
Investment in the period	1.2	1.2
Unrealized/realized gain/(loss)	(0.1)	(0.3)
Closing value of investment	3.8	3.8
Digital Re
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0.5	0.5
Investment in the period	0	0
Unrealized/realized gain/(loss)	0	0
Closing value of investment	0.5	0.5
Crop Re
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	62.5	62.5
Investment in the period	0	0
Unrealized/realized gain/(loss)	0	0
Closing value of investment	$ 62.5	$ 62.5

Investments - Summary of Fixed

Investments - Summary of Fixed Maturities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	$ 5,228.1		$ 5,291.2
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,155.9		5,201.2
Available for sale investments in fixed income maturities, Fair Market Value	5,152.2		5,320.9
Total fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,155.9		5,201.2
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,155.9		5,201.2
Available for sale investments in fixed income maturities, Fair Market Value	5,080		5,231
Agency mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	907.6		941
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	907.6		941
Available for sale investments in fixed income maturities, Fair Market Value	$ 890.6		946.5
Available For Sale Securities Average Ratings By Maturity	AA+	AA+
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	$ 18.7		26.3
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	18.7		26.3
Available for sale investments in fixed income maturities, Fair Market Value	$ 18.4		26.2
Available For Sale Securities Average Ratings By Maturity	AAA	AAA
Total fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
Due one year or less, Cost or Amortized Cost	$ 437		561.7
Due after one year through five years, Cost or Amortized Cost	2,627		2,486.7
Due after five years through ten years, Cost or Amortized Cost	1,062.2		1,092.2
Due after ten years, Cost or Amortized Cost	103.4		93.3
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	4,229.6		4,233.9
Due one year or less, Fair Market Value	437.4		562.4
Due after one year through five years, Fair Market Value	2,593.2		2,492.2
Due after five years through ten years, Fair Market Value	1,031.1		1,097.4
Due after ten years, Fair Market Value	109.3		106.3
Available for sale investments in fixed income maturities, Fair Market Value	$ 4,171		$ 4,258.3
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	Jun. 30, 2018USD ($)Security	Dec. 31, 2017USD ($)Security
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 3,181.3	$ 1,965.3
0-12 months, Gross Unrealized Loss	(60.3)	(12)
Over 12 months, Fair Market Value	916.7	984.3
Over 12 months, Gross Unrealized Loss	(39.1)	(21.9)
Total, Fair Market Value	4,098	2,949.6
Total, Gross Unrealized Loss	$ (99.4)	$ (33.9)
Number of Securities | Security	1,157	769
Total fixed income securities — Available for sale
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 3,139.8	$ 1,918.4
0-12 months, Gross Unrealized Loss	(60.3)	(11.9)
Over 12 months, Fair Market Value	916.7	984.3
Over 12 months, Gross Unrealized Loss	(39.1)	(21.9)
Total, Fair Market Value	4,056.5	2,902.7
Total, Gross Unrealized Loss	$ (99.4)	$ (33.8)
Number of Securities | Security	1,149	761
Total short-term investments — Trading
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 41.5	$ 46.9
0-12 months, Gross Unrealized Loss	0	(0.1)
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	41.5	46.9
Total, Gross Unrealized Loss	$ 0	$ (0.1)
Number of Securities | Security	8	8
U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 808.1	$ 652.1
0-12 months, Gross Unrealized Loss	(15.8)	(5.1)
Over 12 months, Fair Market Value	256.1	259.8
Over 12 months, Gross Unrealized Loss	(9.7)	(6.5)
Total, Fair Market Value	1,064.2	911.9
Total, Gross Unrealized Loss	$ (25.5)	$ (11.6)
Number of Securities | Security	123	101
U.S. agency
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 32.9	$ 20.1
0-12 months, Gross Unrealized Loss	(0.6)	(0.2)
Over 12 months, Fair Market Value	0	6.1
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	32.9	26.2
Total, Gross Unrealized Loss	$ (0.6)	$ (0.2)
Number of Securities | Security	13	10
Municipal
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 90	$ 28.5
0-12 months, Gross Unrealized Loss	(1.1)	(0.2)
Over 12 months, Fair Market Value	0.7	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	90.7	28.5
Total, Gross Unrealized Loss	$ (1.1)	$ (0.2)
Number of Securities | Security	13	9
Corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 1,556.6	$ 699.3
0-12 months, Gross Unrealized Loss	(31.9)	(3.4)
Over 12 months, Fair Market Value	285.3	360.7
Over 12 months, Gross Unrealized Loss	(15.4)	(8.4)
Total, Fair Market Value	1,841.9	1,060
Total, Gross Unrealized Loss	$ (47.3)	$ (11.8)
Number of Securities | Security	682	412
Non-U.S. government-backed corporate
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 42.9	$ 43.5
0-12 months, Gross Unrealized Loss	(0.2)	(0.3)
Over 12 months, Fair Market Value	20.9	13.3
Over 12 months, Gross Unrealized Loss	(0.3)	(0.2)
Total, Fair Market Value	63.8	56.8
Total, Gross Unrealized Loss	$ (0.5)	$ (0.5)
Number of Securities | Security	14	15
Non-U.S. government
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 188.9	$ 206.2
0-12 months, Gross Unrealized Loss	(0.8)	(0.8)
Over 12 months, Fair Market Value	59.3	32
Over 12 months, Gross Unrealized Loss	(0.5)	(0.4)
Total, Fair Market Value	248.2	238.2
Total, Gross Unrealized Loss	$ (1.3)	$ (1.2)
Number of Securities | Security	57	47
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 13.4	$ 11.1
0-12 months, Gross Unrealized Loss	(0.2)	0
Over 12 months, Fair Market Value	5	10.5
Over 12 months, Gross Unrealized Loss	(0.1)	(0.1)
Total, Fair Market Value	18.4	21.6
Total, Gross Unrealized Loss	$ (0.3)	$ (0.1)
Number of Securities | Security	7	11
Agency mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
0-12 months, Fair Market Value	$ 407	$ 257.6
0-12 months, Gross Unrealized Loss	(9.7)	(1.9)
Over 12 months, Fair Market Value	289.4	301.9
Over 12 months, Gross Unrealized Loss	(13.1)	(6.3)
Total, Fair Market Value	696.4	559.5
Total, Gross Unrealized Loss	$ (22.8)	$ (8.2)
Number of Securities | Security	240	156

