Aspen Insurance Holdings Limited (AHL-PE) 10-Q 7 Nov 14 2014 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2014	Oct. 31, 2014
Document Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q3	'
Trading Symbol	'AHL	'
Entity Registrant Name	'ASPEN INSURANCE HOLDINGS LTD	'
Entity Central Index Key	'0001267395	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Large Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	62,232,928

CONDENSED_CONSOLIDATED_BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Investments:	'	'
Fixed income securities, available for sale at fair value (amortized cost — $5,353.6 and $5,449.9)	$5,497.80	$5,569.10
Fixed income securities, trading at fair value (amortized cost — $761.5 and $712.1)	774.1	716.2
Equity securities, available for sale at fair value (cost — $93.9 and $112.2)	125.9	149.5
Equity securities, trading at fair value (cost — $490.2 and $281.6)	521.1	310.9
Short-term investments, available for sale at fair value (amortized cost — $307.2 and $160.3)	307.2	160.3
Short-term investments, trading at fair value (amortized cost — $7.0 and $Nil)	7	0
Catastrophe bonds, trading at fair value (cost — $31.6 and $5.8)	32.2	5.8
Other investments, equity method	8.7	48
Total investments	7,274	6,959.80
Cash and cash equivalents (including $81.1 and $50.0 within consolidated variable interest entities)	1,289.10	1,293.60
Reinsurance recoverables	'	'
Unpaid losses	384.7	332.7
Ceded unearned premiums	229.8	151.9
Receivables	'	'
Underwriting premiums	1,105.30	999
Other	92	90.3
Funds withheld	47.5	46.5
Deferred policy acquisition costs	301.6	262.2
Derivatives at fair value	7	7
Receivable for securities sold	1.6	5.2
Office properties and equipment	64.4	60.1
Deferred taxation	4	1.6
Other assets	10.4	2.2
Intangible assets	18.2	18.4
Total assets	10,829.60	10,230.50
Insurance reserves	'	'
Losses and loss adjustment expenses	4,787.30	4,678.90
Unearned premiums	1,508.70	1,280.60
Total insurance reserves	6,296	5,959.50
Payables	'	'
Reinsurance premiums	146.8	88.2
Current taxation	25.7	15.7
Accrued expenses and other payables	290.3	265.6
Liabilities under derivative contracts	12.7	2.9
Total payables	475.5	372.4
Loan notes issued by variable interest entities, at fair value	64.5	50
Long-term debt	549.1	549
Total liabilities	7,385.10	6,930.90
Commitments and contingent liabilities (see Note 16)	0	0
SHAREHOLDERS’ EQUITY	'	'
63,349,858 shares of par value 0.15144558¢ each (December 31, 2013 — 65,546,976)	0.1	0.1
Preference shares:	'	'
Non-controlling interest	-0.3	-0.3
Additional paid-in capital	1,186.40	1,297.40
Retained earnings	2,005.60	1,783.30
Accumulated other comprehensive income, net of taxes	252.7	219.1
Total shareholders’ equity	3,444.50	3,299.60
Total liabilities and shareholders’ equity	10,829.60	10,230.50
5.950% Preference Shares	'	'
Preference shares:	'	'
Preference shares, value	0	0
7.401% Preference Shares	'	'
Preference shares:	'	'
Preference shares, value	0	0
7.250% preference shares	'	'
Preference shares:	'	'
Preference shares, value	$0	$0

CONDENSED_CONSOLIDATED_BALANCE1

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Fixed income securities, available for sale at fair value (amortized cost — $5,353.6 and $5,449.9)	$5,353,600,000	$5,449,900,000
Fixed income securities, trading at fair value (amortized cost — $761.5 and $712.1)	761,500,000	712,100,000
Equity securities, available for sale at fair value (cost — $93.9 and $112.2)	93,900,000	112,200,000
Equity securities, trading at fair value (cost — $490.2 and $281.6)	490,200,000	281,600,000
Short-term investments, available for sale at fair value (amortized cost — $307.2 and $160.3)	307,200,000	160,300,000
Short-term investments, trading at fair value (amortized cost — $7.0 and $Nil)	7,000,000	0
Catastrophe bonds, trading at fair value (cost — $31.6 and $5.8)	31,600,000	5,800,000
Cash and cash equivalents (including $81.1 and $50.0 within consolidated variable interest entities)	1,289,100,000	1,293,600,000
Ordinary shares, par value (in dollars per share)	$0.15	$0.15
Ordinary shares, issued (in shares)	63,349,858	65,546,976
Preference shares, par value (in dollars per share)	$0.15	$0.15
5.950% Preference Shares	'	'
Preference shares, issued (in shares)	11,000,000	11,000,000
Preference shares, rate	5.95%	5.95%
Preference shares, par value (in dollars per share)	$0.15	$0.15
7.401% Preference Shares	'	'
Preference shares, issued (in shares)	5,327,500	5,327,500
Preference shares, rate	7.40%	7.40%
Preference shares, par value (in dollars per share)	$0.15	$0.15
7.250% preference shares	'	'
Preference shares, issued (in shares)	6,400,000	6,400,000
Preference shares, rate	7.25%	7.25%
Preference shares, par value (in dollars per share)	$0.15	$0.15
Variable Interest Entity, Primary Beneficiary	'	'
Cash and cash equivalents (including $81.1 and $50.0 within consolidated variable interest entities)	$81,100,000	$50,000,000

CONDENSED_CONSOLIDATED_STATEME

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND OTHER COMPREHENSIVE INCOME (USD $)	3 Months Ended		9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Revenues	'	'	'	'
Net earned premium	$610.40	$544.30	$1,793.10	$1,599.20
Net investment income	48	45	143.6	139.2
Realized and unrealized investment gains	1.1	23.6	53.6	54.3
Other income	1	1.6	4.8	3.6
Total revenues	660.5	614.5	1,995.10	1,796.30
Expenses	'	'	'	'
Losses and loss adjustment expenses	342.7	290.2	967.9	892.3
Amortization of deferred policy acquisition costs	115.5	110.5	336.4	322.3
General, administrative and corporate expenses	119.8	98.9	324.2	273.2
Interest on long-term debt	7.4	7.7	22.1	23.2
Change in fair value of derivatives	5.1	-6.6	8.6	0.5
Change in fair value of loan notes issued by variable interest entities	8.5	0	14.5	0
Realized and unrealized investment losses	1.3	5.9	8.9	28
Net realized and unrealized foreign exchange (gains)/losses	21.2	-2.4	10.4	7.1
Other expenses	0.3	0	2.2	0.6
Total expenses	621.8	504.2	1,695.20	1,547.20
Income from operations before income tax	38.7	110.3	299.9	249.1
Income tax expense	-1.3	-2.9	-11.3	-9.8
Net income	37.4	107.4	288.6	239.3
Add: loss attributable to non-controlling interest	0.1	0.3	0	0.3
Net income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	37.5	107.7	288.6	239.6
Available for sale investments:	'	'	'	'
Reclassification adjustment for net realized (gains) on investments included in net income	-3.6	-8.3	-5.9	-20.7
Change in net unrealized gains/(losses) on available for sale securities held	-29.4	4.8	28.6	-138.6
Amortization of loss on derivative contract	0	0	0	0.2
Change in foreign currency translation adjustment	-3.3	3.4	14.7	-21
Other comprehensive income/(loss), gross of tax	-36.3	-0.1	37.4	-180.1
Tax thereon:	'	'	'	'
Reclassification adjustment for net realized gains on investments included in net income	0.1	0.1	0.2	0.5
Change in net unrealized (losses)/gains on available for sale securities held	2	-0.4	-4	11.3
Total tax on other comprehensive income/(loss)	2.1	-0.3	-3.8	11.8
Other comprehensive income/(loss), net of tax	-34.2	-0.4	33.6	-168.3
Total comprehensive income attributable to Aspen Insurance Holdings Limited’s ordinary shareholders	$3.30	$107.30	$322.20	$71.30
Weighted average number of ordinary share and share equivalents	'	'	'	'
Basic (in shares)	65,116,463	66,716,202	65,283,681	67,302,857
Diluted (in shares)	66,513,009	68,561,515	66,598,680	69,959,474
Basic earnings per ordinary share adjusted for preference share dividends	$0.43	$1.47	$3.99	$3.06
Diluted earnings per ordinary share adjusted for preference share dividends	$0.42	$1.43	$3.91	$2.95

CONDENSED_CONSOLIDATED_STATEME1

CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (USD $)

Total

Ordinary shares

Preference shares

Non-controlling interest

Additional paid-in capital

Additional paid-in capital

Additional paid-in capital

Retained earnings

Retained earnings

Retained earnings

Cumulative foreign currency translation adjustments, net of taxes:

Loss on derivatives, net of taxes:

Unrealized appreciation on investments, net of taxes:

In Millions, unless otherwise specified

Ordinary shares

Preference shares

Ordinary shares

Preference shares

Shareholder's equity at Dec. 31, 2012

'

$0.10

$0

$0.20

$1,516.70

'

'

$1,544

'

'

$112.70

($0.50)

$315.20

Increase (Decrease) in Stockholders' Equity [Roll Forward]

'

'

'

'

'

'

'

'

'

'

'

'

'

Net change attributable to non-controlling interest for the period

0.3

'

'

-0.3

'

'

'

0.3

'

'

'

'

'

Shares issued

'

'

'

'

'

16.7

270.4

'

'

'

'

'

'

Ordinary shares repurchased and canceled

'

'

'

'

'

-294.9

'

'

'

'

'

'

'

PIERS redeemed and canceled

'

'

-230

'

'

'

-230

'

'

'

'

'

'

PIERS redemption

-7.1

[1]

'

'

'

'

'

7.1

[2]

'

'

-7.1

[2]

'

'

'

Share-based compensation

'

'

'

'

11.6

'

'

'

'

'

'

'

'

Net income for the period

239.3

'

'

'

'

'

'

239.3

'

'

'

'

'

Dividends on ordinary shares

'

'

'

'

'

'

'

'

-36

'

'

'

'

Dividends on preference shares

-26.1

'

'

'

'

'

'

'

'

-26.1

'

'

'

Change for the period, net of income tax

'

'

'

'

'

'

'

'

'

'

-21

'

-147.5

Reclassification to interest on long-term debt

'

'

'

'

'

'

'

'

'

'

'

0.2

'

Total accumulated other comprehensive income, net of taxes

259.1

'

'

'

'

'

'

'

'

'

'

'

'

Shareholder's equity at Sep. 30, 2013

3,271.10

0.1

0

-0.1

1,297.60

'

'

1,714.40

'

'

91.7

-0.3

167.7

Shareholder's equity at Jun. 30, 2013

'

'

'

'

'

'

'

'

'

'

'

'

'

Increase (Decrease) in Stockholders' Equity [Roll Forward]

'

'

'

'

'

'

'

'

'

'

'

'

'

Net change attributable to non-controlling interest for the period

0.3

'

'

'

'

'

'

'

'

'

'

'

'

PIERS redeemed and canceled

'

'

-230

'

'

'

'

'

'

'

'

'

'

PIERS redemption

[1]

0

'

'

'

'

'

'

'

'

'

'

'

'

Net income for the period

107.4

'

'

'

'

'

'

'

'

'

'

'

'

Dividends on preference shares

-9.5

'

'

'

'

'

'

'

'

'

'

'

'

Total accumulated other comprehensive income, net of taxes

259.1

'

'

'

'

'

'

'

'

'

'

'

'

Shareholder's equity at Sep. 30, 2013

3,271.10

0.1

0

'

'

'

'

'

'

'

'

'

'

Shareholder's equity at Dec. 31, 2013

3,299.60

0.1

0

-0.3

1,297.40

'

'

1,783.30

'

'

88.6

0

130.5

Increase (Decrease) in Stockholders' Equity [Roll Forward]

'

'

'

'

'

'

'

'

'

'

'

'

'

Net change attributable to non-controlling interest for the period

0

'

'

0

'

'

'

0

'

'

'

'

'

Shares issued

'

'

'

'

'

1.9

0

'

'

'

'

'

'

Ordinary shares repurchased and canceled

'

'

'

'

'

-120.9

'

'

'

'

'

'

'

PIERS redeemed and canceled

'

'

-230

'

'

'

0

'

'

'

'

'

'

PIERS redemption

0

[1]

'

'

'

'

'

0

[2]

'

'

0

[2]

'

'

'

Share-based compensation

'

'

'

'

8

'

'

'

'

'

'

'

'

Net income for the period

288.6

'

'

'

'

'

'

288.6

'

'

'

'

'

Dividends on ordinary shares

'

'

'

'

'

'

'

'

-37.9

'

'

'

'

Dividends on preference shares

-28.4

'

'

'

'

'

'

'

'

-28.4

'

'

'

Change for the period, net of income tax

'

'

'

'

'

'

'

'

'

'

14.7

'

18.9

Reclassification to interest on long-term debt

'

'

'

'

'

'

'

'

'

'

'

0

'

Total accumulated other comprehensive income, net of taxes

252.7

'

'

'

'

'

'

'

'

'

'

'

'

Shareholder's equity at Sep. 30, 2014

$3,444.50

$0.10

$0

($0.30)

$1,186.40

'

'

$2,005.60

'

'

$103.30

$0

$149.40

[1]

The $7.1 million reduction in the net income available to ordinary shareholders is the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of $230.0 million of the PIERS on May 30, 3013.

[2]

The $7.1 million reclassification from additional paid-in capital to retained earnings is the difference between the capital raised upon issuance of the 5.625% Perpetual Preferred Income Equity Replacement Securities (“PIERS”), net of the original issuance costs, and the final redemption of $230.0 million of the PIERS on May 30, 2013.

CONDENSED_CONSOLIDATED_STATEME2

CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Parenthetical) (USD $)

3 Months Ended

9 Months Ended

24 Months Ended

0 Months Ended

3 Months Ended

9 Months Ended

0 Months Ended

3 Months Ended

9 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2006

Apr. 25, 2013

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Apr. 25, 2013

Sep. 30, 2013

Sep. 30, 2013

5.625% Preference Shares

5.625% Preference Shares

Preference shares

Preference shares

Preference shares

Preference shares

Reclassification from Additional Paid in Capital to Retained Earnings

Reclassification from Additional Paid in Capital to Retained Earnings

Reclassification from Additional Paid in Capital to Retained Earnings

PIERS redemption

$0

[1]

$0

[1]

$0

[1]

$7.10

[1]

'

'

'

'

'

'

$7.10

$7.10

$7.10

Preferred Stock, Dividend Rate, Percentage

'

'

'

'

5.63%

5.63%

'

'

'

'

'

'

'

PIERS redeemed and canceled

'

'

'

'

'

'

$230

$230

$230

$230

'

'

'

[1]

The $7.1 million reduction in the net income available to ordinary shareholders is the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of $230.0 million of the PIERS on May 30, 3013.

CONDENSED_CONSOLIDATED_STATEME3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013
Cash flows from operating activities:	'	'
Net income	$288.60	$239.30
Proportion due to non-controlling interest	0	0.3
Adjustments to reconcile net income to net cash flows from operating activities:	'	'
Depreciation and amortization	23.9	27.3
Share-based compensation	8	11.6
Realized and unrealized investment (gains)	-53.6	-54.3
Realized and unrealized investment losses	8.9	28
Change in fair value of loan notes issued by variable interest entities	14.5	0
Net realized and unrealized investment foreign exchange losses	31.3	11.5
Loss on derivative contracts	0	0.2
Insurance reserves:	'	'
Losses and loss adjustment expenses	87.3	-46.3
Unearned premiums	226	212.5
Reinsurance recoverables:	'	'
Unpaid losses	-49	54.8
Ceded unearned premiums	-76.7	-57.6
Other receivables	-10.5	7.2
Deferred policy acquisition costs	-39.4	-39.1
Reinsurance premiums payable	32.8	26.3
Funds withheld	-0.9	36.6
Premiums receivable	-106.7	-39.3
Deferred taxes	-1.4	7.8
Income tax payable	10	-4.1
Accrued expenses and other payable	76.4	33.2
Fair value of derivatives and settlement of liabilities under derivatives	9.8	-8.4
Long-term debt	0.1	0.1
Intangible assets	0.1	-0.1
Other assets	-3.2	-14.5
Net cash generated by operating activities	476.3	433
Cash flows (used in) investing activities:	'	'
(Purchases) of fixed income securities — Available for sale	-1,440.90	-1,748.60
(Purchases) of fixed income securities — Trading	-553.6	-555.5
Proceeds from sales and maturities of fixed income securities — Available for sale	1,496.90	1,564.40
Proceeds from sales and maturities of fixed income securities — Trading	515.7	276.3
(Purchases) of equity securities — Available for sale	0	-2.5
(Purchases) of equity securities — Trading	-240.1	-275.4
Net (purchases) of catastrophe bonds — Trading	-26	0
Proceeds from sales of equity securities — Available for sale	27.7	61.8
Proceeds from sales of equity securities — Trading	36	15.1
Net (purchases)/sales of short-term investments — Available for sale	-151	273.5
Net (purchases)/sales of short-term investments — Trading	-7	2.2
Net change in receivable for securities sold	12.8	5.6
Net proceeds from other investments	37.3	0
Purchase/(sale) of equipment	-9.6	-10.1
Net cash (used in) investing activities	-301.8	-393.2
Cash flows (used in) financing activities:	'	'
Proceeds from the issuance of ordinary shares, net of issuance costs	1.9	16.7
Proceeds from the issuance of preference shares, net of issuance costs	0	270.4
PIERS repurchased and cancelled	0	-230
Ordinary shares repurchased	-120.9	-294.9
Dividends paid on ordinary shares	-37.9	-36
Dividends paid on preference shares	-28.4	-26.1
Net cash (used in) financing activities	-185.3	-299.9
Effect of exchange rate movements on cash and cash equivalents	6.3	-5.2
Increase/(decrease) in cash and cash equivalents	-4.5	-265.3
Cash and cash equivalents at beginning of period	1,293.60	1,463.60
Cash and cash equivalents at end of period	1,289.10	1,198.30
Supplemental disclosure of cash flow information:	'	'
Net cash paid/(received) during the period for income tax	7.2	7.3
Cash paid during the period for interest	$21.90	$22.50

History_and_Organization

History and Organization	9 Months Ended
	Sep. 30, 2014
Accounting Policies [Abstract]	'
History and Organization	'
	History and Organization
	Aspen Insurance Holdings Limited (“Aspen Holdings”) was incorporated on May 23, 2002 and holds subsidiaries that provide insurance and reinsurance on a worldwide basis. Its principal operating subsidiaries are Aspen Insurance UK Limited (“Aspen U.K.”), Aspen Bermuda Limited (“Aspen Bermuda”), Aspen Specialty Insurance Company (“Aspen Specialty”), Aspen American Insurance Company (“AAIC”) and Aspen Underwriting Limited (corporate member of Lloyd’s Syndicate 4711, “AUL”) (collectively, the “Operating Subsidiaries”). We have also established Aspen Capital Management, Ltd and other related entities (“ACM”) which are used to leverage our existing franchise and underwriting expertise to offer investors access to diversified products. In such regard, Silverton Re Ltd. (“Silverton”), a Bermuda domiciled special purpose insurer, was established in 2013 to attract third-party capital and to provide additional collateralized capacity to support Aspen Re’s global reinsurance business. References to the “Company,” “we,” “us” or “our” refer to Aspen Holdings or Aspen Holdings and its subsidiaries.

Basis_of_Preparation

Basis of Preparation	9 Months Ended
	Sep. 30, 2014
Accounting Policies [Abstract]	'
Basis of Preparation	'
	Basis of Preparation
	The accompanying unaudited condensed consolidated financial statements have been prepared on the basis of generally accepted accounting principles in the United States (“U.S. GAAP”) for interim financial information and in accordance with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by U.S. GAAP for complete financial statements. In the opinion of management, all adjustments (consisting of normal recurring accruals) considered necessary for a fair presentation have been included. Results for the three and nine months ended September 30, 2014 are not necessarily indicative of the results that may be expected for the year ended December 31, 2014. 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, 2013 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, 2013 contained in the Company’s Annual Report on Form 10-K filed with the United States Securities and Exchange Commission on February 20, 2014 (File No. 001-31909).
	Assumptions and estimates made by management have a significant effect on the amounts reported within the unaudited condensed consolidated financial statements. The most significant of these relate to losses and loss adjustment expenses, the value of investments, reinsurance recoverables and the fair value of derivatives. All material assumptions and estimates are regularly reviewed and adjustments made as necessary, but actual results could be significantly different from those expected when the assumptions or estimates were made.
	Accounting Pronouncements Not Yet Adopted
	On August 5, 2014, the FASB issued ASU 2014-13, “Measuring the Financial Assets and the Financial Liabilities of a Consolidated Collateralized Financing Entity" which addresses the measurement difference in both the initial consolidation and the subsequent measurement of the financial assets and the financial liabilities of a collateralized financing entity. ASU 2014-13 is effective for fiscal years beginning after December 15, 2015 and interim periods beginning after December 15, 2015. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	On August 8, 2014, the FASB issued ASU 2014-14, “Receivables - Troubled debt restructuring by creditors (Subtopic 310-40)" which reduces diversity in practice by addressing the classification of certain foreclosed mortgage loans held by creditors that are either fully or partially guaranteed under government programs. ASU 2014-14 is effective for fiscal years beginning after December 15, 2014 and interim periods beginning after December 15, 2014. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	On August 27, 2014, the FASB issued ASU 2014-15, “Presentation of Financial Statements - Going Concern (Subtopic 204-40)" which provides US GAAP guidance on management’s responsibility to evaluate whether there is substantial doubt about an entity’s ability to continue as a going concern or to provide related footnote disclosures. ASU 2014-15 is effective for fiscal years beginning after December 15, 2016 and interim periods beginning after December 15, 2016. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.

Reclassifications_from_Accumul

Reclassifications from Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 2014
Equity [Abstract]	'
Reclassifications from Accumulated Other Comprehensive Income	'
	Reclassifications from Accumulated Other Comprehensive Income
	The following tables set out the components of the Company’s accumulated other comprehensive income (“AOCI”) that are reclassified into the unaudited condensed consolidated statement of operations for the three and nine months ended September 30, 2014 and 2013:
		Amount Reclassified from AOCI
	Details about the AOCI Components	Three Months Ended September 30, 2014			Three Months Ended September 30, 2013			Affected Line Item in the Unaudited
								Condensed Consolidated Statement
								of Operations
		($ in millions)
	Available for sale securities:
	Realized gains on sale of securities	$	3.9		$	8.6		Realized and unrealized investment gains
	Realized (losses) on sale of securities	(0.3		)	(0.3		)	Realized and unrealized investment losses
		3.6			8.3			Income from operations before income tax
	Tax on net realized gains of securities	(0.1		)	(0.1		)	Income tax expense
		$	3.5		$	8.2		Net income
	Foreign currency translation adjustments:
	Foreign currency translation adjustments, before tax	$	—		$	4.8		Net realized and unrealized foreign exchange gains/(losses)
	Tax on foreign currency translation adjustments	—			(1.1		)	Income tax expense
		$	—		$	3.7		Net income
	Amortization of derivatives:
	Amortization of long-term debt associated expenses, before tax	$	—		$	—		Interest expense
		$	—		$	—		Net income
	Total reclassifications from AOCI to the statement of operations, net of tax	$	3.5		$	11.9		Net income
		Amount Reclassified from AOCI
	Details about the AOCI Components	Nine Months Ended September 30, 2014			Nine Months Ended September 30, 2013			Affected Line Item in the Unaudited
								Condensed Consolidated Statement
								of Operations
		($ in millions)
	Available for sale securities:
	Realized gains on sale of securities	$	10.6		$	21.2		Realized and unrealized investment gains
	Realized (losses) on sale of securities	(4.7		)	(0.5		)	Realized and unrealized investment losses
		5.9			20.7			Income from operations before tax
	Tax on net realized gains of securities	(0.2		)	(0.5		)	Income tax expense
		$	5.7		$	20.2		Net income
	Foreign currency translation adjustments:
	Foreign currency translation adjustments, before tax	$	—		$	0.5		Net realized and unrealized foreign exchange gains/(losses)
	Tax on foreign currency translation adjustments	—			(0.1		)	Income tax expense
		$	—		$	0.4		Net income
	Amortization of derivatives:
	Amortization of long-term debt associated expenses, before tax	$	—		$	(0.2	)	Interest expense
		$	—		$	(0.2	)	Net income
	Total reclassifications from AOCI to the statement of operations, net of tax	$	5.7		$	20.4		Net income

Earnings_per_Ordinary_Share

Earnings per Ordinary Share	9 Months Ended
	Sep. 30, 2014
Earnings Per Share [Abstract]	'
Earnings per Ordinary Share	'
	Earnings per Ordinary Share
	Basic earnings per ordinary share are calculated by dividing net income available to holders of Aspen Holdings’ ordinary shares by the weighted average number of ordinary shares outstanding. Diluted earnings per ordinary share are based on the weighted average number of ordinary shares and dilutive potential ordinary shares outstanding during the period of calculation using the treasury stock method. The following table sets forth the computation of basic and diluted earnings per share for the three and nine months ended September 30, 2014 and 2013, respectively:
		Three Months Ended September 30,						Nine Months Ended September 30,
		2014			2013			2014			2013
		($ in millions, except share and per share amounts)
	Net income	$	37.4		$	107.4		$	288.6		$	239.3
	Preference share dividends	(9.5		)	(9.5		)	(28.4		)	(26.1		)
	Change in redemption value (1)	—			—			—			(7.1		)
	Net profit attributable to non-controlling interest	0.1			0.3			—			0.3
	Basic and diluted net income available to ordinary shareholders	28			98.2			260.2			206.4
	Ordinary shares:
	Basic weighted average ordinary shares	65,116,463			66,716,202			65,283,681			67,302,857
	Weighted average effect of dilutive securities(2)	1,396,546			1,845,313			1,314,999			2,656,617
	Total diluted weighted average ordinary shares	66,513,009			68,561,515			66,598,680			69,959,474
	Earnings per ordinary share:
	Basic	$	0.43		$	1.47		$	3.99		$	3.06
	Diluted	$	0.42		$	1.43		$	3.91		$	2.95
	(1)	The $7.1 million reduction in the net income available to ordinary shareholders is the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of $230.0 million of the PIERS on May 30, 3013.
	(2)	Dilutive securities comprise: employee options, restricted share units and performance shares associated with the Company’s long term incentive plan, employee share purchase plans and director restricted stock units and options as described in Note 14, in addition to the PIERS that were fully redeemed on May 30, 2013.
	Dividends. On October 29, 2014, the Company’s Board of Directors declared the following quarterly dividends:
		Dividend			Payable on:		Record Date:
	Ordinary shares	$	0.2		December 2, 2014		November 13, 2014
	7.401% preference shares	$	0.462563		January 1, 2015		December 15, 2014
	7.250% preference shares	$	0.4531		January 1, 2015		December 15, 2014
	5.95% preference shares	$	0.3719		January 1, 2015		December 15, 2014

