Aspen Insurance Holdings Limited (AHL-PC) 10-Q 5 Nov 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Nov. 01, 2013
Document Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-13	'
Document Fiscal Year Focus	'2013	'
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	'	65,623,898

CONDENSED_CONSOLIDATED_BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Investments:	'	'
Fixed income maturities, available for sale at fair value (amortized cost - $5,379.3 and $5,228.5)	$5,544.90	$5,557.30
Trading investments in fixed income maturities, Fair Market Value	718.6	456.1
Equity securities, available for sale at fair value (cost - $147.5 and $174.0)	163.2	200.1
Equity securities, trading at fair value (cost - $226.5 and $Nil)	278.3	0
Short-term investments, available for sale at fair value (amortized cost - $231.7 and $431.5)	148.3	431.5
Short-term investments, trading at fair value (amortized cost - $10.6 and $2.4)	0.2	2.4
Other investments, equity method	46.1	45
Total investments	6,899.60	6,692.40
Cash and cash equivalents	1,198.30	1,463.60
Reinsurance recoverables	'	'
Unpaid losses	442.2	499
Ceded unearned premiums	179.6	122.6
Receivables	'	'
Underwriting premiums	1,089.10	1,057.50
Other	76.3	68.5
Funds withheld	47.7	84.3
Deferred policy acquisition costs	262.1	223
Derivatives at fair value	5.6	2
Receivable for securities sold	5.9	0.2
Office properties and equipment	60.5	57.9
Tax recoverable	3.1	2.4
Other assets	13.1	18.2
Intangible assets	18.6	19
Total assets	10,301.70	10,310.60
Insurance reserves	'	'
Losses and loss adjustment expenses	4,715.60	4,779.70
Unearned premiums	1,334.60	1,120.80
Total insurance reserves	6,050.20	5,900.50
Payables	'	'
Reinsurance premiums	178.4	154.1
Deferred taxation	17.7	11.8
Accrued expenses and other payables	282.8	249.3
Liabilities under derivative contracts	2.3	7.4
Total payables	481.2	422.6
Long-term debt	499.2	499.1
Total liabilities	7,030.60	6,822.20
Commitments and contingent liabilities (see Note 14)	0	0
SHAREHOLDERS' EQUITY	'	'
Ordinary shares: 67,003,490 shares of par value 0.15144558¢ each (December 31, 2012 - 70,753,723)	0.1	0.1
Preference shares:	'	'
Non-controlling interest	-0.1	0.2
Additional paid-in capital	1,297.60	1,516.70
Retained earnings	1,714.40	1,544
Accumulated other comprehensive income, net of taxes	259.1	427.4
Total shareholders' equity	3,271.10	3,488.40
Total liabilities and shareholders' equity	10,301.70	10,310.60
5.950% Preference Shares [Member]	'	'
Preference shares:	'	'
Preference shares, value	0	0
5.625% Preference Shares [Member]	'	'
Preference shares:	'	'
Preference shares, value	0	0
7.401% Preference Shares [Member]	'	'
Preference shares:	'	'
Preference shares, value	0	0
7.250% Preference Shares [Member]	'	'
Preference shares:	'	'
Preference shares, value	$0	$0

CONDENSED_CONSOLIDATED_BALANCE1

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, except Share data, unless otherwise specified
Fixed income maturities, available for sale, amortized cost	$5,388.20	$5,228.50
Fixed income maturities, trading, amortized cost	713.1	431.6
Equity securities, available for sale, cost	128.7	174
Equity securities, trading at cost	260.4	'
Short-term investments, available for sale, amortized cost	150.3	431.5
Short-term investments, trading, amortized cost	$0.20	$2.40
Ordinary shares, par value	$0.15	$0.15
Ordinary shares, issued	65,700,665	70,753,723
Preference shares, par value	$0.15	$0.15
5.950% Preference Shares [Member]	'	'
Preference shares, issued	11,000,000	0
Preference shares, rate	5.95%	0.00%
Preference shares, par value	$0.15	$0.15
5.625% Preference Shares [Member]	'	'
Preference shares, issued	0	4,600,000
Preference shares, rate	5.63%	5.63%
Preference shares, par value	$0.15	$0.15
7.401% Preference Shares [Member]	'	'
Preference shares, issued	5,327,500	5,327,500
Preference shares, rate	7.40%	7.40%
Preference shares, par value	$0.15	$0.15
7.250% Preference Shares [Member]	'	'
Preference shares, issued	6,400,000	6,400,000
Preference shares, rate	7.25%	7.25%
Preference shares, par value	$0.15	$0.15

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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues	'	'	'	'
Net earned premium	$544.30	$516.20	$1,599.20	$1,525
Net investment income	45	48.6	139.2	153.8
Realized and unrealized investment gains	23.6	13.2	54.3	29.3
Other income	1.6	4.8	3.6	9
Total revenues	614.5	582.8	1,796.30	1,717.10
Expenses	'	'	'	'
Losses and loss adjustment expenses	290.2	255	892.3	801.1
Amortization of deferred policy acquisition costs	110.5	103.1	322.3	301.2
General, administrative and corporate expenses	98.9	90.7	273.2	259
Interest on long-term debt	7.7	7.8	23.2	23.2
Change in fair value of derivatives	-6.6	4.9	0.5	24
Realized and unrealized investment losses	5.9	2.4	28	8.2
Net realized and unrealized foreign exchange losses	-2.4	-4.5	7.1	0.5
Other expenses	0	0.3	0.6	1.9
Total expenses	504.2	459.7	1,547.20	1,419.10
Income from operations before income tax	110.3	123.1	249.1	298
Income tax expense	-2.9	-8	-9.8	-19.6
Net income	107.4	115.1	239.3	278.4
Add: Loss attributable to non-controlling interest	0.3	0	0.3	0.3
Net income attributable to Aspen Insurance Holdings Limited's ordinary shareholders	107.7	115.1	239.6	278.7
Available for sale investments:	'	'	'	'
Reclassification adjustment for net realized losses/(gains) on investments included in net income	-8.3	5	-20.7	5.2
Change in net unrealized gains on available for sale securities held	4.8	27.2	-138.6	52
Amortization of loss on derivative contract	0	0.2	0.2	0.3
Change in foreign currency translation adjustment	3.4	-4.9	-21	-15.8
Other comprehensive income, gross of tax	-0.1	27.5	-180.1	41.7
Tax thereon:	'	'	'	'
Reclassification adjustment for net realized losses on investments included in net income	0.1	0	0.5	0
Change in net unrealized gains on available for sale securities held	-0.4	-1.1	11.3	-0.9
Total tax on other comprehensive income	-0.3	-1.1	11.8	-0.9
Other comprehensive (loss)/income, net of tax	-0.4	26.4	-168.3	40.8
Total comprehensive income attributable to Aspen Insurance Holdings Limited's ordinary shareholders	$107.30	$141.50	$71.30	$319.50
Weighted average number of ordinary share and share equivalents	'	'	'	'
Basic	66,716,202	71,129,102	67,302,857	71,125,664
Diluted	68,561,515	73,397,796	69,959,474	73,703,038
Basic earnings per ordinary share adjusted for preference share dividends	$1.47	$1.50	$3.06	$3.60
Diluted earnings per ordinary share adjusted for preference share dividends	$1.43	$1.45	$2.95	$3.47

CONDENSED_CONSOLIDATED_STATEME1

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

Total

Ordinary Shares [Member]

Preference Shares [Member]

Non-Controlling Interest [Member]

Additional Paid-in Capital [Member]

Additional Paid-in Capital [Member]

Additional Paid-in Capital [Member]

Retained Earnings [Member]

Retained Earnings [Member]

Retained Earnings [Member]

Cumulative Foreign Currency Translation Adjustments, Net of Taxes [Member]

Loss on Derivatives, Net of Taxes [Member]

Unrealized Appreciation on Investments, Net of Taxes [Member]

In Millions, unless otherwise specified

Ordinary Shares [Member]

Preference Shares [Member]

Ordinary Shares [Member]

Preference Shares [Member]

Shareholder's equity at Dec. 31, 2011

'

'

'

$0.30

$1,385

'

'

$1,341.60

'

'

$124.20

($0.70)

$305.40

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

'

'

'

'

'

'

'

'

'

'

'

'

'

Total accumulated other comprehensive income, net of taxes

469.7

'

'

'

'

'

'

'

'

'

'

'

'

Net income for the period

278.4

'

'

'

'

'

'

278.4

'

'

'

'

'

Shares issued

'

'

'

'

'

21.5

154.5

'

'

'

'

'

'

Change for the period, net of income tax

'

'

'

'

'

'

'

'

'

'

-15.8

'

56.3

Reclassification to interest payable

'

'

'

'

'

'

'

'

'

'

'

0.3

'

Dividends

'

'

'

'

'

'

'

'

-35.1

-22.6

'

'

'

Shares repurchased/redeemed and cancelled

'

'

'

'

'

-51.9

'

'

'

'

'

'

'

PIERS redeemed and cancelled

'

'

'

'

'

'

0

'

'

'

'

'

'

PIERS redemption

0

[1]

'

'

'

'

'

0

[2]

0

[2]

'

'

'

'

'

Net (loss) attributable to non-controlling interest for the period

-0.3

'

'

-0.3

'

'

'

-0.3

'

'

'

'

'

Share-based compensation

'

'

'

'

12.8

'

'

'

'

'

'

'

'

Dividends due to non-controlling interest

'

'

'

-0.1

'

'

'

'

'

'

'

'

'

Shareholder's equity at Sep. 30, 2012

3,554.20

0.1

0

-0.1

1,521.90

'

'

1,562.60

'

'

108.4

-0.4

361.7

Shareholder's equity at Dec. 31, 2012

'

'

'

0.2

1,516.70

'

'

1,544

'

'

112.7

-0.5

315.2

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

'

'

'

'

'

'

'

'

'

'

'

'

'

Total accumulated other comprehensive income, net of taxes

259.1

'

'

'

'

'

'

'

'

'

'

'

'

Net income for the period

239.3

'

'

'

'

'

'

239.3

'

'

'

'

'

Shares issued

'

'

'

'

'

16.7

270.4

'

'

'

'

'

'

Change for the period, net of income tax

'

'

'

'

'

'

'

'

'

'

-21

'

-147.5

Reclassification to interest payable

'

'

'

'

'

'

'

'

'

'

'

0.2

'

Dividends

'

'

'

'

'

'

'

'

-36

-26.1

'

'

'

Shares repurchased/redeemed and cancelled

'

'

'

'

'

-294.9

'

'

'

'

'

'

'

PIERS redeemed and cancelled

'

'

-230

'

'

'

-230

'

'

'

'

'

'

PIERS redemption

-7.1

[1]

'

'

'

'

'

7.1

[2]

-7.1

[2]

'

'

'

'

'

Net (loss) attributable to non-controlling interest for the period

-0.3

'

'

-0.3

'

'

'

-0.3

'

'

'

'

'

Share-based compensation

'

'

'

'

11.6

'

'

'

'

'

'

'

'

Dividends due to non-controlling interest

'

'

'

0

'

'

'

'

'

'

'

'

'

Shareholder's equity at Sep. 30, 2013

$3,271.10

$0.10

$0

($0.10)

$1,297.60

'

'

$1,714.40

'

'

$91.70

($0.30)

$167.70

[1]

The $7.1 million deduction to net income is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million.

[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 Perpetual Preferred Income Equity Replacement Securities (“PIERS”), net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million.

CONDENSED_CONSOLIDATED_STATEME2

CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Parenthetical) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
PIERS redemption	$0	[1]	'	[1]	$7.10	[1]	$0	[1]
Reclassification from Additional Paid in Capital to Retained Earnings [Member]	'		'		'		'
PIERS redemption	'		'		7.1		'
Preference Shares [Member]	'		'		'		'
PIERS redeemed and cancelled	'		'		$230		'
[1]	The $7.1 million deduction to net income is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million.

CONDENSED_CONSOLIDATED_STATEME3

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operating activities:	'	'
Net income	$239.30	$278.40
Proportion due to non-controlling interest	0.3	0.3
Adjustments to reconcile net income to net cash flows from operating activities:	'	'
Depreciation and amortization	27.3	31.1
Share-based compensation	11.6	12.8
Realized and unrealized investment (gains)	-54.3	-29.3
Realized and unrealized investment losses	28	8.2
Net realized and unrealized investment foreign exchange losses/(gains)	11.5	1.6
Loss on derivative contracts	0.2	0.3
Insurance reserves:	'	'
Losses and loss adjustment expenses	-46.3	69.5
Unearned premiums	212.5	261.1
Reinsurance recoverables:	'	'
Unpaid losses	54.8	-32.8
Ceded unearned premiums	-57.6	-62.7
Other receivables	7.2	-3.4
Deferred policy acquisition costs	-39.1	-46.7
Reinsurance premiums payable	26.3	-85.1
Funds withheld	36.6	11.2
Premiums receivable	-39.3	-105.2
Deferred taxes	7.8	-2.9
Income tax payable	-4.1	14.5
Accrued expenses and other payable	33.2	30.2
Fair value of derivatives and settlement of liabilities under derivatives	-8.4	-2.2
Long-term debt	0.1	0.1
Intangible assets	-0.1	0
Other assets	-14.5	-3.9
Net cash generated by operating activities	433	345.1
Cash flows (used in)/from investing activities:	'	'
(Purchases) of fixed income maturities - Available for sale	-1,748.60	-1,057.10
(Purchases) of fixed income maturities - Trading	-555.5	-187.7
Proceeds from sales and maturities of fixed income maturities - Available for sale	1,564.40	989.7
Proceeds from sales and maturities of fixed income maturities - Trading	276.3	164.5
(Purchases) of equity securities - Available for sale	-2.5	-41.5
(Purchases) of equity securities - Trading	-275.4	0
Proceeds from sales of equity securities - Available for sale	61.8	35.9
Proceeds from sales of equity securities - Trading	15.1	0
Net (purchases)/sales of short-term investments	275.7	-190.4
Net change in (payable)/receivable for securities (purchased)/sold	5.6	19.3
Purchase of equipment	-10.1	-19.9
Net cash (used in) investing activities	-393.2	-287.2
Cash flows (used in) financing activities:	'	'
Proceeds from the issuance of ordinary shares, net of issuance costs	16.7	21.5
Proceeds from the issuance of preference shares, net of issuance costs	270.4	154.5
PIERS repurchased and cancelled	-230	0
Ordinary shares repurchased	-294.9	-51.9
Dividends paid on ordinary shares	-36	-35.1
Dividends paid on preference shares	-26.1	-22.6
Net cash (used in) financing activities	-299.9	66.4
Effect of exchange rate movements on cash and cash equivalents	-5.2	10.8
Increase/(decrease) in cash and cash equivalents	-265.3	135.1
Cash and cash equivalents at beginning of period	1,463.60	1,239.10
Cash and cash equivalents at end of period	1,198.30	1,374.20
Supplemental disclosure of cash flow information:	'	'
Net cash (received)/paid during the period for income tax	7.3	6.4
Cash paid during the period for interest	$22.50	$22.50