Investments - Narrative (Detail

Investments - Narrative (Details) - USD ($)	Dec. 18, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	May 10, 2018	Mar. 31, 2018	Jan. 01, 2017	Jul. 26, 2016	Jan. 01, 2015
Investment Holdings [Line Items]
Catastrophe bonds		$ 35,500,000		$ 35,500,000		$ 32,400,000
Equity method investment, aggregate cost		67,100,000		67,100,000		66,400,000		$ 66,100,000
Restricted Assets		4,525,900,000		4,525,900,000		4,638,800,000
Other investments		86,200,000		86,200,000		0
Investment Funds		0		0
OTTI charge recognized		0	$ 100,000	0	$ 400,000
MVI
Investment Holdings [Line Items]
Equity method investment, aggregate cost		300,000		300,000		500,000		400,000			$ 800,000
Additional investment						100,000
Bene
Investment Holdings [Line Items]
Equity method investment, aggregate cost		3,800,000		3,800,000		2,900,000		2,700,000		$ 3,300,000
Additional investment						1,200,000
Equity method investment, ownership percentage										20.00%
Digital Re
Investment Holdings [Line Items]
Equity method investment, aggregate cost		500,000		500,000		500,000		500,000	$ 2,300,000
Equity method investment, ownership percentage									49.00%
Crop Re
Investment Holdings [Line Items]
Equity method investment, aggregate cost	$ 62,500,000	62,500,000		62,500,000		62,500,000		$ 62,500,000
Additional investment	$ 5,900,000
Equity method investment, ownership percentage	23.20%
CGB DS | Crop Re
Investment Holdings [Line Items]
Equity method investment, counterparty ownership percentage	76.80%
Limited Partner | Real estate funds
Investment Holdings [Line Items]
Restricted Assets		$ 13,800,000		$ 13,800,000		$ 100,000,000
Other investments							$ 86,200,000

Variable Interest Entities - Na

Variable Interest Entities - Narrative (Details)	Jun. 30, 2018variable_interest_entity
Payables and Accruals [Abstract]
Number of Variable Interest Entities	2