Segment_Reporting

Segment Reporting	9 Months Ended
	Sep. 30, 2014
Segment Reporting [Abstract]	'
Segment Reporting	'
	Segment Reporting
	The Company has two reporting business segments: Insurance and Reinsurance. In arriving at these reporting segments, the Company has considered similarities in economic characteristics, products, customers, distribution, the regulatory environment of the Company’s operating segments and quantitative thresholds to determine the Company’s reportable segments. Segment profit or loss for each of the Company’s operating segments is measured by underwriting profit or loss. Underwriting profit is the excess of net earned premiums over the sum of losses and loss expenses, amortization of deferred policy acquisition costs and general and administrative expenses. Underwriting profit or loss provides a basis for management to evaluate the segment’s underwriting performance.
	Reinsurance Segment. The reinsurance segment consists of property catastrophe reinsurance, other property reinsurance (risk excess, pro rata, facultative and engineering), casualty reinsurance (U.S. treaty, international treaty and global facultative) and specialty reinsurance (credit and surety, agriculture and other specialty). ACM forms part of our property catastrophe reinsurance line of business as it currently focuses entirely on property catastrophe business through the use of alternative capital. For a more detailed description of this segment, see Part I, Item 1, “Business — Business Segments — Reinsurance” in the Company’s 2013 Annual Report on Form 10-K filed with the United States Securities and Exchange Commission.
	Insurance Segment. The insurance segment consists of property and casualty insurance, marine, aviation and energy insurance and financial and professional lines insurance. As previously stated our property and casualty insurance lines of business were integrated into a combined property and casualty line effective November 8, 2013. This includes the programs business, previously reported separately. For a more detailed description of this segment, see Part I, Item 1 “Business — Business Segments — Insurance” in the Company’s 2013 Annual Report on Form 10-K filed with the United States Securities and Exchange Commission.
	Non-underwriting Disclosures. The Company has provided additional disclosures for corporate and other (non-underwriting) income and expenses. Corporate and other income and expenses include net investment income, net realized and unrealized investment gains or losses, expenses associated with managing the group, certain strategic and non-recurring costs, changes in fair value of derivatives and changes in fair value of the loan notes issued by variable interest entities, interest expenses, net realized and unrealized foreign exchange gains or losses and income taxes, which are not allocated to the underwriting segments. Corporate expenses are not allocated to the Company’s operating segments as they typically do not fluctuate with the levels of premiums written and are not directly related to the Company’s segment operations. The Company does not allocate its assets by segment as it evaluates underwriting results of each segment separately from the results of the Company’s investment portfolio.
	The following tables provide a summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of the Company’s business segments for the three months ended September 30, 2014 and 2013:
		Three Months Ended September 30, 2014
		Reinsurance			Insurance			Total
		($ in millions)
	Underwriting Revenues
	Gross written premiums	$	256.9		$	395.6		$	652.5
	Net written premiums	250.9			326.4			577.3
	Gross earned premiums	291			403.9			694.9
	Net earned premiums	279.6			330.8			610.4
	Underwriting Expenses
	Losses and loss adjustment expenses	132			210.7			342.7
	Amortization of deferred policy acquisition costs	52.1			63.4			115.5
	General and administrative expenses	38.4			45.6			84
	Underwriting income	$	57.1		$	11.1		68.2
	Corporate expenses							(35.8		)
	Net investment income							48
	Realized and unrealized investment gains							1.1
	Realized and unrealized investment losses							(1.3		)
	Change in fair value of loan notes issued by variable interest entities							(8.5		)
	Change in fair value of derivatives							(5.1		)
	Interest expense on long term debt							(7.4		)
	Net realized and unrealized foreign exchange (losses)							(21.2		)
	Other income							1
	Other expenses							(0.3		)
	Income before tax							$	38.7
	Net reserves for loss and loss adjustment expenses	$	2,554.20		$	1,848.40		$	4,402.60
	Ratios
	Loss ratio	47.2		%	63.7		%	56.1		%
	Policy acquisition expense ratio	18.6			19.2			18.9
	General and administrative expense ratio	13.7			13.8			19.6			(1)
	Expense ratio	32.3			33			38.5
	Combined ratio	79.5		%	96.7		%	94.6		%
	(1)	The general and administrative expense ratio in the total column includes corporate expenses.
		Three Months Ended September 30, 2013
		Reinsurance			Insurance			Total
		( $ in millions)
	Underwriting Revenues
	Gross written premiums	$	219.5		$	362.1		$	581.6
	Net written premiums	218.4			323.6			542
	Gross earned premiums	268.6			356.5			625.1
	Net earned premiums	255.7			288.6			544.3
	Underwriting Expenses
	Losses and loss adjustment expenses	122.2			168			290.2
	Amortization of deferred policy acquisition costs	49.1			61.4			110.5
	General and administrative expenses	34.6			49.5			84.1
	Underwriting income	$	49.8		$	9.7		59.5
	Corporate expenses							(14.8		)
	Net investment income							45
	Realized and unrealized investment gains							23.6
	Realized and unrealized investment losses							(5.9		)
	Change in fair value of derivatives							6.6
	Interest expense on long term debt							(7.7		)
	Net realized and unrealized foreign exchange gains							2.4
	Other income							1.6
	Other expenses							—
	Income before tax							$	110.3
	Net reserves for loss and loss adjustment expenses	$	2,718.00		$	1,555.40		$	4,273.40
	Ratios
Loss ratio	47.8		%	58.2		%	53.3		%
Policy acquisition expense ratio	19.2			21.3			20.3
General and administrative expense ratio	13.5			17.2			18.2			(1)
Expense ratio	32.7			38.5			38.5
Combined ratio	80.5		%	96.7		%	91.8		%
(1)	The general and administrative expense ratio in the total column includes corporate expenses.
The following tables provide a summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of the Company’s business segments for the nine months ended September 30, 2014 and 2013:
	Nine Months Ended September 30, 2014
	Reinsurance			Insurance			Total
	($ in millions)
Underwriting Revenues
Gross written premiums	$	1,027.50		$	1,259.80		$	2,287.30
Net written premiums	980.4			980.8			1,961.20
Gross earned premiums	859.2			1,182.00			2,041.20
Net earned premiums	825.1			968			1,793.10
Underwriting Expenses
Losses and loss adjustment expenses	367.4			600.5			967.9
Amortization of deferred policy acquisition costs	152.3			184.1			336.4
General and administrative expenses	107			142.6			249.6
Underwriting income	$	198.4		$	40.8		239.2
Corporate expenses							(74.6		)
Net investment income							143.6
Realized and unrealized investment gains							53.6
Realized and unrealized investment losses							(8.9		)
Change in fair value of loan notes issued by variable interest entities							(14.5		)
Change in fair value of derivatives							(8.6		)
Interest expense on long term debt							(22.1		)
Net realized and unrealized foreign exchange (losses)							(10.4		)
Other income							4.8
Other expenses							(2.2		)
Income before tax							$	299.9
Net reserves for loss and loss adjustment expenses	$	2,554.20		$	1,848.40		$	4,402.60
Ratios
Loss ratio	44.5		%	62		%	54		%
Policy acquisition expense ratio	18.5			19			18.8
General and administrative expense ratio	13			14.7			18.1			(1)
Expense ratio	31.5			33.7			36.9
Combined ratio	76		%	95.7		%	90.9		%
(1)	The general and administrative expense ratio in the total column includes corporate expenses.
	Nine Months Ended September 30, 2013
	Reinsurance			Insurance			Total
	( $ in millions)
Underwriting Revenues
Gross written premiums	$	957.7		$	1,084.60		$	2,042.30
Net written premiums	907.5			844.2			1,751.70
Gross earned premiums	828.9			1,000.70			1,829.60
Net earned premiums	788.2			811			1,599.20
Underwriting Expenses
Losses and loss adjustment expenses	394.9			497.4			892.3
Amortization of deferred policy acquisition costs	161			161.3			322.3
General and administrative expenses	97.2			134			231.2
Underwriting income	$	135.1		$	18.3		153.4
Corporate expenses							(42.0		)
Net investment income							139.2
Realized and unrealized investment gains							54.3
Realized and unrealized investment losses							(28.0		)
Change in fair value of derivatives							(0.5		)
Interest expense on long term debt							(23.2		)
Net realized and unrealized foreign exchange (losses)							(7.1		)
Other income							3.6
Other expenses							(0.6		)
Income before tax							$	249.1
Net reserves for loss and loss adjustment expenses	$	2,718.00		$	1,555.40		$	4,273.40
Ratios
Loss ratio	50.1		%	61.3		%	55.8		%
Policy acquisition expense ratio	20.4			19.9			20.2
General and administrative expense ratio	12.3			16.5			17.1			(1)
Expense ratio	32.7			36.4			37.3
Combined ratio	82.8		%	97.7		%	93.1		%
(1)	The general and administrative expense ratio in the total column includes corporate expenses.

Investments

Investments	9 Months Ended
	Sep. 30, 2014
Investments, Debt and Equity Securities [Abstract]	'
Investments	'
	Investments
	Income Statement
	Investment Income. The following table summarizes investment income for the three and nine months ended September 30, 2014 and 2013:
		For the Three Months Ended						For the Nine Months Ended
		30-Sep-14			30-Sep-13			30-Sep-14			30-Sep-13
		($ in millions)						($ in millions)
	Fixed income securities — Available for sale	$	37.6		$	38.2		$	113.4		$	117.2
	Fixed income securities — Trading	6.6			5.3			19.8			13.4
	Short-term investments — Available for sale	0.4			0.3			1			1.9
	Fixed term deposits (included in cash and cash equivalents)	0.5			0.7			2.4			3.4
	Equity securities — Available for sale	1			1.2			3.5			4.7
	Equity securities — Trading	3.5			1.8			10.5			5.5
	Catastrophe bonds — Trading	0.5			—			0.9			—
	Total	$	50.1		$	47.5		$	151.5		$	146.1
	Investment expenses	(2.1		)	(2.5		)	(7.9		)	(6.9		)
	Net investment income	$	48		$	45		$	143.6		$	139.2
	The following table summarizes the net realized and unrealized investment gains and losses recorded in the statement of operations and the change in unrealized gains and losses on investments recorded in other comprehensive income for the three and nine months ended September 30, 2014 and 2013:
		For the Three Months Ended						For the Nine Months Ended
		30-Sep-14			30-Sep-13			30-Sep-14			30-Sep-13
		($ in millions)						($ in millions)
	Available for sale:
	Fixed income securities — gross realized gains	$	1.7		$	6.9		$	8.1		$	15.7
	Fixed income securities — gross realized (losses)	(1.0		)	(5.6		)	(5.5		)	(6.8		)
	Equity securities — gross realized gains	4.2			6.3			9.7			13.8
	Trading:
	Fixed income securities — gross realized gains	1.9			0.8			6.6			6.6
	Fixed income securities — gross realized (losses)	(0.2		)	(1.3		)	(2.4		)	(1.9		)
	Equity securities — gross realized gains	2.8			0.4			4.5			0.8
	Equity securities — gross realized (losses)	(0.1		)	—			(0.3		)	(0.3		)
	Catastrophe bonds — trading	0.5			—			0.5			—
	Net change in gross unrealized gains (losses)	(10.0		)	9			24.2			(2.7		)
	Other investments:
	Gross realized and unrealized gains in Cartesian	—			1.2			—			1.1
	Impairments:
	Total other-than-temporary impairments	—			—			(0.7		)	—
	Total net realized and unrealized investment gains recorded in the statement of operations	$	(0.2	)	$	17.7		$	44.7		$	26.3
	Change in available for sale net unrealized gains (losses):
	Fixed income securities	(28.5		)	(3.3		)	25			(168.7		)
	Equity securities	(4.5		)	(0.2		)	(2.3		)	9.4
	Total change in pre-tax available for sale unrealized gains(losses)	(33.0		)	(3.5		)	22.7			(159.3		)
	Change in taxes	8.1			(0.3		)	(3.8		)	11.8
	Total change in net unrealized gains (losses), net of taxes recorded in other comprehensive income	$	(24.9	)	$	(3.8	)	$	18.9		$	(147.5	)
	Other-than-temporary Impairments. A security is potentially impaired when its fair value is below its amortized cost. The Company reviews its available for sale fixed income and equity portfolios on an individual security basis for potential other-than-temporary impairment (“OTTI”) each quarter based on criteria including issuer-specific circumstances, credit ratings actions and general macro-economic conditions. For a more detailed description of OTTI, please refer to Note 2(c) of the “Notes to the Audited Consolidated Financial Statements” in the Company’s 2013 Annual Report on Form 10-K filed with the United States Securities and Exchange Commission. The OTTI charge recognized for the three and nine months ended September 30, 2014 was $Nil and $0.7 million, respectively (2013 — $Nil and $Nil) and were in relation to the Company's fixed income municipal securities.
	Balance Sheet
	Fixed Income Securities, Short-Term Investments and Equities — Available For Sale. The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income securities, short-term investments and equity securities as at September 30, 2014 and December 31, 2013:
		As at September 30, 2014
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	956.4		$	19.5		$	(2.0	)	$	973.9
	U.S. agency	193.9			8.2			(0.2		)	201.9
	Municipal	29.7			2			—			31.7
	Corporate	2,208.90			77.9			(8.4		)	2,278.40
	Non-U.S. government-backed corporate	74.2			1.3			—			75.5
	Foreign government	743.2			11.1			(1.7		)	752.6
	Asset-backed	141.9			2.3			(0.3		)	143.9
	Non-agency commercial mortgage-backed	47.6			3.9			—			51.5
	Agency mortgage-backed	957.8			34.8			(4.2		)	988.4
	Total fixed income securities — Available for sale	5,353.60			161			(16.8		)	5,497.80
	Total short-term investments — Available for sale	307.2			—			—			307.2
	Total equity securities — Available for sale	93.9			32.7			(0.7		)	125.9
	Total	$	5,754.70		$	193.7		$	(17.5	)	$	5,930.90
	As at December 31, 2013
	Cost or			Gross			Gross			Fair Market
	Amortized Cost			Unrealized			Unrealized			Value
				Gains			Losses
	($ in millions)
U.S. government	$	1,004.70		$	21.2		$	(5.5	)	$	1,020.40
U.S. agency	258.5			11.4			(0.8		)	269.1
Municipal	32.3			0.9			(0.4		)	32.8
Corporate	2,005.60			82.5			(18.7		)	2,069.40
Non-U.S. government-backed corporate	83.4			1.4			(0.2		)	84.6
Foreign government	772			11.2			(4.3		)	778.9
Asset-backed	119.8			2.8			(0.3		)	122.3
Non-agency commercial mortgage-backed	56.9			5.7			—			62.6
Agency mortgage-backed	1,116.70			30.6			(18.3		)	1,129.00
Total fixed income securities — Available for sale	5,449.90			167.7			(48.5		)	5,569.10
Total short-term investments — Available for sale	160.3			—			—			160.3
Total equity securities — Available for sale	112.2			37.8			(0.5		)	149.5
Total	$	5,722.40		$	205.5		$	(49.0	)	$	5,878.90
Fixed Income Securities, Short-Term Investments, Equities and Catastrophe Bonds — Trading. The following tables present the cost or amortized cost, gross unrealized gains and losses, and estimated fair market value of trading investments in fixed income securities, short-term investments, equity securities and catastrophe bonds as at September 30, 2014 and December 31, 2013:
	As at September 30, 2014
	Cost or			Gross			Gross			Fair Market
	Amortized Cost			Unrealized			Unrealized			Value
				Gains			Losses
	($ in millions)
U.S. government	$	16.5		$	—		$	—		$	16.5
U.S. agency	0.2			—			—			0.2
Municipal	1.1			—			—			1.1
Corporate	511.4			12.2			(2.5		)	521.1
Foreign government	135.4			4.1			(0.3		)	139.2
Asset-backed	15.1			0.1			—			15.2
Bank loans	81.8			—			(1.0		)	80.8
Total fixed income securities — Trading	761.5			16.4			(3.8		)	774.1
Total short-term investments — Trading	7			—			—			7
Total equity securities — Trading	490.2			47			(16.1		)	521.1
Total catastrophe bonds — Trading	31.6			0.6			—			32.2
Total	$	1,290.30		$	64		$	(19.9	)	$	1,334.40
	As at December 31, 2013
	Cost or			Gross			Gross			Fair Market
	Amortized Cost			Unrealized			Unrealized			Value
				Gains			Losses
	($ in millions)
U.S. government	$	22.7		$	—		$	(0.7	)	$	22
U.S. agency	0.2			—			—			0.2
Municipal	1.1			—			—			1.1
Corporate	469.8			10.3			(5.3		)	474.8
Foreign government	136.5			1.2			(1.5		)	136.2
Asset-backed	12.7			0.1			—			12.8
Bank loans	69.1			0.3			(0.3		)	69.1
Total fixed income securities — Trading	712.1			11.9			(7.8		)	716.2
Total equity securities — Trading	281.6			34			(4.7		)	310.9
Total catastrophe bonds — Trading	5.8			—			—			5.8
Total	$	999.5		$	45.9		$	(12.5	)	$	1,032.90
The Company classifies these financial instruments as held for trading as this most closely reflects the facts and circumstances of the investments held. Unrealized foreign exchange gains and losses included in the tables above are included within net realized and unrealized foreign exchange gains/(losses) within the statement of operations and other comprehensive income.
As of September 30, 2014, the Company had invested $80.8 million in a U.S. Dollar bank loans trading portfolio and increased its investment in equities by $160.0 million in the nine months ended September 30, 2014. In August 2013, the Company invested in a $200.0 million BBB rated Emerging Market Debt portfolio (“EMD”), which is reported above in corporate and foreign government securities.
Catastrophe Bonds. The Company has invested in catastrophe bonds with a total value of $32.2 million as of September 30, 2014.  The bonds receive quarterly interest payments based on variable interest rates with scheduled maturities ranging from 2016 to 2020.  The redemption value of the bonds will adjust based on the occurrence of a covered event, such as windstorms and earthquakes which occur in the geographic region of the United States, Canada, the North Atlantic, Japan and Australia.
Other Investments. The Company previously had an equity accounted investment in Cartesian Iris Offshore Fund L.P (“Cartesian”), which provided capital to Iris Re, a Class 3 Bermuda reinsurer (“Iris Re”). On June 29, 2013, the Company notified Cartesian Capital Group of its intention to withdraw the Company’s investment in Cartesian and to terminate the services provided to Iris Re. The termination took effect on January 1, 2014. For more information regarding the Company’s investment in Cartesian, refer to Note 6 of the “Notes to Audited Consolidated Financial Statements” in the Company’s 2013 Annual Report on Form 10-K filed with the United States Securities and Exchange Commission.
On October 2, 2012, the Company established a subsidiary, Aspen Recoveries Limited, to take ownership of a 58.5% shareholding in Chaspark Maritime Holdings Ltd., a Singaporean registered company (“Chaspark”), with the remaining shareholding owned by other insurers. The investment is accounted under the equity method. For more information regarding the Company’s investment in Chaspark, refer to Note 7, Variable Interest Entities.
The tables below show the Company’s investments in Cartesian and Chaspark for the three and nine months ended September 30, 2014 :
	For the Three Months Ended September 30, 2014
	Cartesian			Chaspark			Total
	($ in millions)
Opening undistributed value of investment as at June 30, 2014	$	—		$	8.7		$	8.7
Distribution for the three months to September 30, 2014	—			—			—
Closing value of investment as at September 30, 2014	$	—		$	8.7		$	8.7
	For the Nine Months Ended September 30, 2014
	Cartesian			Chaspark			Total
	($ in millions)
Opening undistributed value of investment as at January 1, 2014	$	39.3		$	8.7		$	48
Distribution for the nine months to September 30, 2014	(39.3		)	—			(39.3		)
Closing value of investment as at September 30, 2014	$	—		$	8.7		$	8.7
Fixed Income Securities. The scheduled maturity distribution of available for sale fixed income securities as at September 30, 2014 and December 31, 2013 is set forth below. Actual maturities may differ from contractual maturities because issuers of securities may have the right to call or prepay obligations with or without call or prepayment penalties.
	As at September 30, 2014
	Amortized			Fair Market			Average
	Cost or Cost			Value			S&P Ratings by
							Maturity
	($ in millions)
Due one year or less	$	652.5		$	658.3		AA
Due after one year through five years	2,502.80			2,571.30			AA-
Due after five years through ten years	955			978.8			AA-
Due after ten years	96			105.6			A+
Subtotal	4,206.30			4,314.00
Non-agency commercial mortgage-backed	47.6			51.5			AA+
Agency mortgage-backed	957.8			988.4			AA+
Asset-backed	141.9			143.9			AAA
Total fixed income securities — Available for sale	$	5,353.60		$	5,497.80
	As at December 31, 2013
	Amortized			Fair Market			Average
	Cost or Cost			Value			S&P Ratings by
							Maturity
	($ in millions)
Due one year or less	$	694.8		$	700		AA
Due after one year through five years	2,376.10			2,438.00			AA-
Due after five years through ten years	1,003.90			1,032.80			A+
Due after ten years	81.7			84.4			AA-
Subtotal	4,156.50			4,255.20
Non-agency commercial mortgage-backed	56.9			62.6			AA+
Agency mortgage-backed	1,116.70			1,129.00			AA+
Asset-backed	119.8			122.3			AAA
Total fixed income securities — Available for sale	$	5,449.90		$	5,569.10
Guaranteed Investments. As at September 30, 2014, the Company held $2.4 million (December 31, 2013 — $1.5 million) in investments which are guaranteed by mono-line insurers, excluding those with explicit government guarantees, and the Company’s holding was limited to two municipal securities, both rated Caa2 or higher (December 31, 2013 — two municipal securities, both rated BBB- or higher). The standalone rating (rating without guarantee) is determined as the senior unsecured debt rating of the issuer. Where the credit ratings were split between the two main rating agencies, Standard & Poor’s Financial Services LLC (“S&P”) and Moody’s Investors Service, Inc. (“Moody’s”), the lowest rating was used. The Company’s exposure to other third-party guaranteed debt is primarily to investments backed by non-U.S. government guaranteed issuers.
Gross Unrealized Loss. The following tables summarize as at September 30, 2014 and December 31, 2013 by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position for the Company’s available for sale portfolio:
	September 30, 2014
	0-12 months						Over 12 months						Total
	Fair			Gross			Fair			Gross			Fair		Gross			Number of
	Market			Unrealized			Market			Unrealized			Market		Unrealized			Securities
	Value			Loss			Value			Loss			Value		Loss
	($ in millions)
U.S. government	$	182.2		$	(0.8	)	$	80.1		$	(1.2	)	$	262.3	$	(2.0	)	42
U.S. agency	17.3			—			18.8			(0.2		)	36.1		(0.2		)	9
Municipal	2.2			—			—			—			2.2		—			1
Corporate	474.1			(2.8		)	201.8			(5.6		)	675.9		(8.4		)	274
Non-U.S. government-backed corporate	1.8			—			4.3			—			6.1		—			3
Foreign government	151.7			(0.3		)	110.8			(1.4		)	262.5		(1.7		)	35
Asset-backed	43.2			(0.1		)	11.2			(0.2		)	54.4		(0.3		)	39
Agency mortgage-backed	57.3			(0.4		)	126			(3.8		)	183.3		(4.2		)	54
Total fixed income securities — Available for sale	929.8			(4.4		)	553			(12.4		)	1,482.80		(16.8		)	457
Total short-term investments — Available for sale	2			—			—			—			2		—			1
Total equity securities —	8.8			(0.7		)	—			—			8.8		(0.7		)	7
Available for sale
Total	$	940.6		$	(5.1	)	$	553		$	(12.4	)	$	1,493.60	$	(17.5	)	465
	December 31, 2013
	0-12 months						Over 12 months						Total
	Fair			Gross			Fair			Gross			Fair		Gross			Number of
	Market			Unrealized			Market			Unrealized			Market		Unrealized			Securities
	Value			Loss			Value			Loss			Value		Loss
	($ in millions)
U.S. government	$	293.9		$	(5.5	)	$	—		$	—		$	293.9	$	(5.5	)	51
U.S. agency	72.1			(0.8		)	—			—			72.1		(0.8		)	18
Municipal	5.5			(0.2		)	1.3			(0.2		)	6.8		(0.4		)	7
Corporate	695.4			(16.8		)	23.4			(1.9		)	718.8		(18.7		)	372
Non-U.S. government-backed corporate	21.8			(0.2		)	4.9			—			26.7		(0.2		)	8
Foreign government	239.7			(4.1		)	8.5			(0.2		)	248.2		(4.3		)	44
Asset-backed	50.2			(0.3		)	—			—			50.2		(0.3		)	51
Agency mortgage-backed	491.8			(18.3		)	1.2			—			493		(18.3		)	123
Total fixed income securities — Available for sale	1,870.40			(46.2		)	39.3			(2.3		)	1,909.70		(48.5		)	674
Total short-term investments — Available for sale	7.7			—			—			—			7.7		—			6
Total equity securities —	6			(0.4		)	2.3			(0.1		)	8.3		(0.5		)	7
Available for sale
Total	$	1,884.10		$	(46.6	)	$	41.6		$	(2.4	)	$	1,925.70	$	(49.0	)	687
Investment Purchases and Sales. The following table summarizes investment purchases, sales and maturities for the three and nine months ended September 30, 2014 and 2013:
	For the Three Months Ended						For the Nine Months Ended
	30-Sep-14			30-Sep-13			30-Sep-14			30-Sep-13
	($ in millions)						($ in millions)
(Purchases) of fixed income securities — Available for sale	$	(522.8	)	$	(566.8	)	$	(1,440.9	)	$	(1,748.6	)
(Purchases) of fixed income securities — Trading	(154.7		)	(283.2		)	(553.6		)	(555.5		)
(Purchases) of equity securities — Available for sale	—			—			—			(2.5		)
(Purchases) of equity securities — Trading	(121.2		)	(38.6		)	(240.1		)	(275.4		)
Proceeds from sales and maturities of fixed income securities — Available for sale	481			578.6			1,496.90			1,564.40
Proceeds from sales and maturities of fixed income securities — Trading	143.5			67.8			515.7			276.3
Proceeds from sales of equity securities — Available for sale	12.1			29.7			27.7			61.8
Proceeds from sales of equity securities — Trading	17.8			5			36			15.1
Net change in receivable/(payable) for securities sold/(purchased)	(7.0		)	6.6			12.8			5.6
Net (purchases)/sales of short-term investments — Available for sale	25			95.5			(151.0		)	275.7
Net (purchases)/sales of short-term investments — Trading	7			—			(7.0		)	—
Net (purchases) of catastrophe bonds — Trading	(1.7		)	—			(26.0		)	—
Net sales of other investments	—			—			37.3			—
Net (purchases)/sales for the period	$	(121.0	)	$	(105.4	)	$	(292.2	)	$	(383.1	)

Variable_Interest_Entities_Not

Variable Interest Entities (Notes)	9 Months Ended
	Sep. 30, 2014
Business Combinations [Abstract]	'
Variable Interest Entities	'
	Variable Interest Entities
	As at September 30, 2014, the Company had two investments in two variable interest entities (“VIE”), Chaspark and Silverton.
	Chaspark. On October 2, 2012, the Company established a subsidiary, Aspen Recoveries Limited, to take ownership of a 58.5% shareholding in Chaspark, with the remaining shareholding owned by other insurers. The shareholding in Chaspark was received as a settlement for subrogation rights associated with a contract frustration claim settlement. The Company has determined that Chaspark has the characteristics of a VIE as addressed by the guidance in ASC 810, Consolidation. As discussed further in Note 6 of these unaudited condensed consolidated financial statements, the investment in Chaspark is accounted for under the equity method. In the three and nine months ended September 30, 2014, there was no change in the value of the Company’s investment in Chaspark (September 30, 2013 — $Nil and $Nil). The adjusted carrying value approximates fair value.
	Silverton. On September 10, 2013, the Company established Silverton, a Bermuda domiciled special purpose insurer, to provide additional collateralized capacity to support Aspen Re’s business. In respect of the debt issued by Silverton to investors, Silverton has entered into a retrocession agreement with Aspen Bermuda effective January 1, 2014. Under this agreement, Silverton receives a quota share of Aspen Bermuda’s catastrophe business. 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. Silverton is able to provide investors with access to diversified natural catastrophe risk backed by the distribution, underwriting, analysis and research expertise of Aspen Re.
	The Company has determined that Silverton has the characteristics of a VIE that are addressed by the guidance in ASC 810, Consolidation. The Company concluded that it is the primary beneficiary and has consolidated the subsidiary upon its formation as it owns 100% of the voting shares, 100% of the issued share capital and has a significant financial interest and the power to control Silverton. The Company has no other obligation to provide financial support to Silverton. Neither the creditors nor beneficial interest holders of Silverton have recourse to the Company’s general credit.
	In the event of either an extreme catastrophic property reinsurance event or severe credit related event there is a risk that Aspen Bermuda would be unable to recover losses from Silverton. These two risks are mitigated as follows:
	i.	Silverton has collateralized the aggregate limit provided to Aspen Bermuda by way of a trust in favor of Aspen Bermuda as the beneficiary;
	ii.	the trustee is a large, well-established regulated entity; and
	iii.	all funds within the trust account are bound by investment guidelines restricting investments to one of the institutional class money market funds run by large international investment managers.
	Silverton issued $65.0 million of loan notes on December 27, 2013 (of which $50.0 million was issued to third parties and $15.0 million to Aspen Holdings) which is held as collateral for a fully collateralized quota share of Aspen Re’s global property catastrophe excess of loss reinsurance business. The final amount to be paid to the Silverton noteholders is dependent upon the settlement value of the quota share contract and operating expenses of Silverton. The operations of Silverton commenced on January 1, 2014. The Company’s maximum loss exposure to Silverton is its $15.0 million loan note holdings as at September 30, 2014 (December 31, 2013 — $15.0 million) due to mature on September 16, 2016. The fair value of the loan notes held by the Company increased by $2.6 million and $4.4 million, respectively, for the three and nine months ended September 30, 2014. The total aggregate unpaid balance of the loan notes held by third parties and those held by Aspen Holdings is $83.9 million.
	The tables below show the value of the notes attributable to the third-party investments in Silverton for the three and nine months ended September 30, 2014:
			Three Months Ended September 30, 2014
			($ in millions)
	Beginning balance as at June 30, 2014		$	(56.0	)
	Total change in fair value included in the statement of operations		(8.5		)
	Balance as at September 30, 2014		$	(64.5	)
			Nine Months Ended September 30, 2014
			($ in millions)
	Beginning balance as at January 1, 2014		$	(50.0	)
	Total change in fair value included in the statement of operations		(14.5		)
	Balance as at September 30, 2014		$	(64.5	)