History_and_Organization

History and Organization	9 Months Ended
	Sep. 30, 2013
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 will be used to leverage our existing franchise and underwriting expertise to offer investors access to diversified products. 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, 2013
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, 2013 are not necessarily indicative of the results that may be expected for the year ended December 31, 2013. 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, 2012 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, 2012 contained in the Company’s Annual Report on Form 10-K filed with the United States Securities and Exchange Commission (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.
	New Accounting Policies Adopted in 2013
	In 2011, the Financial Accounting Standard Board (“FASB”) issued ASU 2011-11, “Disclosures about Offsetting Assets and Liabilities.” The ASU 2011-11 retains the existing offsetting model under U.S. GAAP but requires disclosures to allow investors to better compare financial statements prepared under U.S. GAAP with financial statements prepared under International Financial Reporting Standards (“IFRS”) by aligning these requirements. ASU 2011-11 is effective for annual reporting periods beginning January 1, 2013, and interim periods within those annual periods. Retrospective application is required.
	In January 2013, the FASB issued ASU 2013-01, “Clarifying the Scope of Disclosures About Offsetting Assets and Liabilities” which clarifies which instruments and transactions are subject to the offsetting disclosure requirements established by ASU 2011-11. ASU 2013-01 addresses concerns that the scope of the disclosure requirements under ASU 2011-11 was overly broad and imposed unintended costs that were not commensurate with estimated benefits to financial statement users. In choosing to narrow the scope of the offsetting disclosures, the FASB determined that it could make them more operable and cost effective for preparers while still giving financial statement users sufficient information to analyze the most significant presentation differences between financial statements prepared in accordance with U.S. GAAP and those prepared under IFRS. Like ASU 2011-11, ASU 2013-01 is effective for fiscal years beginning on or after January 1, 2013, and interim periods therein. Retrospective application is required for any period presented that begins before the entity’s initial application of the new requirements. The Company adopted ASU 2011-11 and ASU 2013-01 in the first quarter of 2013 and they do not have a material impact on the unaudited condensed consolidated financial statements.
	In February 2013, the FASB issued ASU 2013-02, “Reporting of Amounts Reclassified out of Accumulated Other Comprehensive Income” which adds new disclosure requirements for items reclassified out of accumulated other comprehensive income (“AOCI”). ASU 2013-02 is intended to improve the transparency of changes in other comprehensive income (“OCI”) and items reclassified out of AOCI in the financial statements. It does not amend any existing requirements for reporting net income or OCI in the financial statements. ASU 2013-02 was applied prospectively and was effective for annual reporting periods beginning after December 15, 2012, and interim periods within those years. The Company adopted ASU 2013-02 in the first quarter of 2013 and this information is presented in Note 3 of the accompanying notes to the unaudited condensed consolidated financial statements.
	In July 2013, the FASB issued ASU 2013-10, “Inclusion of the Fed Funds Effective Swap Rate (or Overnight Index Swap Rate) as a Benchmark Interest Rate for Hedge Accounting Purposes” which provides guidance on the risks permitted to be hedged in a fair value or cash flow hedges, specifically, the risk of changes in a hedged item’s fair value or hedged transaction’s cash flows due to the changes in the designated benchmark interest rate. ASU 2013-10 is effective prospectively for qualifying new or redesignated hedging relationships entered into on or after July 17, 2013.  The Company has adopted ASU 2013-10 in the third quarter of 2013 and it does not have a material impact on the unaudited condensed consolidated financial statements.
	Accounting Pronouncements Not Yet Adopted
	In February 2013, the FASB issued ASU 2013-04, “Obligations Resulting from Joint and Several Liability Arrangements for Which the Total Amount of the Obligation is Fixed at the Reporting Date” to provide guidance for the recognition, measurement, and disclosure of obligations resulting from joint and several liability arrangements for which the total amount of the obligation within the scope of this guidance is fixed at the reporting date. Examples of obligations include debt arrangements, other contractual obligations and settled litigation and judicial rulings. ASU 2013-04 will be applied retrospectively and is effective for annual reporting periods beginning after December 15, 2013, and interim periods within those years. The Company does not anticipate that this standard will have a material impact on its unaudited condensed consolidated financial statements.
	In March 2013, the FASB issued ASU 2013-05, “Parent’s Accounting for the Cumulative Translation Adjustment Upon Derecognition of Certain Subsidiaries or Groups of Assets Within a Foreign Entity or of an Investment in a Foreign Entity” to standardize the release of the cumulative translation adjustment into net income when a parent either sells a part or all of its investment in a foreign entity or no longer holds a controlling financial interest in a subsidiary. ASU 2013-05 will be applied prospectively and is effective for annual reporting periods beginning after December 15, 2013, and interim periods within those years. The Company does not expect ASU 2013-05 to have a material impact on its unaudited condensed consolidated financial statements unless it should choose to sell its investments in a foreign entity or cease to hold a controlling financial interest.
	In July 2013, the FASB issued ASU 2013-11, “Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists” which provides guidance on financial statement presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. ASU 2013-11 is effective for fiscal years beginning after December 15, 2013 or interim periods within those years. The Company does not expect ASU 2013-11 to have a material impact on the unaudited condensed consolidated financial statements as its presentation of unrecognized tax benefits is already as required by ASU 2013-11.

Reclassifications_from_Accumul

Reclassifications from Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Reclassifications from Accumulated Other Comprehensive Income	'
	3	Reclassifications from Accumulated Other Comprehensive Income
	The following table sets out the components of the Company’s AOCI that are reclassified into the unaudited condensed consolidated statement of operations for the three and nine months ended September 30, 2013:
			Amount Reclassified from AOCI
	Details about the AOCI Components		Three Months Ended September 30, 2013			Nine Months Ended September 30, 2013			Affected Line Item in the Unaudited
									Condensed Consolidated Statement
									of Operations
			($ in millions)
	Available for sale securities:
	Realized gain on sale of securities		$	8.6		$	21.2		Realized and unrealized investment gains
	Realized (loss) on sale of securities		(0.3		)	(0.5		)	Realized and unrealized investment losses
			8.3			20.7			Income from operations before tax
	Tax on realized gains and (losses) of securities		(0.1		)	(0.5		)	Income tax expense
			$	8.2		$	20.2		Net income
	Foreign currency translation adjustments:
	Foreign currency translation adjustments, before tax		$	4.8		$	0.5		Net realized and unrealized foreign exchange gains/(losses)
	Tax on foreign currency translation adjustments		(1.1		)	(0.1		)	Income tax expense
			$	3.7		$	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		$	11.9		$	20.4		Net income

Earnings_per_Ordinary_Share

Earnings per Ordinary Share	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]	'
Earnings per Ordinary Share	'
	4	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, 2013 and 2012, respectively:
			Three Months Ended September 30,						Nine Months Ended September 30,
			2013			2012			2013			2012
			($ in millions, except share and per share amounts)
	Net income		$	107.4		$	115.1		$	239.3		$	278.4
	PIERS redemption(1)		—			—			(7.1		)	—
	Preference share dividends		(9.5		)	(8.6		)	(26.1		)	(22.6		)
	Basic and diluted net income available to ordinary shareholders		97.9			106.5			206.1			255.8
	Ordinary shares:
	Basic weighted average ordinary shares		66,716,202			71,129,102			67,302,857			71,125,664
	Weighted average effect of dilutive securities(2)		1,845,313			2,268,694			2,656,617			2,577,374
	Total diluted weighted average ordinary shares		68,561,515			73,397,796			69,959,474			73,703,038
	Earnings per ordinary share:
	Basic		$	1.47		$	1.5		$	3.06		$	3.6
	Diluted		$	1.43		$	1.45		$	2.95		$	3.47
	(1)	The $7.1 million deduction to net income is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million.
	(2)	Dilutive securities comprise: investor options, employee options, performance shares associated with the Company’s long term incentive program, employee share purchase plans and restricted stock units as described in Note 13, in addition to the PIERS.
	Dividends. On October 30, 2013, the Company’s Board of Directors declared the following quarterly dividends:
			Dividend			Payable on:		Record Date:
	Ordinary shares		$	0.18		November 27, 2013		November 13, 2013
	7.401% preference shares		$	0.462563		January 1, 2014		December 15, 2013
	7.250% preference shares		$	0.4531		January 1, 2014		December 15, 2013
	5.950% preference shares		$	0.3719		January 1, 2014		December 15, 2013

Segment_Reporting

Segment Reporting	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]	'
Segment Reporting	'
	5	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). For a more detailed description of this segment, see Part I, Item 1, “Business — Business Segment — Reinsurance” in the Company’s 2012 Annual Report on Form 10-K filed with the United States Securities and Exchange Commission.
	Insurance Segment. The insurance segment consists of property insurance, casualty insurance, marine insurance, energy and transportation insurance, financial and professional lines insurance and programs business. For a more detailed description of this segment, see Part I, Item 1 “Business — Business Segment — Insurance” in the Company’s 2012 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, corporate expenses, 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, 2013 and 2012:
			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
	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.
			Three Months Ended September 30, 2012
			Reinsurance			Insurance			Total
			( $ in millions)
	Underwriting Revenues
	Gross written premiums		$	259.5		$	298.9		$	558.4
	Net written premiums		256.9			250.2			507.1
	Gross earned premiums		299.8			302			601.8
	Net earned premiums		279.6			236.6			516.2
	Underwriting Expenses
	Losses and loss adjustment expenses		117.1			137.9			255
	Amortization of deferred policy acquisition costs		55.7			47.4			103.1
	General and administrative expenses		33.6			42.8			76.4
	Underwriting income		73.2			8.5			81.7
	Corporate expenses								(14.3		)
	Net investment income								48.6
	Realized and unrealized investment gains								13.2
	Realized and unrealized investment (losses)								(2.4		)
	Change in fair value of derivatives								(4.9		)
	Interest expense on long term debt								(7.8		)
	Net realized and unrealized foreign exchange gains								4.5
	Other income								4.8
	Other (expenses)								(0.3		)
	Income before tax								$	123.1
	Net reserves for loss and loss adjustment expenses		$	2,755.10		$	1,422.90		$	4,178.00
	Ratios
	Loss ratio		41.9		%	58.3		%	49.4		%
	Policy acquisition expense ratio		19.9			20			20
General and administrative expense ratio		12			18.1			17.6			(1)
Expense ratio		31.9			38.1			37.6
Combined ratio		73.8		%	96.4		%	87		%
(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, 2013 and 2012:
		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.
		Nine Months Ended September 30, 2012
		Reinsurance			Insurance			Total
		( $ in millions)
Underwriting Revenues
Gross written premiums		$	1,033.50		$	973.6		$	2,007.10
Net written premiums		963.2			759.3			1,722.50
Gross earned premiums		890.8			848.8			1,739.60
Net earned premiums		832.6			692.4			1,525.00
Underwriting Expenses
Losses and loss adjustment expenses		386.4			414.7			801.1
Amortization of deferred policy acquisition costs		166.8			134.4			301.2
General and administrative expenses		92.6			126.3			218.9
Underwriting income		186.8			17			203.8
Corporate expenses								(40.1		)
Net investment income								153.8
Realized and unrealized investment gains								29.3
Realized and unrealized investment (losses)								(8.2		)
Change in fair value of derivatives								(24.0		)
Interest expense on long term debt								(23.2		)
Net realized and unrealized foreign exchange (losses)								(0.5		)
Other income								9
Other (expenses)								(1.9		)
Income before tax								$	298
Net reserves for loss and loss adjustment expenses		$	2,755.10		$	1,422.90		$	4,178.00
Ratios
Loss ratio		46.4		%	59.9		%	52.5		%
Policy acquisition expense ratio		20			19.4			19.8
General and administrative expense ratio		11.1			18.2			17			(1)
Expense ratio		31.1			37.6			36.8
Combined ratio		77.5		%	97.5		%	89.3		%
(1)	The general and administrative expense ratio in the total column includes corporate expenses.