Variable Interest Entities - Su

Variable Interest Entities - Summary of Silverton Loan Notes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Movement in Silverton Loan Notes [Roll Forward]
Total liability balance of Loan Notes, Beginning Balance			$ 107.2	$ 277.9
Total change in fair value for the period	$ (3.4)	$ (3.3)	(2.4)	(6.2)
Total distributed in the period			(78.4)	(142.7)
Total liability balance of Loan Notes, Ending Balance	31.8	143	31.8	143
Loan notes (long-term liabilities)					$ 25.5	$ 137.2
Accrued expenses (current liabilities)					6.3	5.8
Total aggregate unpaid balance	31.8	143	107.2	277.9	31.8	143
Third party
Movement in Silverton Loan Notes [Roll Forward]
Total liability balance of Loan Notes, Beginning Balance			86.6	223.4
Total change in fair value for the period			2.4	6.2
Total distributed in the period			(63.7)	(114.1)
Total liability balance of Loan Notes, Ending Balance	25.3	115.5	25.3	115.5
Loan notes (long-term liabilities)					20.3	110.8
Accrued expenses (current liabilities)					5	4.7
Total aggregate unpaid balance	25.3	115.5	86.6	223.4	25.3	115.5
Aspen Holdings
Movement in Silverton Loan Notes [Roll Forward]
Total liability balance of Loan Notes, Beginning Balance			20.6	54.5
Total change in fair value for the period			0.6	1.6
Total distributed in the period			(14.7)	(28.6)
Total liability balance of Loan Notes, Ending Balance	6.5	27.5	6.5	27.5
Loan notes (long-term liabilities)					5.2	26.4
Accrued expenses (current liabilities)					1.3	1.1
Total aggregate unpaid balance	$ 6.5	$ 27.5	20.6	54.5	$ 6.5	$ 27.5
Aspen Holdings | Third party
Movement in Silverton Loan Notes [Roll Forward]
Total change in fair value for the period			$ 3	$ 7.8

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value	$ 35.8	$ 33.5
Derivatives at fair value	14.1	6.4
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value	35.5	32.4
Loan notes issued by variable interest entities, at fair value	(20.3)	(44.2)
Total	6,770.5	7,485.4
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	27.4	73
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	491
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	14.1	6.4
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 at fair value	(49.3)	(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	5,080	5,231
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	1,229.6	1,159.4
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	46.6	52.1
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	105.7	54.9
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	2,267.1	2,415.7
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	91.8	91.3
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	430.2	484.9
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	18.4	26.2
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	890.6	946.5
Short-term investments | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	72.2	89.9
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	1,615.9	1,649.3
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	155.3	161.9
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	77.8	32.2
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	968.5	1,046.3
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	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	219.3	202.5
Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	7.2	9.9
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	187.8	195.5
Real estate funds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	86.2
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	0
Loan notes issued by variable interest entities, at fair value	0	0
Total	1,816.4	2,338.3
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	23.3	73
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	0	491
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 at 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	1,521.2	1,500.6
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	1,229.6	1,159.4
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	291.6	341.2
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 | 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	69.4	87.3
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	202.5	186.4
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	155.3	161.9
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	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	47.2	24.5
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 1 | Real estate funds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0
Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Catastrophe bonds trading, at fair value	35.5	32.4
Loan notes issued by variable interest entities, at fair value	0	0
Total	4,979.4	5,233.7
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	4.1	0
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	14.1	6.4
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 at fair value	(49.3)	(1)
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	3,558.8	3,730.4
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	46.6	52.1
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	105.7	54.9
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	2,267.1	2,415.7
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	91.8	91.3
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	138.6	143.7
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	18.4	26.2
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	890.6	946.5
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	2.8	2.6
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	1,413.4	1,462.9
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | U.S. government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0	0
Level 2 | Held for trading financial assets, at fair value | Fair Value, Measurements, Recurring | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	77.8	32.2
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	968.5	1,046.3
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	0	1
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	172.1	178
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	7.2	9.9
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	187.8	195.5
Level 2 | Real estate funds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0
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	0
Loan notes issued by variable interest entities, at fair value	(20.3)	(44.2)
Total	(25.3)	(86.6)
Level 3 | Fair Value, Measurements, Recurring | Short term investments, trading at fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, 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 at 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 | 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
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 | 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	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
Level 3 | Real estate funds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, at fair value	0
Accrued expenses and other payables | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued by variable interest entities, at fair value	(5)	(42.4)
Accrued expenses and other payables | Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued by variable interest entities, at fair value	0	0
Accrued expenses and other payables | Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued by variable interest entities, at fair value	0	0
Accrued expenses and other payables | Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes issued by variable interest entities, at fair value	$ (5)	$ (42.4)

Fair Value Measurements - Recon

Fair Value Measurements - Reconciliation of Liabilities Using Level 3 Inputs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Accrued expenses (current liabilities)	$ 6.3	$ 5.8	$ 6.3	$ 5.8
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Face Amount, Issued By Third Parties	20.3		20.3		$ 44.2
Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Face Amount, Issued By Third Parties	20.3		20.3		$ 44.2
Level 3 | Fair Value, Measurements, Recurring | 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		(2.9)		(114.1)
Total change in fair value included in the statement of operations		3.3		6.2
Ending balance		115.5		115.5
Level 3 | Fair Value, Measurements, Recurring | Silverton 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	39.8		86.6
Distributed to third party	(17.9)		(63.7)
Total change in fair value included in the statement of operations	3.4		2.4
Ending balance	25.3		25.3
Third party
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Accrued expenses (current liabilities)	$ 5	$ 4.7	$ 5	$ 4.7