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2014
Fair Value Disclosures [Abstract]	'
Fair Value Measurements	'
	Fair Value Measurements
	The Company’s estimates of fair value for financial assets and liabilities are based on the framework established in the fair value accounting guidance included in ASC 820, Fair Value Measurements and Disclosures. The framework prioritizes the inputs, which refer broadly to assumptions market participants would use in pricing an asset or liability, into three levels.
	The Company considers prices for actively traded securities to be derived based on quoted prices in an active market for identical assets, which are Level 1 inputs in the fair value hierarchy. The majority of these securities are valued using prices supplied by index providers.
	The Company considers prices for other securities that may not be as actively traded which are priced via pricing services, index providers, vendors and broker-dealers, or with reference to interest rates and yield curves, to be derived based on inputs that are observable for the asset, either directly or indirectly, which are Level 2 inputs in the fair value hierarchy. The majority of these securities are also valued using prices supplied by index providers.
	The Company considers securities, other financial instruments and derivative insurance contracts subject to fair value measurement whose valuation is derived by internal valuation models to be based largely on unobservable inputs, which are Level 3 inputs in the fair value hierarchy.
	The following tables present the level within the fair value hierarchy at which the Company’s financial assets and liabilities are measured on a recurring basis as at September 30, 2014 and December 31, 2013, respectively:
		As at September 30, 2014
		Level 1				Level 2				Level 3				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	973.9			$	—			$	—			$	973.9
	U.S. agency	—				201.9				—				201.9
	Municipal	—				31.7				—				31.7
	Corporate	—				2,278.40				—				2,278.40
	Non-U.S. government-backed corporate	—				75.5				—				75.5
	Foreign government	538.2				214.4				—				752.6
	Asset-backed	—				143.9				—				143.9
	Non-agency commercial mortgage-backed	—				51.5				—				51.5
	Agency mortgage-backed	—				988.4				—				988.4
	Total fixed income securities available for sale, at fair value	1,512.10				3,985.70				—				5,497.80
	Short-term investments available for sale, at fair value	302.4				4.8				—				307.2
	Equity investments available for sale, at fair value	125.9				—				—				125.9
	Held for trading financial assets, at fair value
	U.S. government	16.5				—				—				16.5
	U.S. agency	—				0.2				—				0.2
	Municipal	—				1.1				—				1.1
	Corporate	—				521.1				—				521.1
	Foreign government	33.9				105.3				—				139.2
	Asset-backed	—				15.2				—				15.2
	Bank loans	—				80.8				—				80.8
	Total fixed income securities trading, at fair value	50.4				723.7				—				774.1
	Short-term investments trading, at fair value	—				7				—				7
	Equity investments trading, at fair value	521.1				—				—				521.1
	Catastrophe bonds trading, at fair value	—				32.2				—				32.2
	Other financial assets and liabilities, at fair value
	Derivatives at fair value — foreign exchange contracts	—				5.7				—				5.7
	Derivatives at fair value— interest rate swaps	—				1.3				—				1.3
	Liabilities under derivative contracts — foreign exchange contracts	—				(12.7		)		—				(12.7		)
	Loan notes issued by variable interest entities, at fair value	—				—				(64.5		)		(64.5		)
	Total	$	2,511.90			$	4,747.70			$	(64.5	)		$	7,195.10
	There were no maturities, settlements or transfers between Level 1, Level 2 and Level 3 during the three and nine months ended September 30, 2014 and no assets or liabilities were classified as Level 3 as at September 30, 2014, except for the loan notes issued by the VIEs.
		As at December 31, 2013
		Level 1				Level 2				Level 3				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	1,020.40			$	—			$	—			$	1,020.40
	U.S. agency	—				269.1				—				269.1
	Municipal	—				32.8				—				32.8
	Corporate	—				2,069.40				—				2,069.40
	Non-U.S. government-backed corporate	—				84.6				—				84.6
	Foreign government	596.2				182.7				—				778.9
	Asset-backed	—				122.3				—				122.3
	Non-agency commercial mortgage-backed	—				62.6				—				62.6
	Agency mortgage-backed	—				1,129.00				—				1,129.00
	Total fixed income securities available for sale, at fair value	1,616.60				3,952.50				—				5,569.10
	Short-term investments available for sale, at fair value	129.5				30.8				—				160.3
	Equity investments available for sale, at fair value	149.5				—				—				149.5
	Held for trading financial assets, at fair value
	U.S. government	22				—				—				22
	U.S. agency	—				0.2				—				0.2
	Municipal	—				1.1				—				1.1
	Corporate	—				474.8				—				474.8
	Foreign government	44.2				92				—				136.2
Asset-backed	—				12.8				—				12.8
Bank loans	—				69.1				—				69.1
Total fixed income securities trading, at fair value	66.2				650				—				716.2
Equity investments trading, at fair value	310.9				—				—				310.9
Catastrophe bonds trading, at fair value	—				5.8				—				5.8
Other financial assets and liabilities, at fair value
Derivatives at fair value – foreign exchange contracts	—				5.9				—				5.9
Derivatives at fair value – interest rate swaps	—				1.1				—				1.1
Liabilities under derivative contracts – foreign exchange contracts	—				(2.9		)		—				(2.9		)
Loan notes issued by variable interest entities, at fair value	—				—				(50.0		)		(50.0		)
Total	$	2,272.70			$	4,643.20			$	(50.0	)		$	6,865.90
There were no maturities, settlements or transfers between Level 1, Level 2 and Level 3 during the twelve months ended December 31, 2013. There were no assets or liabilities that were classified as Level 3 as at December 31, 2013, except for the loan notes issued by the VIEs.
The following tables present a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Level 3 inputs for the three and nine months ended September 30, 2014:
Reconciliation of Liabilities Using Level 3 Inputs		Three Months Ended September 30, 2014
		($ in millions)
Beginning balance as at June 30, 2014		$	(56.0	)
Total change in fair value included in the statement of operations		(8.5		)
Balance as at September 30, 2014		$	(64.5	)
Reconciliation of Liabilities Using Level 3 Inputs		Nine Months Ended September 30, 2014
		($ in millions)
Beginning balance as at January 1, 2014		$	(50.0	)
Total change in fair value included in the statement of operations		(14.5		)
Balance as at September 30, 2014		$	(64.5	)
Valuation of Fixed Income Securities. The Company’s fixed income securities are classified as either available for sale or trading and carried at fair value. As at September 30, 2014 and December 31, 2013, the Company’s fixed income securities were valued by pricing services, index providers or broker-dealers using standard market conventions. The market conventions utilize market quotations, market transactions in comparable instruments and various relationships between instruments including, but not limited to, yield to maturity, dollar prices and spread prices in determining value.
Independent Pricing Services and Index Providers. The underlying methodology used to determine the fair value of securities in the Company’s available for sale and trading portfolios by the pricing services and index providers the Company uses is very similar. Pricing services will gather observable pricing inputs from multiple external sources, including buy and sell-side contacts and broker-dealers, in order to develop their internal prices. Index providers are those firms which provide prices for a range of securities within one or more asset classes, typically using their own in-house market makers (traders) as the primary pricing source for the indices, although ultimate valuations may also rely on other observable data inputs to derive a dollar price for all index-eligible securities. Index providers without in-house trading desks will function similarly to a pricing service in that they will gather their observable pricing inputs from multiple external sources. All prices for the Company’s securities attributed to index providers are for an individual security within the respective indices.
Pricing services and index providers provide pricing for less complex, liquid securities based on market quotations in active markets. Pricing services and index providers supply prices for a broad range of securities including those for actively traded securities, such as Treasury and other Government securities, in addition to those that trade less frequently or where valuation includes reference to credit spreads, pay down and pre-pay features and other observable inputs. These securities include Government Agency, Municipals, Corporate and Asset-Backed Securities.
For securities that may trade less frequently or do not trade on a listed exchange, these pricing services and index providers may use matrix pricing consisting of observable market inputs to estimate the fair value of a security. These observable market inputs include: reported trades, benchmark yields, broker-dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers, reference data, and industry and economic factors. Additionally, pricing services and index providers may use a valuation model such as an option adjusted spread model commonly used for estimating fair values of mortgage-backed and asset-backed securities. Neither the Company, nor its index providers, derives dollar prices using an index as a pricing input for any individual security.
Broker-Dealers. The Company obtains quotes from broker-dealers who are active in the corresponding markets when prices are unavailable from independent pricing services or index providers. Generally, broker-dealers value securities through their trading desks based on observable market inputs. Their pricing methodologies include mapping securities based on trade data, bids or offers, observed spreads and performance of newly issued securities. They may also establish pricing through observing secondary trading of similar securities. Quotes from broker-dealers are non-binding.
The Company obtains prices for all of its fixed income investment securities via its third-party accounting service provider, and in the majority of cases receiving a number of quotes so as to obtain the most comprehensive information available to determine a security’s fair value. A single valuation is applied to each security based on the vendor hierarchy maintained by the Company’s third-party accounting service provider.
As at September 30, 2014, the Company obtained an average of 2.0 quotes per fixed income investment, consistent with 2.6 quotes as at December 31, 2013. Pricing sources used in pricing fixed income investments as at September 30, 2014 and December 31, 2013 were as follows:
	As at September 30, 2014			As at December 31, 2013
Index providers	84	%		85	%
Pricing services	12			12
Broker-dealers	4			3
Total	100	%		100	%
A summary of securities priced using pricing information from index providers as at September 30, 2014 and December 31, 2013 is provided below:
	As at September 30, 2014							As at December 31, 2013
	Fair Market				% of Total			Fair Market				% of Total
	Value Determined				Fair Value by			Value Determined				Fair Value by
	using Prices from				Security Type			using Prices from				Security Type
	Index Providers							Index Providers
	($ in millions, except for percentages)
U.S. government	$	911.9			92	%		$	998.5			96	%
U.S. agency	191.3				95	%		255.3				95	%
Municipal	13.4				41	%		14.5				43	%
Corporate	2,675.20				96	%		2,400.80				94	%
Non-U.S. government-backed corporate	57.1				76	%		55.9				66	%
Foreign government	559.1				63	%		605.8				66	%
Asset-backed	146				92	%		130.6				97	%
Non-agency commercial mortgage-backed	51.5				100	%		61				97	%
Agency mortgage-backed	650.5				66	%		830.6				74	%
Total fixed income securities	$	5,256.00			84	%		$	5,353.00			85	%
Equities	647				100	%		460.4				100	%
Total fixed income securities and equity investments	$	5,903.00			85	%		$	5,813.40			86	%
The Company, in conjunction with its third-party accounting service provider, obtains an understanding of the methods, models and inputs used by the third-party pricing service and index provider to assess the ongoing appropriateness of vendors’ prices. The Company and its third-party accounting service provider also have controls in place to validate that amounts provided represent fair values. Processes to validate and review pricing include, but are not limited to:
•	quantitative analysis (e.g., comparing the quarterly return for each managed portfolio to its target benchmark, with significant differences identified and investigated);
•	comparison of market values obtained from pricing services, index providers and broker-dealers against alternative price sources for each security where further investigation is completed when significant differences exist for pricing of individual securities between pricing sources;
•	initial and ongoing evaluation of methodologies used by outside parties to calculate fair value; and
•	comparison of the fair value estimates to the Company’s knowledge of the current market.
Prices obtained from pricing services, index providers and broker-dealers are not adjusted by us; however, prices provided by a pricing service, index provider or broker-dealer in certain instances may be challenged based on market or information available from internal sources, including those available to the Company’s third-party investment accounting service provider. Subsequent to any challenge, revisions made by the pricing service, index provider or broker-dealer to the quotes are supplied to the Company’s investment accounting service provider.
Management reviews the vendor hierarchy maintained by the Company’s third-party accounting service provider in order to determine which price source provides the most appropriate fair value (i.e., a price obtained from a pricing service with more seniority in the hierarchy will be used over a less senior one in all cases). The hierarchy level assigned to each security in the Company’s available for sale and trading portfolios is based upon its assessment of the transparency and reliability of the inputs used in the valuation as of the measurement date. The hierarchy of index providers and pricing services is determined using various qualitative and quantitative points arising from reviews of the vendors conducted by the Company’s third-party accounting service provider. Vendor reviews include annual onsite due diligence meetings with index providers and pricing services vendors covering valuation methodology, operational walkthroughs and legal and compliance updates. Index providers are assigned the highest priority in the pricing hierarchy due primarily to availability and reliability of pricing information.
Fixed Income Securities. The Company’s fixed income securities are traded on the over-the-counter (“OTC”) market based on prices provided by one or more market makers in each security. Securities such as U.S. Government, U.S. Agency, Foreign Government and investment grade corporate bonds have multiple market makers in addition to readily observable market value indicators such as expected credit spread, except for Treasury securities, over the yield curve. The Company uses a variety of pricing sources to value fixed income securities including those securities that have pay down/prepay features such as mortgage-backed securities and asset-backed securities in order to ensure fair and accurate pricing. The fair value estimates for the investment grade securities in the Company’s portfolio do not use significant unobservable inputs or modeling techniques.
U.S. Government and Agency. U.S. government and agency securities consist primarily of bonds issued by the U.S. Treasury and corporate debt issued by agencies such as the Federal National Mortgage Association (“FNMA”), the Federal Home Loan Mortgage Corporation (“FHLMC”) and the Federal Home Loan Bank. As the fair values of U.S. Treasury securities are based on unadjusted market prices in active markets, they are classified within Level 1. The fair values of U.S. government agency securities are priced using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of U.S. government agency securities are classified within Level 2.
Municipals. The Company’s municipal portfolio comprises bonds issued by U.S. domiciled state and municipality entities. The fair value of these securities is determined using spreads obtained from broker-dealers, trade prices and the new issue market which are Level 2 inputs in the fair value hierarchy. Consequently, these securities are classified within Level 2.
Foreign Government. The issuers for securities in this category are non-U.S. governments and their agencies. The fair values of non-U.S. government bonds, primarily sourced from international indices, are based on unadjusted market prices in active markets and are therefore classified within Level 1. The fair values of the non-U.S. agency securities, again primarily sourced from international indices, are priced using the spread above the risk-free yield curve. As the yields for the risk-free yield curve and the spreads for these securities are observable market inputs, the fair values of non-U.S. agency securities are classified within Level 2. In addition, foreign government securities include a portion of the EMD portfolio which is also classified within Level 2.
Corporate. Corporate securities consist primarily of U.S. and foreign corporations covering a variety of industries and are for the most part priced by index providers and pricing vendors. Some issuers may participate in the FDIC program or other similar non-U.S. government programs which guarantee timely payment of principal and interest in the event of a default. The fair values of these securities are generally determined using the spread above the risk-free yield curve. Inputs used in the evaluation of these securities include credit data, interest rate data, market observations and sector news, broker-dealer quotes and trade volumes. In addition, corporate securities include the Company’s BB High Yield Bonds and a portion of the BBB Emerging Market Debt portfolio. The Company classifies all of these securities within Level 2.
Mortgage-backed Securities. The Company’s residential and commercial mortgage-backed securities consist of bonds issued by the Government National Mortgage Association, the FNMA and the FHLMC as well as private non-agency issuers. The fair values of these securities are determined through the use of a pricing model (including Option Adjusted Spread) which uses prepayment speeds and spreads to determine the appropriate average life of the mortgage-backed security. These spreads are generally obtained from broker-dealers, trade prices and the new issue market. As the significant inputs used to price mortgage-backed securities are observable market inputs, these securities are classified within Level 2.
Asset-backed Securities. The underlying collateral for the Company’s asset-backed securities consists mainly of student loans, automobile loans and credit card receivables. These securities are primarily priced by index providers and pricing vendors. Inputs to the valuation process include broker-dealer quotes and other available trade information, prepayment speeds, interest rate data and credit spreads. The Company classifies these securities within Level 2.
Bank Loans. These are variable rate, senior secured debt instruments issued by non-investment grade companies that are not publicly registered but are the most senior debt in a capital structure and are generally secured by company assets. Although these assets do not trade in as liquid a market as traditional fixed income instruments, they are valued in similar fashion to other fixed maturities, using similar inputs such as yield curves, interest rates and credit spreads. These securities are primarily priced by a third party pricing vendor. Bank loans are therefore classified within Level 2.
Short-term Investments. Short-term investments comprise highly liquid debt securities with a maturity greater than three months but less than one year from the date of purchase. Short-term investments are valued in a manner similar to the Company’s fixed maturity investments and are classified within Levels 1 and 2.
Equity Securities. Equity securities include U.S. and foreign common stocks and are classified either as trading or available for sale and carried at fair value. These securities are classified within Level 1 as their fair values are based on quoted market prices in active markets from independent pricing sources. As at September 30, 2014, the Company obtained an average of 4.0 quotes per equity investment, compared to 4.9 quotes as at December 31, 2013. Pricing sources used in pricing equities as at September 30, 2014 and December 31, 2013 were all provided by index providers.
Catastrophe Bonds. Catastrophe bonds held by the Company are variable rate fixed income instruments with redemption values adjusted based on the occurrence of a covered event, usually windstorms and earthquakes.  These bonds have been classified as trading and carried at fair value.  Bonds are priced using an average of multiple broker-dealer quotes and, as such, are classified as Level 2.
Foreign Exchange Contracts. The foreign exchange contracts which the Company uses to mitigate currency risk are characterized as OTC due to their customized nature and the fact that they do not trade on a major exchange. These instruments trade in a deep liquid market, providing substantial price transparency and accordingly are classified as Level 2.
Interest Rate Swaps. The interest rate swaps which the Company uses to mitigate interest rate risk are also characterized as OTC and are valued by the counterparty using quantitative models with multiple market inputs. The market inputs, such as interest rates and yield curves, are observable and the valuation can be compared for reasonableness with third party pricing services. Consequently, these instruments are classified as Level 2.
Loan Notes Issued by Variable Interest Entities. Silverton, a licensed special purpose insurer, is consolidated into the Company’s group accounts as a VIE. In the fourth quarter of 2013, Silverton issued $65.0 million ($50.0 million third-party funded) loan notes with a maturity date of September 16, 2016. The Company has elected to account for the loan notes at fair value using the guidance as prescribed under ASC 825, Financial Instruments as the Company believes it represents the most meaningful measurement basis for these liabilities. The loan notes are recorded at fair value at each reporting period and, as they are not quoted on an active market and contain significant unobservable inputs, they have been classified as a Level 3 instrument in the fair value hierarchy. The loan notes are unique because they are linked to the specific risks of the Company’s property catastrophe book.
To determine the fair value of the loan notes, the Company runs an internal model which considers the seasonality of the risk assumed under the retrocessional agreement between Aspen Bermuda and Silverton. The seasonality used in the model is determined by applying the percentage of property catastrophe losses planned by the Company’s actuaries to the estimated written premium to determine earned premium for each quarter. The inputs to the internal valuation model are based on Company specific data due to the lack of availability of observable market inputs. Reserves for losses are the most significant unobservable input. An increase in reserves for losses would normally result in a decrease in the fair value of the loan notes while a decrease in reserves would normally result in an increase in the fair value of the loan notes. The observable and unobservable inputs used to determine the fair value of the loan notes as at September 30, 2014 are presented in the table below:
At September 30, 2014		Fair Value				Valuation Method		Observable (O) and		Low				High
		Level 3						Unobservable (U) inputs
	($ in millions)									($ in millions)
Loan notes held by third parties		$	64.5			Internal Valuation Model		Gross premiums written (O)		$	—			$	40
								Reserve for losses (U)		$	—			$	2.8
								Contract period (O)		N/A				365 days
								Initial value of issuance (O)		$	50			$	50

Reinsurance

Reinsurance	9 Months Ended
	Sep. 30, 2014
Insurance [Abstract]	'
Reinsurance	'
	Reinsurance
	The Company purchases retrocession and reinsurance to limit and diversify the Company’s risk exposure and increase its own insurance and reinsurance underwriting capacity. These agreements provide for recovery of a portion of losses and loss adjustment expenses from reinsurers. As is the case with most reinsurance contracts, the Company remains liable to the extent that reinsurers do not meet their obligations under these agreements, and therefore, in line with its risk management objectives, the Company evaluates the financial condition of its reinsurers and monitors concentrations of credit risk. The largest concentrations of reinsurance recoverables as at September 30, 2014 were 24.9% with Lloyd’s syndicates rated A+ by S&P (December 31, 2013 — 23.5%) and 16.9% with Munich Re rated AA- by S&P (December 31, 2013  — 17.1%).

Derivative_Contracts

Derivative Contracts	9 Months Ended
	Sep. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]	'
Derivative Contracts	'
	Derivative Contracts
	The following table summarizes information on the location and amounts of derivative fair values on the consolidated balance sheet as at September 30, 2014 and December 31, 2013:
				As at September 30, 2014						As at December 31, 2013
	Derivatives Not Designated as Hedging Instruments		Balance Sheet Location	Notional			Fair			Notional		Fair
	Under ASC 815			Amount			Value			Amount		Value
				($ in millions)						($ in millions)
	Interest Rate Swaps		Derivatives at Fair Value	$	981.2		$	1.3		$	1,000.00	$	1.1		(1)
	Foreign Exchange Contracts		Derivatives at Fair Value	$	161.4		$	5.7		$	224.4	$	5.9
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	205.6		$	(12.7	)	$	57.5	$	(2.9	)
	(1)	Net of $23.4 million of cash collateral provided to counterparties, Goldman Sachs International ($481.2 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2013 — $34.3 million).
	The following tables provide the unrealized and realized gains/(losses) recorded in the statement of operations for the three and nine months ended September 30, 2014 and 2013, respectively:
				Amount of Income/(Loss)
				Recognized in the Statement
				of Operations for the
				Three Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	30-Sep-14			30-Sep-13
	ASC 815		Statement of Operations
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(6.4	)	$	10.8
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	1.3		$	(4.2	)
				Amount of Income/(Loss)
				Recognized in the Statement
				of Operations for the
				Nine Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	30-Sep-14			30-Sep-13
	ASC 815		Statement of Operations
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(4.5	)	$	(3.7	)
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	(4.1	)	$	3.2
	Foreign Exchange Contracts. The Company uses foreign exchange contracts to manage foreign currency risk. A foreign exchange contract involves an obligation to purchase or sell a specified currency at a future date at a price set at the time of the contract. Foreign exchange contracts will not eliminate fluctuations in the value of the Company’s assets and liabilities denominated in foreign currencies but rather allow it to establish a rate of exchange for a future point in time.
	As at September 30, 2014, the Company held foreign exchange contracts with an aggregate notional value of $367.0 million. The foreign exchange contracts are recorded as derivatives at fair value with changes recorded as a change in fair value of derivatives in the statement of operations. For the three and nine months ended September 30, 2014, the impact of foreign exchange contracts on net income was a charge of $6.4 million (September 30, 2013 — gain of $10.8 million) and a charge of $4.5 million (September 30, 2013 — charge of $3.7 million), respectively.
	Interest Rate Swaps. As at September 30, 2014, the Company held fixed for floating interest rate swaps with a total notional amount of $981.2 million (December 31, 2013 — $1.0 billion) that are due to mature between October 8, 2014 and November 9, 2020. The interest rate swaps are used in the ordinary course of the Company’s investment activities to partially mitigate the negative impact of rises in interest rates on the market value of the Company’s fixed income portfolio. For the three and nine months ended September 30, 2014, there was a gain in respect of the interest rate swaps of $1.3 million (September 30, 2013 — charge of $4.2 million) and a charge of $4.1 million (September 30, 2013 — gain of $3.2 million), respectively.
	As at September 30, 2014, cash collateral with a fair value of $23.4 million was held by the Company’s counterparties to support the current valuation of the interest rate swaps (December 31, 2013 — $34.3 million). As at September 30, 2014, no non-cash collateral was transferred to the Company by its counterparties (December 31, 2013 — $Nil). Transfers of cash collateral are recorded on the consolidated balance sheet within Derivatives at Fair Value, while transfers in respect of non-cash collateral are disclosed but not recorded. As at September 30, 2014, no amount was recorded in the consolidated balance sheet for the pledged assets.
	None of the derivatives mentioned above meet the requirements for hedge accounting as per ASC 815 Derivatives and Hedging and therefore changes in the estimated fair value are included in the consolidated statement of operations.

Deferred_Policy_Acquisition_Co

Deferred Policy Acquisition Costs	9 Months Ended
	Sep. 30, 2014
Insurance [Abstract]	'
Deferred Policy Acquisition Costs	'
	Deferred Policy Acquisition Costs
	The following table represents a reconciliation of beginning and ending deferred policy acquisition costs for the three and nine months ended September 30, 2014 and 2013:
			Three Months Ended September 30,						Nine Months Ended September 30,
			2014			2013			2014			2013
			($ in millions)						($ in millions)
	Balance at the beginning of the period		$	306.4		$	264.4		$	262.2		$	223
		Acquisition costs deferred	110.7			108.2			375.8			361.4
		Amortization of deferred policy acquisition costs	(115.5		)	(110.5		)	(336.4		)	(322.3		)
	Balance at the end of the period		$	301.6		$	262.1		$	301.6		$	262.1

Reserves_for_Losses_and_Loss_A

Reserves for Losses and Loss Adjustment Expenses	9 Months Ended
	Sep. 30, 2014
Insurance [Abstract]	'
Reserves for Losses and Loss Adjustment Expenses	'
	Reserves for Losses and Loss Adjustment Expenses
	The following table represents a reconciliation of beginning and ending consolidated loss and loss adjustment expenses (“LAE”) reserves for the nine months ended September 30, 2014 and twelve months ended December 31, 2013:
		Nine Months Ended September 30, 2014			Twelve Months Ended December 31, 2013
		($ in millions)
	Provision for losses and LAE at the start of the year	$	4,678.90		$	4,779.70
	Less reinsurance recoverable	(332.7		)	(499.0		)
	Net loss and LAE at the start of the year	4,346.20			4,280.70
	Net loss and LAE expenses assumed/(disposed)	(24.2		)	(34.6		)
	Provision for losses and LAE for claims incurred:
	Current year	1,060.50			1,331.40
	Prior years	(92.6		)	(107.7		)
	Total incurred	967.9			1,223.70
	Losses and LAE payments for claims incurred:
	Current year	(56.6		)	(172.8		)
	Prior years	(785.7		)	(912.3		)
	Total paid	(842.3		)	(1,085.1		)
	Foreign exchange (gains)	(45.0		)	(38.5		)
	Net losses and LAE reserves at period end	4,402.60			4,346.20
	Plus reinsurance recoverable on unpaid losses at period end	384.7			332.7
	Provision for losses and LAE at the end of the relevant period	$	4,787.30		$	4,678.90
	For the nine months ended September 30, 2014, there was a reduction of $92.6 million in the Company’s estimate of the ultimate claims to be paid in respect of prior accident years compared to a reduction of $87.2 million for the nine months ended September 30, 2013. The Company had $24.2 million worth of disposals relating to commutations during the nine months ended September 30, 2014 (nine months ended September 30, 2013 — $34.5 million). For additional information on the reserve releases, please refer to “Management’s Discussion and Analysis of Financial Condition and Results of Operations — Reserves for Losses and Loss Adjustment Expenses” below.