Investments

Investments	9 Months Ended
	Sep. 30, 2013
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, 2013 and 2012:
		For the Three Months Ended						For the Nine Months Ended
		30-Sep-13			30-Sep-12			30-Sep-13			30-Sep-12
		($ in millions)						($ in millions)
	Fixed income maturities — Available for sale	$	38.2		$	43.7		$	117.2		$	136.9
	Fixed income maturities — Trading	5.3			4			13.4			12.2
	Short-term investments — Available for sale	0.3			0.5			1.9			1.8
	Fixed term deposits (included in cash and cash equivalents)	0.7			1.4			3.4			4.7
	Equity securities — Available for sale	1.2			1.9			4.7			5.1
	Equity securities — Trading	1.8			—			5.5			—
	Total	$	47.5		$	51.5		$	146.1		$	160.7
	Investment expenses	(2.5		)	(2.9		)	(6.9		)	(6.9		)
	Net investment income	$	45		$	48.6		$	139.2		$	153.8
	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 Months Ended						For the Nine Months Ended
		30-Sep-13			30-Sep-12			30-Sep-13			30-Sep-12
		($ in millions)						($ in millions)
	Available for sale:
	Fixed income maturities — gross realized gains	$	6.9		$	1.5		$	15.7		$	5.3
	Fixed income maturities — gross realized (losses)	(5.6		)	(0.1		)	(6.8		)	(0.4		)
	Equity securities — gross realized gains	6.3			1			13.8			2.9
	Equity securities — gross realized (losses)	—			(0.1		)	—			(4.5		)
	Trading:
	Fixed income maturities — gross realized gains	0.8			2.3			6.6			6.7
	Fixed income maturities — gross realized (losses)	(1.3		)	(0.1		)	(1.9		)	(0.3		)
	Equity securities — gross realized gains	0.4			—			0.8			—
	Equity securities — gross realized (losses)	—			—			(0.3		)	—
	Net change in gross unrealized gains/(losses)	9			6.7			(2.7		)	12.7
	Impairments:
	Total other-than-temporary impairments	—			(2.1		)	—			(3.0		)
	Gross realized and unrealized gains in Cartesian Iris	1.2			1.7			1.1			1.7
	Total net realized and unrealized investment gains recorded in the statement of operations	$	17.7		$	10.8		$	26.3		$	21.1
	Change in available for sale net unrealized gains/(losses):
	Fixed income maturities	(3.3		)	25.5			(168.7		)	40.9
	Short-term investments	—			0.2			—			0.2
	Equity securities	(0.2		)	6.5			9.4			16.1
	Total change in pre-tax available for sale unrealized gains/(losses)	(3.5		)	32.2			(159.3		)	57.2
	Change in taxes	(0.3		)	(1.1		)	11.8			(0.9		)
	Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income	$	(3.8	)	$	31.1		$	(147.5	)	$	56.3
	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 2012 Annual Report on Form 10-K, filed with the United States Securities and Exchange Commission. The total OTTI charge for the three and nine months ended September 30, 2013 was $Nil (2012 — $2.1 million) and $Nil (2012 — $3.0 million), respectively.
	Balance Sheet
	Fixed Income Maturities, Short-Term Investments and Equities — Available For Sale. The following tables present the cost or amortized cost, gross unrealized gains and losses and estimated fair market value of available for sale investments in fixed income maturities, short-term investments and equity securities as at September 30, 2013 and December 31, 2012:
		As at September 30, 2013
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	995.3		$	28.4		$	(2.6	)	$	1,021.10
	U.S. agency	269.2			13			(0.7		)	281.5
	Municipal	29.1			1.1			(0.3		)	29.9
	Corporate	1,942.60			92.2			(16.7		)	2,018.10
	Non-U.S. government-backed corporate	77.5			1.4			—			78.9
	Foreign government	815.5			9.9			(3.2		)	822.2
	Asset-backed	113.7			3.1			(0.3		)	116.5
	Non-agency commercial mortgage-backed	58.1			6.3			—			64.4
	Agency mortgage-backed	1,087.20			35.1			(10.0		)	1,112.30
	Total fixed income maturities — Available for sale	5,388.20			190.5			(33.8		)	5,544.90
	Total short-term investments — Available for sale	150.3			—			(2.0		)	148.3
	Total equity securities — Available for sale	128.7			35.3			(0.8		)	163.2
	Total	$	5,667.20		$	225.8		$	(36.6	)	$	5,856.40
		As at December 31, 2012
	Cost or			Gross			Gross			Fair Market
	Amortized Cost			Unrealized			Unrealized			Value
				Gains			Losses
	($ in millions)
U.S. government	$	1,071.80		$	54.8		$	(0.3	)	$	1,126.30
U.S. agency	288.3			20.3			—			308.6
Municipal	37.2			2.6			(0.1		)	39.7
Corporate	1,889.20			149.9			(0.6		)	2,038.50
Non-U.S. government-backed corporate	98			3.1			—			101.1
Foreign government	617			24.1			(0.1		)	641
Asset-backed	49.2			4.6			—			53.8
Non-agency commercial mortgage-backed	61.7			9.4			—			71.1
Agency mortgage-backed	1,116.10			61.2			(0.1		)	1,177.20
Total fixed income maturities — Available for sale	5,228.50			330			(1.2		)	5,557.30
Total short-term investments — Available for sale	431.5			—			—			431.5
Total equity securities — Available for sale	174			28.2			(2.1		)	200.1
Total	$	5,834.00		$	358.2		$	(3.3	)	$	6,188.90
Unrealized foreign exchange gains and losses included in the tables above are included within change in foreign currency translation adjustment within the statement of operations and other comprehensive income.
Fixed Income Maturities, Short Term Investments and Equities — 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 maturities, short-term investments and equity securities as at September 30, 2013 and December 31, 2012:
	As at September 30, 2013
	Cost or			Gross			Gross			Fair Market
	Amortized Cost			Unrealized			Unrealized			Value
				Gains			Losses
	($ in millions)
U.S. government	$	32		$	—		$	(0.9	)	$	31.1
U.S. agency	0.2			—			—			0.2
Municipal	0.6			—			—			0.6
Corporate	464.9			12.1			(6.0		)	471
Foreign government	135.9			1.3			(0.9		)	136.3
Asset-backed	6.6			—			—			6.6
Bank loans	72.9			0.1			(0.2		)	72.8
Total fixed income maturities — Trading	713.1			13.5			(8.0		)	718.6
Total short-term investments — Trading	0.2			—			—			0.2
Total equity securities — Trading	260.4			20.5			(2.6		)	278.3
Total	$	973.7		$	34		$	(10.6	)	$	997.1
	As at December 31, 2012
	Cost or			Gross			Gross			Fair Market
	Amortized Cost			Unrealized			Unrealized			Value
				Gains			Losses
	($ in millions)
U.S. government	$	9.3		$	0.2		$	(0.1	)	$	9.4
U.S. agency	0.2			—			—			0.2
Municipal	2.8			0.1			—			2.9
Corporate	392			22.7			(0.3		)	414.4
Foreign government	24.4			1.9			—			26.3
Asset-backed	2.9			—			—			2.9
Total fixed income maturities — Trading	431.6			24.9			(0.4		)	456.1
Total short-term investments — Trading	2.4			—			—			2.4
Total	$	434		$	24.9		$	(0.4	)	$	458.5
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, 2013, the Company had invested $25.7 million in BB high yield bonds classified within corporate securities and $72.8 million in its U.S. Dollar bank loans trading portfolio. 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.
Other Investments. The Company has invested in Cartesian Iris Offshore Fund L.P. (“Cartesian”), which provides capital to Iris Re, a Class 3 Bermudian reinsurer. The Company has accounted for its investment in Cartesian in accordance with the equity method of accounting. The Company is not committed to making further investments in Cartesian; accordingly, the carrying value of the investment represents the Company’s maximum exposure to a loss as a result of its involvement with the partnership at each balance sheet date. In addition to returns on its investment, the Company provides services on risk selection, pricing and portfolio design in return for a percentage of profits from Iris Re. Adjustments to the carrying value of this investment are made based on the Company’s share of capital including share of income and expenses, which is provided in the quarterly management accounts of the partnership. The adjusted carrying value approximates fair value. 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. This will take effect on January 1, 2014.
In the three and nine months ended September 30, 2013, the change in the value of the Company’s investment in Cartesian was an increase of $1.2 million (September 30, 2012 — increase of $1.7 million) and an increase of $1.1 million (September 30, 2012 — gain of $1.7 million), respectively. Changes in the value are recognized in realized and unrealized investment gains and losses in the unaudited condensed consolidated statement of operations. For more information regarding the Company’s investment in Cartesian, refer to Note 6 of the “Notes to Audited Consolidated Financial Statements” in the Company’s 2012 Annual Report filed on Form 10-K filed with the United States Securities and Exchange Commission.
On October 2, 2012, the Company established a subsidiary, Aspen Recoveries Limited, to take ownership of a 58.5% shareholding in Chaspark Maritime Holdings Ltd., a Singaporean registered company (“Chaspark”), with the remaining shareholding owned by other insurers. The shareholding in Chaspark was received as a settlement for subrogation rights associated with a contract frustration claim settlement. The Company has determined that Chaspark has the characteristics of a variable interest entity as addressed by the guidance in ASC 810-10, Consolidation. However, having considered the provisions of ASC 810-10, the Company’s investment in Chaspark does not permit the Company to direct the activities which most significantly impact Chaspark’s economic performance and the Company is not acting as principal or agent for a related party group of investors. Under these circumstances, the Company is not required to consolidate Chaspark. The investment is therefore accounted for under the equity method. Adjustments to the carrying value of this investment are made based on the Company’s share of capital including share of income and expenses, which is provided in the quarterly management accounts. The adjusted carrying value approximates fair value.
The table below shows the Company’s investments in Cartesian and Chaspark for the nine months ended September 30, 2013 and twelve months ended December 31, 2012:
	Cartesian			Chaspark			Total
	($ in millions)
Opening undistributed value of investment as at January 1, 2013	$	36.3		$	8.7		$	45
Unrealized gain for the nine months to September 30, 2013	1.1			—			1.1
Closing value of investment as at September 30, 2013	$	37.4		$	8.7		$	46.1
Opening undistributed value of investment as at January 1, 2012	$	33.1		$	—		$	33.1
Investment in Chaspark Maritime Holdings Ltd.	—			8.7			8.7
Unrealized gain for the twelve months to December 31, 2012	3.2			—			3.2
Closing value of investment as at December 31, 2012	$	36.3		$	8.7		$	45
Fixed Maturities. The scheduled maturity distribution of available for sale fixed income maturity securities as at September 30, 2013 and December 31, 2012 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, 2013
	Amortized			Fair Market			Average
	Cost or Cost			Value			S&P Ratings by
							Maturity
	($ in millions)
Due one year or less	$	649.6		$	652.5		AA
Due after one year through five years	2,385.80			2,461.20			AA
Due after five years through ten years	992.5			1,033.50			A+
Due after ten years	101.3			104.5			AA-
Subtotal	4,129.20			4,251.70
Non-agency commercial mortgage-backed	58.1			64.4			AA+
Agency mortgage-backed	1,087.20			1,112.30			AA+
Other asset-backed	113.7			116.5			AAA
Total fixed income maturities — Available for sale	$	5,388.20		$	5,544.90
	As at December 31, 2012
	Amortized			Fair Market			Average
	Cost or Cost			Value			S&P Ratings by
							Maturity
	($ in millions)
Due one year or less	$	554.6		$	561		AA
Due after one year through five years	2,270.00			2,378.80			AA
Due after five years through ten years	1,077.80			1,199.30			AA-
Due after ten years	99.1			116.1			AA+
Subtotal	4,001.50			4,255.20
Non-agency commercial mortgage-backed	61.7			71.1			AA+
Agency mortgage-backed	1,116.10			1,177.20			AA+
Other asset-backed	49.2			53.8			AAA
Total fixed income maturities — Available for sale	$	5,228.50		$	5,557.30
Guaranteed Investments. As at September 30, 2013, the Company held $2.3 million (December 31, 2012 — $1.5 million) in investments which are guaranteed by mono-line insurers, excluding those with explicit government guarantees, and the Company’s holding was limited to two municipal securities, both rated BBB- or higher (December 31, 2012 — two municipal securities, both rated BBB or higher). The standalone rating (rating without guarantee) is determined as the senior unsecured debt rating of the issuer. Where the credit ratings were split between the two main rating agencies, Standard & Poor’s Financial Services LLC (“S&P”) and Moody’s Investors Service, Inc. (“Moody’s”), the lowest rating was used. The Company’s exposure to other third-party guaranteed debt is primarily to investments backed by non-U.S. government guaranteed issuers.
Gross Unrealized Loss. The following tables summarize as at September 30, 2013 and December 31, 2012, by type of security, the aggregate fair value and gross unrealized loss by length of time the security has been in an unrealized loss position for the Company’s available for sale portfolio:
	September 30, 2013
	0-12 months						Over 12 months						Total
	Fair			Gross			Fair			Gross			Fair		Gross			Number of
	Market			Unrealized			Market			Unrealized			Market		Unrealized			Securities
	Value			Loss			Value			Loss			Value		Loss
	($ in millions)
U.S. government	$	193.3		$	(2.6	)	$	—		$	—		$	193.3	$	(2.6	)	38
U.S. agency	67.1			(0.7		)	—			—			67.1		(0.7		)	14
Municipal	2.5			(0.1		)	1.3			(0.2		)	3.8		(0.3		)	3
Corporate	611.6			(15.9		)	7.5			(0.8		)	619.1		(16.7		)	331
Non-U.S. government-backed corporate	6.9			—			4.8			—			11.7		—			3
Foreign government	234.5			(3.1		)	4			(0.1		)	238.5		(3.2		)	40
Asset-backed	43.8			(0.3		)	—			—			43.8		(0.3		)	39
Non-agency commercial mortgage-backed	—			—			—			—			—		—			—
Agency mortgage-backed	384.2			(10.0		)	—			—			384.2		(10.0		)	85
Total fixed income maturities — Available for sale	1,543.90			(32.7		)	17.6			(1.1		)	1,561.50		(33.8		)	553
Total short-term investments — Available for sale	99			(2.0		)	—			—			99		(2.0		)	1
Total equity securities — Available for sale	8.5			(0.5		)	3.8			(0.3		)	12.3		(0.8		)	7
Total	$	1,651.40		$	(35.2	)	$	21.4		$	(1.4	)	$	1,672.80	$	(36.6	)	561
	December 31, 2012
	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	$	72.7		$	(0.3	)	$	—		$	—		$	72.7	$	(0.3	)	9
Municipal	1.4			(0.1		)	—			—			1.4		(0.1		)	1
Corporate	170.8			(0.6		)	—			—			170.8		(0.6		)	63
Non-U.S. government-backed corporate	10.1			—			2			—			12.1		—			3
Foreign government	87.7			(0.1		)	4			—			91.7		(0.1		)	20
Asset-backed	0.7			—			—			—			0.7		—			1
Agency mortgage-backed	26.7			(0.1		)	—			—			26.7		(0.1		)	14
Total fixed income maturities — Available for sale	370.1			(1.2		)	6			—			376.1		(1.2		)	111
Total short-term investments — Available for sale	9.4			—			—			—			9.4		—			4
Total equity securities — Available for sale	28.6			(1.9		)	2.2			(0.2		)	30.8		(2.1		)	18
Total	$	408.1		$	(3.1	)	$	8.2		$	(0.2	)	$	416.3	$	(3.3	)	133
Investment Purchases and Sales. The following table summarizes investment purchases, sales and maturities for the three and nine months ended September 30, 2013 and 2012:
	For the Three Months Ended						For the Nine Months Ended
	30-Sep-13			30-Sep-12			30-Sep-13			30-Sep-12
	($ in millions)						($ in millions)
(Purchases) of fixed income maturities — Available for sale	$	(566.8	)	$	(351.8	)	$	(1,748.6	)	$	(1,057.1	)
(Purchases) of fixed income maturities — Trading	(283.2		)	(73.0		)	(555.5		)	(187.7		)
(Purchases) of equity securities — Available for sale	—			(12.0		)	(2.5		)	(41.5		)
(Purchases) of equity securities — Trading	(38.6		)	—			(275.4		)	—
Proceeds from sales and maturities of fixed income maturities — Available for sale	578.6			240.4			1,564.40			989.7
Proceeds from sales and maturities of fixed income maturities — Trading	67.8			49.8			276.3			164.5
Proceeds from sales of equity securities — Available for sale	29.7			10.1			61.8			35.9
Proceeds from sales of equity securities — Trading	5			—			15.1			—
Net change in (payable)/receivable for securities (purchased) /sold	6.6			(22.8		)	5.6			19.3
Net (purchases)/sales of short-term investments	95.5			13.2			275.7			(190.4		)
Net (purchases)/sales for the period	$	(105.4	)	$	(146.1	)	$	(383.1	)	$	(267.3	)

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2013
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 Topic 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 at September 30, 2013 and December 31, 2012 respectively:
			As at September 30, 2013
			Level 1			Level 2				Total
			($ in millions)
	Available for sale financial assets, at fair value
	U.S. government		$	1,021.10		$	—			$	1,021.10
	U.S. agency		—			281.5				281.5
	Municipal		—			29.9				29.9
	Corporate		—			2,018.10				2,018.10
	Non-U.S. government-backed corporate		—			78.9				78.9
	Foreign government		612.6			209.6				822.2
	Asset-backed		—			116.5				116.5
	Non-agency commercial mortgage-backed		—			64.4				64.4
	Agency mortgage-backed		—			1,112.30				1,112.30
	Total fixed income maturities available for sale, at fair value		1,633.70			3,911.20				5,544.90
	Short-term investments available for sale, at fair value		118			30.3				148.3
	Equity investments available for sale, at fair value		163.2			—				163.2
	Held for trading financial assets, at fair value
	U.S. government		$	31.1		$	—			$	31.1
	U.S. agency		—			0.2				0.2
	Municipal		—			0.6				0.6
	Corporate		—			471				471
	Foreign government		42.9			93.4				136.3
	Asset-backed		—			6.6				6.6
	Bank loans		—			72.8				72.8
	Total fixed income maturities trading, at fair value		74			644.6				718.6
	Short-term investments trading, at fair value		—			0.2				0.2
	Equity investments trading, at fair value		278.3			—				278.3
	Other financial assets and liabilities, at fair value
	Derivatives at fair value — foreign exchange contracts		—			5.6				5.6
	Liabilities under derivative contracts — foreign exchange contracts		—			(0.9		)		(0.9		)
	Liabilities under derivative contracts — interest rate swaps		—			(1.4		)		(1.4		)
	Total		$	2,267.20		$	4,589.60			$	6,856.80
	There were no transfers between Level 1 and Level 2 during the three and nine months ended September 30, 2013 and no assets or liabilities were classified as Level 3.
			As at December 31, 2012
			Level 1			Level 2				Total
			($ in millions)
	Available for sale financial assets, at fair value
	U.S. government		$	1,126.30		$	—			1,126.30
	U.S. agency		—			308.6				308.6
	Municipal		—			39.7				39.7
	Corporate		—			2,038.50				2,038.50
	Non-U.S. government-backed corporate		—			101.1				101.1
	Foreign government		487.2			153.8				641
	Asset-backed		—			53.8				53.8
	Non-agency commercial mortgage-backed		—			71.1				71.1
	Agency mortgage-backed		—			1,177.20				1,177.20
	Total fixed income maturities available for sale, at fair value		1,613.50			3,943.80				5,557.30
	Short-term investments available for sale, at fair value		426.2			5.3				431.5
	Equity investments available for sale, at fair value		200.1			—				200.1
	Held for trading financial assets, at fair value
	U.S. government		$	9.4		$	—			$	9.4
	U.S. agency		—			0.2				0.2
	Municipal		—			2.9				2.9
	Corporate		—			414.4				414.4
	Foreign government		4.2			22.1				26.3
	Asset-backed		—			2.9				2.9
	Total fixed income maturities trading, at fair value		13.6			442.5				456.1
Short-term investments trading, at fair value		2.4			—				2.4
Other financial assets and liabilities, at fair value
Derivatives at fair value — foreign exchange contracts		—			2				2
Liabilities under derivative contracts — foreign exchange contracts		—			(6.1		)		(6.1		)
Liabilities under derivative contracts — interest rate swaps		—			(1.3		)		(1.3		)
Total		$	2,255.80		$	4,386.20			$	6,642.00
There were no transfers between Level 1 and Level 2 during the twelve months ended December 31, 2012 and no assets or liabilities were classified as Level 3 as at December 31, 2012.
Valuation of Fixed Maturities. The Company’s fixed income maturity securities are classified as either available for sale or trading and carried at fair value. At September 30, 2013 and December 31, 2012, 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, 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.
At September 30, 2013, the Company obtained an average of 2.6 quotes per fixed income investment, compared to 2.9 quotes at December 31, 2012. Pricing sources used in pricing fixed income investments at September 30, 2013 and December 31, 2012 were as follows:
		As at September 30, 2013		As at December 31, 2012
Index providers		87	%	89	%
Pricing services		10		9
Broker-dealers		3		2
Total		100	%	100	%
Fixed Income Maturities. A summary of securities priced using pricing information from index providers at September 30, 2013 and December 31, 2012 is provided below:
		As at September 30, 2013						As at December 31, 2012
		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		$	1,013.90		96	%		$	1,034.50			91	%
U.S. agency		267.7			95	%		291.9				95	%
Municipal		13.3			43	%		20.5				48	%
Corporate		2,358.30			95	%		2,339.80				95	%
Non-U.S. government-backed corporate		66.4			84	%		84.5				84	%
Foreign government		685.5			72	%		516.9				77	%
Asset-backed		117.8			96	%		44.6				79	%
Non-agency commercial mortgage-backed		62.7			97	%		70.4				99	%
Agency mortgage-backed		836.5			75	%		957.8				81	%
Total fixed income maturities		$	5,422.10		87	%		$	5,360.90			89	%
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.
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.
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. The Company classifies 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, who is considered the leader in this asset class. 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 and are classified as either trading or available for sale and carried at estimated fair value. 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.
At September 30, 2013, the Company obtained an average of 4.9 quotes per equity investment, compared to 4.9 quotes as at December 31, 2012. Pricing sources used in pricing equities at September 30, 2013 and December 31, 2012 were all provided by index providers.
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 very 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.