Fair Value Measurements - Prici

Fair Value Measurements - Pricing Sources Used in Pricing Fixed Income Investments (Details)	Jun. 30, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Index providers	84.00%	84.00%
Pricing services	14.00%	11.00%
Broker-dealers	2.00%	5.00%
Total	100.00%	100.00%

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 5,604.5	$ 6,254.6
% of Total Fair Value by Security Type	84.00%	85.00%
Fixed Income Maturities
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 5,604.5	$ 5,763.6
% of Total Fair Value by Security Type	84.00%	84.00%
Fixed Income Maturities | U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 1,384.9	$ 1,321.3
% 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	$ 44.8	$ 43.4
% of Total Fair Value by Security Type	96.00%	83.00%
Fixed Income Maturities | Municipal
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 133.6	$ 37.4
% of Total Fair Value by Security Type	73.00%	43.00%
Fixed Income Maturities | Corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 3,075.1	$ 3,299.6
% of Total Fair Value by Security Type	95.00%	83.00%
Fixed Income Maturities | Non-U.S. government-backed corporate
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 40.4	$ 44
% of Total Fair Value by Security Type	44.00%	48.00%
Fixed Income Maturities | Non-U.S. government
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 342.9	$ 399.4
% of Total Fair Value by Security Type	53.00%	58.00%
Fixed Income Maturities | Asset-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 12.5	$ 13.5
% of Total Fair Value by Security Type	49.00%	37.00%
Fixed Income Maturities | Agency mortgage-backed
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 570.3	$ 605
% of Total Fair Value by Security Type	53.00%	53.00%
Equity securities — Trading
Fair Value [Line Items]
Fair Market Value Determined using Prices from Index Providers	$ 0	$ 491
% of Total Fair Value by Security Type	0.00%	100.00%

Fair Value Measurements - Loan

Fair Value Measurements - Loan Notes Issued by Variable Interest Entities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums	$ 853.8	$ 822.1	$ 1,970.6	$ 1,820.1
Maximum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums			61.1		$ 61.1
Reserve for losses			$ 61.9		$ 61.9
Contract period			365 days		365 days
Loan notes held by third parties	325		$ 325		$ 325
Minimum | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross written premiums			50.1		50.1
Reserve for losses			4.2		4.2
Loan notes held by third parties	325		325		325
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	6,770.5		6,770.5		7,485.4
Loan notes held by third parties	20.3		20.3		44.2
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	(25.3)		(25.3)		(86.6)
Loan notes held by third parties	20.3		20.3		44.2
Fair Value, Measurements, Recurring | Level 3 | Silverton 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total					86.6
Amount of Level 3 liabilities settled		2.9		114.1
Total change in fair value included in the statement of operations		$ 3.3		$ 6.2
Fair Value, Measurements, Recurring | Level 3 | Silverton 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	25.3		25.3
Amount of Level 3 liabilities settled	17.9		63.7
Total change in fair value included in the statement of operations	3.4		2.4
Accrued expenses and other payables | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes held by third parties	5		5		42.4
Accrued expenses and other payables | Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan notes held by third parties	$ 5		$ 5		$ 42.4

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) $ in Millions	3 Months Ended					6 Months Ended		12 Months Ended
	Jun. 30, 2018USD ($)quote / Investment	Jun. 30, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Jun. 30, 2018USD ($)quote / Investment	Jun. 30, 2017USD ($)	Dec. 31, 2017USD ($)quote / Investment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Quotes per fixed income investment | quote / Investment	2.1					2.1		2
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
Fair Value, Measurements, Recurring | Level 3 | Silverton
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repayments of debt								$ 115.6
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		$ 2.9					$ 114.1
Fair Value, Measurements, Recurring | Level 3 | Silverton 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of Level 3 liabilities settled	$ 17.9					$ 63.7

Reinsurance - Narrative (Detail

Reinsurance - Narrative (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Plus reinsurance recoverable on unpaid losses at period end	$ 1,656.4	$ 1,515.2	$ 779.4	$ 560.7
Reinsurance recoverables, uncollateralized	$ 1,126.5	$ 1,001.9
Munich Re
Concentration Risk [Line Items]
Concentration Risk, Percentage	16.70%	17.00%
Lloyd's
Concentration Risk [Line Items]
Concentration Risk, Percentage	12.10%	13.80%
Aioi Nissay Dowa
Concentration Risk [Line Items]
Concentration Risk, Percentage	8.60%	7.60%