Capital_Structure

Capital Structure	9 Months Ended
	Sep. 30, 2014
Equity [Abstract]	'
Capital Structure	'
	Capital Structure
	The following table provides a summary of the Company’s authorized and issued share capital as at September 30, 2014 and December 31, 2013:
		As at September 30, 2014			As at December 31, 2013
		Number		$ in	Number	$ in
				Thousands		Thousands
	Authorized share capital:
	Ordinary Shares 0.15144558¢ per share	969,629,030		1,469	969,629,030	1,469
	Non-Voting Shares 0.15144558¢ per share	6,787,880		10	6,787,880	10
	Preference Shares 0.15144558¢ per share	100,000,000		152	100,000,000	152
	Total authorized share capital			1,631		1,631
	Issued share capital:
	Issued ordinary shares of 0.15144558¢ per share	63,349,858		96	65,546,976	99
	Issued 7.401% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	5,327,500		8	5,327,500	8
	Issued 7.250% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	6,400,000		10	6,400,000	10
	Issued 5.95% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	11,000,000		17	11,000,000	17
	Total issued share capital			131		134
	Additional paid-in capital as at September 30, 2014 was $1,186.4 million (December 31, 2013 — $1,297.4 million). Additional paid-in capital includes the aggregate liquidation preferences of the Company’s preference shares of $568.2 million (December 31, 2013 — $568.2 million) less issue costs of $12.4 million (December 31, 2013 — $12.4 million).
	Ordinary Shares. The following table summarizes transactions in the Company’s ordinary shares during the nine months ended September 30, 2014:
		Number of Ordinary Shares
	Ordinary shares in issue at December 31, 2013	65,546,976
	Ordinary share transactions in the nine months ended September 30, 2014
	Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or	692,929
	2008 share purchase plan
	Ordinary shares issued to non-employee directors	1,309
	Ordinary shares repurchased	(2,891,356	)
	Ordinary shares in issue at September 30, 2014	63,349,858
	Ordinary Share Repurchases. On February 7, 2013, the Company’s Board of Directors replaced the then existing share repurchase authorization of $400.0 million with a new share repurchase authorization of $500.0 million. The ordinary share repurchase authorization, which was effective immediately through February 7, 2015, permits the Company to effect repurchases from time to time through a combination of transactions, including open market repurchases, privately negotiated transactions and accelerated share repurchase transactions.
	Under open market repurchases, the Company acquired and cancelled 2,120,625 and 2,891,130 ordinary shares, respectively, for the three and nine months ended September 30, 2014. The total consideration paid for the three and nine months ended September 30, 2014 was $90.0 million and $120.9 million, respectively, with the average price for the three and nine months ended September 30, 2014 being $42.46 and $41.82, respectively. Under open market repurchases, the Company acquired and cancelled 1,498,451 and 3,979,826 ordinary shares, respectively, for the three and nine months ended September 30, 2013. The total consideration paid for the three and nine months ended September 30, 2013 was $54.8 million and $143.0 million, respectively; and the average price was $36.59 and $35.92, respectively.
	On February 26, 2013, the Company entered into an Accelerated Share Repurchase Agreement (“ASR”) with Goldman Sachs & Co. (“Goldman”) to repurchase an aggregate of $150.0 million of the Company’s ordinary shares. Under this arrangement, the Company initially acquired and cancelled 3,348,214 ordinary shares on March 1, 2013.
	The ASR commenced on February 27, 2013, and was terminated on August 29, 2013. Settlement was made entirely in the Company’s ordinary shares and was accounted for as an equity transaction under the guidelines specified under ASC 815 Derivatives and Hedging. On August 29, 2013, Goldman terminated the ASR and delivered to the Company an additional 705,062 ordinary shares. The total amount repurchased under the ASR was 4,053,276 ordinary shares at an average price of $37.01.
	In addition to the share repurchase program, the Company purchases shares offered from time to time by Appleby Services (Bermuda) Ltd. (the “Names’ Trustee”). On March 4, 2013, the Company entered into a share repurchase agreement with the Names’ Trustee for the purchase of 54,437 ordinary shares for a total purchase price of $2.0 million. This share repurchase closed on March 21, 2013.
	Preference Shares Redemption. During 2005 and 2006, the Company issued 4.6 million 5.625% Perpetual Preferred Income Equity Replacement Securities (“PIERS”). The PIERS were convertible at the Company’s option if, at any time on or after January 1, 2009, the closing sale price of the Company’s ordinary shares equalled or exceeded 130% of the then prevailing conversion price for 20 trading days during any consecutive 30-trading day period, as well as the last day of such 30-day period.
	The PIERS were dilutive to the Company’s ordinary shares when the Company’s share price exceeded the prevailing conversion price and therefore, as the Company’s share price was generally above the 130% conversion price test, they were included in the Company’s fully diluted share count until the Company announced it would mandatorily redeem the PIERS.
	On April 25, 2013, the Company announced it would mandatorily redeem all of its PIERS outstanding based on the terms of the PIERS. Each holder of a PIERS unit received $50.00, equating to a total payment of $230.0 million in cash plus a number of the Company’s ordinary shares based on the conversion rate calculated in accordance with the average trading price of the Company’s ordinary shares over a 20-trading day settlement period following the Company’s issuance of the press release announcing the mandatory conversion. The conversion rate was 1.7121 shares of the Company’s ordinary shares per $50.00 liquidation preference of the PIERS equating to a total issuance of 1,835,860 ordinary shares. The Company settled the amount on May 30, 2013. In accordance with the terms of the PIERS, no further dividends were paid on the PIERS following the announcement of their mandatory redemption. Additionally, upon redemption of the PIERS, the $7.1 million difference between the capital raised upon issuance of the PIERS, net of original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million was reclassified from additional paid-in capital to retained earnings.
	Preference Shares Issuance. On May 2, 2013, the Company issued 11.0 million shares of 5.95% of Fixed-to-Floating Perpetual Non-Cumulative Preference Shares (the “5.95% Preference Shares”). Each preference shareholder will receive dividends on a non-cumulative basis only when declared by the Board of Directors of the Company initially at an annual fixed rate of 5.95% until July 1, 2023 at which time a floating rate, reset quarterly, of 3-month LIBOR plus 4.06% will commence per annum. The 5.95% Preference Shares have a liquidation preference of $25.00 per share and net proceeds were $270.4 million (comprising $275.0 million of total liquidation preference less $4.6 million of issue expenses). The Company used $230.0 million of the net proceeds from this offering for settling the cash portion of the mandatory conversion of the PIERS.
	The 5.95% Preference Shares rank equally with preference shares previously issued by the Company and have no fixed maturity date. The Company may redeem all or a portion of the 5.95% Preference Shares at a redemption price of $25.00 per share on or after July 1, 2023. The Company has listed the 5.95% Preference Shares on the New York Stock Exchange under the symbol “AHLPRC.”
	Rights Agreement. On April 17, 2014, the Board of Directors of the Company resolved to issue one preferred share purchase right (a “Right”) for each outstanding ordinary share, and adopted a shareholder rights plan, as set forth in the Rights Agreement dated as of April 17, 2014. Each Right will allow its holder to purchase from the Company one one-thousandth of a share of Series A Junior Participating Preference Shares for $160, once the Rights become exercisable. The Rights will not be exercisable until 10 business days after the public announcement that a person or group has acquired the beneficial ownership of 10% or more of the outstanding ordinary shares of the Company (or 15% in the case of passive institutional investors). The Rights may be redeemed at any time at the discretion of the Board of Directors of the Company. As of September 30, 2014, no Rights have been exercisable or exercised.

ShareBased_Payments

Share-Based Payments	9 Months Ended
	Sep. 30, 2014
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 two arrangements: the employee incentive plans and the non-employee director plan. When options are exercised or other equity awards have vested, new shares are issued as the Company does not currently hold treasury shares.
	Employee and Non-Employee Director Awards. Employee options and other awards were granted under the Aspen 2003 Share Incentive Plan, as amended (the “2003 Share Incentive Plan”), prior to April 24, 2013 and thereafter under the 2013 Share Incentive Plan (the “2013 Share Incentive Plan”). The total number of ordinary shares that may be issued under the 2013 Share Incentive Plan is 2,845,683 ordinary shares, which includes 595,683 ordinary shares available to grant under the 2003 Share Incentive Plan as of February 25, 2013. The number of ordinary shares that may be issued under the 2013 Share Incentive Plan is adjusted per the number of awards that may be forfeited under the 2003 Share Incentive Plan. The non-employee director awards are granted under the 2006 Stock Option Plan for Non-Employee Directors.
	Stock options were granted with an exercise price equivalent to the fair value of the share on the grant date. The weighted average value at grant date is determined using the Black-Scholes option pricing model. Stock options typically vest over a three-year period with a ten-year exercise period (except for options granted in 2007 which have a seven-year exercise period) with vesting dependent on time and performance conditions established at the time of grant. No options were granted during the three and nine months ended September 30, 2014 (2013 — Nil and Nil) and Nil and 53,436 options, respectively, were exercised and issued in the three and nine months ended September 30, 2014 (2013 — 204,328 and 747,023 options). No charges against income were made in respect of employee options for the three and nine months ended September 30, 2014 (2013 — $Nil and $Nil).
	Restricted share units (“RSUs”) granted to employees typically vest over a three-year period, based on continued service. Some of the RSU grants vest at year-end, while others vest on the anniversary of the date of grant or when the Compensation Committee of the Board of Directors agrees to deliver the RSUs. The fair value of the RSUs is based on the closing price on the date of the grant adjusted for illiquidity and is expensed through the income statement evenly over the vesting period. In the three and nine months ended September 30, 2014, the Company granted Nil and 259,640 RSUs to its employees, respectively (2013 — 8,571 and 307,441). Compensation costs charged against income in respect of employee RSUs for the three and nine months ended September 30, 2014 were $1.9 million and $7.7 million, respectively (2013 — $2.4 million and $7.0 million).
	In the case of non-employee directors, generally one-twelfth of the RSUs vest on each one month anniversary of the date of grant, with 100% of the RSUs vesting on the first anniversary of the date of grant. On February 6, 2014 (with a grant date of February 10, 2014), the Board of Directors approved a total of 27,180 RSUs for the non-employee directors (February 11, 2013 — 25,533 and April 26, 2013 — 2,110) and 13,590 RSUs to the Chairman (February 11, 2013 — 14,188). Compensation costs charged against income in respect of non-employee director RSUs for the three and nine months ended September 30, 2014 were $0.3 million and $1.1 million, respectively (2013 — $0.3 million and $0.6 million).
	The total fair value adjustment for all RSUs for the three and nine months ended September 30, 2014 was $0.3 million and $0.9 million respectively (2013 — $0.1 million and $0.3 million). The total tax credit recognized by the Company in relation to RSUs in the three and nine months ended September 30, 2014 was $0.5 million and $1.8 million, respectively (2013 — $0.5 million and $1.0 million).
	Performance Shares. During the three and nine months ended September 30, 2014, the Company granted Nil and 315,389 performance shares, respectively (three and nine months ended September 30, 2013 — Nil and 250,066). The performance shares will be subject to a three-year vesting period with a separate annual diluted book value per share (“BVPS”) growth test for each year, adjusted to add back ordinary dividends and movements in AOCI to shareholders’ equity at the end of the relevant year. One-third of the grant will be eligible for vesting each year based on a formula, and will only be issuable at the end of the three-year period.
	If the diluted BVPS growth achieved in 2014 is:
	•	less than 5.2%, then the portion of the performance shares subject to the vesting conditions in such year will be forfeited (i.e., one-third of the initial grant);
	•	between 5.2% and 10.4%, then the percentage of the performance shares eligible for vesting in such year will be between 10% and 100% on a straight-line basis; or
	•	between 10.4% and 20.8%, then the percentage of the performance shares eligible for vesting in such year will be between 100% and 200% on a straight-line basis.
	In calculating BVPS for 2014, the entire movement in AOCI will be excluded. Interest rate movements and credit spread movements in AOCI can be fairly significant and adversely impact growth in BVPS which management does not have any control over. The Compensation Committee also agreed that it will review the impact of any capital management actions undertaken during 2014, including share repurchases and special dividends, and consider whether any further adjustments to growth in BVPS should be may be made in the context of such actions. The Compensation Committee also further agreed to exclude from the calculation of BVPS for 2014 the costs payable to third-party service providers resulting from the Company’s response to the proposals received from Endurance Specialty Holdings Ltd. (“Endurance”). The Compensation Committee believes that it would not be appropriate for employee’s performance-related compensation to be impacted by these costs.
	The Compensation Committee will determine the vesting conditions for the 2015 and 2016 portions of the grant in such years taking into consideration the market conditions and the Company’s business plans at the commencement of the years concerned. Notwithstanding the vesting criteria for each given year, if the shares eligible for vesting in 2015 and 2016 are greater than 100% for the portion of such year’s grant and the average diluted BVPS growth over such year and the preceding year is less than the average of the minimum vesting thresholds for such year and the preceding year, then only 100% (and no more) of the ordinary shares that are eligible for vesting in such year shall vest. Notwithstanding the foregoing, if in the judgment of the Compensation Committee the main reason for the BVPS metric in the earlier year falling below the minimum threshold is due to the impact of rising interest rates and bond yields, then the Compensation Committee may, in its discretion, disapply this limitation on 100% vesting.
	The fair value of performance share awards is based on the value of the closing share price on the date of the grant adjusted for illiquidity less a deduction for expected dividends which would not accrue during the vesting period. Compensation costs charged against income in the three and nine months ended September 30, 2014 in respect of performance shares were $0.3 million and $4.5 million, respectively (2013 — $1.4 million and $4.2 million). The total tax recognized by the Company in relation to performance shares in the three and nine months ended September 30, 2014 was a tax credit of $0.1 million and $1.0 million, respectively (2013 — $0.7 million and $1.3 million).
	Phantom Shares. On April 22, 2014, the Compensation Committee approved the grant of 154,512, phantom shares with a grant date of April 25, 2014 to its employees (February 6, 2013 — 152,541 with additional grants of 6,521 and 542 phantom shares made on April 8, 2013 and June 11, 2013, respectively.) The phantom shares are subject to a three-year vesting period with a separate annual diluted BVPS growth test for each year, in accordance with the test described above for the 2014 performance shares, with the difference being that any vested amount would be paid in cash in lieu of ordinary shares. As ordinary shares are not issued, these instruments have no dilutive effect.
	The fair value of the phantom shares is based on the closing share price on the date of the grant adjusted for illiquidity, less estimated dividends payable over the vesting period. The fair value is expensed through the consolidated income statement evenly over the vesting period, but as the payment to beneficiaries will ultimately be in cash rather than ordinary shares, an adjustment is required each quarter to revalue the accumulated liability to the balance sheet date fair value. Compensation costs charged against income in the three and nine months ended September 30, 2014 in respect of phantom shares were $1.2 million and $3.6 million, respectively (2013 — a credit of $1.6 million and a charge of $0.0 million) with a fair value adjustment for the three and nine months ended September 30, 2014 of $0.8 million and $1.3 million, respectively (2013 — $0.9 million and $1.7 million, respectively). The total tax credit recognized by the Company in relation to phantom shares in the three and nine months ended September 30, 2014 was $0.2 million and $0.7 million, respectively (2013 — $0.1 million and $0.1 million, respectively).
	Employee Share Purchase Plans. On April 30, 2008, the shareholders of the Company approved the Employee Share Purchase Plan, the 2008 Sharesave Scheme and the International Employee Share Purchase Plan (collectively, the “ESPP”), which are implemented by a series of consecutive offering periods as determined by the Board of Directors. In respect of the Employee Share Purchase Plan, employees can save up to $500 per month over a two-year period, at the end of which they will be eligible to purchase Company shares at a discounted price, subject to a further one year holding period. In respect of the 2008 Sharesave Scheme, employees can save up to £250 per month over a three-year period (and effective April 6, 2014, £500 per month), at the end of which they will be eligible to purchase Company shares at a discounted price. The purchase price will be eighty-five percent (85%) of the fair market value of a share on the offering date which may be adjusted upon changes in capitalization of the Company. Under the ESPP, 627 and 11,821 ordinary shares, respectively, were issued during the three and nine months ended September 30, 2014 (2013 — 1,985 and 16,339 shares). Compensation costs charged against income in the three and nine months ended September 30, 2014 in respect of the ESPP were $0.1 million and $0.2 million, respectively (2013 — $0.1 million and $0.2 million).

Intangible_Assets

Intangible Assets	9 Months Ended
	Sep. 30, 2014
Goodwill and Intangible Assets Disclosure [Abstract]	'
Intangible Assets	'
	Intangible Assets
	The following tables provide a summary of the Company’s intangible assets for the three and nine months ended September 30, 2014 and 2013:
		Three Months Ended September 30, 2014										Three Months Ended September 30, 2013
		Trade		Insurance		Other			Total			Trade		Insurance		Other			Total
		Mark		Licenses								Mark		Licenses
		($ in millions)										($ in millions)
	Intangible Assets
	Beginning of the period	$	1.6	$	16.6	$	—		$	18.2		$	1.6	$	16.6	$	0.5		$	18.7
	Amortization	—		—		—			—			—		—		(0.1		)	(0.1		)
	End of the period	$	1.6	$	16.6	$	—		$	18.2		$	1.6	$	16.6	$	0.4		$	18.6
		Nine Months Ended September 30, 2014										Nine Months Ended September 30, 2013
		Trade		Insurance		Other			Total			Trade		Insurance		Other			Total
		Mark		Licenses								Mark		Licenses
		($ in millions)										($ in millions)
	Intangible Assets
	Beginning of the period	$	1.6	$	16.6	$	0.2		$	18.4		$	1.6	$	16.6	$	0.8		$	19
	Amortization	—		—		(0.2		)	(0.2		)	—		—		(0.4		)	(0.4		)
	End of the period	$	1.6	$	16.6	$	—		$	18.2		$	1.6	$	16.6	$	0.4		$	18.6
	License to use the “Aspen” Trademark. On April 5, 2005, the Company entered into an agreement with Aspen (Actuaries and Pension Consultants) Plc to acquire the right to use the Aspen trademark in the United Kingdom. The consideration paid was approximately $1.6 million. As at September 30, 2014, the value of the license to use the Aspen trademark was $1.6 million (December 31, 2013 — $1.6 million). The trademark has an indefinite useful life and is tested for impairment annually or when events or changes in circumstances indicate that the asset might be impaired.
	Insurance Licenses. The total value of the licenses as at September 30, 2014 was $16.6 million (December 31, 2013 — $16.6 million). This includes $10.0 million of acquired licenses held by AAIC, $4.5 million of acquired licenses held by Aspen Specialty and $2.1 million of acquired licenses held by Aspen U.K. The insurance licenses are considered to have an indefinite life and are not being amortized. The licenses have an indefinite life and are tested for impairment annually or when events or changes in circumstances indicate that the asset might be impaired.
	Other. In 2010, the Company purchased APJ Continuation Limited and its subsidiaries (“APJ”) for an aggregate consideration of $4.8 million. The directors of Aspen Holdings assessed the fair value of the net tangible and financial assets acquired at $1.2 million. The $3.6 million intangible asset represented the Company’s assessment of the value of renewal rights and distribution channels ($2.2 million) and the lock-in period for employees associated with the business ($1.4 million). The asset was amortized over a five-year period and the value as at September 30, 2014 was $Nil (December 31, 2013 — $0.2 million).

Commitments_and_Contingent_Lia

Commitments and Contingent Liabilities	9 Months Ended
	Sep. 30, 2014
Commitments and Contingencies Disclosure [Abstract]	'
Commitments and Contingent Liabilities	'
	Commitments and Contingent Liabilities
	(a)	Restricted assets
	The Company is obliged by the terms of its contractual obligations to U.S. policyholders and by obligations to certain regulatory authorities to facilitate issue of letters of credit or maintain certain balances in trust funds for the benefit of policyholders.
	The following table details the forms and value of Company’s restricted assets as at September 30, 2014 and December 31, 2013:
		As at September 30, 2014			As at December 31, 2013
		($ in millions, except percentages)
	Regulatory trusts and deposits:
	Affiliated transactions	$	741.4		$	685.8
	Third party	2,146.10			2,236.40
	Letters of credit / guarantees(1)	776.4			830.4
	Total restricted assets	$	3,663.90		$	3,752.60
	Total as percent of cash and invested assets	42.8		%	45.5		%
	(1)	As of September 30, 2014, the Company had pledged funds of $749.4 million and £16.2 million (December 31, 2013 — $803.7 million and £16.1 million) as collateral for the secured letters of credit.
	Funds at Lloyd’s. AUL operates at Lloyd’s as the corporate member for Syndicate 4711. Lloyd’s determines Syndicate 4711’s required regulatory capital principally through the syndicate’s annual business plan. Such capital, called Funds at Lloyd’s, comprises cash and investments.
	The amounts provided as Funds at Lloyd’s will be drawn upon and become a liability of the Company in the event of Syndicate 4711 declaring a loss at a level that cannot be funded from other resources, or if Syndicate 4711 requires funds to cover a short term liquidity gap. The amount which the Company provides as Funds at Lloyd’s is not available for distribution to the Company for the payment of dividends. Aspen Managing Agency Limited, the managing agent to Syndicate 4711, is also required by Lloyd’s to maintain a minimum level of capital which as at September 30, 2014 was $0.7 million (December 31, 2013 — $0.7 million). This is not available for distribution by the Company for the payment of dividends.
	Letters of Credit Facility. On June 30, 2014, Aspen Bermuda and Citibank Europe plc (“Citi Europe”) replaced an existing letter of credit facility, dated July 30, 2012, in a maximum aggregate amount of up to $950.0 million (the “LOC Facility”) comprised of two maturity tranches (Tranche I with a limit of $650.0 million and Tranche II with a limit of $300.0 million) which expired on its own terms on June 30, 2014. The LOC Facility was replaced with a new letter of credit facility in a maximum aggregate amount of up to $575.0 million (the “New LOC Facility”). Under the New LOC Facility, which will expire on June 30, 2016, Aspen Bermuda will pay to Citi Europe (a) a letter of credit fee based on the available amounts of each letter of credit and (b) a commitment fee, which varies based upon usage, on the unutilized portion of the New LOC Facility. Aspen Bermuda will also pay interest on the amount drawn by any beneficiary under a credit provided under the New LOC Facility at a rate per annum of LIBOR plus 1% (plus reserve asset costs, if any) from the date of drawing until the date of reimbursement by Aspen Bermuda. The New LOC Facility is used to secure obligations of Aspen Bermuda to its policyholders. In addition to the New LOC Facility, we also use regulatory trusts to secure our obligations to policyholders.
	The terms of a Pledge Agreement between Aspen Bermuda and Citi Europe (pursuant to an Assignment Agreement dated October 11, 2006) dated January 17, 2006, as amended, were also amended on June 30, 2014 to change the types of securities or other assets that are acceptable as collateral under the New LOC Facility. All other agreements relating to Aspen Bermuda’s LOC Facility, which now apply to the New LOC Facility with Citi Europe, as previously filed with the United States Securities and Exchange Commission, remain in full force and effect.
	Aspen U.K. and Aspen Bermuda have a $100.0 million secured letter of credit facility agreement with Barclays Bank PLC. As at September 30, 2014, the Company had $12.3 million of outstanding collateralized letters of credit under this facility (December 31, 2013 — $18.9 million).
	Interest Rate Swaps. As at September 30, 2014, cash collateral with a fair value of $23.4 million was held by the Company’s counterparties to support the current valuation of the interest rate swaps (December 31, 2013 — $34.3 million). For more information, please refer to Note 10 of these unaudited condensed consolidated financial statements.
	(b)	Operating leases
	Amounts outstanding under operating leases net of subleases as of September 30, 2014 were:
		2014			2015			2016		2017		2018		Later		Total
														Years
		($ in millions)
	Operating Lease Obligations	$	1.8		$	13.6		$	9.7	$	8.5	$	7.2	$	20.3	$	61.1
	(c)	Contingent liabilities
	In common with the rest of the insurance and reinsurance industry, the Company is also subject to litigation and arbitration in the ordinary course of business. The Company’s Operating Subsidiaries are regularly engaged in the investigation, conduct and defense of disputes, or potential disputes, resulting from questions of insurance or reinsurance coverage or claims activities. Pursuant to insurance and reinsurance arrangements, many of these disputes are resolved by arbitration or other forms of alternative dispute resolution. Such legal proceedings are considered in connection with estimating the Company’s Insurance Reserves — Loss and Loss Adjustment Expenses, as provided on the Company’s consolidated balance sheet.
	In some jurisdictions, 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 of September 30, 2014, based on available information, it was the opinion of the Company’s management that the probability of the ultimate resolution of pending or threatened litigation or arbitrations having a material effect on the Company’s financial condition, results of operations or liquidity would be remote.

Basis_of_Preparation_Policies

Basis of Preparation (Policies)	9 Months Ended
	Sep. 30, 2014
Accounting Policies [Abstract]	'
Accounting Policies Not Yet Adopted	'
	Accounting Pronouncements Not Yet Adopted
	On August 5, 2014, the FASB issued ASU 2014-13, “Measuring the Financial Assets and the Financial Liabilities of a Consolidated Collateralized Financing Entity" which addresses the measurement difference in both the initial consolidation and the subsequent measurement of the financial assets and the financial liabilities of a collateralized financing entity. ASU 2014-13 is effective for fiscal years beginning after December 15, 2015 and interim periods beginning after December 15, 2015. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	On August 8, 2014, the FASB issued ASU 2014-14, “Receivables - Troubled debt restructuring by creditors (Subtopic 310-40)" which reduces diversity in practice by addressing the classification of certain foreclosed mortgage loans held by creditors that are either fully or partially guaranteed under government programs. ASU 2014-14 is effective for fiscal years beginning after December 15, 2014 and interim periods beginning after December 15, 2014. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.
	On August 27, 2014, the FASB issued ASU 2014-15, “Presentation of Financial Statements - Going Concern (Subtopic 204-40)" which provides US GAAP guidance on management’s responsibility to evaluate whether there is substantial doubt about an entity’s ability to continue as a going concern or to provide related footnote disclosures. ASU 2014-15 is effective for fiscal years beginning after December 15, 2016 and interim periods beginning after December 15, 2016. Early application for a public business entity is permitted. The Company does not expect this ASU to have a material impact on its consolidated financial statements.