Reinsurance

Reinsurance	9 Months Ended
	Sep. 30, 2013
Insurance [Abstract]	'
Reinsurance	'
	Reinsurance
	The Company purchases retrocession and reinsurance to limit and diversify the Company’s risk exposure and to 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, 2013 were with Lloyd’s syndicates and with Munich Re. Balances with Lloyd’s and Munich Re represented 28.1% and 9.4%, respectively, of reinsurance recoverables (December 31, 2012 — Lloyd’s 20.2% and Munich Re 10.5%).

Derivative_Contracts

Derivative Contracts	9 Months Ended
	Sep. 30, 2013
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, 2013 and December 31, 2012:
				As at September 30, 2013							As at December 31, 2012
	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		Liabilities under Derivative Contracts	$	1,000.00		$	(1.4	)	(1)	$	1,000.00	$	(1.3	)	(1)
	Foreign Exchange Contracts		Derivatives at Fair Value	$	275.9		$	5.6			$	335.4	$	2
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	38.6		$	(0.9	)		$	88.6	$	(6.1	)
	(1)	Net of $34.1 million of cash collateral provided to counterparties as security for the Company’s net liability position (December 31, 2012 — $52.0 million).
	The following tables provide the unrealized and realized gains/(losses) recorded in the statement of operations for the three and nine months ended September 30, 2013 and 2012 respectively:
				Amount of Income/(Loss)
				Recognized in the Statement
				of Operations
				Three Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	30-Sep-13			30-Sep-12
	ASC 815		Statement of Operations
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	10.8		$	3.2
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	(4.2	)	$	(8.1	)
				Amount of Income/(Loss)
				Recognized in the Statement
				of  Operations
				Nine Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	30-Sep-13			30-Sep-12
	ASC 815		Statement of Operations
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(3.7	)	$	(1.1	)
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	3.2		$	(22.9	)
	Foreign Exchange Contracts. The Company uses foreign exchange contracts to manage foreign currency risk. A foreign exchange contract involves an obligation to purchase or sell a specified currency at a future date at a price set at the time of the contract. Foreign exchange contracts will not eliminate fluctuations in the value of the Company’s assets and liabilities denominated in foreign currencies but rather allow it to establish a rate of exchange for a future point in time.
	As at September 30, 2013, the Company held foreign exchange contracts with an aggregate value of $314.5 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, 2013, the impact of foreign exchange contracts on net income was a gain of $10.8 million (September 30, 2012 — gain of $3.2 million) and a charge of $3.7 million (September 30, 2012 — charge of $1.1 million), respectively.
	Interest Rate Swaps. As at September 30, 2013, the Company held fixed for floating interest rate swaps with a total notional amount of $1.0 billion (December 31, 2012 — $1.0 billion) that are due to mature between August 2, 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, 2013, there was a charge in respect of the interest rate swaps of $4.2 million (September 30, 2012 — charge of $8.1 million) and a gain of $3.2 million (September 30, 2012 — charge of $22.9 million), respectively.
	As at September 30, 2013, cash collateral with a fair value of $34.1 million was held by the Company’s counterparties to support the current valuation of the interest rate swaps (December 31, 2012 — $52.0 million). As at September 30, 2013, no non-cash collateral was transferred to the Company by its counterparties (December 31, 2012 — $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, 2013, 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 FASB ASU 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, 2013
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 three and nine months ended September 30, 2013 and 2012:
			Three Months Ended September 30, 2013			Three Months Ended September 30, 2012
			($ in millions)
	Balance at the beginning of the period		$	264.4		$	233.2
		Acquisition costs deferred	108.2			101.9
		Amortization of deferred policy acquisition costs	(110.5		)	(103.1		)
	Balance at the end of the period		$	262.1		$	232
			Nine Months Ended			Nine Months Ended September 30, 2012
			September 30, 2013
			($ in millions)
	Balance at the beginning of the period		$	223		$	184.5
		Acquisition costs deferred	361.4			348.7
		Amortization of deferred policy acquisition costs	(322.3		)	(301.2		)
	Balance at the end of the period		$	262.1		$	232

Reserves_for_Losses_and_Adjust

Reserves for Losses and Adjustment Expenses	9 Months Ended
	Sep. 30, 2013
Text Block [Abstract]	'
Reserves for Losses and Adjustment Expenses	'
	Reserves for Losses and Loss Adjustment Expenses
	The following table represents a reconciliation of beginning and ending consolidated loss and loss adjustment expenses (“LAE”) reserves for the nine months ended September 30, 2013 and twelve months ended December 31, 2012:
		Nine Months Ended September 30, 2013			Twelve Months Ended December 31, 2012
		($ in millions)
	Provision for losses and LAE at the start of the year	$	4,779.70		$	4,525.20
	Less reinsurance recoverable	(499.0		)	(426.6		)
	Net loss and LAE at the start of the year	4,280.70			4,098.60
	Net loss and LAE expenses (disposed)	(34.5		)	(9.0		)
	Provision for losses and LAE for claims incurred:
	Current year	979.5			1,375.90
	Prior years	(87.2		)	(137.4		)
	Total incurred	892.3			1,238.50
	Losses and LAE payments for claims incurred:
	Current year	(156.8		)	(244.3		)
	Prior years	(671.0		)	(835.7		)
	Total paid	(827.8		)	(1,080.0		)
	Foreign exchange losses	(37.3		)	32.6
	Net losses and LAE reserves at period end	4,273.40			4,280.70
	Plus reinsurance recoverable on unpaid losses at period end	442.2			499
	Provision for losses and LAE at the end of the relevant period	$	4,715.60		$	4,779.70
	For the nine months ended September 30, 2013, there was a reduction of $87.2 million in the Company’s estimate of the ultimate claims to be paid in respect of prior accident years compared to $95.4 million for the nine months ended September 30, 2012. The Company disposed of $34.5 million of its reserves relating to commutations during the nine months ended September 30, 2013 (September 30, 2012 — $8.8 million relating to commutations). 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, 2013
Equity [Abstract]	'
Capital Structure	'
	Capital Structure
	The following table provides a summary of the Company’s authorized and issued share capital at September 30, 2013 and December 31, 2012:
			As at September 30, 2013		As at December 31, 2012
			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		65,700,665	100	70,753,723	107
	Issued 5.625% preference shares of 0.15144558¢ each with a liquidation preference of $50 per share		—	—	4,600,000	7
	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 5.950% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share		11,000,000	17	—	—
	Total issued share capital			135		132
	Additional paid-in capital includes the aggregate liquidation preferences of the Company’s preference shares of $568.2 million (December 31, 2012 — $523.2 million) less issue costs of $12.6 million (December 31, 2012 — $15.1 million).
	Ordinary Shares. The following table summarizes transactions in the Company’s ordinary shares during the nine-months ended September 30, 2013:
		Number of Ordinary Shares
	Shares in issue at December 31, 2012	70,753,723
	Share transactions in the nine months ended September 30, 2013:
	Shares issued to employees under the 2003 share incentive plan and/or 2008 share purchase plan	1,188,079
	Shares issued to non-employee directors	10,542
	Shares repurchased from ordinary shareholders	(8,087,539	)
	Shares issued in respect of the redemption of the PIERS	1,835,860
	Shares in issue at September 30, 2013	65,700,665
	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 authorization of $500.0 million. The total share repurchase authorization, which was effective immediately through February 7, 2015, permits the Company to effect the repurchases from time to time through a combination of transactions, including open market repurchases, privately negotiated transactions and accelerated share repurchase transactions.
	Under the open market repurchases, the Company acquired and cancelled for the three and nine months ended September 30, 2013, 1,498,451 and 3,979,826 ordinary shares, respectively. The total consideration paid for the three and nine months ended September 30, 2013 was $54.8 million and $143.0 million, respectively, with the average prices for the three and nine months ended September 30, 2013 being $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 equaled 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. The $7.1 million reclassification from additional paid-in capital to retained earnings is the 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.
	Preference Shares Issuance. On May 2, 2013, the Company issued 11.0 million shares of 5.950% of Fixed-to-Floating Perpetual Non-Cumulative Preference Shares (the “5.950% Preference Shares”). Each preference shareholder will receive dividends on a non-cumulative basis only when declared by the Board of Directors initially at an annual fixed rate of 5.950% until July 1, 2023 at which time a floating rate, reset quarterly, of 3-month LIBOR plus 4.06% will commence per annum. The 5.950% 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 5.950% 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 5.950% Preference Shares at a redemption price of $25.00 per share on or after July 1, 2023. The Company has listed the 5.950% Preference Shares on the New York Stock Exchange under the symbol “AHLPRC”.

ShareBased_Payments

Share-Based Payments	9 Months Ended
	Sep. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'
Share-Based Payments	'
	Share-Based Payments
	The Company issued options and other equity incentives under three arrangements: investor options, the employee incentive plan 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.
	Investor Options. The investor options were issued on June 21, 2002 in connection with the transfer to Aspen Holdings of part of the operations of Wellington Underwriting plc (“Wellington”), the Company’s predecessor company. The Company conferred the option to subscribe for up to 6,787,880 ordinary shares of Aspen Holdings to Wellington and members of Syndicate 2020 who were not corporate members of the Lloyd’s syndicate managed by Wellington (the “Wellington Names”). All of the options issued to Wellington were exercised on March 28, 2007 resulting in the issuance of 426,083 ordinary shares by the Company.
	The options issued to the Wellington Names were held for their benefit by the Names’ Trustee. The subscription price payable under the options was initially £10 and increased by 5% per annum, less any dividends paid. Option holders were not entitled to participate in any dividends prior to exercise and would not rank as a creditor in the event of liquidation. All options were exercised prior to the expiry date of June 21, 2012.
	Employee and Non-Employee Director Awards. Employee options and other awards are granted under the Aspen 2003 Share Incentive Plan prior to April 24, 2013 and thereafter, the new 2013 Share Incentive Plan. The total number of ordinary shares that may be issued under the 2013 Share Incentive Plan is 2,845,683 shares, which includes 595,683 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 contract 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, 2013 (2012 —Nil and Nil) and 204,328 and 747,023 options, respectively, were exercised and issued in the three and nine months ended September 30, 2013 (2012 — 1,165,201 and 1,221,152 options). No charges against income were made in respect of employee options for the three and nine months ended September 30, 2013 (2012 — $Nil and $Nil).
	Restricted share units (“RSUs”) granted to employees vest over a three-year period typically, based on continued service. Some of the grants vest at year-end, while some other grants vest on the anniversary of the date of grant or when the Compensation Committee of the Board of Directors agrees to deliver them. The fair value of the RSUs is based on the closing price on the date of the grant. The fair value is expensed through the income statement evenly over the vesting period. During the three and nine months ended September 30, 2013, the Company granted to employees 8,571 and 307,441 RSUs, respectively (2012 — Nil and 345,852). 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 becoming vested on the first anniversary of the date of grant. On February 7, 2013 (with a grant date of February 11, 2013), the Board of Directors approved a total of 25,533 RSUs for the non-employee directors (February 2, 2012 — 29,071) and 14,188 RSUs to the Chairman (February 2, 2012 — 17,705). On April 24, 2013 (with a grant date of April 26, 2013), a further award was made to a newly appointed non-employee director of 2,110 RSUs. On July 24, 2013, (with a grant date of July 26, 2013) a further award of 1,449 RSUs was made to a newly appointed non-employee director. Compensation costs charged against income in respect of RSUs for the three and nine months ended September 30, 2013 were $2.4 million and $7.0 million, respectively (2012 — $1.8 million and $5.8 million).
	Performance Shares. On February 6, 2013, the Compensation Committee approved the grant of 250,066 performance shares with a grant date of February 11, 2013 (February 2, 2012 — 334,125 with 18,597 subsequently forfeited; November 1, 2012 — 10,006). 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 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 2013 is:
	•	less than 5%, then the portion of the performance shares subject to the vesting conditions in such year will be forfeited (i.e., 33.33% of the initial grant);
	•	between 5% and 10%, 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% and 20%, then the percentage of the performance shares eligible for vesting in such year will be between 100% and 200% on a straight-line basis.
	The Compensation Committee will determine the vesting conditions for the 2014 and 2015 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 in any given year, the shares eligible for vesting 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 (which in the case of the 2013 portion of the grant, the average BVPS is less than 5%), then only 100% (and no more) of the shares that are eligible for vesting in such year shall vest. Notwithstanding the foregoing, if in the judgment of the Compensation Committee the main reason for the BVPS metric in the earlier year falling below the minimum threshold (or below 5% in the case of 2013) 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 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, 2013 in respect of performance shares were a credit of $1.4 million and a charge of $4.2 million, respectively (2012 — $3.3 million and $9.9 million).
	Phantom Shares. On February 6, 2013, the Compensation Committee approved the grant of 152,541 phantom shares with a grant date of February 11, 2013 (February 2, 2012 — 278,143). Additional grants of 6,521 and 542 phantom shares were made on April 8, 2013 and June 11, 2013, respectively. The phantom shares will be 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 2013 performance shares, with the difference being that any vested amount would be paid in cash in lieu of shares. As 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, 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 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, 2013 in respect of phantom shares were a credit of $1.6 million and $Nil, respectively (2012 — a charge of $0.5 million and $1.6 million).
	Employee Share Purchase Plans. On April 30, 2008, the shareholders of the Company approved the Employee Share Purchase Plan (the “ESPP”), the 2008 Sharesave Scheme and the International Employee Share Purchase Plan, which are implemented by a series of consecutive offering periods as determined by the Board of Directors. In respect of the ESPP, employees can save up to $500 per month over a two-year period, at the end of which they 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, 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 plan, 1,985 and 16,339 ordinary shares were issued during the three and nine months ended September 30, 2013, respectively (2012 — 200 and 59,184 shares). Compensation costs charged against income in the three and nine months ended September 30, 2013 in respect of the ESPP were $0.1 million and $0.2 million, respectively (2012 — $Nil and $Nil).

Intangible_Assets

Intangible Assets	9 Months Ended
	Sep. 30, 2013
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, 2013 and 2012:
		Three Months Ended September 30, 2013										Three Months Ended September 30, 2012
		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	$	0.5		$	18.7		$	1.6	$	16.6	$	1.3		$	19.5
	Amortization	—		—		(0.1		)	(0.1		)	—		—		(0.3		)	(0.3		)
	End of the period	$	1.6	$	16.6	$	0.4		$	18.6		$	1.6	$	16.6	$	1		$	19.2
		Nine Months Ended September 30, 2013										Nine Months Ended September 30, 2012
		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	$	0.8		$	19		$	1.6	$	16.6	$	1.8		$	20
	Amortization	—		—		(0.4		)	(0.4		)	—		—		(0.8		)	(0.8		)
	End of the period	$	1.6	$	16.6	$	0.4		$	18.6		$	1.6	$	16.6	$	1		$	19.2
	License to use the “Aspen” Trademark. On April 5, 2005, the Company entered into an agreement with Aspen (Actuaries and Pension Consultants) Plc to acquire the right to use the Aspen trademark in the United Kingdom. The consideration paid was approximately $1.6 million. As at September 30, 2013, the value of the license to use the Aspen trademark was $1.6 million (December 31, 2012 — $1.6 million).
	Insurance Licenses. The total value of the licenses as at September 30, 2013, was $16.6 million (December 31, 2012 — $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 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 is being amortized over a five-year period and the value as at September 30, 2013 was $0.4 million (December 31, 2012 — $1.5 million).