Derivative Contracts - Fair Val

Derivative Contracts - Fair Value of Derivative Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivatives at fair value	$ 14.1	$ 6.4
Foreign Exchange Contracts | Not Designated as Hedging Instruments
Derivatives, Fair Value [Line Items]
Derivative notional amount	1,153.3	751.6
Cash collateral provided	2.3	0.6
Foreign Exchange Contracts | Not Designated as Hedging Instruments | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivative notional amount	340.9	577.7
Foreign Exchange Contracts | Not Designated as Hedging Instruments | Liabilities under Derivative Contracts
Derivatives, Fair Value [Line Items]
Liability notional amount	812.4	173.9
Foreign Exchange Contracts | Designated as Hedging Instrument | Derivatives at Fair Value
Derivatives, Fair Value [Line Items]
Derivative notional amount	114.3	60.6
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	14.2	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	(49.3)	(1)
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	$ (0.1)	$ 1.4

Derivative Contracts - Gain_(Lo

Derivative Contracts - Gain/(Loss) Recognized in Income on Derivative (Details) - Foreign Exchange Contracts - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
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	$ (46.1)	$ 17.6	$ (22.6)	$ 20.7
Designated as Hedging Instrument | General and Administrative Expense
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contract settlement	1.3	0.9	3	1.2
Designated as Hedging Instrument | Net change from current period hedged transactions
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contracts, net change	$ (2.6)	$ 2.4	$ (1.7)	$ 3.7

Derivative Contracts - Narrativ

Derivative Contracts - Narrative (Details) - Foreign Exchange Contracts - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	$ 1,153.3		$ 1,153.3		$ 751.6
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	(46.1)	$ 17.6	(22.6)	$ 20.7
Designated as Hedging Instrument | General and Administrative Expense
Derivative Instruments, Gain (Loss) [Line Items]
Foreign exchange contract settlement	1.3	0.9	3	1.2
Designated as Hedging Instrument | Net change from current period hedged transactions
Derivative Instruments, Gain (Loss) [Line Items]
Net unrealized gain (loss)	(2.6)	$ 2.4	(1.7)	$ 3.7
Derivatives at Fair Value | Not Designated as Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	340.9		340.9		577.7
Derivatives at Fair Value | Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	$ 114.3		$ 114.3		$ 60.6

Deferred Policy Acquisition C63

Deferred Policy Acquisition Costs - Reconciliation of Beginning and Ending Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance at the beginning of the period	$ 319.9	$ 367.5	$ 294.3	$ 358.4
Acquisition costs deferred	64.6	93.4	181	216.2
Amortization of deferred policy acquisition costs	(85.9)	(96.3)	(176.7)	(210)
Balance at the end of the period	$ 298.6	$ 364.6	$ 298.6	$ 364.6

Reserves for Losses and Loss 64

Reserves for Losses and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses ("LAE") Reserves (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Provision for losses and LAE at the start of the year	$ 6,749.5	$ 5,319.9	$ 5,319.9
Net loss and LAE at the start of the year	5,234.3	4,759.2	4,759.2
Net loss and LAE expenses (disposed)	0	0	(125.5)
Provision for losses and LAE for claims incurred:
Current year	700.8	749.2	2,100.1
Prior years	(80.2)	(74.9)	(105.4)
Total incurred	620.6	674.3	1,994.7
Losses and LAE payments for claims incurred:
Current year	(49.7)	(54.4)	(397.5)
Prior years	(874.7)	(686.7)	(1,157.6)
Total paid	(924.4)	(741.1)	(1,555.1)
Foreign exchange losses/(gains)	(54.1)	99.6	161
Net losses and LAE reserves at period end	4,876.4	4,792	5,234.3
Plus reinsurance recoverable on unpaid losses at period end	1,656.4	779.4	1,515.2	$ 560.7
Provision for losses and LAE at the end of the relevant period	$ 6,532.8	$ 5,571.4	$ 6,749.5