Reclassifications_from_Accumul1

Reclassifications from Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2014
Equity [Abstract]	'
Schedule of Accumulated Other Comprehensive Income Reclassification	'
	The following tables set out the components of the Company’s accumulated other comprehensive income (“AOCI”) that are reclassified into the unaudited condensed consolidated statement of operations for the three and nine months ended September 30, 2014 and 2013:
		Amount Reclassified from AOCI
	Details about the AOCI Components	Three Months Ended September 30, 2014			Three Months Ended September 30, 2013			Affected Line Item in the Unaudited
								Condensed Consolidated Statement
								of Operations
		($ in millions)
	Available for sale securities:
	Realized gains on sale of securities	$	3.9		$	8.6		Realized and unrealized investment gains
	Realized (losses) on sale of securities	(0.3		)	(0.3		)	Realized and unrealized investment losses
		3.6			8.3			Income from operations before income tax
	Tax on net realized gains of securities	(0.1		)	(0.1		)	Income tax expense
		$	3.5		$	8.2		Net income
	Foreign currency translation adjustments:
	Foreign currency translation adjustments, before tax	$	—		$	4.8		Net realized and unrealized foreign exchange gains/(losses)
	Tax on foreign currency translation adjustments	—			(1.1		)	Income tax expense
		$	—		$	3.7		Net income
	Amortization of derivatives:
	Amortization of long-term debt associated expenses, before tax	$	—		$	—		Interest expense
		$	—		$	—		Net income
	Total reclassifications from AOCI to the statement of operations, net of tax	$	3.5		$	11.9		Net income
		Amount Reclassified from AOCI
	Details about the AOCI Components	Nine Months Ended September 30, 2014			Nine Months Ended September 30, 2013			Affected Line Item in the Unaudited
								Condensed Consolidated Statement
								of Operations
		($ in millions)
	Available for sale securities:
	Realized gains on sale of securities	$	10.6		$	21.2		Realized and unrealized investment gains
	Realized (losses) on sale of securities	(4.7		)	(0.5		)	Realized and unrealized investment losses
		5.9			20.7			Income from operations before tax
	Tax on net realized gains of securities	(0.2		)	(0.5		)	Income tax expense
		$	5.7		$	20.2		Net income
	Foreign currency translation adjustments:
	Foreign currency translation adjustments, before tax	$	—		$	0.5		Net realized and unrealized foreign exchange gains/(losses)
	Tax on foreign currency translation adjustments	—			(0.1		)	Income tax expense
		$	—		$	0.4		Net income
	Amortization of derivatives:
	Amortization of long-term debt associated expenses, before tax	$	—		$	(0.2	)	Interest expense
		$	—		$	(0.2	)	Net income
	Total reclassifications from AOCI to the statement of operations, net of tax	$	5.7		$	20.4		Net income

Earnings_per_Ordinary_Share_Ta

Earnings per Ordinary Share (Tables)	9 Months Ended
	Sep. 30, 2014
Earnings Per Share [Abstract]	'
Computation of Basic and Diluted Earnings per Share	'
	The following table sets forth the computation of basic and diluted earnings per share for the three and nine months ended September 30, 2014 and 2013, respectively:
		Three Months Ended September 30,						Nine Months Ended September 30,
		2014			2013			2014			2013
		($ in millions, except share and per share amounts)
	Net income	$	37.4		$	107.4		$	288.6		$	239.3
	Preference share dividends	(9.5		)	(9.5		)	(28.4		)	(26.1		)
	Change in redemption value (1)	—			—			—			(7.1		)
	Net profit attributable to non-controlling interest	0.1			0.3			—			0.3
	Basic and diluted net income available to ordinary shareholders	28			98.2			260.2			206.4
	Ordinary shares:
	Basic weighted average ordinary shares	65,116,463			66,716,202			65,283,681			67,302,857
	Weighted average effect of dilutive securities(2)	1,396,546			1,845,313			1,314,999			2,656,617
	Total diluted weighted average ordinary shares	66,513,009			68,561,515			66,598,680			69,959,474
	Earnings per ordinary share:
	Basic	$	0.43		$	1.47		$	3.99		$	3.06
	Diluted	$	0.42		$	1.43		$	3.91		$	2.95
	(1)	The $7.1 million reduction in the net income available to ordinary shareholders is the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of $230.0 million of the PIERS on May 30, 3013.
	(2)	Dilutive securities comprise: employee options, restricted share units and performance shares associated with the Company’s long term incentive plan, employee share purchase plans and director restricted stock units and options as described in Note 14, in addition to the PIERS that were fully redeemed on May 30, 2013.
Summary of Declared Dividends	'
	Dividends. On October 29, 2014, the Company’s Board of Directors declared the following quarterly dividends:
		Dividend			Payable on:		Record Date:
	Ordinary shares	$	0.2		December 2, 2014		November 13, 2014
	7.401% preference shares	$	0.462563		January 1, 2015		December 15, 2014
	7.250% preference shares	$	0.4531		January 1, 2015		December 15, 2014
	5.95% preference shares	$	0.3719		January 1, 2015		December 15, 2014

Segment_Reporting_Tables

Segment Reporting (Tables)	9 Months Ended
	Sep. 30, 2014
Segment Reporting [Abstract]	'
Summary of Gross and Net Written and Earned Premiums, Underwriting Results, Ratios and Reserves for Each of Company's Business Segments	'
	The following tables provide a summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of the Company’s business segments for the three months ended September 30, 2014 and 2013:
		Three Months Ended September 30, 2014
		Reinsurance			Insurance			Total
		($ in millions)
	Underwriting Revenues
	Gross written premiums	$	256.9		$	395.6		$	652.5
	Net written premiums	250.9			326.4			577.3
	Gross earned premiums	291			403.9			694.9
	Net earned premiums	279.6			330.8			610.4
	Underwriting Expenses
	Losses and loss adjustment expenses	132			210.7			342.7
	Amortization of deferred policy acquisition costs	52.1			63.4			115.5
	General and administrative expenses	38.4			45.6			84
	Underwriting income	$	57.1		$	11.1		68.2
	Corporate expenses							(35.8		)
	Net investment income							48
	Realized and unrealized investment gains							1.1
	Realized and unrealized investment losses							(1.3		)
	Change in fair value of loan notes issued by variable interest entities							(8.5		)
	Change in fair value of derivatives							(5.1		)
	Interest expense on long term debt							(7.4		)
	Net realized and unrealized foreign exchange (losses)							(21.2		)
	Other income							1
	Other expenses							(0.3		)
	Income before tax							$	38.7
	Net reserves for loss and loss adjustment expenses	$	2,554.20		$	1,848.40		$	4,402.60
	Ratios
	Loss ratio	47.2		%	63.7		%	56.1		%
	Policy acquisition expense ratio	18.6			19.2			18.9
	General and administrative expense ratio	13.7			13.8			19.6			(1)
	Expense ratio	32.3			33			38.5
	Combined ratio	79.5		%	96.7		%	94.6		%
	(1)	The general and administrative expense ratio in the total column includes corporate expenses.
		Three Months Ended September 30, 2013
		Reinsurance			Insurance			Total
		( $ in millions)
	Underwriting Revenues
	Gross written premiums	$	219.5		$	362.1		$	581.6
	Net written premiums	218.4			323.6			542
	Gross earned premiums	268.6			356.5			625.1
	Net earned premiums	255.7			288.6			544.3
	Underwriting Expenses
	Losses and loss adjustment expenses	122.2			168			290.2
	Amortization of deferred policy acquisition costs	49.1			61.4			110.5
	General and administrative expenses	34.6			49.5			84.1
	Underwriting income	$	49.8		$	9.7		59.5
	Corporate expenses							(14.8		)
	Net investment income							45
	Realized and unrealized investment gains							23.6
	Realized and unrealized investment losses							(5.9		)
	Change in fair value of derivatives							6.6
	Interest expense on long term debt							(7.7		)
	Net realized and unrealized foreign exchange gains							2.4
	Other income							1.6
	Other expenses							—
	Income before tax							$	110.3
	Net reserves for loss and loss adjustment expenses	$	2,718.00		$	1,555.40		$	4,273.40
	Ratios
	Loss ratio	47.8		%	58.2		%	53.3		%
	Policy acquisition expense ratio	19.2			21.3			20.3
	General and administrative expense ratio	13.5			17.2			18.2			(1)
	Expense ratio	32.7			38.5			38.5
	Combined ratio	80.5		%	96.7		%	91.8		%
(1)	The general and administrative expense ratio in the total column includes corporate expenses.
The following tables provide a summary of gross and net written and earned premiums, underwriting results, ratios and reserves for each of the Company’s business segments for the nine months ended September 30, 2014 and 2013:
	Nine Months Ended September 30, 2014
	Reinsurance			Insurance			Total
	($ in millions)
Underwriting Revenues
Gross written premiums	$	1,027.50		$	1,259.80		$	2,287.30
Net written premiums	980.4			980.8			1,961.20
Gross earned premiums	859.2			1,182.00			2,041.20
Net earned premiums	825.1			968			1,793.10
Underwriting Expenses
Losses and loss adjustment expenses	367.4			600.5			967.9
Amortization of deferred policy acquisition costs	152.3			184.1			336.4
General and administrative expenses	107			142.6			249.6
Underwriting income	$	198.4		$	40.8		239.2
Corporate expenses							(74.6		)
Net investment income							143.6
Realized and unrealized investment gains							53.6
Realized and unrealized investment losses							(8.9		)
Change in fair value of loan notes issued by variable interest entities							(14.5		)
Change in fair value of derivatives							(8.6		)
Interest expense on long term debt							(22.1		)
Net realized and unrealized foreign exchange (losses)							(10.4		)
Other income							4.8
Other expenses							(2.2		)
Income before tax							$	299.9
Net reserves for loss and loss adjustment expenses	$	2,554.20		$	1,848.40		$	4,402.60
Ratios
Loss ratio	44.5		%	62		%	54		%
Policy acquisition expense ratio	18.5			19			18.8
General and administrative expense ratio	13			14.7			18.1			(1)
Expense ratio	31.5			33.7			36.9
Combined ratio	76		%	95.7		%	90.9		%
(1)	The general and administrative expense ratio in the total column includes corporate expenses.
	Nine Months Ended September 30, 2013
	Reinsurance			Insurance			Total
	( $ in millions)
Underwriting Revenues
Gross written premiums	$	957.7		$	1,084.60		$	2,042.30
Net written premiums	907.5			844.2			1,751.70
Gross earned premiums	828.9			1,000.70			1,829.60
Net earned premiums	788.2			811			1,599.20
Underwriting Expenses
Losses and loss adjustment expenses	394.9			497.4			892.3
Amortization of deferred policy acquisition costs	161			161.3			322.3
General and administrative expenses	97.2			134			231.2
Underwriting income	$	135.1		$	18.3		153.4
Corporate expenses							(42.0		)
Net investment income							139.2
Realized and unrealized investment gains							54.3
Realized and unrealized investment losses							(28.0		)
Change in fair value of derivatives							(0.5		)
Interest expense on long term debt							(23.2		)
Net realized and unrealized foreign exchange (losses)							(7.1		)
Other income							3.6
Other expenses							(0.6		)
Income before tax							$	249.1
Net reserves for loss and loss adjustment expenses	$	2,718.00		$	1,555.40		$	4,273.40
Ratios
Loss ratio	50.1		%	61.3		%	55.8		%
Policy acquisition expense ratio	20.4			19.9			20.2
General and administrative expense ratio	12.3			16.5			17.1			(1)
Expense ratio	32.7			36.4			37.3
Combined ratio	82.8		%	97.7		%	93.1		%
(1)	The general and administrative expense ratio in the total column includes corporate expenses.

Investments_Tables

Investments (Tables)	9 Months Ended
	Sep. 30, 2014
Investments, Debt and Equity Securities [Abstract]	'
Summary of Investment Income	'
	Investment Income. The following table summarizes investment income for the three and nine months ended September 30, 2014 and 2013:
		For the Three Months Ended							For the Nine Months Ended
		30-Sep-14				30-Sep-13			30-Sep-14			30-Sep-13
		($ in millions)							($ in millions)
	Fixed income securities — Available for sale	$	37.6			$	38.2		$	113.4		$	117.2
	Fixed income securities — Trading	6.6				5.3			19.8			13.4
	Short-term investments — Available for sale	0.4				0.3			1			1.9
	Fixed term deposits (included in cash and cash equivalents)	0.5				0.7			2.4			3.4
	Equity securities — Available for sale	1				1.2			3.5			4.7
	Equity securities — Trading	3.5				1.8			10.5			5.5
	Catastrophe bonds — Trading	0.5				—			0.9			—
	Total	$	50.1			$	47.5		$	151.5		$	146.1
	Investment expenses	(2.1		)		(2.5		)	(7.9		)	(6.9		)
	Net investment income	$	48			$	45		$	143.6		$	139.2
Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments	'
	The following table summarizes the net realized and unrealized investment gains and losses recorded in the statement of operations and the change in unrealized gains and losses on investments recorded in other comprehensive income for the three and nine months ended September 30, 2014 and 2013:
		For the Three Months Ended							For the Nine Months Ended
		30-Sep-14				30-Sep-13			30-Sep-14			30-Sep-13
		($ in millions)							($ in millions)
	Available for sale:
	Fixed income securities — gross realized gains	$	1.7			$	6.9		$	8.1		$	15.7
	Fixed income securities — gross realized (losses)	(1.0		)		(5.6		)	(5.5		)	(6.8		)
	Equity securities — gross realized gains	4.2				6.3			9.7			13.8
	Trading:
	Fixed income securities — gross realized gains	1.9				0.8			6.6			6.6
	Fixed income securities — gross realized (losses)	(0.2		)		(1.3		)	(2.4		)	(1.9		)
	Equity securities — gross realized gains	2.8				0.4			4.5			0.8
	Equity securities — gross realized (losses)	(0.1		)		—			(0.3		)	(0.3		)
	Catastrophe bonds — trading	0.5				—			0.5			—
	Net change in gross unrealized gains (losses)	(10.0		)		9			24.2			(2.7		)
	Other investments:
	Gross realized and unrealized gains in Cartesian	—				1.2			—			1.1
	Impairments:
	Total other-than-temporary impairments	—				—			(0.7		)	—
	Total net realized and unrealized investment gains recorded in the statement of operations	$	(0.2	)		$	17.7		$	44.7		$	26.3
	Change in available for sale net unrealized gains (losses):
	Fixed income securities	(28.5		)		(3.3		)	25			(168.7		)
	Equity securities	(4.5		)		(0.2		)	(2.3		)	9.4
	Total change in pre-tax available for sale unrealized gains(losses)	(33.0		)		(3.5		)	22.7			(159.3		)
	Change in taxes	8.1				(0.3		)	(3.8		)	11.8
	Total change in net unrealized gains (losses), net of taxes recorded in other comprehensive income	$	(24.9	)		$	(3.8	)	$	18.9		$	(147.5	)
Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities, Short-Term Investments and Equities	'
	Fixed Income Securities, Short-Term Investments and Equities — Available For Sale. The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income securities, short-term investments and equity securities as at September 30, 2014 and December 31, 2013:
		As at September 30, 2014
		Cost or				Gross			Gross			Fair Market
		Amortized Cost				Unrealized			Unrealized			Value
						Gains			Losses
		($ in millions)
	U.S. government	$	956.4			$	19.5		$	(2.0	)	$	973.9
	U.S. agency	193.9				8.2			(0.2		)	201.9
	Municipal	29.7				2			—			31.7
	Corporate	2,208.90				77.9			(8.4		)	2,278.40
	Non-U.S. government-backed corporate	74.2				1.3			—			75.5
	Foreign government	743.2				11.1			(1.7		)	752.6
	Asset-backed	141.9				2.3			(0.3		)	143.9
	Non-agency commercial mortgage-backed	47.6				3.9			—			51.5
	Agency mortgage-backed	957.8				34.8			(4.2		)	988.4
	Total fixed income securities — Available for sale	5,353.60				161			(16.8		)	5,497.80
	Total short-term investments — Available for sale	307.2				—			—			307.2
	Total equity securities — Available for sale	93.9				32.7			(0.7		)	125.9
	Total	$	5,754.70			$	193.7		$	(17.5	)	$	5,930.90
		As at December 31, 2013
		Cost or				Gross			Gross			Fair Market
		Amortized Cost				Unrealized			Unrealized			Value
						Gains			Losses
		($ in millions)
	U.S. government	$	1,004.70			$	21.2		$	(5.5	)	$	1,020.40
	U.S. agency	258.5				11.4			(0.8		)	269.1
	Municipal	32.3				0.9			(0.4		)	32.8
	Corporate	2,005.60				82.5			(18.7		)	2,069.40
	Non-U.S. government-backed corporate	83.4				1.4			(0.2		)	84.6
	Foreign government	772				11.2			(4.3		)	778.9
	Asset-backed	119.8				2.8			(0.3		)	122.3
	Non-agency commercial mortgage-backed	56.9				5.7			—			62.6
	Agency mortgage-backed	1,116.70				30.6			(18.3		)	1,129.00
	Total fixed income securities — Available for sale	5,449.90				167.7			(48.5		)	5,569.10
	Total short-term investments — Available for sale	160.3				—			—			160.3
	Total equity securities — Available for sale	112.2				37.8			(0.5		)	149.5
	Total	$	5,722.40			$	205.5		$	(49.0	)	$	5,878.90
Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities	'
	Fixed Income Securities, Short-Term Investments, Equities and Catastrophe Bonds — Trading. The following tables present the cost or amortized cost, gross unrealized gains and losses, and estimated fair market value of trading investments in fixed income securities, short-term investments, equity securities and catastrophe bonds as at September 30, 2014 and December 31, 2013:
		As at September 30, 2014
		Cost or				Gross			Gross			Fair Market
		Amortized Cost				Unrealized			Unrealized			Value
						Gains			Losses
		($ in millions)
	U.S. government	$	16.5			$	—		$	—		$	16.5
	U.S. agency	0.2				—			—			0.2
	Municipal	1.1				—			—			1.1
	Corporate	511.4				12.2			(2.5		)	521.1
	Foreign government	135.4				4.1			(0.3		)	139.2
	Asset-backed	15.1				0.1			—			15.2
	Bank loans	81.8				—			(1.0		)	80.8
	Total fixed income securities — Trading	761.5				16.4			(3.8		)	774.1
	Total short-term investments — Trading	7				—			—			7
	Total equity securities — Trading	490.2				47			(16.1		)	521.1
	Total catastrophe bonds — Trading	31.6				0.6			—			32.2
	Total	$	1,290.30			$	64		$	(19.9	)	$	1,334.40
		As at December 31, 2013
		Cost or				Gross			Gross			Fair Market
		Amortized Cost				Unrealized			Unrealized			Value
						Gains			Losses
		($ in millions)
	U.S. government	$	22.7			$	—		$	(0.7	)	$	22
	U.S. agency	0.2				—			—			0.2
	Municipal	1.1				—			—			1.1
	Corporate	469.8				10.3			(5.3		)	474.8
	Foreign government	136.5				1.2			(1.5		)	136.2
	Asset-backed	12.7				0.1			—			12.8
	Bank loans	69.1				0.3			(0.3		)	69.1
	Total fixed income securities — Trading	712.1				11.9			(7.8		)	716.2
	Total equity securities — Trading	281.6				34			(4.7		)	310.9
	Total catastrophe bonds — Trading	5.8				—			—			5.8
	Total	$	999.5			$	45.9		$	(12.5	)	$	1,032.90
Other Investments	'
	The tables below show the Company’s investments in Cartesian and Chaspark for the three and nine months ended September 30, 2014 :
		For the Three Months Ended September 30, 2014
		Cartesian				Chaspark			Total
		($ in millions)
	Opening undistributed value of investment as at June 30, 2014	$	—			$	8.7		$	8.7
	Distribution for the three months to September 30, 2014	—				—			—
	Closing value of investment as at September 30, 2014	$	—			$	8.7		$	8.7
		For the Nine Months Ended September 30, 2014
		Cartesian				Chaspark			Total
		($ in millions)
	Opening undistributed value of investment as at January 1, 2014	$	39.3			$	8.7		$	48
	Distribution for the nine months to September 30, 2014	(39.3		)		—			(39.3		)
	Closing value of investment as at September 30, 2014	$	—			$	8.7		$	8.7
	The tables below show the value of the notes attributable to the third-party investments in Silverton for the three and nine months ended September 30, 2014:
			Three Months Ended September 30, 2014
			($ in millions)
	Beginning balance as at June 30, 2014		$	(56.0	)
	Total change in fair value included in the statement of operations		(8.5		)
	Balance as at September 30, 2014		$	(64.5	)
			Nine Months Ended September 30, 2014
			($ in millions)
	Beginning balance as at January 1, 2014		$	(50.0	)
	Total change in fair value included in the statement of operations		(14.5		)
	Balance as at September 30, 2014		$	(64.5	)
Summary of Fixed Maturities	'
	Fixed Income Securities. The scheduled maturity distribution of available for sale fixed income securities as at September 30, 2014 and December 31, 2013 is set forth below. Actual maturities may differ from contractual maturities because issuers of securities may have the right to call or prepay obligations with or without call or prepayment penalties.
		As at September 30, 2014
		Amortized				Fair Market			Average
		Cost or Cost				Value			S&P Ratings by
									Maturity
		($ in millions)
	Due one year or less	$	652.5			$	658.3		AA
	Due after one year through five years	2,502.80				2,571.30			AA-
	Due after five years through ten years	955				978.8			AA-
	Due after ten years	96				105.6			A+
	Subtotal	4,206.30				4,314.00
	Non-agency commercial mortgage-backed	47.6				51.5			AA+
	Agency mortgage-backed	957.8				988.4			AA+
	Asset-backed	141.9				143.9			AAA
	Total fixed income securities — Available for sale	$	5,353.60			$	5,497.80
		As at December 31, 2013
		Amortized				Fair Market			Average
		Cost or Cost				Value			S&P Ratings by
									Maturity
		($ in millions)
	Due one year or less	$	694.8			$	700		AA
	Due after one year through five years	2,376.10				2,438.00			AA-
	Due after five years through ten years	1,003.90				1,032.80			A+
	Due after ten years	81.7				84.4			AA-
	Subtotal	4,156.50				4,255.20
	Non-agency commercial mortgage-backed	56.9				62.6			AA+
	Agency mortgage-backed	1,116.70				1,129.00			AA+
	Asset-backed	119.8				122.3			AAA
	Total fixed income securities — Available for sale	$	5,449.90			$	5,569.10
Aggregate Fair Value and Gross Unrealized Loss by Type of Security	'
	Gross Unrealized Loss. The following tables summarize as at September 30, 2014 and December 31, 2013 by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position for the Company’s available for sale portfolio:
		September 30, 2014
		0-12 months							Over 12 months						Total
		Fair				Gross			Fair			Gross			Fair		Gross			Number of
		Market				Unrealized			Market			Unrealized			Market		Unrealized			Securities
		Value				Loss			Value			Loss			Value		Loss
		($ in millions)
	U.S. government	$	182.2			$	(0.8	)	$	80.1		$	(1.2	)	$	262.3	$	(2.0	)	42
	U.S. agency	17.3				—			18.8			(0.2		)	36.1		(0.2		)	9
	Municipal	2.2				—			—			—			2.2		—			1
	Corporate	474.1				(2.8		)	201.8			(5.6		)	675.9		(8.4		)	274
	Non-U.S. government-backed corporate	1.8				—			4.3			—			6.1		—			3
	Foreign government	151.7				(0.3		)	110.8			(1.4		)	262.5		(1.7		)	35
	Asset-backed	43.2				(0.1		)	11.2			(0.2		)	54.4		(0.3		)	39
	Agency mortgage-backed	57.3				(0.4		)	126			(3.8		)	183.3		(4.2		)	54
	Total fixed income securities — Available for sale	929.8				(4.4		)	553			(12.4		)	1,482.80		(16.8		)	457
	Total short-term investments — Available for sale	2				—			—			—			2		—			1
	Total equity securities —	8.8				(0.7		)	—			—			8.8		(0.7		)	7
	Available for sale
	Total	$	940.6			$	(5.1	)	$	553		$	(12.4	)	$	1,493.60	$	(17.5	)	465
		December 31, 2013
		0-12 months							Over 12 months						Total
		Fair				Gross			Fair			Gross			Fair		Gross			Number of
		Market				Unrealized			Market			Unrealized			Market		Unrealized			Securities
		Value				Loss			Value			Loss			Value		Loss
		($ in millions)
	U.S. government	$	293.9			$	(5.5	)	$	—		$	—		$	293.9	$	(5.5	)	51
	U.S. agency	72.1				(0.8		)	—			—			72.1		(0.8		)	18
	Municipal	5.5				(0.2		)	1.3			(0.2		)	6.8		(0.4		)	7
	Corporate	695.4				(16.8		)	23.4			(1.9		)	718.8		(18.7		)	372
	Non-U.S. government-backed corporate	21.8				(0.2		)	4.9			—			26.7		(0.2		)	8
	Foreign government	239.7				(4.1		)	8.5			(0.2		)	248.2		(4.3		)	44
	Asset-backed	50.2				(0.3		)	—			—			50.2		(0.3		)	51
	Agency mortgage-backed	491.8				(18.3		)	1.2			—			493		(18.3		)	123
	Total fixed income securities — Available for sale	1,870.40				(46.2		)	39.3			(2.3		)	1,909.70		(48.5		)	674
	Total short-term investments — Available for sale	7.7				—			—			—			7.7		—			6
	Total equity securities —	6				(0.4		)	2.3			(0.1		)	8.3		(0.5		)	7
	Available for sale
	Total	$	1,884.10			$	(46.6	)	$	41.6		$	(2.4	)	$	1,925.70	$	(49.0	)	687
Analysis of Investment Purchases/Sales and Maturities	'
	Investment Purchases and Sales. The following table summarizes investment purchases, sales and maturities for the three and nine months ended September 30, 2014 and 2013:
		For the Three Months Ended							For the Nine Months Ended
		30-Sep-14				30-Sep-13			30-Sep-14			30-Sep-13
		($ in millions)							($ in millions)
	(Purchases) of fixed income securities — Available for sale	$	(522.8	)		$	(566.8	)	$	(1,440.9	)	$	(1,748.6	)
	(Purchases) of fixed income securities — Trading	(154.7		)		(283.2		)	(553.6		)	(555.5		)
	(Purchases) of equity securities — Available for sale	—				—			—			(2.5		)
	(Purchases) of equity securities — Trading	(121.2		)		(38.6		)	(240.1		)	(275.4		)
	Proceeds from sales and maturities of fixed income securities — Available for sale	481				578.6			1,496.90			1,564.40
	Proceeds from sales and maturities of fixed income securities — Trading	143.5				67.8			515.7			276.3
	Proceeds from sales of equity securities — Available for sale	12.1				29.7			27.7			61.8
	Proceeds from sales of equity securities — Trading	17.8				5			36			15.1
	Net change in receivable/(payable) for securities sold/(purchased)	(7.0		)		6.6			12.8			5.6
	Net (purchases)/sales of short-term investments — Available for sale	25				95.5			(151.0		)	275.7
	Net (purchases)/sales of short-term investments — Trading	7				—			(7.0		)	—
	Net (purchases) of catastrophe bonds — Trading	(1.7		)		—			(26.0		)	—
	Net sales of other investments	—				—			37.3			—
	Net (purchases)/sales for the period	$	(121.0	)		$	(105.4	)	$	(292.2	)	$	(383.1	)

Variable_Interest_Entities_Tab

Variable Interest Entities (Tables)	9 Months Ended
	Sep. 30, 2014
Business Combinations [Abstract]	'
Schedule of Variable Interest Entities	'
	The tables below show the Company’s investments in Cartesian and Chaspark for the three and nine months ended September 30, 2014 :
		For the Three Months Ended September 30, 2014
		Cartesian				Chaspark		Total
		($ in millions)
	Opening undistributed value of investment as at June 30, 2014	$	—			$	8.7	$	8.7
	Distribution for the three months to September 30, 2014	—				—		—
	Closing value of investment as at September 30, 2014	$	—			$	8.7	$	8.7
		For the Nine Months Ended September 30, 2014
		Cartesian				Chaspark		Total
		($ in millions)
	Opening undistributed value of investment as at January 1, 2014	$	39.3			$	8.7	$	48
	Distribution for the nine months to September 30, 2014	(39.3		)		—		(39.3		)
	Closing value of investment as at September 30, 2014	$	—			$	8.7	$	8.7
	The tables below show the value of the notes attributable to the third-party investments in Silverton for the three and nine months ended September 30, 2014:
			Three Months Ended September 30, 2014
			($ in millions)
	Beginning balance as at June 30, 2014		$	(56.0	)
	Total change in fair value included in the statement of operations		(8.5		)
	Balance as at September 30, 2014		$	(64.5	)
			Nine Months Ended September 30, 2014
			($ in millions)
	Beginning balance as at January 1, 2014		$	(50.0	)
	Total change in fair value included in the statement of operations		(14.5		)
	Balance as at September 30, 2014		$	(64.5	)