Commitments_and_Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2013
Commitments and Contingencies Disclosure [Abstract]	'
Commitments and Contingencies	'
	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, 2013 and December 31, 2012:
			As at September 30, 2013			As at December 31, 2012
			($ in millions, except percentages)
	Regulatory trusts and deposits:
	Affiliated transactions		$	696.3		$	598.7
	Third party		2,197.90			1,933.50
	Letters of credit / guarantees(1)		827.2			1,189.20
	Total restricted assets		$	3,721.40		$	3,721.40
	Total as percent of cash and invested assets		46		%	45.6		%
	(1)	As of September 30, 2013, the Company had pledged funds of $801.1 million and £16.1 million (December 31, 2012 — $1,163.2 million and £16.0 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, investments and a fully collateralized letter of credit. The fully collateralized letter of credit has been provided by Aspen Bermuda and is disclosed in the above table as an affiliated transaction.
	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 is also required by Lloyd’s to maintain a minimum level of capital which as at September 30, 2013 was $0.6 million (December 31, 2012 — $0.6 million). This is not available for distribution by the Company for the payment of dividends.
	Interest Rate Swaps. As at September 30, 2013, cash collateral with a fair value of $34.1 million was held by the Company’s counterparties to support the current valuation of the interest rate swaps (December 31, 2012 — $52.0 million). For more information, please refer to Note 9 of these unaudited condensed consolidated financial statements.
	(b)	Operating leases
	Amounts outstanding under operating leases net of subleases as of September 30, 2013 were:
			2013			2014			2015		2016		2017		Later		Total
															Years
			($ in millions)
	Operating Lease Obligations		$	5.5		$	12.2		$	11.6	$	8.5	$	6.9	$	11.7	$	56.4
	(c)	Variable interest entities
	As at September 30, 2013, the Company had two investments in variable interest entities, Cartesian Iris Offshore Fund L.P, and Chaspark Maritime Holdings Ltd., as disclosed in Note 6 of these unaudited condensed consolidated financial statements.
	(d)	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, 2013, 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, 2013
Accounting Policies [Abstract]	'
New Accounting Policies Adopted in 2013	'
	New Accounting Policies Adopted in 2013
	In 2011, the Financial Accounting Standard Board (“FASB”) issued ASU 2011-11, “Disclosures about Offsetting Assets and Liabilities.” The ASU 2011-11 retains the existing offsetting model under U.S. GAAP but requires disclosures to allow investors to better compare financial statements prepared under U.S. GAAP with financial statements prepared under International Financial Reporting Standards (“IFRS”) by aligning these requirements. ASU 2011-11 is effective for annual reporting periods beginning January 1, 2013, and interim periods within those annual periods. Retrospective application is required.
	In January 2013, the FASB issued ASU 2013-01, “Clarifying the Scope of Disclosures About Offsetting Assets and Liabilities” which clarifies which instruments and transactions are subject to the offsetting disclosure requirements established by ASU 2011-11. ASU 2013-01 addresses concerns that the scope of the disclosure requirements under ASU 2011-11 was overly broad and imposed unintended costs that were not commensurate with estimated benefits to financial statement users. In choosing to narrow the scope of the offsetting disclosures, the FASB determined that it could make them more operable and cost effective for preparers while still giving financial statement users sufficient information to analyze the most significant presentation differences between financial statements prepared in accordance with U.S. GAAP and those prepared under IFRS. Like ASU 2011-11, ASU 2013-01 is effective for fiscal years beginning on or after January 1, 2013, and interim periods therein. Retrospective application is required for any period presented that begins before the entity’s initial application of the new requirements. The Company adopted ASU 2011-11 and ASU 2013-01 in the first quarter of 2013 and they do not have a material impact on the unaudited condensed consolidated financial statements.
	In February 2013, the FASB issued ASU 2013-02, “Reporting of Amounts Reclassified out of Accumulated Other Comprehensive Income” which adds new disclosure requirements for items reclassified out of accumulated other comprehensive income (“AOCI”). ASU 2013-02 is intended to improve the transparency of changes in other comprehensive income (“OCI”) and items reclassified out of AOCI in the financial statements. It does not amend any existing requirements for reporting net income or OCI in the financial statements. ASU 2013-02 was applied prospectively and was effective for annual reporting periods beginning after December 15, 2012, and interim periods within those years. The Company adopted ASU 2013-02 in the first quarter of 2013 and this information is presented in Note 3 of the accompanying notes to the unaudited condensed consolidated financial statements.
	In July 2013, the FASB issued ASU 2013-10, “Inclusion of the Fed Funds Effective Swap Rate (or Overnight Index Swap Rate) as a Benchmark Interest Rate for Hedge Accounting Purposes” which provides guidance on the risks permitted to be hedged in a fair value or cash flow hedges, specifically, the risk of changes in a hedged item’s fair value or hedged transaction’s cash flows due to the changes in the designated benchmark interest rate. ASU 2013-10 is effective prospectively for qualifying new or redesignated hedging relationships entered into on or after July 17, 2013.  The Company has adopted ASU 2013-10 in the third quarter of 2013 and it does not have a material impact on the unaudited condensed consolidated financial statements.
Accounting Pronouncements Not Yet Adopted	'
	Accounting Pronouncements Not Yet Adopted
	In February 2013, the FASB issued ASU 2013-04, “Obligations Resulting from Joint and Several Liability Arrangements for Which the Total Amount of the Obligation is Fixed at the Reporting Date” to provide guidance for the recognition, measurement, and disclosure of obligations resulting from joint and several liability arrangements for which the total amount of the obligation within the scope of this guidance is fixed at the reporting date. Examples of obligations include debt arrangements, other contractual obligations and settled litigation and judicial rulings. ASU 2013-04 will be applied retrospectively and is effective for annual reporting periods beginning after December 15, 2013, and interim periods within those years. The Company does not anticipate that this standard will have a material impact on its unaudited condensed consolidated financial statements.
	In March 2013, the FASB issued ASU 2013-05, “Parent’s Accounting for the Cumulative Translation Adjustment Upon Derecognition of Certain Subsidiaries or Groups of Assets Within a Foreign Entity or of an Investment in a Foreign Entity” to standardize the release of the cumulative translation adjustment into net income when a parent either sells a part or all of its investment in a foreign entity or no longer holds a controlling financial interest in a subsidiary. ASU 2013-05 will be applied prospectively and is effective for annual reporting periods beginning after December 15, 2013, and interim periods within those years. The Company does not expect ASU 2013-05 to have a material impact on its unaudited condensed consolidated financial statements unless it should choose to sell its investments in a foreign entity or cease to hold a controlling financial interest.
	In July 2013, the FASB issued ASU 2013-11, “Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists” which provides guidance on financial statement presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. ASU 2013-11 is effective for fiscal years beginning after December 15, 2013 or interim periods within those years. The Company does not expect ASU 2013-11 to have a material impact on the unaudited condensed consolidated financial statements as its presentation of unrecognized tax benefits is already as required by ASU 2013-11.

Reclassifications_from_Accumul1

Reclassifications from Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Schedule of Accumulated Other Comprehensive Income Reclassification	'
	The following table sets out the components of the Company’s AOCI that are reclassified into the unaudited condensed consolidated statement of operations for the three and nine months ended September 30, 2013:
		Amount Reclassified from AOCI
	Details about the AOCI Components	Three Months Ended September 30, 2013			Nine Months Ended September 30, 2013			Affected Line Item in the Unaudited
								Condensed Consolidated Statement
								of Operations
		($ in millions)
	Available for sale securities:
	Realized gain on sale of securities	$	8.6		$	21.2		Realized and unrealized investment gains
	Realized (loss) on sale of securities	(0.3		)	(0.5		)	Realized and unrealized investment losses
		8.3			20.7			Income from operations before tax
	Tax on realized gains and (losses) of securities	(0.1		)	(0.5		)	Income tax expense
		$	8.2		$	20.2		Net income
	Foreign currency translation adjustments:
	Foreign currency translation adjustments, before tax	$	4.8		$	0.5		Net realized and unrealized foreign exchange gains/(losses)
	Tax on foreign currency translation adjustments	(1.1		)	(0.1		)	Income tax expense
		$	3.7		$	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	$	11.9		$	20.4		Net income

Earnings_per_Ordinary_Share_Ta

Earnings per Ordinary Share (Tables)	9 Months Ended
	Sep. 30, 2013
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, 2013 and 2012, respectively:
			Three Months Ended September 30,						Nine Months Ended September 30,
			2013			2012			2013			2012
			($ in millions, except share and per share amounts)
	Net income		$	107.4		$	115.1		$	239.3		$	278.4
	PIERS redemption(1)		—			—			(7.1		)	—
	Preference share dividends		(9.5		)	(8.6		)	(26.1		)	(22.6		)
	Basic and diluted net income available to ordinary shareholders		97.9			106.5			206.1			255.8
	Ordinary shares:
	Basic weighted average ordinary shares		66,716,202			71,129,102			67,302,857			71,125,664
	Weighted average effect of dilutive securities(2)		1,845,313			2,268,694			2,656,617			2,577,374
	Total diluted weighted average ordinary shares		68,561,515			73,397,796			69,959,474			73,703,038
	Earnings per ordinary share:
	Basic		$	1.47		$	1.5		$	3.06		$	3.6
	Diluted		$	1.43		$	1.45		$	2.95		$	3.47
	(1)	The $7.1 million deduction to net income is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million.
	(2)	Dilutive securities comprise: investor options, employee options, performance shares associated with the Company’s long term incentive program, employee share purchase plans and restricted stock units as described in Note 13, in addition to the PIERS.
	The following table sets forth the computation of basic and diluted earnings per share for the three and nine months ended September 30, 2013 and 2012, respectively:
			Three Months Ended September 30,						Nine Months Ended September 30,
			2013			2012			2013			2012
			($ in millions, except share and per share amounts)
	Net income		$	107.4		$	115.1		$	239.3		$	278.4
	PIERS redemption(1)		—			—			(7.1		)	—
	Preference share dividends		(9.5		)	(8.6		)	(26.1		)	(22.6		)
	Basic and diluted net income available to ordinary shareholders		97.9			106.5			206.1			255.8
	Ordinary shares:
	Basic weighted average ordinary shares		66,716,202			71,129,102			67,302,857			71,125,664
	Weighted average effect of dilutive securities(2)		1,845,313			2,268,694			2,656,617			2,577,374
	Total diluted weighted average ordinary shares		68,561,515			73,397,796			69,959,474			73,703,038
	Earnings per ordinary share:
	Basic		$	1.47		$	1.5		$	3.06		$	3.6
	Diluted		$	1.43		$	1.45		$	2.95		$	3.47
Summary of Declared Dividends	'
	Dividends. On October 30, 2013, the Company’s Board of Directors declared the following quarterly dividends:
			Dividend			Payable on:		Record Date:
	Ordinary shares		$	0.18		November 27, 2013		November 13, 2013
	7.401% preference shares		$	0.462563		January 1, 2014		December 15, 2013
	7.250% preference shares		$	0.4531		January 1, 2014		December 15, 2013
	5.950% preference shares		$	0.3719		January 1, 2014		December 15, 2013

Segment_Reporting_Tables

Segment Reporting (Tables)	9 Months Ended
	Sep. 30, 2013
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, 2013 and 2012:
			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
	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.
			Three Months Ended September 30, 2012
			Reinsurance			Insurance			Total
			( $ in millions)
	Underwriting Revenues
	Gross written premiums		$	259.5		$	298.9		$	558.4
	Net written premiums		256.9			250.2			507.1
	Gross earned premiums		299.8			302			601.8
	Net earned premiums		279.6			236.6			516.2
	Underwriting Expenses
	Losses and loss adjustment expenses		117.1			137.9			255
	Amortization of deferred policy acquisition costs		55.7			47.4			103.1
	General and administrative expenses		33.6			42.8			76.4
	Underwriting income		73.2			8.5			81.7
	Corporate expenses								(14.3		)
	Net investment income								48.6
	Realized and unrealized investment gains								13.2
	Realized and unrealized investment (losses)								(2.4		)
	Change in fair value of derivatives								(4.9		)
	Interest expense on long term debt								(7.8		)
	Net realized and unrealized foreign exchange gains								4.5
	Other income								4.8
	Other (expenses)								(0.3		)
	Income before tax								$	123.1
	Net reserves for loss and loss adjustment expenses		$	2,755.10		$	1,422.90		$	4,178.00
	Ratios
	Loss ratio		41.9		%	58.3		%	49.4		%
	Policy acquisition expense ratio		19.9			20			20
	General and administrative expense ratio		12			18.1			17.6			(1)
	Expense ratio		31.9			38.1			37.6
	Combined ratio		73.8		%	96.4		%	87		%
	(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, 2013 and 2012:
		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.
		Nine Months Ended September 30, 2012
		Reinsurance			Insurance			Total
		( $ in millions)
Underwriting Revenues
Gross written premiums		$	1,033.50		$	973.6		$	2,007.10
Net written premiums		963.2			759.3			1,722.50
Gross earned premiums		890.8			848.8			1,739.60
Net earned premiums		832.6			692.4			1,525.00
Underwriting Expenses
Losses and loss adjustment expenses		386.4			414.7			801.1
Amortization of deferred policy acquisition costs		166.8			134.4			301.2
General and administrative expenses		92.6			126.3			218.9
Underwriting income		186.8			17			203.8
Corporate expenses								(40.1		)
Net investment income								153.8
Realized and unrealized investment gains								29.3
Realized and unrealized investment (losses)								(8.2		)
Change in fair value of derivatives								(24.0		)
Interest expense on long term debt								(23.2		)
Net realized and unrealized foreign exchange (losses)								(0.5		)
Other income								9
Other (expenses)								(1.9		)
Income before tax								$	298
Net reserves for loss and loss adjustment expenses		$	2,755.10		$	1,422.90		$	4,178.00
Ratios
Loss ratio		46.4		%	59.9		%	52.5		%
Policy acquisition expense ratio		20			19.4			19.8
General and administrative expense ratio		11.1			18.2			17			(1)
Expense ratio		31.1			37.6			36.8
Combined ratio		77.5		%	97.5		%	89.3		%
(1)	The general and administrative expense ratio in the total column includes corporate expenses.