Reserves for Losses and Loss 65

Reserves for Losses and Loss Adjustment Expenses - Short-duration Insurance Contracts, Claims Development by Accident Year (Details) $ in Millions	Jun. 30, 2018USD ($)Claim	Dec. 31, 2017USD ($)	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	$ 4,830.5
Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	4,884.2
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	2,885.9
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	1,998.3
All outstanding liabilities before 2012, net of reinsurance	114
Liability for claims and claims adjustment expenses, net of reinsurance	2,112.3
Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	4,521.2
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	2,422.9
All outstanding liabilities for 2012 and subsequent years, net of reinsurance	2,098.3
All outstanding liabilities before 2012, net of reinsurance	619.9
Liability for claims and claims adjustment expenses, net of reinsurance	2,718.2
Accident Year 2012 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	647.2	$ 638.2	$ 654.7	$ 669.9	$ 655.9	$ 626.8	$ 605
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 45.9
Number of Reported Claims | Claim	15,682
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 531.7	524.7	502.8	452.9	378.5	307.9	117.3
Accident Year 2012 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	686.7	687.9	689.4	690.6	720.9	737.3	692.8
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 71.2
Number of Reported Claims | Claim	3,838
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 511.4	496.5	458.7	414.8	360.9	248.2	$ 63.2
Accident Year 2013 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	651.1	650.6	660.4	679	667.9	689.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 32.4
Number of Reported Claims | Claim	14,631
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 532.7	502.8	455	364.9	254.5	90.7
Accident Year 2013 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	499.7	504.9	524.3	550.2	572.2	579.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 66.2
Number of Reported Claims | Claim	3,689
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 371.6	359.9	320.1	278.4	186.8	$ 63.2
Accident Year 2014 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	699.5	695.1	702.4	730.1	755.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 68
Number of Reported Claims | Claim	18,836
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 481	465.9	389.8	247.5	99.5
Accident Year 2014 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	477.5	487.1	514.2	529.9	550.6
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 82.8
Number of Reported Claims | Claim	3,696
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 308.1	289.2	248.5	172.6	$ 57
Accident Year 2015 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	858.1	855.1	905.9	916
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 154.4
Number of Reported Claims | Claim	21,132
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 523.4	471.9	326.9	119.2
Accident Year 2015 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	536.9	558.2	564.5	580.3
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 117.6
Number of Reported Claims | Claim	3,899
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 306.2	272.1	171.4	$ 57.5
Accident Year 2016 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	873.6	881.6	909.3
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 284.6
Number of Reported Claims | Claim	21,052
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 415.4	346.1	117.3
Accident Year 2016 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	768.9	762.8	749.1
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 218.1
Number of Reported Claims | Claim	3,709
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 400	350.7	$ 125.5
Accident Year 2017 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	847.7	896.1
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 175.9
Number of Reported Claims | Claim	20,622
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 369.5	167.8
Accident Year 2017 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	1,189	1,190.2
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 420.5
Number of Reported Claims | Claim	3,746
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 508.1	$ 227.9
Accident Year 2018 | Insurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	307
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 192.9
Number of Reported Claims | Claim	6,929
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 32.2
Accident Year 2018 | Reinsurance
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expense, Net	362.5
Incurred but Not Reported (IBNR) Claims Liability, Net	$ 289.9
Number of Reported Claims | Claim	1,415
Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 17.5

Reserves for Losses and Loss 66

Reserves for Losses and Loss Adjustment Expenses - Short-duration Insurance Contracts Reconciliation Of Claims Development To Liability (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claims adjustment expenses, net of reinsurance	$ 4,830.5
Reinsurance recoverable on unpaid losses	1,656.4
Unallocated claims incurred	49.7
Other	(3.8)
Liability for unpaid claims and claim adjustment expense, aggregate reconciling items	45.9
Losses and loss adjustment expenses	6,532.8	$ 6,749.5	$ 5,571.4	$ 5,319.9
Insurance lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claims adjustment expenses, net of reinsurance	2,112.3
Reinsurance recoverable on unpaid losses	1,382.8
Reinsurance lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claims adjustment expenses, net of reinsurance	2,718.2
Reinsurance recoverable on unpaid losses	273.6
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 67

Reserves for Losses and Loss Adjustment Expenses - Narrative (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Insurance [Abstract]
Reserve releases	$ (80.2)	$ (74.9)	$ (105.4)
Amount of reserves ceded			$ 125.5

Capital Structure - Summary of

Capital Structure - Summary of Authorized and Issued Share Capital (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	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:
Ordinary shares, issued (in shares)			59,687,717	59,474,085
Issued ordinary shares of 0.15144558¢ per share			$ 90	$ 90
Total issued share capital			$ 122	$ 122
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
Stock Repurchased and Retired During Period, Shares	0	197,673
Stock Repurchased and Retired During Period, Value	$ 10,000	$ 10,000
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
Preferred Stock Dividends Rate Percentage			5.625%	5.625%

Capital Structure - Summary o69

Capital Structure - Summary of Ordinary Shares (Details)	6 Months Ended
	Jun. 30, 2018shares
Ordinary Shares [Roll Forward]
Ordinary shares in issue as at December 31, 2017	59,474,085
Ordinary shares issued to employees under the 2013 share incentive plan and/or 2008 share purchase plan	208,954
Ordinary shares issued to non-employee directors	4,678
Ordinary shares in issue as at June 30, 2018	59,687,717