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2014
Fair Value Disclosures [Abstract]	'
Financial Assets Measured on Recurring Basis	'
	The following tables present the level within the fair value hierarchy at which the Company’s financial assets and liabilities are measured on a recurring basis as at September 30, 2014 and December 31, 2013, respectively:
		As at September 30, 2014
		Level 1				Level 2				Level 3				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	973.9			$	—			$	—			$	973.9
	U.S. agency	—				201.9				—				201.9
	Municipal	—				31.7				—				31.7
	Corporate	—				2,278.40				—				2,278.40
	Non-U.S. government-backed corporate	—				75.5				—				75.5
	Foreign government	538.2				214.4				—				752.6
	Asset-backed	—				143.9				—				143.9
	Non-agency commercial mortgage-backed	—				51.5				—				51.5
	Agency mortgage-backed	—				988.4				—				988.4
	Total fixed income securities available for sale, at fair value	1,512.10				3,985.70				—				5,497.80
	Short-term investments available for sale, at fair value	302.4				4.8				—				307.2
	Equity investments available for sale, at fair value	125.9				—				—				125.9
	Held for trading financial assets, at fair value
	U.S. government	16.5				—				—				16.5
	U.S. agency	—				0.2				—				0.2
	Municipal	—				1.1				—				1.1
	Corporate	—				521.1				—				521.1
	Foreign government	33.9				105.3				—				139.2
	Asset-backed	—				15.2				—				15.2
	Bank loans	—				80.8				—				80.8
	Total fixed income securities trading, at fair value	50.4				723.7				—				774.1
	Short-term investments trading, at fair value	—				7				—				7
	Equity investments trading, at fair value	521.1				—				—				521.1
	Catastrophe bonds trading, at fair value	—				32.2				—				32.2
	Other financial assets and liabilities, at fair value
	Derivatives at fair value — foreign exchange contracts	—				5.7				—				5.7
	Derivatives at fair value— interest rate swaps	—				1.3				—				1.3
	Liabilities under derivative contracts — foreign exchange contracts	—				(12.7		)		—				(12.7		)
	Loan notes issued by variable interest entities, at fair value	—				—				(64.5		)		(64.5		)
	Total	$	2,511.90			$	4,747.70			$	(64.5	)		$	7,195.10
	There were no maturities, settlements or transfers between Level 1, Level 2 and Level 3 during the three and nine months ended September 30, 2014 and no assets or liabilities were classified as Level 3 as at September 30, 2014, except for the loan notes issued by the VIEs.
		As at December 31, 2013
		Level 1				Level 2				Level 3				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	1,020.40			$	—			$	—			$	1,020.40
	U.S. agency	—				269.1				—				269.1
	Municipal	—				32.8				—				32.8
	Corporate	—				2,069.40				—				2,069.40
	Non-U.S. government-backed corporate	—				84.6				—				84.6
	Foreign government	596.2				182.7				—				778.9
	Asset-backed	—				122.3				—				122.3
	Non-agency commercial mortgage-backed	—				62.6				—				62.6
	Agency mortgage-backed	—				1,129.00				—				1,129.00
	Total fixed income securities available for sale, at fair value	1,616.60				3,952.50				—				5,569.10
	Short-term investments available for sale, at fair value	129.5				30.8				—				160.3
	Equity investments available for sale, at fair value	149.5				—				—				149.5
	Held for trading financial assets, at fair value
	U.S. government	22				—				—				22
	U.S. agency	—				0.2				—				0.2
	Municipal	—				1.1				—				1.1
	Corporate	—				474.8				—				474.8
	Foreign government	44.2				92				—				136.2
	Asset-backed	—				12.8				—				12.8
	Bank loans	—				69.1				—				69.1
	Total fixed income securities trading, at fair value	66.2				650				—				716.2
	Equity investments trading, at fair value	310.9				—				—				310.9
	Catastrophe bonds trading, at fair value	—				5.8				—				5.8
Other financial assets and liabilities, at fair value
Derivatives at fair value – foreign exchange contracts	—				5.9				—				5.9
Derivatives at fair value – interest rate swaps	—				1.1				—				1.1
Liabilities under derivative contracts – foreign exchange contracts	—				(2.9		)		—				(2.9		)
Loan notes issued by variable interest entities, at fair value	—				—				(50.0		)		(50.0		)
Total	$	2,272.70			$	4,643.20			$	(50.0	)		$	6,865.90
Reconciliation of Liabilities Using Level 3 Inputs	'
	The following tables present a reconciliation of the beginning and ending balances for all assets and liabilities measured at fair value on a recurring basis using Level 3 inputs for the three and nine months ended September 30, 2014:
	Reconciliation of Liabilities Using Level 3 Inputs		Three Months Ended September 30, 2014
			($ in millions)
	Beginning balance as at June 30, 2014		$	(56.0	)
	Total change in fair value included in the statement of operations		(8.5		)
	Balance as at September 30, 2014		$	(64.5	)
	Reconciliation of Liabilities Using Level 3 Inputs		Nine Months Ended September 30, 2014
			($ in millions)
	Beginning balance as at January 1, 2014		$	(50.0	)
	Total change in fair value included in the statement of operations		(14.5		)
	Balance as at September 30, 2014		$	(64.5	)
Pricing Sources Used in Pricing Fixed Income Investments	'
	Pricing sources used in pricing fixed income investments as at September 30, 2014 and December 31, 2013 were as follows:
		As at September 30, 2014			As at December 31, 2013
	Index providers	84	%		85	%
	Pricing services	12			12
	Broker-dealers	4			3
	Total	100	%		100	%
Summary of Securities Priced Using Pricing Information from Index Providers	'
	A summary of securities priced using pricing information from index providers as at September 30, 2014 and December 31, 2013 is provided below:
		As at September 30, 2014							As at December 31, 2013
		Fair Market				% of Total			Fair Market				% of Total
		Value Determined				Fair Value by			Value Determined				Fair Value by
		using Prices from				Security Type			using Prices from				Security Type
		Index Providers							Index Providers
		($ in millions, except for percentages)
	U.S. government	$	911.9			92	%		$	998.5			96	%
	U.S. agency	191.3				95	%		255.3				95	%
	Municipal	13.4				41	%		14.5				43	%
	Corporate	2,675.20				96	%		2,400.80				94	%
	Non-U.S. government-backed corporate	57.1				76	%		55.9				66	%
	Foreign government	559.1				63	%		605.8				66	%
	Asset-backed	146				92	%		130.6				97	%
	Non-agency commercial mortgage-backed	51.5				100	%		61				97	%
	Agency mortgage-backed	650.5				66	%		830.6				74	%
	Total fixed income securities	$	5,256.00			84	%		$	5,353.00			85	%
	Equities	647				100	%		460.4				100	%
	Total fixed income securities and equity investments	$	5,903.00			85	%		$	5,813.40			86	%
Long-term Debt Issued by Silverton	'
	The observable and unobservable inputs used to determine the fair value of the loan notes as at September 30, 2014 are presented in the table below:
	At September 30, 2014		Fair Value				Valuation Method		Observable (O) and		Low				High
			Level 3						Unobservable (U) inputs
		($ in millions)									($ in millions)
	Loan notes held by third parties		$	64.5			Internal Valuation Model		Gross premiums written (O)		$	—			$	40
									Reserve for losses (U)		$	—			$	2.8
									Contract period (O)		N/A				365 days
									Initial value of issuance (O)		$	50			$	50

Derivative_Contracts_Tables

Derivative Contracts (Tables)	9 Months Ended
	Sep. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]	'
Fair Value of Derivative Instruments	'
	The following table summarizes information on the location and amounts of derivative fair values on the consolidated balance sheet as at September 30, 2014 and December 31, 2013:
				As at September 30, 2014						As at December 31, 2013
	Derivatives Not Designated as Hedging Instruments		Balance Sheet Location	Notional			Fair			Notional		Fair
	Under ASC 815			Amount			Value			Amount		Value
				($ in millions)						($ in millions)
	Interest Rate Swaps		Derivatives at Fair Value	$	981.2		$	1.3		$	1,000.00	$	1.1		(1)
	Foreign Exchange Contracts		Derivatives at Fair Value	$	161.4		$	5.7		$	224.4	$	5.9
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	205.6		$	(12.7	)	$	57.5	$	(2.9	)
	(1)	Net of $23.4 million of cash collateral provided to counterparties, Goldman Sachs International ($481.2 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2013 — $34.3 million).
Gain/(Loss) Recognized in Income on Derivative	'
	The following tables provide the unrealized and realized gains/(losses) recorded in the statement of operations for the three and nine months ended September 30, 2014 and 2013, respectively:
				Amount of Income/(Loss)
				Recognized in the Statement
				of Operations for the
				Three Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	30-Sep-14			30-Sep-13
	ASC 815		Statement of Operations
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(6.4	)	$	10.8
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	1.3		$	(4.2	)
				Amount of Income/(Loss)
				Recognized in the Statement
				of Operations for the
				Nine Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	30-Sep-14			30-Sep-13
	ASC 815		Statement of Operations
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(4.5	)	$	(3.7	)
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	(4.1	)	$	3.2

Deferred_Policy_Acquisition_Co1

Deferred Policy Acquisition Costs (Tables)	9 Months Ended
	Sep. 30, 2014
Insurance [Abstract]	'
Reconciliation of beginning and ending deferred policy acquisition costs	'
	The following table represents a reconciliation of beginning and ending deferred policy acquisition costs for the three and nine months ended September 30, 2014 and 2013:
			Three Months Ended September 30,						Nine Months Ended September 30,
			2014			2013			2014			2013
			($ in millions)						($ in millions)
	Balance at the beginning of the period		$	306.4		$	264.4		$	262.2		$	223
		Acquisition costs deferred	110.7			108.2			375.8			361.4
		Amortization of deferred policy acquisition costs	(115.5		)	(110.5		)	(336.4		)	(322.3		)
	Balance at the end of the period		$	301.6		$	262.1		$	301.6		$	262.1

Reserves_for_Losses_and_Loss_A1

Reserves for Losses and Loss Adjustment Expenses (Tables)	9 Months Ended
	Sep. 30, 2014
Insurance [Abstract]	'
Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses ("LAE") Reserves	'
	The following table represents a reconciliation of beginning and ending consolidated loss and loss adjustment expenses (“LAE”) reserves for the nine months ended September 30, 2014 and twelve months ended December 31, 2013:
		Nine Months Ended September 30, 2014			Twelve Months Ended December 31, 2013
		($ in millions)
	Provision for losses and LAE at the start of the year	$	4,678.90		$	4,779.70
	Less reinsurance recoverable	(332.7		)	(499.0		)
	Net loss and LAE at the start of the year	4,346.20			4,280.70
	Net loss and LAE expenses assumed/(disposed)	(24.2		)	(34.6		)
	Provision for losses and LAE for claims incurred:
	Current year	1,060.50			1,331.40
	Prior years	(92.6		)	(107.7		)
	Total incurred	967.9			1,223.70
	Losses and LAE payments for claims incurred:
	Current year	(56.6		)	(172.8		)
	Prior years	(785.7		)	(912.3		)
	Total paid	(842.3		)	(1,085.1		)
	Foreign exchange (gains)	(45.0		)	(38.5		)
	Net losses and LAE reserves at period end	4,402.60			4,346.20
	Plus reinsurance recoverable on unpaid losses at period end	384.7			332.7
	Provision for losses and LAE at the end of the relevant period	$	4,787.30		$	4,678.90

Capital_Structure_Tables

Capital Structure (Tables)	9 Months Ended
	Sep. 30, 2014
Equity [Abstract]	'
Summary of Authorized and Issued Share Capital	'
	The following table provides a summary of the Company’s authorized and issued share capital as at September 30, 2014 and December 31, 2013:
		As at September 30, 2014			As at December 31, 2013
		Number		$ in	Number	$ in
				Thousands		Thousands
	Authorized share capital:
	Ordinary Shares 0.15144558¢ per share	969,629,030		1,469	969,629,030	1,469
	Non-Voting Shares 0.15144558¢ per share	6,787,880		10	6,787,880	10
	Preference Shares 0.15144558¢ per share	100,000,000		152	100,000,000	152
	Total authorized share capital			1,631		1,631
	Issued share capital:
	Issued ordinary shares of 0.15144558¢ per share	63,349,858		96	65,546,976	99
	Issued 7.401% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	5,327,500		8	5,327,500	8
	Issued 7.250% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	6,400,000		10	6,400,000	10
	Issued 5.95% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share	11,000,000		17	11,000,000	17
	Total issued share capital			131		134
Summary of Ordinary Shares	'
	The following table summarizes transactions in the Company’s ordinary shares during the nine months ended September 30, 2014:
		Number of Ordinary Shares
	Ordinary shares in issue at December 31, 2013	65,546,976
	Ordinary share transactions in the nine months ended September 30, 2014
	Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or	692,929
	2008 share purchase plan
	Ordinary shares issued to non-employee directors	1,309
	Ordinary shares repurchased	(2,891,356	)
	Ordinary shares in issue at September 30, 2014	63,349,858

Intangible_Assets_Tables

Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2014
Goodwill and Intangible Assets Disclosure [Abstract]	'
Summary of Changes in Intangible Assets	'
	The following tables provide a summary of the Company’s intangible assets for the three and nine months ended September 30, 2014 and 2013:
		Three Months Ended September 30, 2014										Three Months Ended September 30, 2013
		Trade		Insurance		Other			Total			Trade		Insurance		Other			Total
		Mark		Licenses								Mark		Licenses
		($ in millions)										($ in millions)
	Intangible Assets
	Beginning of the period	$	1.6	$	16.6	$	—		$	18.2		$	1.6	$	16.6	$	0.5		$	18.7
	Amortization	—		—		—			—			—		—		(0.1		)	(0.1		)
	End of the period	$	1.6	$	16.6	$	—		$	18.2		$	1.6	$	16.6	$	0.4		$	18.6
		Nine Months Ended September 30, 2014										Nine Months Ended September 30, 2013
		Trade		Insurance		Other			Total			Trade		Insurance		Other			Total
		Mark		Licenses								Mark		Licenses
		($ in millions)										($ in millions)
	Intangible Assets
	Beginning of the period	$	1.6	$	16.6	$	0.2		$	18.4		$	1.6	$	16.6	$	0.8		$	19
	Amortization	—		—		(0.2		)	(0.2		)	—		—		(0.4		)	(0.4		)
	End of the period	$	1.6	$	16.6	$	—		$	18.2		$	1.6	$	16.6	$	0.4		$	18.6

Commitments_and_Contingent_Lia1

Commitments and Contingent Liabilities (Tables)	9 Months Ended
	Sep. 30, 2014
Commitments and Contingencies Disclosure [Abstract]	'
Schedule of Company's Restricted Assets	'
	The following table details the forms and value of Company’s restricted assets as at September 30, 2014 and December 31, 2013:
		As at September 30, 2014			As at December 31, 2013
		($ in millions, except percentages)
	Regulatory trusts and deposits:
	Affiliated transactions	$	741.4		$	685.8
	Third party	2,146.10			2,236.40
	Letters of credit / guarantees(1)	776.4			830.4
	Total restricted assets	$	3,663.90		$	3,752.60
	Total as percent of cash and invested assets	42.8		%	45.5		%
	(1)	As of September 30, 2014, the Company had pledged funds of $749.4 million and £16.2 million (December 31, 2013 — $803.7 million and £16.1 million) as collateral for the secured letters of credit.
Amounts Outstanding under Operating Leases	'
	Amounts outstanding under operating leases net of subleases as of September 30, 2014 were:
		2014			2015			2016		2017		2018		Later		Total
														Years
		($ in millions)
	Operating Lease Obligations	$	1.8		$	13.6		$	9.7	$	8.5	$	7.2	$	20.3	$	61.1

Reclassifications_from_Accumul2

Reclassifications from Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income Reclassification (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Available for sale securities:	'	'	'	'
Realized gains on sale of securities	$3.90	$8.60	$10.60	$21.20
Realized (losses) on sale of securities	-0.3	-0.3	-4.7	-0.5
Net realized and unrealized investment gains reclassified to the statement of operations, gross of tax	3.6	8.3	5.9	20.7
Tax on net realized gains of securities	0.1	0.1	0.2	0.5
Net realized and unrealized investment gains reclassified to the statement of operations, net of tax	3.5	8.2	5.7	20.2
Foreign currency translation adjustments:	'	'	'	'
Foreign currency translation adjustments, before tax	0	4.8	0	0.5
Tax on foreign currency translation adjustments	0	-1.1	0	-0.1
Net realized and unrealized foreign exchange investment gains reclassified to the statement of operations, net of tax	0	3.7	0	0.4
Amortization of derivatives:	'	'	'	'
Amortization of long-term debt associated expenses, before tax	0	0	0	-0.2
Amortization of long-term debt associated expenses, net of tax	0	0	0	-0.2
Total reclassifications from AOCI to the statement of operations, net of tax	$3.50	$11.90	$5.70	$20.40

Earnings_per_Ordinary_Share_Co

Earnings per Ordinary Share - Computation of Basic and Diluted Earnings per Share (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Earnings Per Share [Abstract]	'		'		'		'
Net income	$37.40		$107.40		$288.60		$239.30
Preference share dividends	-9.5		-9.5		-28.4		-26.1
Change in redemption value	0	[1]	0	[1]	0	[1]	-7.1	[1]
Proportion due to non-controlling interest	0.1		0.3		0		0.3
Basic and diluted net income available to ordinary shareholders	$28		$98.20		$260.20		$206.40
Ordinary shares:	'		'		'		'
Basic weighted average ordinary shares	65,116,463		66,716,202		65,283,681		67,302,857
Weighted average effect of dilutive securities	1,396,546	[2]	1,845,313	[2]	1,314,999	[2]	2,656,617	[2]
Total diluted weighted average ordinary shares	66,513,009		68,561,515		66,598,680		69,959,474
Earnings per ordinary share:	'		'		'		'
Basic, dollars per share	$0.43		$1.47		$3.99		$3.06
Diluted, dollars per share	$0.42		$1.43		$3.91		$2.95
[1]	The $7.1 million reduction in the net income available to ordinary shareholders is the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of $230.0 million of the PIERS on May 30, 3013.
[2]	Dilutive securities comprise: employee options, restricted share units and performance shares associated with the Company’s long term incentive plan, employee share purchase plans and director restricted stock units and options as described in Note 14, in addition to the PIERS that were fully redeemed on May 30, 2013.

Earnings_per_Ordinary_Share_Ad

Earnings per Ordinary Share - Additional Information (Details) (USD $)

3 Months Ended

9 Months Ended

0 Months Ended

3 Months Ended

9 Months Ended

0 Months Ended

3 Months Ended

9 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Apr. 25, 2013

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Apr. 25, 2013

Sep. 30, 2013

Sep. 30, 2013

Preference shares

Preference shares

Preference shares

Preference shares

Reclassification from Additional Paid in Capital to Retained Earnings

Reclassification from Additional Paid in Capital to Retained Earnings

Reclassification from Additional Paid in Capital to Retained Earnings

Class of Stock [Line Items]

'

'

'

'

'

'

'

'

'

'

'

PIERS redemption

$0

[1]

$0

[1]

$0

[1]

$7.10

[1]

'

'

'

'

$7.10

$7.10

$7.10

PIERS redeemed and canceled

'

'

'

'

$230

$230

$230

$230

'

'

'

[1]

The $7.1 million reduction in the net income available to ordinary shareholders is the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of $230.0 million of the PIERS on May 30, 3013.

Earnings_per_Ordinary_Share_Su

Earnings per Ordinary Share - Summary of Declared Dividends (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014
Ordinary Shares	'
Dividends Payable [Line Items]	'
Dividend	$0.20
Payable on:	2-Dec-14
Record Date:	13-Nov-14
7.401% preference shares	'
Dividends Payable [Line Items]	'
Dividend	$0.46
Payable on:	1-Jan-15
Record Date:	15-Dec-14
7.250% preference shares	'
Dividends Payable [Line Items]	'
Dividend	$0.45
Payable on:	1-Jan-15
Record Date:	15-Dec-14
5.95% preference shares	'
Dividends Payable [Line Items]	'
Dividend	$0.37
Payable on:	1-Jan-15
Record Date:	15-Dec-14

Earnings_per_Ordinary_Share_Su1

Earnings per Ordinary Share - Summary of Declared Dividends (Parenthetical) (Detail)	9 Months Ended
	Sep. 30, 2014
5.95% preference shares	'
Dividends Payable [Line Items]	'
Preferred Stock, Dividend Rate, Percentage	5.95%
7.401% preference shares	'
Dividends Payable [Line Items]	'
Preferred Stock, Dividend Rate, Percentage	7.40%
7.250% preference shares	'
Dividends Payable [Line Items]	'
Preferred Stock, Dividend Rate, Percentage	7.25%

Segment_Reporting_Additional_I

Segment Reporting - Additional Information (Detail)	Sep. 30, 2014
	Segment
Segment Reporting [Abstract]	'
Number of business segments	2

Segment_Reporting_Summary_of_G

Segment Reporting - Summary of Gross and Net Written and Earned Premiums, Underwriting Results, Ratios and Reserves for Each of Company's Business Segments (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013	Dec. 31, 2012
Underwriting Revenues	'		'		'		'		'	'
Gross written premiums	$652.50		$581.60		$2,287.30		$2,042.30		'	'
Net written premiums	577.3		542		1,961.20		1,751.70		'	'
Gross earned premiums	694.9		625.1		2,041.20		1,829.60		'	'
Net earned premiums	610.4		544.3		1,793.10		1,599.20		'	'
Underwriting Expenses	'		'		'		'		'	'
Losses and loss adjustment expenses	342.7		290.2		967.9		892.3		'	'
Amortization of deferred policy acquisition costs	115.5		110.5		336.4		322.3		'	'
General and administrative expenses	84		84.1		249.6		231.2		'	'
Underwriting income	68.2		59.5		239.2		153.4		'	'
Corporate expenses	-35.8		-14.8		-74.6		-42		'	'
Net investment income	48		45		143.6		139.2		'	'
Realized and unrealized investment gains	1.1		23.6		53.6		54.3		'	'
Realized and unrealized investment losses	-1.3		-5.9		-8.9		-28		'	'
Change in fair value of loan notes issued by variable interest entities	-8.5		0		-14.5		0		'	'
Change in fair value of derivatives	-5.1		6.6		-8.6		-0.5		'	'
Interest expense on long term debt	-7.4		-7.7		-22.1		-23.2		'	'
Net realized and unrealized foreign exchange (losses)	-21.2		2.4		-10.4		-7.1		'	'
Other income	1		1.6		4.8		3.6		'	'
Other expenses	-0.3		0		-2.2		-0.6		'	'
Income from operations before income tax	38.7		110.3		299.9		249.1		'	'
Net reserves for loss and loss adjustment expenses	4,402.60		4,273.40		4,402.60		4,273.40		4,346.20	4,280.70
Ratios	'		'		'		'		'	'
Loss ratio	56.10%		53.30%		54.00%		55.80%		'	'
Policy acquisition expense ratio	18.90%		20.30%		18.80%		20.20%		'	'
General and administrative expense ratio	19.60%	[1]	18.20%	[1]	18.10%	[1]	17.10%	[1]	'	'
Expense ratio	38.50%		38.50%		36.90%		37.30%		'	'
Combined ratio	94.60%		91.80%		90.90%		93.10%		'	'
Reinsurance	'		'		'		'		'	'
Underwriting Revenues	'		'		'		'		'	'
Gross written premiums	256.9		219.5		1,027.50		957.7		'	'
Net written premiums	250.9		218.4		980.4		907.5		'	'
Gross earned premiums	291		268.6		859.2		828.9		'	'
Net earned premiums	279.6		255.7		825.1		788.2		'	'
Underwriting Expenses	'		'		'		'		'	'
Losses and loss adjustment expenses	132		122.2		367.4		394.9		'	'
Amortization of deferred policy acquisition costs	52.1		49.1		152.3		161		'	'
General and administrative expenses	38.4		34.6		107		97.2		'	'
Underwriting income	57.1		49.8		198.4		135.1		'	'
Net reserves for loss and loss adjustment expenses	2,554.20		2,718		2,554.20		2,718		'	'
Ratios	'		'		'		'		'	'
Loss ratio	47.20%		47.80%		44.50%		50.10%		'	'
Policy acquisition expense ratio	18.60%		19.20%		18.50%		20.40%		'	'
General and administrative expense ratio	13.70%		13.50%		13.00%		12.30%		'	'
Expense ratio	32.30%		32.70%		31.50%		32.70%		'	'
Combined ratio	79.50%		80.50%		76.00%		82.80%		'	'
Insurance	'		'		'		'		'	'
Underwriting Revenues	'		'		'		'		'	'
Gross written premiums	395.6		362.1		1,259.80		1,084.60		'	'
Net written premiums	326.4		323.6		980.8		844.2		'	'
Gross earned premiums	403.9		356.5		1,182		1,000.70		'	'
Net earned premiums	330.8		288.6		968		811		'	'
Underwriting Expenses	'		'		'		'		'	'
Losses and loss adjustment expenses	210.7		168		600.5		497.4		'	'
Amortization of deferred policy acquisition costs	63.4		61.4		184.1		161.3		'	'
General and administrative expenses	45.6		49.5		142.6		134		'	'
Underwriting income	11.1		9.7		40.8		18.3		'	'
Net reserves for loss and loss adjustment expenses	$1,848.40		$1,555.40		$1,848.40		$1,555.40		'	'
Ratios	'		'		'		'		'	'
Loss ratio	63.70%		58.20%		62.00%		61.30%		'	'
Policy acquisition expense ratio	19.20%		21.30%		19.00%		19.90%		'	'
General and administrative expense ratio	13.80%		17.20%		14.70%		16.50%		'	'
Expense ratio	33.00%		38.50%		33.70%		36.40%		'	'
Combined ratio	96.70%		96.70%		95.70%		97.70%		'	'
[1]	The general and administrative expense ratio in the total column includes corporate expenses.