Investments_Tables

Investments (Tables)	9 Months Ended
	Sep. 30, 2013
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, 2013 and 2012:
		For the Three Months Ended						For the Nine Months Ended
		30-Sep-13			30-Sep-12			30-Sep-13			30-Sep-12
		($ in millions)						($ in millions)
	Fixed income maturities — Available for sale	$	38.2		$	43.7		$	117.2		$	136.9
	Fixed income maturities — Trading	5.3			4			13.4			12.2
	Short-term investments — Available for sale	0.3			0.5			1.9			1.8
	Fixed term deposits (included in cash and cash equivalents)	0.7			1.4			3.4			4.7
	Equity securities — Available for sale	1.2			1.9			4.7			5.1
	Equity securities — Trading	1.8			—			5.5			—
	Total	$	47.5		$	51.5		$	146.1		$	160.7
	Investment expenses	(2.5		)	(2.9		)	(6.9		)	(6.9		)
	Net investment income	$	45		$	48.6		$	139.2		$	153.8
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 Months Ended						For the Nine Months Ended
		30-Sep-13			30-Sep-12			30-Sep-13			30-Sep-12
		($ in millions)						($ in millions)
	Available for sale:
	Fixed income maturities — gross realized gains	$	6.9		$	1.5		$	15.7		$	5.3
	Fixed income maturities — gross realized (losses)	(5.6		)	(0.1		)	(6.8		)	(0.4		)
	Equity securities — gross realized gains	6.3			1			13.8			2.9
	Equity securities — gross realized (losses)	—			(0.1		)	—			(4.5		)
	Trading:
	Fixed income maturities — gross realized gains	0.8			2.3			6.6			6.7
	Fixed income maturities — gross realized (losses)	(1.3		)	(0.1		)	(1.9		)	(0.3		)
	Equity securities — gross realized gains	0.4			—			0.8			—
	Equity securities — gross realized (losses)	—			—			(0.3		)	—
	Net change in gross unrealized gains/(losses)	9			6.7			(2.7		)	12.7
	Impairments:
	Total other-than-temporary impairments	—			(2.1		)	—			(3.0		)
	Gross realized and unrealized gains in Cartesian Iris	1.2			1.7			1.1			1.7
	Total net realized and unrealized investment gains recorded in the statement of operations	$	17.7		$	10.8		$	26.3		$	21.1
	Change in available for sale net unrealized gains/(losses):
	Fixed income maturities	(3.3		)	25.5			(168.7		)	40.9
	Short-term investments	—			0.2			—			0.2
	Equity securities	(0.2		)	6.5			9.4			16.1
	Total change in pre-tax available for sale unrealized gains/(losses)	(3.5		)	32.2			(159.3		)	57.2
	Change in taxes	(0.3		)	(1.1		)	11.8			(0.9		)
	Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income	$	(3.8	)	$	31.1		$	(147.5	)	$	56.3
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 Maturities, 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 maturities, short-term investments and equity securities as at September 30, 2013 and December 31, 2012:
		As at September 30, 2013
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	995.3		$	28.4		$	(2.6	)	$	1,021.10
	U.S. agency	269.2			13			(0.7		)	281.5
	Municipal	29.1			1.1			(0.3		)	29.9
	Corporate	1,942.60			92.2			(16.7		)	2,018.10
	Non-U.S. government-backed corporate	77.5			1.4			—			78.9
	Foreign government	815.5			9.9			(3.2		)	822.2
	Asset-backed	113.7			3.1			(0.3		)	116.5
	Non-agency commercial mortgage-backed	58.1			6.3			—			64.4
	Agency mortgage-backed	1,087.20			35.1			(10.0		)	1,112.30
	Total fixed income maturities — Available for sale	5,388.20			190.5			(33.8		)	5,544.90
	Total short-term investments — Available for sale	150.3			—			(2.0		)	148.3
	Total equity securities — Available for sale	128.7			35.3			(0.8		)	163.2
	Total	$	5,667.20		$	225.8		$	(36.6	)	$	5,856.40
		As at December 31, 2012
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	1,071.80		$	54.8		$	(0.3	)	$	1,126.30
	U.S. agency	288.3			20.3			—			308.6
	Municipal	37.2			2.6			(0.1		)	39.7
	Corporate	1,889.20			149.9			(0.6		)	2,038.50
	Non-U.S. government-backed corporate	98			3.1			—			101.1
	Foreign government	617			24.1			(0.1		)	641
	Asset-backed	49.2			4.6			—			53.8
	Non-agency commercial mortgage-backed	61.7			9.4			—			71.1
	Agency mortgage-backed	1,116.10			61.2			(0.1		)	1,177.20
	Total fixed income maturities — Available for sale	5,228.50			330			(1.2		)	5,557.30
	Total short-term investments — Available for sale	431.5			—			—			431.5
	Total equity securities — Available for sale	174			28.2			(2.1		)	200.1
	Total	$	5,834.00		$	358.2		$	(3.3	)	$	6,188.90
Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities	'
	Fixed Income Maturities, Short Term Investments and Equities — 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 maturities, short-term investments and equity securities as at September 30, 2013 and December 31, 2012:
		As at September 30, 2013
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	32		$	—		$	(0.9	)	$	31.1
	U.S. agency	0.2			—			—			0.2
	Municipal	0.6			—			—			0.6
	Corporate	464.9			12.1			(6.0		)	471
	Foreign government	135.9			1.3			(0.9		)	136.3
	Asset-backed	6.6			—			—			6.6
	Bank loans	72.9			0.1			(0.2		)	72.8
	Total fixed income maturities — Trading	713.1			13.5			(8.0		)	718.6
	Total short-term investments — Trading	0.2			—			—			0.2
	Total equity securities — Trading	260.4			20.5			(2.6		)	278.3
	Total	$	973.7		$	34		$	(10.6	)	$	997.1
		As at December 31, 2012
		Cost or			Gross			Gross			Fair Market
		Amortized Cost			Unrealized			Unrealized			Value
					Gains			Losses
		($ in millions)
	U.S. government	$	9.3		$	0.2		$	(0.1	)	$	9.4
	U.S. agency	0.2			—			—			0.2
	Municipal	2.8			0.1			—			2.9
	Corporate	392			22.7			(0.3		)	414.4
	Foreign government	24.4			1.9			—			26.3
	Asset-backed	2.9			—			—			2.9
	Total fixed income maturities — Trading	431.6			24.9			(0.4		)	456.1
	Total short-term investments — Trading	2.4			—			—			2.4
	Total	$	434		$	24.9		$	(0.4	)	$	458.5
Other Investments	'
	The table below shows the Company’s investments in Cartesian and Chaspark for the nine months ended September 30, 2013 and twelve months ended December 31, 2012:
		Cartesian			Chaspark			Total
		($ in millions)
	Opening undistributed value of investment as at January 1, 2013	$	36.3		$	8.7		$	45
	Unrealized gain for the nine months to September 30, 2013	1.1			—			1.1
	Closing value of investment as at September 30, 2013	$	37.4		$	8.7		$	46.1
	Opening undistributed value of investment as at January 1, 2012	$	33.1		$	—		$	33.1
	Investment in Chaspark Maritime Holdings Ltd.	—			8.7			8.7
	Unrealized gain for the twelve months to December 31, 2012	3.2			—			3.2
	Closing value of investment as at December 31, 2012	$	36.3		$	8.7		$	45
Summary of Fixed Maturities	'
	Fixed Maturities. The scheduled maturity distribution of available for sale fixed income maturity securities as at September 30, 2013 and December 31, 2012 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, 2013
		Amortized			Fair Market			Average
		Cost or Cost			Value			S&P Ratings by
								Maturity
		($ in millions)
	Due one year or less	$	649.6		$	652.5		AA
	Due after one year through five years	2,385.80			2,461.20			AA
	Due after five years through ten years	992.5			1,033.50			A+
	Due after ten years	101.3			104.5			AA-
	Subtotal	4,129.20			4,251.70
	Non-agency commercial mortgage-backed	58.1			64.4			AA+
	Agency mortgage-backed	1,087.20			1,112.30			AA+
	Other asset-backed	113.7			116.5			AAA
	Total fixed income maturities — Available for sale	$	5,388.20		$	5,544.90
		As at December 31, 2012
		Amortized			Fair Market			Average
		Cost or Cost			Value			S&P Ratings by
								Maturity
		($ in millions)
	Due one year or less	$	554.6		$	561		AA
	Due after one year through five years	2,270.00			2,378.80			AA
	Due after five years through ten years	1,077.80			1,199.30			AA-
	Due after ten years	99.1			116.1			AA+
	Subtotal	4,001.50			4,255.20
	Non-agency commercial mortgage-backed	61.7			71.1			AA+
	Agency mortgage-backed	1,116.10			1,177.20			AA+
	Other asset-backed	49.2			53.8			AAA
	Total fixed income maturities — Available for sale	$	5,228.50		$	5,557.30
Aggregate Fair Value and Gross Unrealized Loss by Type of Security	'
	Gross Unrealized Loss. The following tables summarize as at September 30, 2013 and December 31, 2012, 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, 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	$	193.3		$	(2.6	)	$	—		$	—		$	193.3	$	(2.6	)	38
	U.S. agency	67.1			(0.7		)	—			—			67.1		(0.7		)	14
	Municipal	2.5			(0.1		)	1.3			(0.2		)	3.8		(0.3		)	3
	Corporate	611.6			(15.9		)	7.5			(0.8		)	619.1		(16.7		)	331
	Non-U.S. government-backed corporate	6.9			—			4.8			—			11.7		—			3
	Foreign government	234.5			(3.1		)	4			(0.1		)	238.5		(3.2		)	40
	Asset-backed	43.8			(0.3		)	—			—			43.8		(0.3		)	39
	Non-agency commercial mortgage-backed	—			—			—			—			—		—			—
	Agency mortgage-backed	384.2			(10.0		)	—			—			384.2		(10.0		)	85
	Total fixed income maturities — Available for sale	1,543.90			(32.7		)	17.6			(1.1		)	1,561.50		(33.8		)	553
	Total short-term investments — Available for sale	99			(2.0		)	—			—			99		(2.0		)	1
	Total equity securities — Available for sale	8.5			(0.5		)	3.8			(0.3		)	12.3		(0.8		)	7
	Total	$	1,651.40		$	(35.2	)	$	21.4		$	(1.4	)	$	1,672.80	$	(36.6	)	561
		December 31, 2012
		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	$	72.7		$	(0.3	)	$	—		$	—		$	72.7	$	(0.3	)	9
	Municipal	1.4			(0.1		)	—			—			1.4		(0.1		)	1
	Corporate	170.8			(0.6		)	—			—			170.8		(0.6		)	63
	Non-U.S. government-backed corporate	10.1			—			2			—			12.1		—			3
	Foreign government	87.7			(0.1		)	4			—			91.7		(0.1		)	20
	Asset-backed	0.7			—			—			—			0.7		—			1
	Agency mortgage-backed	26.7			(0.1		)	—			—			26.7		(0.1		)	14
	Total fixed income maturities — Available for sale	370.1			(1.2		)	6			—			376.1		(1.2		)	111
	Total short-term investments — Available for sale	9.4			—			—			—			9.4		—			4
	Total equity securities — Available for sale	28.6			(1.9		)	2.2			(0.2		)	30.8		(2.1		)	18
	Total	$	408.1		$	(3.1	)	$	8.2		$	(0.2	)	$	416.3	$	(3.3	)	133
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, 2013 and 2012:
		For the Three Months Ended						For the Nine Months Ended
		30-Sep-13			30-Sep-12			30-Sep-13			30-Sep-12
		($ in millions)						($ in millions)
	(Purchases) of fixed income maturities — Available for sale	$	(566.8	)	$	(351.8	)	$	(1,748.6	)	$	(1,057.1	)
	(Purchases) of fixed income maturities — Trading	(283.2		)	(73.0		)	(555.5		)	(187.7		)
	(Purchases) of equity securities — Available for sale	—			(12.0		)	(2.5		)	(41.5		)
	(Purchases) of equity securities — Trading	(38.6		)	—			(275.4		)	—
	Proceeds from sales and maturities of fixed income maturities — Available for sale	578.6			240.4			1,564.40			989.7
	Proceeds from sales and maturities of fixed income maturities — Trading	67.8			49.8			276.3			164.5
	Proceeds from sales of equity securities — Available for sale	29.7			10.1			61.8			35.9
	Proceeds from sales of equity securities — Trading	5			—			15.1			—
	Net change in (payable)/receivable for securities (purchased) /sold	6.6			(22.8		)	5.6			19.3
	Net (purchases)/sales of short-term investments	95.5			13.2			275.7			(190.4		)
	Net (purchases)/sales for the period	$	(105.4	)	$	(146.1	)	$	(383.1	)	$	(267.3	)

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2013
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 at September 30, 2013 and December 31, 2012 respectively:
		As at September 30, 2013
		Level 1			Level 2				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	1,021.10		$	—			$	1,021.10
	U.S. agency	—			281.5				281.5
	Municipal	—			29.9				29.9
	Corporate	—			2,018.10				2,018.10
	Non-U.S. government-backed corporate	—			78.9				78.9
	Foreign government	612.6			209.6				822.2
	Asset-backed	—			116.5				116.5
	Non-agency commercial mortgage-backed	—			64.4				64.4
	Agency mortgage-backed	—			1,112.30				1,112.30
	Total fixed income maturities available for sale, at fair value	1,633.70			3,911.20				5,544.90
	Short-term investments available for sale, at fair value	118			30.3				148.3
	Equity investments available for sale, at fair value	163.2			—				163.2
	Held for trading financial assets, at fair value
	U.S. government	$	31.1		$	—			$	31.1
	U.S. agency	—			0.2				0.2
	Municipal	—			0.6				0.6
	Corporate	—			471				471
	Foreign government	42.9			93.4				136.3
	Asset-backed	—			6.6				6.6
	Bank loans	—			72.8				72.8
	Total fixed income maturities trading, at fair value	74			644.6				718.6
	Short-term investments trading, at fair value	—			0.2				0.2
	Equity investments trading, at fair value	278.3			—				278.3
	Other financial assets and liabilities, at fair value
	Derivatives at fair value — foreign exchange contracts	—			5.6				5.6
	Liabilities under derivative contracts — foreign exchange contracts	—			(0.9		)		(0.9		)
	Liabilities under derivative contracts — interest rate swaps	—			(1.4		)		(1.4		)
	Total	$	2,267.20		$	4,589.60			$	6,856.80
	There were no transfers between Level 1 and Level 2 during the three and nine months ended September 30, 2013 and no assets or liabilities were classified as Level 3.
		As at December 31, 2012
		Level 1			Level 2				Total
		($ in millions)
	Available for sale financial assets, at fair value
	U.S. government	$	1,126.30		$	—			1,126.30
	U.S. agency	—			308.6				308.6
	Municipal	—			39.7				39.7
	Corporate	—			2,038.50				2,038.50
	Non-U.S. government-backed corporate	—			101.1				101.1
	Foreign government	487.2			153.8				641
	Asset-backed	—			53.8				53.8
	Non-agency commercial mortgage-backed	—			71.1				71.1
	Agency mortgage-backed	—			1,177.20				1,177.20
	Total fixed income maturities available for sale, at fair value	1,613.50			3,943.80				5,557.30
	Short-term investments available for sale, at fair value	426.2			5.3				431.5
	Equity investments available for sale, at fair value	200.1			—				200.1
	Held for trading financial assets, at fair value
	U.S. government	$	9.4		$	—			$	9.4
	U.S. agency	—			0.2				0.2
	Municipal	—			2.9				2.9
	Corporate	—			414.4				414.4
	Foreign government	4.2			22.1				26.3
	Asset-backed	—			2.9				2.9
	Total fixed income maturities trading, at fair value	13.6			442.5				456.1
	Short-term investments trading, at fair value	2.4			—				2.4
	Other financial assets and liabilities, at fair value
	Derivatives at fair value — foreign exchange contracts	—			2				2
	Liabilities under derivative contracts — foreign exchange contracts	—			(6.1		)		(6.1		)
	Liabilities under derivative contracts — interest rate swaps	—			(1.3		)		(1.3		)
Total	$	2,255.80		$	4,386.20			$	6,642.00
Pricing Sources Used in Pricing Fixed Income Investments	'
	Pricing sources used in pricing fixed income investments at September 30, 2013 and December 31, 2012 were as follows:
		As at September 30, 2013		As at December 31, 2012
	Index providers	87	%	89	%
	Pricing services	10		9
	Broker-dealers	3		2
	Total	100	%	100	%
Summary of Securities Priced Using Pricing Information from Index Providers	'
	Fixed Income Maturities. A summary of securities priced using pricing information from index providers at September 30, 2013 and December 31, 2012 is provided below:
		As at September 30, 2013						As at December 31, 2012
		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	$	1,013.90		96	%		$	1,034.50			91	%
	U.S. agency	267.7			95	%		291.9				95	%
	Municipal	13.3			43	%		20.5				48	%
	Corporate	2,358.30			95	%		2,339.80				95	%
	Non-U.S. government-backed corporate	66.4			84	%		84.5				84	%
	Foreign government	685.5			72	%		516.9				77	%
	Asset-backed	117.8			96	%		44.6				79	%
	Non-agency commercial mortgage-backed	62.7			97	%		70.4				99	%
	Agency mortgage-backed	836.5			75	%		957.8				81	%
	Total fixed income maturities	$	5,422.10		87	%		$	5,360.90			89	%