Capital Structure - Narrative (

Capital Structure - Narrative (Details) - USD ($)	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Feb. 08, 2017
Class of Stock [Line Items]
Additional paid-in capital		$ 965,400,000		$ 954,700,000
Aggregate liquidation preferences		525,000,000		525,000,000
Issuance costs		$ 13,100,000		$ 13,100,000
Stock Repurchased and Retired During Period, Shares	0		197,673
Stock Repurchased and Retired During Period, Value	$ 10,000,000		$ 10,000,000
Repurchase Program 2017
Class of Stock [Line Items]
Existing share repurchase authorization amount					$ 250,000,000

Share-Based Payments - Share-ba

Share-Based Payments - Share-based Payments - Non-Employee Directors RSUs (Details) - Restricted Share Units - shares	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
RSUs granted, number of shares (in shares)	230,766	174,488.707095856
Non-Employee Directors
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
RSUs granted, number of shares (in shares)	29,025	22,230
Board of Directors Chairman
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
RSUs granted, number of shares (in shares)	12,900	8,892

Share-Based Payments - Narrativ

Share-Based Payments - Narrative (Details)	6 Months Ended
	Jun. 30, 2018USD ($)Agreementshares	Jun. 30, 2018GBP (£)Agreementshares	Jun. 30, 2017USD ($)shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of arrangements under options and other equity incentives | Agreement	3	3
Vesting period	3 years	3 years
Award vesting rights, percentage	100.00%	100.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights, percentage	200.00%	200.00%
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, number of shares (in shares) | shares	215,273	215,273	206,076
Compensation costs	$ 2,700,000		$ 1,200,000
Tax benefit (expense) from compensation expense	$ 600,000		$ 300,000
Award vesting rights, percentage	33.33%	33.33%
Phantom Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, number of shares (in shares) | shares	150,185	150,185	173,619
Compensation costs	$ 1,600,000		$ 2,100,000
Tax benefit (expense) from compensation expense	$ 600,000		$ 100,000
Restricted Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years	3 years
Granted, number of shares (in shares) | shares	230,766	230,766	174,488.707095856
Compensation costs	$ 4,600,000		$ 5,000,000
Tax benefit (expense) from compensation expense	1,100,000		1,200,000
Tax charge, non-employee directors	$ 0		$ 0
2013 Share Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available to grant under incentive plan (in shares) | shares	2,845,683	2,845,683
2016 Share Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available to grant under incentive plan (in shares) | shares	263,695	263,695
Sharesave Scheme
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee contribution period	3 years	3 years
Purchase price of fair market value	85.00%	85.00%
Employee savings under ESPP | £		£ 500
International Employee Share Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee contribution period	2 years	2 years
Employee savings under ESPP	$ 500
Shares issued (in shares) | shares	31,684	31,684	9,979
Employee Share Purchase Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Tax benefit (expense) from compensation expense	$ 0		$ 0
Compensation costs charged in ESPP	$ 100,000		$ 200,000
Non-Employee Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights, percentage	8.33%	8.33%
Non-Employee Directors | Restricted Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, number of shares (in shares) | shares	29,025	29,025	22,230
Compensation costs	$ 800,000		$ 700,000

Intangible Assets and Goodwil73

Intangible Assets and Goodwill - Summary of Changes in Intangible Assets (Details) - USD ($) $ in Millions	Jan. 19, 2016	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Amortization		$ (0.5)	$ (1.1)	$ (0.9)	$ (2)
Goodwill [Roll Forward]
Goodwill, Beginning of the period		3.9	26	3.9	24.2
Goodwill, Additions		0	0	0	1.8
Goodwill, End of the period		3.9	26	3.9	26
Intangible assets and goodwill, Beginning of the period		27.5	80.5	27.9	79.6
Intangible Assets and Goodwill, Acquired		0	0	0	1.8
Intangible assets and goodwill, End of the period		27	79.4	27	79.4
Trademark
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		2.8	6.4	2.9	6.6
Intangible Assets, Additions		0	0	0	0
Amortization		(0.1)	(0.1)	(0.2)	(0.3)
Intangible Assets, End of the period	$ 4	2.7	6.3	2.7	6.3
Insurance Licenses
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		16.7	16.7	16.7	16.7
Intangible Assets, Additions		0	0	0	0
Amortization		0	0	0	0
Intangible Assets, End of the period		16.7	16.7	16.7	16.7
Agency Relationships
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		2.2	25.8	2.3	26.2
Intangible Assets, Additions		0	0	0	0
Amortization		(0.2)	(0.7)	(0.3)	(1.1)
Intangible Assets, End of the period	25	2	25.1	2	25.1
Non-compete Agreements
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		0.6	3.1	0.7	3.3
Intangible Assets, Additions	2.9	0	0	0	0
Amortization		(0.1)	(0.1)	(0.2)	(0.3)
Intangible Assets, End of the period		0.5	3	0.5	3
Consulting Relationships
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		0	0.9	0	0.9
Intangible Assets, Additions		0	0	0	0
Amortization		0	(0.1)	0	(0.1)
Intangible Assets, End of the period	$ 1	0	0.8	0	0.8
Contractual Rights
Finite and Indefinite Lived Intangible Assets [Roll Forward]
Intangible Assets, Beginning of the period		1.3	1.6	1.4	1.7
Intangible Assets, Additions		0	0	0	0
Amortization		(0.1)	(0.1)	(0.2)	(0.2)
Intangible Assets, End of the period		$ 1.2	$ 1.5	$ 1.2	$ 1.5