Investments_Summary_of_Investm

Investments - Summary of Investment Income (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	$50.10	$47.50	$151.50	$146.10
Investment expenses	-2.1	-2.5	-7.9	-6.9
Net investment income	48	45	143.6	139.2
Total fixed income securities — Available for sale	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	37.6	38.2	113.4	117.2
Fixed income maturities — Trading	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	6.6	5.3	19.8	13.4
Short-term investments — Available for sale	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	0.4	0.3	1	1.9
Fixed term deposits (included in cash and cash equivalents)	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	0.5	0.7	2.4	3.4
Equity securities — Available for sale	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	1	1.2	3.5	4.7
Equity securities — Trading	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	3.5	1.8	10.5	5.5
Catastrophe bonds — Trading	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	$0.50	$0	$0.90	$0

Investments_Net_Realized_and_U

Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Fixed income maturities — gross realized gains	$3,900,000	$8,600,000	$10,600,000	$21,200,000
Fixed income maturities — gross realized (losses)	-300,000	-300,000	-4,700,000	-500,000
Catastrophe bonds — trading	500,000	0	500,000	0
Net change in gross unrealized gains (losses)	-10,000,000	9,000,000	24,200,000	-2,700,000
Other-than-temporary impairment charge	0	0	-700,000	0
Total net realized and unrealized investment gains recorded in the statement of operations	-200,000	17,700,000	44,700,000	26,300,000
Change in available for sale net unrealized gains/(losses):	'	'	'	'
Fixed income maturities	-28,500,000	-3,300,000	25,000,000	-168,700,000
Equity securities	-4,500,000	-200,000	-2,300,000	9,400,000
Total change in pre-tax available for sale unrealized gains (losses)	-33,000,000	-3,500,000	22,700,000	-159,300,000
Change in taxes	8,100,000	-300,000	-3,800,000	11,800,000
Total change in net unrealized gains (losses), net of taxes recorded in other comprehensive income	-24,900,000	-3,800,000	18,900,000	-147,500,000
Fixed income maturities - Available for sale	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Fixed income maturities — gross realized gains	1,700,000	6,900,000	8,100,000	15,700,000
Fixed income maturities — gross realized (losses)	-1,000,000	-5,600,000	-5,500,000	-6,800,000
Equity Securities - Available for Sale	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Equity securities — gross realized gains	4,200,000	6,300,000	9,700,000	13,800,000
Fixed Income Maturities - Trading	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Fixed income maturities — gross realized gains	1,900,000	800,000	6,600,000	6,600,000
Fixed income maturities — gross realized (losses)	-200,000	-1,300,000	-2,400,000	-1,900,000
Equity Securities - Trading	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Equity securities — gross realized gains	2,800,000	400,000	4,500,000	800,000
Equity securities — gross realized (losses)	-100,000	0	-300,000	-300,000
Cartesian Iris Offshore Fund Limited Partner [Member]	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Income (Loss) from Equity Method Investments	$0	$1,200,000	$0	$1,100,000

Investments_Additional_Informa

Investments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Aug. 31, 2013	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014	Oct. 02, 2012
			insurer		insurer	Corporate and Foreign Government Debt Securities	Bank Loans	Bank Loans	Equities	Chaspark Maritime Holdings Ltd
Investment Securities [Line Items]	'	'	'	'	'	'	'	'	'	'
Other-than-temporary impairment charge	$0	$0	($0.70)	$0	'	'	'	'	'	'
Fair market value	774.1	'	774.1	'	716.2	200	80.8	69.1	'	'
Increase in investments	'	'	'	'	'	'	'	'	160	'
Catastrophe bonds	32.2	'	32.2	'	5.8	'	'	'	'	'
Percentage of ownership acquired	'	'	'	'	'	'	'	'	'	58.50%
Investment funded	$2.40	'	$2.40	'	$1.50	'	'	'	'	'
Number of municipal holdings	'	'	2	'	2	'	'	'	'	'

Investments_Cost_Gross_Unreali

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities, Short-Term Investments and Equities (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	$5,754.70	$5,722.40
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	193.7	205.5
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	-17.5	-49
Fair Market Value	5,930.90	5,878.90
Total fixed income securities — Available for sale	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	5,353.60	5,449.90
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	161	167.7
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	-16.8	-48.5
Fair Market Value	5,497.80	5,569.10
Total short-term investments — Available for sale	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	307.2	160.3
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Fair Market Value	307.2	160.3
Total equity securities — Available for sale	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	93.9	112.2
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	32.7	37.8
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	-0.7	-0.5
Fair Market Value	125.9	149.5
U.S. government	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	956.4	1,004.70
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	19.5	21.2
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	-2	-5.5
Fair Market Value	973.9	1,020.40
U.S. agency	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	193.9	258.5
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	8.2	11.4
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	-0.2	-0.8
Fair Market Value	201.9	269.1
Municipal	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	29.7	32.3
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	2	0.9
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	-0.4
Fair Market Value	31.7	32.8
Corporate	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	2,208.90	2,005.60
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	77.9	82.5
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	-8.4	-18.7
Fair Market Value	2,278.40	2,069.40
Non-U.S. government-backed corporate	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	74.2	83.4
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	1.3	1.4
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	-0.2
Fair Market Value	75.5	84.6
Foreign government	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	743.2	772
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	11.1	11.2
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	-1.7	-4.3
Fair Market Value	752.6	778.9
Asset-backed	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	141.9	119.8
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	2.3	2.8
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	-0.3	-0.3
Fair Market Value	143.9	122.3
Non-agency commercial mortgage-backed	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	47.6	56.9
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	3.9	5.7
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Fair Market Value	51.5	62.6
Agency mortgage-backed	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	957.8	1,116.70
Available For Sale Debt Securities Gross Unrealized Gain Accumulated In Investments	34.8	30.6
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	-4.2	-18.3
Fair Market Value	$988.40	$1,129

Investments_Cost_Gross_Unreali1

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	$761.50	$712.10
Fair Market Value	774.1	716.2
U.S. government	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	16.5	22.7
Trading Securities, Gross Unrealized Gains	0	0
Trading Securities, Gross Unrealized Losses	0	-0.7
Fair Market Value	16.5	22
U.S. agency	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	0.2	0.2
Trading Securities, Gross Unrealized Gains	0	0
Trading Securities, Gross Unrealized Losses	0	0
Fair Market Value	0.2	0.2
Municipal	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	1.1	1.1
Trading Securities, Gross Unrealized Gains	0	0
Trading Securities, Gross Unrealized Losses	0	0
Fair Market Value	1.1	1.1
Corporate	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	511.4	469.8
Trading Securities, Gross Unrealized Gains	12.2	10.3
Trading Securities, Gross Unrealized Losses	-2.5	-5.3
Fair Market Value	521.1	474.8
Foreign government	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	135.4	136.5
Trading Securities, Gross Unrealized Gains	4.1	1.2
Trading Securities, Gross Unrealized Losses	-0.3	-1.5
Fair Market Value	139.2	136.2
Asset-backed	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	15.1	12.7
Trading Securities, Gross Unrealized Gains	0.1	0.1
Trading Securities, Gross Unrealized Losses	0	0
Fair Market Value	15.2	12.8
Bank loans	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	81.8	69.1
Trading Securities, Gross Unrealized Gains	0	0.3
Trading Securities, Gross Unrealized Losses	-1	-0.3
Fair Market Value	80.8	69.1
Total fixed income securities — Trading	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	761.5	712.1
Trading Securities, Gross Unrealized Gains	16.4	11.9
Trading Securities, Gross Unrealized Losses	-3.8	-7.8
Fair Market Value	774.1	716.2
Short-term investments - Trading	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	7	'
Trading Securities, Gross Unrealized Gains	0	'
Trading Securities, Gross Unrealized Losses	0	'
Fair Market Value	7	'
Total equity securities — Trading	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	490.2	281.6
Trading Securities, Gross Unrealized Gains	47	34
Trading Securities, Gross Unrealized Losses	-16.1	-4.7
Fair Market Value	521.1	310.9
Total catastrophe bonds — Trading	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	31.6	5.8
Trading Securities, Gross Unrealized Gains	0.6	0
Trading Securities, Gross Unrealized Losses	0	0
Fair Market Value	32.2	5.8
Total	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Cost or Amortized Cost	1,290.30	999.5
Trading Securities, Gross Unrealized Gains	64	45.9
Trading Securities, Gross Unrealized Losses	-19.9	-12.5
Fair Market Value	$1,334.40	$1,032.90

Investments_Other_Investments_

Investments - Other Investments (Detail) (USD $)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2014
Equity Method Investment, Aggregate Cost [Roll Forward]	'	'
Opening undistributed value of investment, balance	$8.70	$48
Distribution	0	-39.3
Closing value of investment, balance	8.7	8.7
Cartesian	'	'
Equity Method Investment, Aggregate Cost [Roll Forward]	'	'
Opening undistributed value of investment, balance	0	39.3
Distribution	0	-39.3
Closing value of investment, balance	0	0
Chaspark	'	'
Equity Method Investment, Aggregate Cost [Roll Forward]	'	'
Opening undistributed value of investment, balance	8.7	8.7
Distribution	0	0
Closing value of investment, balance	$8.70	$8.70

Investments_Summary_of_Fixed_M

Investments - Summary of Fixed Maturities (Detail) (USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	$5,353.60	$5,449.90
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,754.70	5,722.40
Available for sale investments in fixed income maturities, Fair Market Value	5,497.80	5,569.10
Non-agency commercial mortgage-backed	'	'
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	47.6	56.9
Available for sale investments in fixed income maturities, Fair Market Value	51.5	62.6
Available for sale investments Average Ratings by Maturity	'AA+	'AA+
Agency mortgage-backed	'	'
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	957.8	1,116.70
Available for sale investments in fixed income maturities, Fair Market Value	988.4	1,129
Available for sale investments Average Ratings by Maturity	'AA+	'AA+
Asset-backed	'	'
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	141.9	119.8
Available for sale investments in fixed income maturities, Fair Market Value	143.9	122.3
Available for sale investments Average Ratings by Maturity	'AAA	'AAA
Total fixed income securities — Available for sale	'	'
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]	'	'
Due one year or less, Cost or Amortized Cost	652.5	694.8
Due after one year through five years, Cost or Amortized Cost	2,502.80	2,376.10
Due after five years through ten years, Cost or Amortized Cost	955	1,003.90
Due after ten years, Cost or Amortized Cost	96	81.7
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	4,206.30	4,156.50
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,353.60	5,449.90
Due one year or less, Fair Market Value	658.3	700
Due after one year through five years, Fair Market Value	2,571.30	2,438
Due after five years through ten years, Fair Market Value	978.8	1,032.80
Due after ten years, Fair Market Value	105.6	84.4
Available for sale investments in fixed income maturities, Fair Market Value	5,497.80	5,569.10
Due one year or less, Average Ratings by Maturity	'AA	'AA
Due after one year through five years, Average Ratings by Maturity	'AA-	'AA-
Due after five years through ten years, Average Ratings by Maturity	'AA-	'A+
Due after ten years, Average Ratings by Maturity	'A+	'AA-
Other Fixed Income Securities [Member]	'	'
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]	'	'
Available for sale investments in fixed income maturities, Fair Market Value	$4,314	$4,255.20

Investments_Aggregate_Fair_Val

Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified	Security	Security
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	$940.60	$1,884.10
0-12 months, Gross Unrealized Loss	-5.1	-46.6
Over 12 months, Fair Market Value	553	41.6
Over 12 months, Gross Unrealized Loss	-12.4	-2.4
Total, Fair Market Value	1,493.60	1,925.70
Total, Gross Unrealized Loss	-17.5	-49
Number of Securities	465	687
Total fixed income securities — Available for sale	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	929.8	1,870.40
0-12 months, Gross Unrealized Loss	-4.4	-46.2
Over 12 months, Fair Market Value	553	39.3
Over 12 months, Gross Unrealized Loss	-12.4	-2.3
Total, Fair Market Value	1,482.80	1,909.70
Total, Gross Unrealized Loss	-16.8	-48.5
Number of Securities	457	674
Total short-term investments — Available for sale	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	2	7.7
0-12 months, Gross Unrealized Loss	0	0
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	2	7.7
Total, Gross Unrealized Loss	0	0
Number of Securities	1	6
Total equity securities — Available for sale	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	8.8	6
0-12 months, Gross Unrealized Loss	-0.7	-0.4
Over 12 months, Fair Market Value	0	2.3
Over 12 months, Gross Unrealized Loss	0	-0.1
Total, Fair Market Value	8.8	8.3
Total, Gross Unrealized Loss	-0.7	-0.5
Number of Securities	7	7
U.S. government	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	182.2	293.9
0-12 months, Gross Unrealized Loss	-0.8	-5.5
Over 12 months, Fair Market Value	80.1	0
Over 12 months, Gross Unrealized Loss	-1.2	0
Total, Fair Market Value	262.3	293.9
Total, Gross Unrealized Loss	-2	-5.5
Number of Securities	42	51
U.S. agency	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	17.3	72.1
0-12 months, Gross Unrealized Loss	0	-0.8
Over 12 months, Fair Market Value	18.8	0
Over 12 months, Gross Unrealized Loss	-0.2	0
Total, Fair Market Value	36.1	72.1
Total, Gross Unrealized Loss	-0.2	-0.8
Number of Securities	9	18
Municipal	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	2.2	5.5
0-12 months, Gross Unrealized Loss	0	-0.2
Over 12 months, Fair Market Value	0	1.3
Over 12 months, Gross Unrealized Loss	0	-0.2
Total, Fair Market Value	2.2	6.8
Total, Gross Unrealized Loss	0	-0.4
Number of Securities	1	7
Corporate	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	474.1	695.4
0-12 months, Gross Unrealized Loss	-2.8	-16.8
Over 12 months, Fair Market Value	201.8	23.4
Over 12 months, Gross Unrealized Loss	-5.6	-1.9
Total, Fair Market Value	675.9	718.8
Total, Gross Unrealized Loss	-8.4	-18.7
Number of Securities	274	372
Non-U.S. government-backed corporate	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	1.8	21.8
0-12 months, Gross Unrealized Loss	0	-0.2
Over 12 months, Fair Market Value	4.3	4.9
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	6.1	26.7
Total, Gross Unrealized Loss	0	-0.2
Number of Securities	3	8
Foreign government	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	151.7	239.7
0-12 months, Gross Unrealized Loss	-0.3	-4.1
Over 12 months, Fair Market Value	110.8	8.5
Over 12 months, Gross Unrealized Loss	-1.4	-0.2
Total, Fair Market Value	262.5	248.2
Total, Gross Unrealized Loss	-1.7	-4.3
Number of Securities	35	44
Asset-backed	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	43.2	50.2
0-12 months, Gross Unrealized Loss	-0.1	-0.3
Over 12 months, Fair Market Value	11.2	0
Over 12 months, Gross Unrealized Loss	-0.2	0
Total, Fair Market Value	54.4	50.2
Total, Gross Unrealized Loss	-0.3	-0.3
Number of Securities	39	51
Agency mortgage-backed	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	57.3	491.8
0-12 months, Gross Unrealized Loss	-0.4	-18.3
Over 12 months, Fair Market Value	126	1.2
Over 12 months, Gross Unrealized Loss	-3.8	0
Total, Fair Market Value	183.3	493
Total, Gross Unrealized Loss	($4.20)	($18.30)
Number of Securities	54	123

Investments_Analysis_of_Invest

Investments - Analysis of Investment Purchases/Sales and Maturities (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Gain (Loss) on Investments [Line Items]	'	'	'	'
(Purchases) of fixed income securities — Available for sale	($522.80)	($566.80)	($1,440.90)	($1,748.60)
(Purchases) of fixed income securities — Trading	-154.7	-283.2	-553.6	-555.5
(Purchases) of equity securities — Available for sale	0	0	0	-2.5
(Purchases) of equity securities - Trading	-121.2	-38.6	-240.1	-275.4
Proceeds from sales and maturities of fixed income maturities - Available for sale	481	578.6	1,496.90	1,564.40
Proceeds from sales and maturities of fixed income securities — Trading	143.5	67.8	515.7	276.3
Proceeds from sales of equity securities - Available for sale	12.1	29.7	27.7	61.8
Proceeds from sales of equity securities - Trading	17.8	5	36	15.1
Net change in receivable/(payable)/ for securities sold/(purchased)	-7	6.6	12.8	5.6
Net (purchases) of catastrophe bonds - Trading	-1.7	0	-26	0
Net sales of other investments	0	0	37.3	0
Net (purchases)/sales for the period	-121	-105.4	-292.2	-383.1
Available for sale	'	'	'	'
Gain (Loss) on Investments [Line Items]	'	'	'	'
Net (purchases)/sales of short-term investments	25	95.5	-151	275.7
Trading	'	'	'	'
Gain (Loss) on Investments [Line Items]	'	'	'	'
Net (purchases)/sales of short-term investments	$7	$0	($7)	$0

Variable_Interest_Entities_Add

Variable Interest Entities - Additional Information (Details) (USD $)

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Oct. 02, 2012

Sep. 30, 2014

Sep. 30, 2014

Dec. 27, 2013

Sep. 10, 2013

Sep. 30, 2014

Dec. 27, 2013

Sep. 30, 2014

Sep. 30, 2014

Dec. 27, 2013

Sep. 30, 2014

Sep. 30, 2014

Dec. 31, 2013

variable_interest_entity

Chaspark

Chaspark

Chaspark

Chaspark

Chaspark

Silverton

Silverton

Silverton

Silverton

Silverton

Silverton

Silverton

Silverton

Silverton

Silverton

Silverton

Silverton

Investment

Level 3

Third party

Aspen Holdings

Aspen Holdings

Aspen Holdings

Silverton Loan Notes

Silverton Loan Notes

Silverton Loan Notes

Fair Value, Measurements, Recurring

Aspen Holdings

Aspen Holdings

Variable Interest Entity [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number Of Investments In Variable Interest Entities

2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of Variable Interest Entities

2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of ownership acquired

'

'

'

'

'

'

58.50%

'

'

'

'

'

'

'

'

'

'

'

'

Income (Loss) from Equity Method Investments

'

'

$0

$0

$0

$0

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of interests acquired

'

'

'

'

'

'

'

'

'

'

100.00%

'

'

'

'

'

'

'

'

Loans issued

'

'

'

'

'

'

'

'

'

65,000,000

'

'

50,000,000

'

'

15,000,000

'

'

'

Maximum loss exposure, amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

15,000,000

15,000,000

Debt instrument, change in fair value

'

'

'

'

'

'

'

'

'

'

'

'

'

2,600,000

4,400,000

'

'

'

'

Total aggregate unpaid balance

549,100,000

549,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

83,900,000

'

'

Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Unobservable Input [Roll Forward]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Beginning balance

'

'

'

'

'

'

'

-56,000,000

-50,000,000

'

'

-64,500,000

'

'

'

'

'

'

'

Total change in fair value included in the statement of operations

'

'

'

'

'

'

'

-8,500,000

-14,500,000

'

'

'

'

'

'

'

'

'

'

Ending balance

'

'

'

'

'

'

'

($64,500,000)

($64,500,000)

'

'

($64,500,000)

'

'

'

'

'

'

'

Fair_Value_Measurements_Financ

Fair Value Measurements - Financial Assets Measured on Recurring Basis (Detail) (USD $)	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Catastrophe bonds, trading at fair value (cost — $31.6 and $5.8)	$31.60	'	$5.80
Derivatives at fair value	7	'	7
Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Catastrophe bonds, trading at fair value (cost — $31.6 and $5.8)	32.2	'	5.8
Loan notes issued by variable interest entities, at fair value	-64.5	'	-50
Total	7,195.10	'	6,865.90
Recurring | Short-term investments trading, at fair value	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	7	'	'
Recurring | Equity investments trading, at fair value	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	521.1	'	310.9
Recurring | Derivatives at fair value | Foreign Exchange Contracts	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Derivatives at fair value	5.7	'	5.9
Recurring | Derivatives at fair value | Interest Rate Swaps	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Derivatives at fair value	'	'	1.1
Recurring | Liabilities under derivative contracts | Foreign Exchange Contracts	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Liabilities under derivative contracts, fair value	-1.3	'	-2.9
Recurring | Liabilities under derivative contracts | Interest Rate Swaps	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Liabilities under derivative contracts, fair value	-12.7	'	'
Available for sale financial assets, at fair value | Recurring | U.S. government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	973.9	'	1,020.40
Available for sale financial assets, at fair value | Recurring | U.S. agency	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	201.9	'	269.1
Available for sale financial assets, at fair value | Recurring | Municipal	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	31.7	'	32.8
Available for sale financial assets, at fair value | Recurring | Corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	2,278.40	'	2,069.40
Available for sale financial assets, at fair value | Recurring | Non-U.S. government-backed corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	75.5	'	84.6
Available for sale financial assets, at fair value | Recurring | Foreign government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	752.6	'	778.9
Available for sale financial assets, at fair value | Recurring | Asset-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	143.9	'	122.3
Available for sale financial assets, at fair value | Recurring | Non-agency commercial mortgage-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	51.5	'	62.6
Available for sale financial assets, at fair value | Recurring | Agency mortgage-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	988.4	'	1,129
Total fixed income securities — Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	5,497.80	'	5,569.10
Short-term investments — Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	307.2	'	160.3
Equity investments - Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	125.9	'	149.5
Held for trading financial assets, at fair value | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	774.1	'	716.2
Held for trading financial assets, at fair value | Recurring | U.S. government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	16.5	'	22
Held for trading financial assets, at fair value | Recurring | U.S. agency	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0.2	'	0.2
Held for trading financial assets, at fair value | Recurring | Municipal	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	1.1	'	1.1
Held for trading financial assets, at fair value | Recurring | Corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	521.1	'	474.8
Held for trading financial assets, at fair value | Recurring | Foreign government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	139.2	'	136.2
Held for trading financial assets, at fair value | Recurring | Asset-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	15.2	'	12.8
Held for trading financial assets, at fair value | Recurring | Bank Loans	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	80.8	'	69.1
Level 1 | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Catastrophe bonds, trading at fair value (cost — $31.6 and $5.8)	0	'	0
Loan notes issued by variable interest entities, at fair value	0	'	0
Total	2,511.90	'	2,272.70
Level 1 | Recurring | Short-term investments trading, at fair value	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	'
Level 1 | Recurring | Equity investments trading, at fair value	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	521.1	'	310.9
Level 1 | Recurring | Derivatives at fair value | Foreign Exchange Contracts	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Derivatives at fair value	0	'	0
Level 1 | Recurring | Derivatives at fair value | Interest Rate Swaps	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Derivatives at fair value	'	'	0
Level 1 | Recurring | Liabilities under derivative contracts | Foreign Exchange Contracts	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Liabilities under derivative contracts, fair value	0	'	0
Level 1 | Recurring | Liabilities under derivative contracts | Interest Rate Swaps	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Liabilities under derivative contracts, fair value	0	'	'
Level 1 | Available for sale financial assets, at fair value | Recurring | U.S. government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	973.9	'	1,020.40
Level 1 | Available for sale financial assets, at fair value | Recurring | U.S. agency	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 1 | Available for sale financial assets, at fair value | Recurring | Municipal	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 1 | Available for sale financial assets, at fair value | Recurring | Corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 1 | Available for sale financial assets, at fair value | Recurring | Non-U.S. government-backed corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 1 | Available for sale financial assets, at fair value | Recurring | Foreign government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	538.2	'	596.2
Level 1 | Available for sale financial assets, at fair value | Recurring | Asset-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 1 | Available for sale financial assets, at fair value | Recurring | Non-agency commercial mortgage-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 1 | Available for sale financial assets, at fair value | Recurring | Agency mortgage-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 1 | Total fixed income securities — Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	1,512.10	'	1,616.60
Level 1 | Short-term investments — Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	302.4	'	129.5
Level 1 | Equity investments - Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	125.9	'	149.5
Level 1 | Held for trading financial assets, at fair value | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	50.4	'	66.2
Level 1 | Held for trading financial assets, at fair value | Recurring | U.S. government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	16.5	'	22
Level 1 | Held for trading financial assets, at fair value | Recurring | U.S. agency	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 1 | Held for trading financial assets, at fair value | Recurring | Municipal	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 1 | Held for trading financial assets, at fair value | Recurring | Corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 1 | Held for trading financial assets, at fair value | Recurring | Foreign government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	33.9	'	44.2
Level 1 | Held for trading financial assets, at fair value | Recurring | Asset-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 1 | Held for trading financial assets, at fair value | Recurring | Bank Loans	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 2 | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Catastrophe bonds, trading at fair value (cost — $31.6 and $5.8)	32.2	'	5.8
Loan notes issued by variable interest entities, at fair value	0	'	0
Total	4,747.70	'	4,643.20
Level 2 | Recurring | Short-term investments trading, at fair value	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	7	'	'
Level 2 | Recurring | Equity investments trading, at fair value	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 2 | Available for sale financial assets, at fair value | Recurring | U.S. government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 2 | Available for sale financial assets, at fair value | Recurring | U.S. agency	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	201.9	'	269.1
Level 2 | Available for sale financial assets, at fair value | Recurring | Municipal	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	31.7	'	32.8
Level 2 | Available for sale financial assets, at fair value | Recurring | Corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	2,278.40	'	2,069.40
Level 2 | Available for sale financial assets, at fair value | Recurring | Non-U.S. government-backed corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	75.5	'	84.6
Level 2 | Available for sale financial assets, at fair value | Recurring | Foreign government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	214.4	'	182.7
Level 2 | Available for sale financial assets, at fair value | Recurring | Asset-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	143.9	'	122.3
Level 2 | Available for sale financial assets, at fair value | Recurring | Non-agency commercial mortgage-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	51.5	'	62.6
Level 2 | Available for sale financial assets, at fair value | Recurring | Agency mortgage-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	988.4	'	1,129
Level 2 | Total fixed income securities — Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	3,985.70	'	3,952.50
Level 2 | Short-term investments — Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	4.8	'	30.8
Level 2 | Equity investments - Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 2 | Held for trading financial assets, at fair value | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	723.7	'	650
Level 2 | Held for trading financial assets, at fair value | Recurring | U.S. government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 2 | Held for trading financial assets, at fair value | Recurring | U.S. agency	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0.2	'	0.2
Level 2 | Held for trading financial assets, at fair value | Recurring | Municipal	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	1.1	'	1.1
Level 2 | Held for trading financial assets, at fair value | Recurring | Corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	521.1	'	474.8
Level 2 | Held for trading financial assets, at fair value | Recurring | Foreign government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	105.3	'	92
Level 2 | Held for trading financial assets, at fair value | Recurring | Asset-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	15.2	'	12.8
Level 2 | Held for trading financial assets, at fair value | Recurring | Bank Loans	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	80.8	'	69.1
Level 3 | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Catastrophe bonds, trading at fair value (cost — $31.6 and $5.8)	0	'	0
Loan notes issued by variable interest entities, at fair value	-64.5	'	-50
Total	-64.5	'	-50
Level 3 | Recurring | Short-term investments trading, at fair value	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	'
Level 3 | Recurring | Equity investments trading, at fair value	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Recurring | Derivatives at fair value | Foreign Exchange Contracts	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Derivatives at fair value	0	'	0
Level 3 | Recurring | Derivatives at fair value | Interest Rate Swaps	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Derivatives at fair value	'	'	0
Level 3 | Recurring | Liabilities under derivative contracts | Foreign Exchange Contracts	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Liabilities under derivative contracts, fair value	0	'	0
Level 3 | Recurring | Liabilities under derivative contracts | Interest Rate Swaps	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Liabilities under derivative contracts, fair value	0	'	'
Level 3 | Available for sale financial assets, at fair value | Recurring | U.S. government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Available for sale financial assets, at fair value | Recurring | U.S. agency	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Available for sale financial assets, at fair value | Recurring | Municipal	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Available for sale financial assets, at fair value | Recurring | Corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Available for sale financial assets, at fair value | Recurring | Non-U.S. government-backed corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Available for sale financial assets, at fair value | Recurring | Foreign government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Available for sale financial assets, at fair value | Recurring | Asset-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Available for sale financial assets, at fair value | Recurring | Non-agency commercial mortgage-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Available for sale financial assets, at fair value | Recurring | Agency mortgage-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Total fixed income securities — Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Short-term investments — Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Equity investments - Available for sale | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Held for trading financial assets, at fair value | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Held for trading financial assets, at fair value | Recurring | U.S. government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Held for trading financial assets, at fair value | Recurring | U.S. agency	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Held for trading financial assets, at fair value | Recurring | Municipal	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Held for trading financial assets, at fair value | Recurring | Corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Held for trading financial assets, at fair value | Recurring | Foreign government	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Held for trading financial assets, at fair value | Recurring | Asset-backed	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Level 3 | Held for trading financial assets, at fair value | Recurring | Bank Loans	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Investments, fair value	0	'	0
Derivatives at Fair Value | Derivatives Not Designated as Hedging Instruments Under ASC 815 | Level 2 | Recurring | Derivatives at fair value | Foreign Exchange Contracts	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Fair Value	5.7	'	5.9
Derivatives at Fair Value | Derivatives Not Designated as Hedging Instruments Under ASC 815 | Level 2 | Recurring | Derivatives at fair value | Interest Rate Swaps	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Fair Value	'	'	1.1	[1]
Liabilities under Derivative Contracts | Derivatives Not Designated as Hedging Instruments Under ASC 815 | Level 2 | Recurring | Liabilities under derivative contracts | Foreign Exchange Contracts	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Fair Value	'	'	-2.9
Liabilities under Derivative Contracts | Derivatives Not Designated as Hedging Instruments Under ASC 815 | Level 2 | Recurring | Liabilities under derivative contracts | Interest Rate Swaps	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Fair Value	-12.7	'	'
Silverton	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	64.5	56	50
Silverton | Level 3 | Recurring	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	$64.50	'	'
[1]	Net of $23.4 million of cash collateral provided to counterparties, Goldman Sachs International ($481.2 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2013 — $34.3 million).