Derivative_Contracts_Tables

Derivative Contracts (Tables)	9 Months Ended
	Sep. 30, 2013
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, 2013 and December 31, 2012:
				As at September 30, 2013							As at December 31, 2012
	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		Liabilities under Derivative Contracts	$	1,000.00		$	(1.4	)	(1)	$	1,000.00	$	(1.3	)	(1)
	Foreign Exchange Contracts		Derivatives at Fair Value	$	275.9		$	5.6			$	335.4	$	2
	Foreign Exchange Contracts		Liabilities under Derivative Contracts	$	38.6		$	(0.9	)		$	88.6	$	(6.1	)
	(1)	Net of $34.1 million of cash collateral provided to counterparties as security for the Company’s net liability position (December 31, 2012 — $52.0 million).
Gain/(Loss) Recognized in Income on Derivative	'
	The following tables provide the unrealized and realized gains/(losses) recorded in the statement of operations for the three and nine months ended September 30, 2013 and 2012 respectively:
				Amount of Income/(Loss)
				Recognized in the Statement
				of Operations
				Three Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	30-Sep-13			30-Sep-12
	ASC 815		Statement of Operations
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	10.8		$	3.2
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	(4.2	)	$	(8.1	)
				Amount of Income/(Loss)
				Recognized in the Statement
				of  Operations
				Nine Months Ended
	Derivatives Not Designated as Hedging Instruments Under		Location of Income/(Loss) Recognized in the	30-Sep-13			30-Sep-12
	ASC 815		Statement of Operations
				($ in millions)
	Foreign Exchange Contracts		Change in Fair Value of Derivatives	$	(3.7	)	$	(1.1	)
	Interest Rate Swaps		Change in Fair Value of Derivatives	$	3.2		$	(22.9	)

Deferred_Policy_Acquisition_Co1

Deferred Policy Acquisition Costs (Tables)	9 Months Ended
	Sep. 30, 2013
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 three and nine months ended September 30, 2013 and 2012:
			Three Months Ended September 30, 2013			Three Months Ended September 30, 2012
			($ in millions)
	Balance at the beginning of the period		$	264.4		$	233.2
		Acquisition costs deferred	108.2			101.9
		Amortization of deferred policy acquisition costs	(110.5		)	(103.1		)
	Balance at the end of the period		$	262.1		$	232
			Nine Months Ended			Nine Months Ended September 30, 2012
			September 30, 2013
			($ in millions)
	Balance at the beginning of the period		$	223		$	184.5
		Acquisition costs deferred	361.4			348.7
		Amortization of deferred policy acquisition costs	(322.3		)	(301.2		)
	Balance at the end of the period		$	262.1		$	232

Reserves_for_Losses_and_Adjust1

Reserves for Losses and Adjustment Expenses (Tables)	9 Months Ended
	Sep. 30, 2013
Text Block [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, 2013 and twelve months ended December 31, 2012:
		Nine Months Ended September 30, 2013			Twelve Months Ended December 31, 2012
		($ in millions)
	Provision for losses and LAE at the start of the year	$	4,779.70		$	4,525.20
	Less reinsurance recoverable	(499.0		)	(426.6		)
	Net loss and LAE at the start of the year	4,280.70			4,098.60
	Net loss and LAE expenses (disposed)	(34.5		)	(9.0		)
	Provision for losses and LAE for claims incurred:
	Current year	979.5			1,375.90
	Prior years	(87.2		)	(137.4		)
	Total incurred	892.3			1,238.50
	Losses and LAE payments for claims incurred:
	Current year	(156.8		)	(244.3		)
	Prior years	(671.0		)	(835.7		)
	Total paid	(827.8		)	(1,080.0		)
	Foreign exchange losses	(37.3		)	32.6
	Net losses and LAE reserves at period end	4,273.40			4,280.70
	Plus reinsurance recoverable on unpaid losses at period end	442.2			499
	Provision for losses and LAE at the end of the relevant period	$	4,715.60		$	4,779.70

Capital_Structure_Tables

Capital Structure (Tables)	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Summary of Authorized and Issued Share Capital	'
	The following table provides a summary of the Company’s authorized and issued share capital at September 30, 2013 and December 31, 2012:
			As at September 30, 2013		As at December 31, 2012
			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		65,700,665	100	70,753,723	107
	Issued 5.625% preference shares of 0.15144558¢ each with a liquidation preference of $50 per share		—	—	4,600,000	7
	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 5.950% preference shares of 0.15144558¢ each with a liquidation preference of $25 per share		11,000,000	17	—	—
	Total issued share capital			135		132
Summary of Ordinary Shares	'
	Ordinary Shares. The following table summarizes transactions in the Company’s ordinary shares during the nine-months ended September 30, 2013:
		Number of Ordinary Shares
	Shares in issue at December 31, 2012	70,753,723
	Share transactions in the nine months ended September 30, 2013:
	Shares issued to employees under the 2003 share incentive plan and/or 2008 share purchase plan	1,188,079
	Shares issued to non-employee directors	10,542
	Shares repurchased from ordinary shareholders	(8,087,539	)
	Shares issued in respect of the redemption of the PIERS	1,835,860
	Shares in issue at September 30, 2013	65,700,665

Intangible_Assets_Tables

Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2013
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, 2013 and 2012:
		Three Months Ended September 30, 2013										Three Months Ended September 30, 2012
		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	$	0.5		$	18.7		$	1.6	$	16.6	$	1.3		$	19.5
	Amortization	—		—		(0.1		)	(0.1		)	—		—		(0.3		)	(0.3		)
	End of the period	$	1.6	$	16.6	$	0.4		$	18.6		$	1.6	$	16.6	$	1		$	19.2
		Nine Months Ended September 30, 2013										Nine Months Ended September 30, 2012
		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	$	0.8		$	19		$	1.6	$	16.6	$	1.8		$	20
	Amortization	—		—		(0.4		)	(0.4		)	—		—		(0.8		)	(0.8		)
	End of the period	$	1.6	$	16.6	$	0.4		$	18.6		$	1.6	$	16.6	$	1		$	19.2

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2013
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, 2013 and December 31, 2012:
			As at September 30, 2013			As at December 31, 2012
			($ in millions, except percentages)
	Regulatory trusts and deposits:
	Affiliated transactions		$	696.3		$	598.7
	Third party		2,197.90			1,933.50
	Letters of credit / guarantees(1)		827.2			1,189.20
	Total restricted assets		$	3,721.40		$	3,721.40
	Total as percent of cash and invested assets		46		%	45.6		%
	(1)	As of September 30, 2013, the Company had pledged funds of $801.1 million and £16.1 million (December 31, 2012 — $1,163.2 million and £16.0 million) as collateral for the secured letters of credit.
Amounts Outstanding under Operating Leases	'
	Amounts outstanding under operating leases net of subleases as of September 30, 2013 were:
			2013			2014			2015		2016		2017		Later		Total
															Years
			($ in millions)
	Operating Lease Obligations		$	5.5		$	12.2		$	11.6	$	8.5	$	6.9	$	11.7	$	56.4

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, 2013	Sep. 30, 2013
Available for sale securities:	'	'
Realized gain on sale of securities	$8.60	$21.20
Realized (loss) on sale of securities	-0.3	-0.5
Net realized and unrealized investment gains reclassified to the statement of operations, gross of tax	8.3	20.7
Tax on realized gains and (losses) of securities	-0.1	-0.5
Net realized and unrealized investment gains reclassified to the statement of operations, net of tax	8.2	20.2
Foreign currency translation adjustments:	'	'
Realized translation adjustments, before tax	4.8	0.5
Tax credit on realized translation adjustments	-1.1	-0.1
Net realized and unrealized foreign exchange investment gains reclassified to the statement of operations, net of tax	3.7	0.4
Amortization of derivatives:	'	'
Amortization of long-term debt associated expenses, before tax	0	-0.2
Amortization of long-term debt associated expenses, net of tax	0	-0.2
Total reclassifications from AOCI to the statement of operations, net of tax	$11.90	$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, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Earnings Per Share [Abstract]	'		'		'		'
Net income	$107.40		$115.10		$239.30		$278.40
PIERS redemption	0	[1]	'	[1]	-7.1	[1]	0	[1]
Preference share dividends	-9.5		-8.6		-26.1		-22.6
Basic and diluted net income available to ordinary shareholders	$97.90		$106.50		$206.10		$255.80
Ordinary shares:	'		'		'		'
Basic weighted average ordinary shares	66,716,202		71,129,102		67,302,857		71,125,664
Weighted average effect of dilutive securities	1,845,313	[2]	2,268,694	[2]	2,656,617	[2]	2,577,374	[2]
Total diluted weighted average ordinary shares	68,561,515		73,397,796		69,959,474		73,703,038
Earnings per ordinary share:	'		'		'		'
Basic	$1.47		$1.50		$3.06		$3.60
Diluted	$1.43		$1.45		$2.95		$3.47
[1]	The $7.1 million deduction to net income is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million.
[2]	Dilutive securities comprise: investor options, employee options, performance shares associated with the Company’s long term incentive program, employee share purchase plans and restricted stock units as described in Note 13, in addition to the PIERS.

Earnings_per_Ordinary_Share_Co1

Earnings per Ordinary Share - Computation of Basic and Diluted Earnings per Share (Parenthetical) (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Earnings Per Share [Line Items]	'		'		'		'
PIERS redemption	$0	[1]	'	[1]	$7.10	[1]	$0	[1]
Reclassification from Additional Paid in Capital to Retained Earnings [Member]	'		'		'		'
Earnings Per Share [Line Items]	'		'		'		'
PIERS redemption	'		'		7.1		'
Preference Shares [Member]	'		'		'		'
Earnings Per Share [Line Items]	'		'		'		'
PIERS redeemed and cancelled	'		'		$230		'
[1]	The $7.1 million deduction to net income is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million.

Earnings_per_Ordinary_Share_Su

Earnings per Ordinary Share - Summary of Declared Dividends (Detail) (USD $)	3 Months Ended
	Sep. 30, 2013
Ordinary Shares [Member]	'
Dividends Payable [Line Items]	'
Dividend	$0.18
Payable on	27-Nov-13
Record Date	13-Nov-13
7.401% Preference Shares [Member]	'
Dividends Payable [Line Items]	'
Dividend	$0.46
Payable on	1-Jan-14
Record Date	15-Dec-13
7.250% Preference Shares [Member]	'
Dividends Payable [Line Items]	'
Dividend	$0.45
Payable on	1-Jan-14
Record Date	15-Dec-13
5.950% Preference Shares [Member]	'
Dividends Payable [Line Items]	'
Dividend	$0.37
Payable on	1-Jan-14
Record Date	15-Dec-13

Earnings_per_Ordinary_Share_Su1

Earnings per Ordinary Share - Summary of Declared Dividends (Parenthetical) (Detail)	3 Months Ended
	Sep. 30, 2013
7.401% Preference Shares [Member]	'
Dividends Payable [Line Items]	'
Preference shares, rate	7.40%
7.250% Preference Shares [Member]	'
Dividends Payable [Line Items]	'
Preference shares, rate	7.25%
5.950% Preference Shares [Member]	'
Dividends Payable [Line Items]	'
Preference shares, rate	5.95%

Segment_Reporting_Additional_I

Segment Reporting - Additional Information (Detail)	Sep. 30, 2013
	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, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Underwriting Revenues	'		'		'		'
Gross written premiums	$581.60		$558.40		$2,042.30		$2,007.10
Net written premiums	542		507.1		1,751.70		1,722.50
Gross earned premiums	625.1		601.8		1,829.60		1,739.60
Net earned premiums	544.3		516.2		1,599.20		1,525
Underwriting Expenses	'		'		'		'
Losses and loss adjustment expenses	290.2		255		892.3		801.1
Amortization of deferred policy acquisition costs	110.5		103.1		322.3		301.2
General and administrative expenses	84.1		76.4		231.2		218.9
Underwriting income	59.5		81.7		153.4		203.8
Corporate expenses	-14.8		-14.3		-42		-40.1
Net investment income	45		48.6		139.2		153.8
Realized and unrealized investment gains	23.6		13.2		54.3		29.3
Realized and unrealized investment (losses)	-5.9		-2.4		-28		-8.2
Change in fair value of derivatives	6.6		-4.9		-0.5		-24
Interest expense on long term debt	-7.7		-7.8		-23.2		-23.2
Net realized and unrealized foreign exchange (losses)	2.4		4.5		-7.1		-0.5
Other income	1.6		4.8		3.6		9
Other (expenses)	'		-0.3		-0.6		-1.9
Income from operations before income tax	110.3		123.1		249.1		298
Net reserves for loss and loss adjustment expenses	4,273.40		4,178		4,273.40		4,178
Ratios	'		'		'		'
Loss ratio	53.30%		49.40%		55.80%		52.50%
Policy acquisition expense ratio	20.30%		20.00%		20.20%		19.80%
General and administrative expense ratio	18.20%	[1]	17.60%	[1]	17.10%	[1]	17.00%	[1]
Expense ratio	38.50%		37.60%		37.30%		36.80%
Combined ratio	91.80%		87.00%		93.10%		89.30%
Reinsurance [Member]	'		'		'		'
Underwriting Revenues	'		'		'		'
Gross written premiums	219.5		259.5		957.7		1,033.50
Net written premiums	218.4		256.9		907.5		963.2
Gross earned premiums	268.6		299.8		828.9		890.8
Net earned premiums	255.7		279.6		788.2		832.6
Underwriting Expenses	'		'		'		'
Losses and loss adjustment expenses	122.2		117.1		394.9		386.4
Amortization of deferred policy acquisition costs	49.1		55.7		161		166.8
General and administrative expenses	34.6		33.6		97.2		92.6
Underwriting income	49.8		73.2		135.1		186.8
Net reserves for loss and loss adjustment expenses	2,718		2,755.10		2,718		2,755.10
Ratios	'		'		'		'
Loss ratio	47.80%		41.90%		50.10%		46.40%
Policy acquisition expense ratio	19.20%		19.90%		20.40%		20.00%
General and administrative expense ratio	13.50%	[1]	12.00%	[1]	12.30%	[1]	11.10%	[1]
Expense ratio	32.70%		31.90%		32.70%		31.10%
Combined ratio	80.50%		73.80%		82.80%		77.50%
Insurance [Member]	'		'		'		'
Underwriting Revenues	'		'		'		'
Gross written premiums	362.1		298.9		1,084.60		973.6
Net written premiums	323.6		250.2		844.2		759.3
Gross earned premiums	356.5		302		1,000.70		848.8
Net earned premiums	288.6		236.6		811		692.4
Underwriting Expenses	'		'		'		'
Losses and loss adjustment expenses	168		137.9		497.4		414.7
Amortization of deferred policy acquisition costs	61.4		47.4		161.3		134.4
General and administrative expenses	49.5		42.8		134		126.3
Underwriting income	9.7		8.5		18.3		17
Net reserves for loss and loss adjustment expenses	$1,555.40		$1,422.90		$1,555.40		$1,422.90
Ratios	'		'		'		'
Loss ratio	58.20%		58.30%		61.30%		59.90%
Policy acquisition expense ratio	21.30%		20.00%		19.90%		19.40%
General and administrative expense ratio	17.20%	[1]	18.10%	[1]	16.50%	[1]	18.20%	[1]
Expense ratio	38.50%		38.10%		36.40%		37.60%
Combined ratio	96.70%		96.40%		97.70%		97.50%
[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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	$47.50	$51.50	$146.10	$160.70
Investment expenses	-2.5	-2.9	-6.9	-6.9
Net investment income	45	48.6	139.2	153.8
Total Fixed Income Maturities - Available For Sale [Member]	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	38.2	43.7	117.2	136.9
Fixed Income Maturities - Trading [Member]	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	5.3	4	13.4	12.2
Short-term Investments - Available for Sale, at Fair Value [Member]	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	0.3	0.5	1.9	1.8
Fixed Term Deposits (Included in Cash and Cash Equivalents) [Member]	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	0.7	1.4	3.4	4.7
Equity Securities - Available for Sale [Member]	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	1.2	1.9	4.7	5.1
Equity Securities - Trading [Member]	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'
Total	$1.80	$0	$5.50	$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		12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'	'
Fixed income maturities - gross realized gains	$8.60	'	$21.20	'	'
Fixed income maturities - gross realized (losses)	-0.3	'	-0.5	'	'
Net change in gross unrealized gains	9	6.7	-2.7	12.7	'
Total other-than-temporary impairments	0	-2.1	0	-3	'
Income (Loss) from Equity Method Investments	'	'	1.1	'	3.2
Total net realized and unrealized investment gains recorded in the statement of operations	17.7	10.8	26.3	21.1	'
Change in available for sale net unrealized gains:	'	'	'	'	'
Fixed income maturities	-3.3	25.5	-168.7	40.9	'
Available For Sale Securities Short Term Investments Change In Unrealized Gains	0	0.2	0	0.2	'
Equity securities	-0.2	6.5	9.4	16.1	'
Total change in pre-tax available for sale unrealized gains	-3.5	32.2	-159.3	57.2	'
Change in taxes	-0.3	-1.1	11.8	-0.9	'
Total change in net unrealized gains, net of taxes recorded in other comprehensive income	-3.8	31.1	-147.5	56.3	'
Total Fixed Income Maturities - Available For Sale [Member]	'	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'	'
Fixed income maturities - gross realized gains	6.9	1.5	15.7	5.3	'
Fixed income maturities - gross realized (losses)	-5.6	0	-6.8	-0.4	'
Available for Sale Equity Securities [Member]	'	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'	'
Equity securities - gross realized gains	6.3	1	13.8	2.9	'
Equity securities - gross realized (losses)	0	-0.1	0	-4.5	'
Fixed Income Maturities - Trading [Member]	'	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'	'
Fixed income maturities - gross realized gains	0.8	2.3	6.6	6.7	'
Fixed income maturities - gross realized (losses)	-1.3	-0.1	-1.9	-0.3	'
Equity Securities - Trading [Member]	'	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'	'
Equity securities - gross realized gains	0.4	0	0.8	0	'
Equity securities - gross realized (losses)	0	0	-0.3	0	'
Cartesian Iris Offshore Fund Limited Partner [Member]	'	'	'	'	'
Schedule of Investment Income, Reported Amounts, by Category [Line Items]	'	'	'	'	'
Income (Loss) from Equity Method Investments	$1.20	$1.70	$1.10	$1.70	'