Intangible Assets and Goodwil74

Intangible Assets and Goodwill - Narrative (Details) - USD ($) $ in Thousands	Dec. 18, 2017	Jan. 01, 2017	Oct. 31, 2016	Sep. 22, 2016	Jan. 19, 2016	Apr. 05, 2005	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2016
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	$ 0	$ 1,800
Impairment charge											$ 3,400
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		2,000	25,100	2,000	25,100	2,300	$ 2,200	$ 25,800	$ 26,200
Intangible Assets, Additions							0	0	$ 0	0
Asset amortization period									15 years
Trademark
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset					4,000		2,700	6,300	$ 2,700	6,300	2,900	2,800	6,400	6,600
Intangible Assets, Additions							0	0	$ 0	0
Asset amortization period									10 years
Trademark | 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	16,700	$ 16,700	16,700	16,700	16,700	16,700	16,700
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							1,200	1,500	1,200	1,500	1,400	1,300	1,600	1,700
Intangible Assets, Additions							0	0	0	0
Consulting Relationships
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Value of the asset					1,000		0	800	0	800	0	0	900	900
Intangible Assets, Additions							0	0	$ 0	0
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							500	3,000	$ 500	3,000	700	$ 600	$ 3,100	$ 3,300
Intangible Assets, Additions					2,900		0	$ 0	$ 0	$ 0
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 | AgriLogic
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Intangible Assets, Additions					$ 1,800
Crop Re
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Equity method investment, ownership percentage	23.20%
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
LSML | Contractual Rights
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Asset amortization period				5 years
Aspen U.K.
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Aggregate consideration for sale and purchase agreement						$ 1,600
Aspen U.K. | Trademark
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
Disposed of by Sale | AgriLogic
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Sale of goodwill	$ (20,600)
Disposed of by Sale | AgriLogic | Trademark
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Intangible assets (disposed)	(3,100)
Disposed of by Sale | AgriLogic | Customer Lists
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Intangible assets (disposed)	(21,800)
Disposed of by Sale | AgriLogic | Non-compete Agreements
Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]
Intangible assets (disposed)	$ (900)

Commitments and Contingent Li75

Commitments and Contingent Liabilities - Schedule of Company's Restricted Assets (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Restricted assets	$ 4,525.9	$ 4,638.8
Total as percent of investable assets(2)	56.00%	53.40%
Securities and cash as collateral secured letters of credit	$ 840.3	$ 658.5
Letters of credit / guarantees
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Restricted assets	840.3	658.5
Affiliated transactions
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Restricted assets	1,154.5	1,455
Third party
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Restricted assets	2,431.1	2,425.3
Limited Partner | Real estate funds
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Restricted assets	13.8	100
Purchase Commitment | Limited Partner | Real estate funds
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Restricted assets	$ 86.2	$ 0

Commitments and Contingent Li76

Commitments and Contingent Liabilities - Narrative (Details) £ in Millions	Mar. 27, 2017USD ($)	Jun. 30, 2016	Jun. 30, 2018USD ($)	Jun. 30, 2018GBP (£)	May 10, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2017GBP (£)	Jun. 30, 2017USD ($)
Line of Credit Facility [Line Items]
Restricted assets			$ 4,525,900,000			$ 4,638,800,000
Other investments			86,200,000			0
Assets held by insurance regulators			494,200,000			458,700,000
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%
Letter of Credit
Line of Credit Facility [Line Items]
Restricted assets			$ 840,300,000			658,500,000
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%
Limited Partner | Real estate funds
Line of Credit Facility [Line Items]
Restricted assets			$ 13,800,000			$ 100,000,000
Other investments					$ 86,200,000

Commitments and Contingent Li77

Commitments and Contingent Liabilities - Amounts Outstanding under Operating Leases (Details) $ in Millions	Jun. 30, 2018USD ($)
Leases [Abstract]
2,017	$ 11
2,018	15.9
2,019	15.4
2,020	10.9
2,021	8.9
Later Years	74.6
Total	$ 136.7