Fair_Value_Measurements_Reconc

Fair Value Measurements - Reconciliation of Liabilities Using Level 3 Inputs (Details) (Silverton, USD $)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2014
Silverton	'	'
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'
Beginning balance	($56)	($50)
Total change in fair value included in the statement of operations	-8.5	-14.5
Ending balance	($64.50)	($64.50)

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Quotes per fixed income investment	2	2.6
Quotes per equity investment	4	4.9
Silverton	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Debt Instrument, Face Amount	'	65
Loan notes held by third parties	'	50

Fair_Value_Measurements_Pricin

Fair Value Measurements - Pricing Sources Used in Pricing Fixed Income Investments (Detail)	Sep. 30, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]	'	'
Index providers	84.00%	85.00%
Pricing services	12.00%	12.00%
Broker-dealers	4.00%	3.00%
Total	100.00%	100.00%

Fair_Value_Measurements_Summar

Fair Value Measurements - Summary of Securities Priced Using Pricing Information from Index Providers (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	$5,903	$5,813.40
% of Total Fair Value by Security Type	85.00%	86.00%
Fixed Income Maturities	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	5,256	5,353
% of Total Fair Value by Security Type	84.00%	85.00%
Fixed Income Maturities | U.S. government	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	911.9	998.5
% of Total Fair Value by Security Type	92.00%	96.00%
Fixed Income Maturities | U.S. agency	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	191.3	255.3
% of Total Fair Value by Security Type	95.00%	95.00%
Fixed Income Maturities | Municipal	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	13.4	14.5
% of Total Fair Value by Security Type	41.00%	43.00%
Fixed Income Maturities | Corporate	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	2,675.20	2,400.80
% of Total Fair Value by Security Type	96.00%	94.00%
Fixed Income Maturities | Non-U.S. government-backed corporate	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	57.1	55.9
% of Total Fair Value by Security Type	76.00%	66.00%
Fixed Income Maturities | Foreign government	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	559.1	605.8
% of Total Fair Value by Security Type	63.00%	66.00%
Fixed Income Maturities | Asset-backed	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	146	130.6
% of Total Fair Value by Security Type	92.00%	97.00%
Fixed Income Maturities | Non-agency commercial mortgage-backed	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	51.5	61
% of Total Fair Value by Security Type	100.00%	97.00%
Fixed Income Maturities | Agency mortgage-backed	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	650.5	830.6
% of Total Fair Value by Security Type	66.00%	74.00%
Equities	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	$647	$460.40
% of Total Fair Value by Security Type	100.00%	100.00%

Fair_Value_Measurements_Loan_N

Fair Value Measurements - Loan Notes Issued by Variable Interest Entities (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014	Dec. 31, 2013
					Fair Value Level 3	Fair Value Level 3	Recurring	Recurring	Recurring	Recurring
					Low	High			Fair Value Level 3	Fair Value Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'	'	'	'	'	'	'	'
Gross premiums written	$652.50	$581.60	$2,287.30	$2,042.30	$0	$40	'	'	'	'
Reserve for losses	'	'	'	'	0	2.8	'	'	'	'
Contract period	'	'	'	'	'	'365 days	'	'	'	'
Loan notes held by third parties	'	'	'	'	$50	$50	$64.50	$50	$64.50	$50

Reinsurance_Additional_Informa

Reinsurance - Additional Information (Detail)	Sep. 30, 2014	Dec. 31, 2013
Lloyd's	'	'
Reinsurance [Line Items]	'	'
Reinsurance recoverables	24.90%	23.50%
Munich Re	'	'
Reinsurance [Line Items]	'	'
Reinsurance recoverables	16.90%	17.10%

Derivative_Contracts_Fair_Valu

Derivative Contracts - Fair Value of Derivative Instruments (Detail) (USD $)	Sep. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Interest Rate Swaps	'		'
Derivatives, Fair Value [Line Items]	'		'
Derivative, Notional Amount	$981.20	[1]	$1,000
Interest Rate Swaps | Derivatives Not Designated as Hedging Instruments Under ASC 815 | Derivatives at Fair Value	'		'
Derivatives, Fair Value [Line Items]	'		'
Notional Amount	'		1,000	[1]
Fair Value	1.3	[1]	'
Foreign Exchange Contracts	'		'
Derivatives, Fair Value [Line Items]	'		'
Derivative, Notional Amount	367		'
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments Under ASC 815 | Derivatives at Fair Value	'		'
Derivatives, Fair Value [Line Items]	'		'
Notional Amount	161.4		224.4
Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments Under ASC 815 | Liabilities under Derivative Contracts	'		'
Derivatives, Fair Value [Line Items]	'		'
Notional Amount	205.6		57.5
Derivatives at fair value | Level 2 | Fair Value, Measurements, Recurring | Interest Rate Swaps | Derivatives Not Designated as Hedging Instruments Under ASC 815 | Derivatives at Fair Value	'		'
Derivatives, Fair Value [Line Items]	'		'
Fair Value	'		1.1	[1]
Derivatives at fair value | Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments Under ASC 815 | Derivatives at Fair Value	'		'
Derivatives, Fair Value [Line Items]	'		'
Fair Value	5.7		5.9
Liabilities under derivative contracts | Level 2 | Fair Value, Measurements, Recurring | Interest Rate Swaps | Derivatives Not Designated as Hedging Instruments Under ASC 815 | Liabilities under Derivative Contracts	'		'
Derivatives, Fair Value [Line Items]	'		'
Fair Value	-12.7		'
Liabilities under derivative contracts | Level 2 | Fair Value, Measurements, Recurring | Foreign Exchange Contracts | Derivatives Not Designated as Hedging Instruments Under ASC 815 | Liabilities under Derivative Contracts	'		'
Derivatives, Fair Value [Line Items]	'		'
Fair Value	'		($2.90)
[1]	Net of $23.4 million of cash collateral provided to counterparties, Goldman Sachs International ($481.2 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2013 — $34.3 million).

Derivative_Contracts_Fair_Valu1

Derivative Contracts - Fair Value of Derivative Instruments (Parenthetical) (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Derivatives, Fair Value [Line Items]	'	'
Collateral already posted, aggregate fair value	$23.40	$34.30
Goldman Sachs International	'	'
Derivatives, Fair Value [Line Items]	'	'
Notional Amount	481.2	'
Credit Agricole CIB	'	'
Derivatives, Fair Value [Line Items]	'	'
Notional Amount	$500	'

Derivative_Contracts_GainLoss_

Derivative Contracts - Gain/(Loss) Recognized in Income on Derivative (Detail) (Change in Fair Value of Derivatives, USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Foreign Exchange Contracts	'	'	'	'
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'
Amount of Income/(Loss) Recognized in the Statement of Operations	($6.40)	$10.80	($4.50)	($3.70)
Interest Rate Swaps	'	'	'	'
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'
Amount of Income/(Loss) Recognized in the Statement of Operations	$1.30	($4.20)	($4.10)	$3.20

Derivative_Contracts_Additiona

Derivative Contracts - Additional Information (Detail) (USD $)

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Foreign Exchange Contracts

Interest Rate Swaps

Interest Rate Swaps

Change In Fair Value Of Derivatives Net Foreign Exchange Gains And Losses

Change In Fair Value Of Derivatives Net Foreign Exchange Gains And Losses

Change In Fair Value Of Derivatives Net Foreign Exchange Gains And Losses

Change In Fair Value Of Derivatives Net Foreign Exchange Gains And Losses

Change In Fair Value Of Derivatives Net Foreign Exchange Gains And Losses

Change In Fair Value Of Derivatives Net Foreign Exchange Gains And Losses

Change In Fair Value Of Derivatives Net Foreign Exchange Gains And Losses

Change In Fair Value Of Derivatives Net Foreign Exchange Gains And Losses

Foreign Exchange Contracts

Foreign Exchange Contracts

Foreign Exchange Contracts

Foreign Exchange Contracts

Interest Rate Swaps

Interest Rate Swaps

Interest Rate Swaps

Interest Rate Swaps

Derivative Instruments, Gain (Loss) [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

Notional Amount

'

'

$367,000,000

$981,200,000

[1]

$1,000,000,000

'

'

'

'

'

'

'

'

Derivative instruments not designated as hedging instruments, gain (loss), net

'

'

'

'

'

-6,400,000

10,800,000

-4,500,000

-3,700,000

1,300,000

-4,200,000

-4,100,000

3,200,000

Collateral already posted, aggregate fair value

23,400,000

34,300,000

'

'

'

'

'

'

'

'

'

'

'

Pledged assets

$0

$0

'

'

'

'

'

'

'

'

'

'

'

[1]

Net of $23.4 million of cash collateral provided to counterparties, Goldman Sachs International ($481.2 million notional) and Crédit Agricole CIB ($500.0 million notional) under respective International Swap Dealers Association agreements, as security for the Company’s net liability position (December 31, 2013 — $34.3 million).

Deferred_Policy_Acquisition_Co2

Deferred Policy Acquisition Costs (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]	'	'	'	'
Balance at the beginning of the period	$306.40	$264.40	$262.20	$223
Acquisition costs deferred	110.7	108.2	375.8	361.4
Amortization of deferred policy acquisition costs	-115.5	-110.5	-336.4	-322.3
Balance at the end of the period	$301.60	$262.10	$301.60	$262.10

Reserves_for_Losses_and_Loss_A2

Reserves for Losses and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses ("LAE") Reserves (Detail) (USD $)	9 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2012
Insurance [Abstract]	'	'	'	'
Provision for losses and LAE at the start of the year	$4,678.90	$4,779.70	$4,779.70	'
Less reinsurance recoverable	'	'	-332.7	-499
Net loss and LAE at the start of the year	4,346.20	4,280.70	4,280.70	'
Net loss and LAE expenses assumed/(disposed)	-24.2	-34.5	-34.6	'
Provision for losses and LAE for claims incurred:	'	'	'	'
Current year	1,060.50	'	1,331.40	'
Prior years	-92.6	-87.2	-107.7	'
Total incurred	967.9	'	1,223.70	'
Losses and LAE payments for claims incurred:	'	'	'	'
Current year	-56.6	'	-172.8	'
Prior years	-785.7	'	-912.3	'
Total paid	-842.3	'	-1,085.10	'
Foreign exchange losses/(gains)	-45	'	-38.5	'
Net losses and LAE reserves at period end	4,402.60	4,273.40	4,346.20	'
Plus reinsurance recoverable on unpaid losses at period end	384.7	'	332.7	'
Provision for losses and LAE at the end of the relevant period	$4,787.30	'	$4,678.90	'

Reserves_for_Losses_and_Loss_A3

Reserves for Losses and Loss Adjustment Expenses - Additional Information (Detail) (USD $)	9 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Insurance [Abstract]	'	'	'
Reserve releases	$92.60	$87.20	$107.70
Loss portfolio transfers (disposed)	$24.20	$34.50	$34.60

Capital_Structure_Summary_of_A

Capital Structure - Summary of Authorized and Issued Share Capital (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Thousands, except Share data, unless otherwise specified
Authorized share capital:	'	'
Number of ordinary shares	969,629,030	969,629,030
Number of non-voting shares	6,787,880	6,787,880
Number of preference shares	100,000,000	100,000,000
Ordinary Shares 0.15144558¢ per share	$1,469	$1,469
Non-Voting Shares 0.15144558¢ per share	10	10
Preference Shares 0.15144558¢ per share	152	152
Total authorized share capital	1,631	1,631
Issued share capital:	'	'
Number of issued ordinary shares	63,349,858	65,546,976
Issued ordinary shares of 0.15144558¢ per share	96	99
Total issued share capital	131	134
7.401% Preference Shares	'	'
Issued share capital:	'	'
Number of issued preference shares of 0.15144558¢ each with a liquidation preference	5,327,500	5,327,500
Issued preference shares of 0.15144558¢ each with a liquidation preference	8	8
7.250% preference shares	'	'
Issued share capital:	'	'
Number of issued preference shares of 0.15144558¢ each with a liquidation preference	6,400,000	6,400,000
Issued preference shares of 0.15144558¢ each with a liquidation preference	10	10
5.950% Preference Shares	'	'
Issued share capital:	'	'
Number of issued preference shares of 0.15144558¢ each with a liquidation preference	11,000,000	11,000,000
Issued preference shares of 0.15144558¢ each with a liquidation preference	$17	$17

Capital_Structure_Summary_of_A1

Capital Structure - Summary of Authorized and Issued Share Capital (Parenthetical) (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Class of Stock [Line Items]	'	'
Ordinary shares, par value (in dollars per share)	$0.15	$0.15
Non-voting shares, par value	$0.15	$0.15
Preference shares, par value (in dollars per share)	$0.15	$0.15
Redemption price per share	$25	'
7.401% Preference Shares	'	'
Class of Stock [Line Items]	'	'
Preference shares, par value (in dollars per share)	$0.15	$0.15
Preference shares, rate	7.40%	7.40%
Redemption price per share	$25	$25
7.250% preference shares	'	'
Class of Stock [Line Items]	'	'
Preference shares, par value (in dollars per share)	$0.15	$0.15
Preference shares, rate	7.25%	7.25%
Redemption price per share	$25	$25
5.950% Preference Shares	'	'
Class of Stock [Line Items]	'	'
Preference shares, par value (in dollars per share)	$0.15	$0.15
Preference shares, rate	5.95%	5.95%
Redemption price per share	$25	$25

Capital_Structure_Additional_I

Capital Structure - Additional Information (Detail) (USD $)

0 Months Ended

1 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

9 Months Ended

0 Months Ended

3 Months Ended

9 Months Ended

0 Months Ended

9 Months Ended

24 Months Ended

0 Months Ended

3 Months Ended

9 Months Ended

0 Months Ended

0 Months Ended

1 Months Ended

3 Months Ended

9 Months Ended

0 Months Ended

1 Months Ended

Apr. 17, 2014

Apr. 25, 2013

Apr. 17, 2014

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2013

Sep. 30, 2014

Apr. 25, 2013

Sep. 30, 2013

Sep. 30, 2013

Apr. 25, 2013

Sep. 30, 2013

Dec. 31, 2006

Apr. 25, 2013

Apr. 25, 2013

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

2-May-13

2-May-13

Feb. 07, 2013

Feb. 07, 2013

Aug. 29, 2013

Feb. 26, 2013

Feb. 07, 2013

Mar. 31, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Mar. 04, 2013

Mar. 21, 2013

Apr. 17, 2014

Apr. 17, 2014

Minimum

Reclassification from Additional Paid in Capital to Retained Earnings

Reclassification from Additional Paid in Capital to Retained Earnings

Reclassification from Additional Paid in Capital to Retained Earnings

5.625% Preference Shares

5.625% Preference Shares

5.625% Preference Shares

5.625% Preference Shares

Preference shares

Preference shares

Preference shares

Preference shares

Perpetual Non-Cumulative Preference Shares

Perpetual Non-Cumulative Preference Shares

Existing Share Repurchase Program

New Repurchase Program

Accelerated Share Repurchase Agreement

Accelerated Share Repurchase Agreement

Accelerated Share Repurchase Agreement

Accelerated Share Repurchase Agreement

Open Markets

Open Markets

Open Markets

Open Markets

Names Trustee [Member]

Names Trustee [Member]

Passive Institutional Investors

Preferred Stock [Member]

Class of Stock [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Additional Paid in Capital

'

'

'

$1,186,400,000

'

$1,186,400,000

'

$1,297,400,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Aggregate liquidation preferences

'

'

'

568,200,000

'

568,200,000

'

568,200,000

'

'

'

'

'

'

'

'

'

'

'

'

'

270,400,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Issuance costs

'

'

'

'

'

12,400,000

'

12,400,000

'

'

'

'

'

'

'

'

'

'

'

'

4,600,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

ASR, commenced date

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

7-Feb-13

'

'

'

'

'

'

'

'

'

Existing share repurchase authorization amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

400,000,000

500,000,000

'

'

'

'

'

'

'

'

'

'

'

'

Shares repurchase and cancelled during period, shares

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

3,348,214

2,120,625

1,498,451

2,891,130

3,979,826

'

'

'

'

Common Stock Additional Shares Receivable

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

705,062

'

'

'

'

'

'

'

'

'

'

'

Stock Repurchased During Period, Shares

'

'

'

'

'

2,891,356

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

4,053,276

'

'

'

'

'

'

'

54,437

'

'

'

Shares repurchase and cancelled during period, value

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

150,000,000

'

'

90,000,000

54,800,000

120,900,000

143,000,000

'

2,000,000

'

'

Average price

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$37.01

'

'

'

$42.46

$36.59

$41.82

$35.92

'

'

'

'

Preference shares, issued (in shares)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

4,600,000

'

'

'

'

'

'

11,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Minimum conversion price

'

'

'

'

'

'

'

'

130.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Conversion price, number of trading days in period

'

'20 days

'

'

'

'20 days

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of consecutive trading days in period

'

'

'

'

'

'30 days

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Conversion rate of ordinary shares

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1.7121

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Preference shares, liquidation preference

'

'

'

$25

'

$25

'

'

'

'

'

'

'

'

'

$50

'

'

'

'

'

$25

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Shares issued in respect of the redemption of the PIERS

'

'

'

'

'

'

'

'

'

'

'

'

1,835,860

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

PIERS redemption

'

'

'

0

[1]

0

[1]

0

[1]

7,100,000

[1]

'

'

7,100,000

7,100,000

7,100,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

PIERS redeemed and canceled

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

230,000,000

230,000,000

230,000,000

230,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Annual rate of dividend

'

'

'

'

'

'

'

'

'

'

'

'

'

5.63%

5.63%

'

'

'

'

'

5.95%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Preference shares dividend, floating rate

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'3-month LIBOR

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Preference shares dividend, variable spread

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

4.06%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Preference shares

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$275,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Preference shares, redemption date

'

'

'

'

'

1-Jul-23

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Convertible preferred stock, shares issued upon conversion

'

'

0.001

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1

Class of warrant or right, exercise price of warrants or rights

'

'

$160

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Equity method investment, ownership percentage

'

'

10.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

15.00%

'

Minimum period before exercisable

'10 days

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

[1]

The $7.1 million reduction in the net income available to ordinary shareholders is the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of $230.0 million of the PIERS on May 30, 3013.

Capital_Structure_Summary_of_O

Capital Structure - Summary of Ordinary Shares (Detail)	9 Months Ended
	Sep. 30, 2014
Ordinary Shares [Roll Forward]	'
Ordinary shares in issue at the beginning of the year	65,546,976
Ordinary shares issued to employees under the 2003 and 2013 share incentive plans and/or 2008 share purchase plan	692,929
Ordinary shares issued to non-employee directors	1,309
Ordinary shares repurchased	-2,891,356
Ordinary shares in issue at the end of the period	63,349,858

ShareBased_Payments_Additional

Share-Based Payments - Additional Information (Detail)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

0 Months Ended

9 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

0 Months Ended

3 Months Ended

9 Months Ended

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Apr. 06, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Feb. 06, 2014

Apr. 26, 2013

Feb. 11, 2013

Feb. 07, 2014

Feb. 11, 2013

Apr. 22, 2014

Jun. 11, 2013

Apr. 08, 2013

Feb. 06, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2014

USD ($)

USD ($)

USD ($)

USD ($)

Achievement One

Achievement Two

Achievement Two

Achievement Three

Achievement Three

Employee Share Purchase Plans

Employee Share Purchase Plans

Employee Share Purchase Plans

Employee Share Purchase Plans

Sharesave Scheme

Sharesave Scheme

2013 Share Incentive Plan

2003 Share Incentive Plan

International Employee Share Purchase Plan

Investor Options [Member]

Performance Shares

Performance Shares

Performance Shares

Performance Shares

Stock Options

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Restricted Share Units

Phantom Shares

Phantom Shares

Phantom Shares

Phantom Shares

Phantom Shares

Phantom Shares

Phantom Shares

Phantom Shares

Grants in 2007

Grant Year 2015 and 2016 [Member]

Minimum

Maximum

Minimum

Maximum

USD ($)

USD ($)

USD ($)

USD ($)

GBP (£)

GBP (£)

USD ($)

Agreement

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

Director

Director

Director

Director

Director

Director

Director

Board of Directors Chairman

Board of Directors Chairman

USD ($)

USD ($)

USD ($)

USD ($)

Stock Options

Performance Shares

USD ($)

USD ($)

USD ($)

USD ($)

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

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of arrangements under options and other equity incentives

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Shares available to grant under incentive plan

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2,845,683

595,683

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Vesting period

'

'

'3 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'3 years

'

'3 years

'

'

'3 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'7 years

'

Exercise period

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'10 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Options grants in period

0

0

0

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Options exercised

0

204,328

53,436

747,023

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Compensation costs

$0

$0

$0

$0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$300,000

($1,400,000)

$4,500,000

$4,200,000

'

$1,900,000

$2,400,000

$7,700,000

$7,000,000

$300,000

$300,000

$1,100,000

$600,000

'

'

'

'

'

'

'

'

'

$1,200,000

($1,600,000)

$3,600,000

$0

'

'

Granted, number of shares

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0

0

315,389

250,066

'

0

8,571

259,640

307,441

'

'

'

'

'

'

'

'

'

154,512

542

6,521

152,541

'

'

'

'

'

'

Percentage of restricted share units vested and issued

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

"RSUs" for non-employee directors

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

27,180

2,110

25,533

13,590

14,188

'

'

'

'

'

'

'

'

'

'

Achieved and required performance growth, percentage

'

'

'

'

5.20%

5.20%

10.40%

10.40%

20.80%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of initial shares granted

'

'

'

'

33.33%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Eligible for vesting, percent

'

'

'

'

'

10.00%

100.00%

100.00%

200.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Vesting rule, threshold percent of eligible shares

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

100.00%

Vesting rule, maximum vesting percent of eligible shares

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

100.00%

Employee savings under ESPP

'

'

'

'

'

'

'

'

'

'

'

'

'

500

250

'

'

500

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Employee contribution period

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'3 years

'

'

'2 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Fair value adjustment capitalized amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

-300,000

100,000

900,000

300,000

'

'

'

'

'

'

'

'

'

'

'

'

'

800,000

900,000

1,300,000

1,700,000

'

'

Tax benefit from compensation expense

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

-100,000

700,000

-1,000,000

1,300,000

'

-500,000

-500,000

-1,800,000

-1,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

-200,000

-100,000

-700,000

-100,000

'

'

Additional employee contribution period

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'1 year

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Shares issued

627

1,985

11,821

16,339

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Purchase price of fair market value

'

'

'

'

'

'

'

'

'

'

'

'

'

'

85.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Compensation costs charged in ESPP

'

'

'

'

'

'

'

'

'

$100,000

$100,000

$200,000

$200,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Intangible_Assets_Summary_of_C

Intangible Assets - Summary of Changes in Intangible Assets (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Finite and Indefinite Lived Intangible Assets [Roll Forward]	'	'	'	'
Beginning of the period	$18.20	$18.70	$18.40	$19
Amortization	0	-0.1	-0.2	-0.4
End of the period	18.2	18.6	18.2	18.6
Trade Mark	'	'	'	'
Finite and Indefinite Lived Intangible Assets [Roll Forward]	'	'	'	'
Beginning of the period	1.6	1.6	1.6	1.6
Amortization	0	0	0	0
End of the period	1.6	1.6	1.6	1.6
Insurance Licenses	'	'	'	'
Finite and Indefinite Lived Intangible Assets [Roll Forward]	'	'	'	'
Beginning of the period	16.6	16.6	16.6	16.6
Amortization	0	0	0	0
End of the period	16.6	16.6	16.6	16.6
Other	'	'	'	'
Finite and Indefinite Lived Intangible Assets [Roll Forward]	'	'	'	'
Beginning of the period	0	0.5	0.2	0.8
Amortization	0	-0.1	-0.2	-0.4
End of the period	$0	$0.40	$0	$0.40

Intangible_Assets_Additional_I

Intangible Assets - Additional Information (Detail) (USD $)

9 Months Ended

0 Months Ended

12 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2014

Jun. 30, 2014

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Dec. 31, 2012

Dec. 31, 2010

Apr. 05, 2005

Dec. 31, 2010

Sep. 30, 2014

Jun. 30, 2014

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Dec. 31, 2012

Sep. 30, 2014

Jun. 30, 2014

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Dec. 31, 2012

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Dec. 31, 2010

Dec. 31, 2010

Sep. 30, 2014

Jun. 30, 2014

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Dec. 31, 2012

Aspen U.K.

APJ Continuation Limited

Trade Mark

Trade Mark

Trade Mark

Trade Mark

Trade Mark

Trade Mark

Insurance Licenses

Insurance Licenses

Insurance Licenses

Insurance Licenses

Insurance Licenses

Insurance Licenses

Insurance Licenses

Insurance Licenses

Insurance Licenses

Intangible Asset: Renewal Rights and Distribution Channels

Intangible Asset: Lock-in Period

Other

Other

Other

Other

Other

Other

AAIC

Aspen Specialty

Aspen U.K.

Finite And Indefinite Lived Intangible Assets By Major Class [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Aggregate consideration for sale and purchase agreement

'

'

'

'

'

'

'

$1.60

$4.80

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Value of the asset

18.2

18.2

18.4

18.6

18.7

19

'

'

'

1.6

1.6

1.6

1.6

1.6

1.6

16.6

16.6

16.6

16.6

16.6

16.6

'

'

'

'

'

0

0

0.2

0.4

0.5

0.8

Acquired licenses

'

'

'

'

'

'

3.6

'

'

'

'

'

'

'

'

'

'

'

'

'

'

10

4.5

2.1

2.2

1.4

'

'

'

'

'

'

Fair value of tangible and financial assets, net

'

'

'

'

'

'

$1.20

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Asset amortization period

'5 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Commitments_and_Contingent_Lia2

Commitments and Contingent Liabilities - Schedule of Company's Restricted Assets (Detail) (USD $)	Sep. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Regulatory trusts and deposits:	'		'
Total restricted assets	$3,663.90		$3,752.60
Total as percent of cash and invested assets	42.80%		45.50%
Affiliated transactions	'		'
Regulatory trusts and deposits:	'		'
Total restricted assets	741.4		685.8
Third party	'		'
Regulatory trusts and deposits:	'		'
Total restricted assets	2,146.10		2,236.40
Letters of credit / guarantees	'		'
Regulatory trusts and deposits:	'		'
Total restricted assets	$776.40	[1]	$830.40	[1]
[1]	As of September 30, 2014, the Company had pledged funds of $749.4 million and £16.2 million (December 31, 2013 — $803.7 million and £16.1 million) as collateral for the secured letters of credit.

Commitments_and_Contingent_Lia3

Commitments and Contingent Liabilities - Schedule of Company's Restricted Assets (Parenthetical) (Detail)	Sep. 30, 2014	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2013
In Millions, unless otherwise specified	USD ($)	GBP (£)	USD ($)	GBP (£)
Commitments and Contingencies Disclosure [Abstract]	'	'	'	'
Securities and cash as collateral secured letters of credit	$749.40	£ 16.2	$803.70	£ 16.1

Commitments_and_Contingent_Lia4

Commitments and Contingent Liabilities - Additional Information (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Jun. 30, 2014	Jul. 30, 2012	Jul. 30, 2012	Jul. 30, 2012	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014
			Citi Europe	Citi Europe	Citi Europe	Citi Europe	Barclays Bank PLC	Barclays Bank PLC	LIBOR
			Letter of Credit	Letter of Credit	Letter of Credit, Tranche I	Letter of Credit, Tranche II	Letter of Credit	Letter of Credit	Citi Europe
				tranche					Letter of Credit
Line of Credit Facility [Line Items]	'	'	'	'	'	'	'	'	'
Minimum capital required	$700,000	$700,000	'	'	'	'	'	'	'
Maximum borrowing capacity	'	'	575,000,000	950,000,000	650,000,000	300,000,000	100,000,000	'	'
Line of Credit Facility, Number of Maturity Tranches	'	'	'	2	'	'	'	'	'
Debt Instrument, Basis Spread on Variable Rate	'	'	'	'	'	'	'	'	1.00%
Letters of Credit Outstanding, Amount	'	'	'	'	'	'	12,300,000	18,900,000	'
Collateral already posted, aggregate fair value	$23,400,000	$34,300,000	'	'	'	'	'	'	'

Commitments_and_Contingent_Lia5

Commitments and Contingent Liabilities - Amounts Outstanding under Operating Leases (Detail) (USD $)	Sep. 30, 2014
In Millions, unless otherwise specified
Leases [Abstract]	'
2014	$1.80
2015	13.6
2016	9.7
2017	8.5
2018	7.2
Later Years	20.3
Total	$61.10