Investments_Additional_Informa

Investments - Additional Information (Detail) (USD $)

3 Months Ended

9 Months Ended

12 Months Ended

3 Months Ended

9 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Oct. 02, 2012

Sep. 30, 2013

Aug. 31, 2013

Sep. 30, 2013

Insurers

Insurers

Cartesian Iris Offshore Fund Limited Partner [Member]

Cartesian Iris Offshore Fund Limited Partner [Member]

Cartesian Iris Offshore Fund Limited Partner [Member]

Cartesian Iris Offshore Fund Limited Partner [Member]

Chaspark Maritime Holdings Ltd. [Member]

BB High Yield Bonds [Member]

Corporate and Foreign Government Debt Securities [Member]

Bank Loans [Member]

Investment Securities [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

Other-than-temporary impairment charge

$0

($2.10)

$0

($3)

'

'

'

'

'

'

'

'

'

Investment funded

2.3

'

2.3

'

1.5

'

'

'

'

'

25.7

'

'

Trading investments in fixed income maturities, Fair Market Value

718.6

'

718.6

'

456.1

'

'

'

'

'

'

200

72.8

Income (loss) from Cartesian iris

'

'

$1.10

'

$3.20

$1.20

$1.70

$1.10

$1.70

'

'

'

'

Percentage of ownership acquired

'

'

'

'

'

'

'

'

'

58.50%

'

'

'

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, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	$5,667.20	$5,834
Available for sale investments in fixed income maturities, Gross Unrealized Gains	225.8	358.2
Available for sale investments in fixed income maturities, Gross Unrealized Losses	-36.6	-3.3
Total, Available for sale investments in fixed income maturities, Fair Market Value	5,856.40	6,188.90
Total Fixed Income Maturities - Available for Sale [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,388.20	5,228.50
Available for sale investments in fixed income maturities, Gross Unrealized Gains	190.5	330
Available for sale investments in fixed income maturities, Gross Unrealized Losses	-33.8	-1.2
Total, Available for sale investments in fixed income maturities, Fair Market Value	5,544.90	5,557.30
Total Short-Term Investments Available for Sale [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	150.3	431.5
Available for sale investments in fixed income maturities, Gross Unrealized Gains	0	0
Available for sale investments in fixed income maturities, Gross Unrealized Losses	-2	0
Total, Available for sale investments in fixed income maturities, Fair Market Value	148.3	431.5
Total Equity Securities - Available for Sale [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	128.7	174
Available for sale investments in fixed income maturities, Gross Unrealized Gains	35.3	28.2
Available for sale investments in fixed income maturities, Gross Unrealized Losses	-0.8	-2.1
Total, Available for sale investments in fixed income maturities, Fair Market Value	163.2	200.1
U.S. Government [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	995.3	1,071.80
Available for sale investments in fixed income maturities, Gross Unrealized Gains	28.4	54.8
Available for sale investments in fixed income maturities, Gross Unrealized Losses	-2.6	-0.3
Total, Available for sale investments in fixed income maturities, Fair Market Value	1,021.10	1,126.30
U.S. Agency [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	269.2	288.3
Available for sale investments in fixed income maturities, Gross Unrealized Gains	13	20.3
Available for sale investments in fixed income maturities, Gross Unrealized Losses	-0.7	0
Total, Available for sale investments in fixed income maturities, Fair Market Value	281.5	308.6
Municipal [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	29.1	37.2
Available for sale investments in fixed income maturities, Gross Unrealized Gains	1.1	2.6
Available for sale investments in fixed income maturities, Gross Unrealized Losses	-0.3	-0.1
Total, Available for sale investments in fixed income maturities, Fair Market Value	29.9	39.7
Corporate [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	1,942.60	1,889.20
Available for sale investments in fixed income maturities, Gross Unrealized Gains	92.2	149.9
Available for sale investments in fixed income maturities, Gross Unrealized Losses	-16.7	-0.6
Total, Available for sale investments in fixed income maturities, Fair Market Value	2,018.10	2,038.50
Non-U.S. Government-Backed Corporate [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	77.5	98
Available for sale investments in fixed income maturities, Gross Unrealized Gains	1.4	3.1
Available for sale investments in fixed income maturities, Gross Unrealized Losses	0	0
Total, Available for sale investments in fixed income maturities, Fair Market Value	78.9	101.1
Foreign Government [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	815.5	617
Available for sale investments in fixed income maturities, Gross Unrealized Gains	9.9	24.1
Available for sale investments in fixed income maturities, Gross Unrealized Losses	-3.2	-0.1
Total, Available for sale investments in fixed income maturities, Fair Market Value	822.2	641
Asset-Backed [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	113.7	49.2
Available for sale investments in fixed income maturities, Gross Unrealized Gains	3.1	4.6
Available for sale investments in fixed income maturities, Gross Unrealized Losses	-0.3	0
Total, Available for sale investments in fixed income maturities, Fair Market Value	116.5	53.8
Non-Agency Commercial Mortgage-Backed [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	58.1	61.7
Available for sale investments in fixed income maturities, Gross Unrealized Gains	6.3	9.4
Available for sale investments in fixed income maturities, Gross Unrealized Losses	0	0
Total, Available for sale investments in fixed income maturities, Fair Market Value	64.4	71.1
Agency Mortgage-Backed [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	1,087.20	1,116.10
Available for sale investments in fixed income maturities, Gross Unrealized Gains	35.1	61.2
Available for sale investments in fixed income maturities, Gross Unrealized Losses	-10	-0.1
Total, Available for sale investments in fixed income maturities, Fair Market Value	$1,112.30	$1,177.20

Investments_Cost_Gross_Unreali1

Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Detail) (USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Fixed income maturities, trading at fair value, cost or Amortized Cost	$713.10	$431.60
Total Cost or Amortized Cost	973.7	434
Trading investments in fixed income maturities, Fair Market Value	718.6	456.1
Short-term investments, trading at fair value, Cost or Amortized Cost	5,667.20	5,834
Gross Unrealized Gains	34	24.9
Gross Unrealized Losses	-10.6	-0.4
Total trading short-term investments, Fair Market Value	5,856.40	6,188.90
Total, trading investments in fixed income maturities, Fair Market Value	997.1	458.5
U.S. Government [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Fixed income maturities, trading at fair value, cost or Amortized Cost	32	9.3
Trading investments in fixed income maturities, Gross Unrealized Gains	0	0.2
Trading investments in fixed income maturities, Gross Unrealized Losses	-0.9	-0.1
Trading investments in fixed income maturities, Fair Market Value	31.1	9.4
U.S. Agency [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Fixed income maturities, trading at fair value, cost or Amortized Cost	0.2	0.2
Trading investments in fixed income maturities, Gross Unrealized Gains	0	0
Trading investments in fixed income maturities, Gross Unrealized Losses	0	0
Trading investments in fixed income maturities, Fair Market Value	0.2	0.2
Municipal [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Fixed income maturities, trading at fair value, cost or Amortized Cost	0.6	2.8
Trading investments in fixed income maturities, Gross Unrealized Gains	0	0.1
Trading investments in fixed income maturities, Gross Unrealized Losses	0	0
Trading investments in fixed income maturities, Fair Market Value	0.6	2.9
Corporate [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Fixed income maturities, trading at fair value, cost or Amortized Cost	464.9	392
Trading investments in fixed income maturities, Gross Unrealized Gains	12.1	22.7
Trading investments in fixed income maturities, Gross Unrealized Losses	-6	-0.3
Trading investments in fixed income maturities, Fair Market Value	471	414.4
Foreign Government [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Fixed income maturities, trading at fair value, cost or Amortized Cost	135.9	24.4
Trading investments in fixed income maturities, Gross Unrealized Gains	1.3	1.9
Trading investments in fixed income maturities, Gross Unrealized Losses	-0.9	0
Trading investments in fixed income maturities, Fair Market Value	136.3	26.3
Asset-Backed Securities [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Fixed income maturities, trading at fair value, cost or Amortized Cost	6.6	2.9
Trading investments in fixed income maturities, Gross Unrealized Gains	0	0
Trading investments in fixed income maturities, Gross Unrealized Losses	0	0
Trading investments in fixed income maturities, Fair Market Value	6.6	2.9
Bank Loans [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Fixed income maturities, trading at fair value, cost or Amortized Cost	72.9	'
Trading investments in fixed income maturities, Gross Unrealized Gains	0.1	'
Trading investments in fixed income maturities, Gross Unrealized Losses	-0.2	'
Trading investments in fixed income maturities, Fair Market Value	72.8	'
Equity Securities - Trading [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Equity securities, trading at fair value, cost or Amortized cost	260.4	'
Gross Unrealized Gains	20.5	'
Gross Unrealized Losses	-2.6	'
Total equity securities - Trading, Fair Market Value	278.3	'
Short-Term Investments - Trading [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Short-term investments, trading at fair value, Cost or Amortized Cost	0.2	2.4
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Total trading short-term investments, Fair Market Value	0.2	2.4
Fixed Income Maturities - Trading [Member]	'	'
Schedule of Trading Securities and Other Trading Items [Line Items]	'	'
Fixed income maturities, trading at fair value, cost or Amortized Cost	713.1	431.6
Trading investments in fixed income maturities, Gross Unrealized Gains	13.5	24.9
Trading investments in fixed income maturities, Gross Unrealized Losses	-8	-0.4
Trading investments in fixed income maturities, Fair Market Value	$718.60	$456.10

Investments_Other_Investments_

Investments - Other Investments (Detail) (USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule Of Equity Method Investments Joint Ventures [Line Items]	'	'
Opening undistributed value of investment, balance	$45	$33.10
Investment in Chaspark Maritime Holdings Ltd.	'	8.7
Income (Loss) from Equity Method Investments	1.1	3.2
Closing value of investment, balance	46.1	45
Cartesian Iris Offshore Fund L.P. [Member]	'	'
Schedule Of Equity Method Investments Joint Ventures [Line Items]	'	'
Opening undistributed value of investment, balance	36.3	33.1
Investment in Chaspark Maritime Holdings Ltd.	'	0
Income (Loss) from Equity Method Investments	1.1	3.2
Closing value of investment, balance	37.4	36.3
Chaspark Maritime Holdings Ltd. [Member]	'	'
Schedule Of Equity Method Investments Joint Ventures [Line Items]	'	'
Opening undistributed value of investment, balance	8.7	0
Investment in Chaspark Maritime Holdings Ltd.	'	8.7
Income (Loss) from Equity Method Investments	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, 2013	Dec. 31, 2012
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	$5,667.20	$5,834
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	5,388.20	5,228.50
Available for sale investments in fixed income maturities, Fair Market Value	5,544.90	5,557.30
Non-Agency Commercial Mortgage-Backed [Member]	'	'
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	58.1	61.7
Available for sale investments in fixed income maturities, Fair Market Value	64.4	71.1
Available for sale investments Average Ratings by Maturity	'AA+	'AA+
Agency Mortgage-Backed [Member]	'	'
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]	'	'
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	1,087.20	1,116.10
Available for sale investments in fixed income maturities, Fair Market Value	1,112.30	1,177.20
Available for sale investments Average Ratings by Maturity	'AA+	'AA+
Other Asset-Backed [Member]	'	'
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	113.7	49.2
Available for sale investments in fixed income maturities, Fair Market Value	116.5	53.8
Available for sale investments Average Ratings by Maturity	'AAA	'AAA
Total Fixed Income Maturities - Available for Sale [Member]	'	'
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	5,388.20	5,228.50
Available for sale investments in fixed income maturities, Fair Market Value	5,544.90	5,557.30
Other Fixed Income Securities [Member]	'	'
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]	'	'
Due one year or less, Cost or Amortized Cost	649.6	554.6
Due after one year through five years, Cost or Amortized Cost	2,385.80	2,270
Due after five years through ten years, Cost or Amortized Cost	992.5	1,077.80
Due after ten years, Cost or Amortized Cost	101.3	99.1
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	4,129.20	4,001.50
Due one year or less, Fair Market Value	652.5	561
Due after one year through five years, Fair Market Value	2,461.20	2,378.80
Due after five years through ten years, Fair Market Value	1,033.50	1,199.30
Due after ten years, Fair Market Value	104.5	116.1
Available for sale investments in fixed income maturities, Fair Market Value	$4,251.70	$4,255.20
Due one year or less, Average Ratings by Maturity	'AA	'AA
Due after one year through five years, Average Ratings by Maturity	'AA	'AA
Due after five years through ten years, Average Ratings by Maturity	'A+	'AA-
Due after ten years, Average Ratings by Maturity	'AA-	'AA+

Investments_Aggregate_Fair_Val

Investments - Aggregate Fair Value and Gross Unrealized Loss by Type of Security (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Security	Security
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	$1,651.40	$408.10
0-12 months, Gross Unrealized Loss	-35.2	-3.1
Over 12 months, Fair Market Value	21.4	8.2
Over 12 months, Gross Unrealized Loss	-1.4	-0.2
Total, Fair Market Value	1,672.80	416.3
Total, Gross Unrealized Loss	-36.6	-3.3
Number of Securities	561	133
Total Fixed Income Maturities - Available For Sale [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	1,543.90	370.1
0-12 months, Gross Unrealized Loss	-32.7	-1.2
Over 12 months, Fair Market Value	17.6	6
Over 12 months, Gross Unrealized Loss	-1.1	0
Total, Fair Market Value	1,561.50	376.1
Total, Gross Unrealized Loss	-33.8	-1.2
Number of Securities	553	111
Short-term Investments - Available for Sale, at Fair Value [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	99	9.4
0-12 months, Gross Unrealized Loss	-2	0
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	99	9.4
Total, Gross Unrealized Loss	-2	0
Number of Securities	1	4
Equity Securities - Available for Sale, at Fair Value [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	8.5	28.6
0-12 months, Gross Unrealized Loss	-0.5	-1.9
Over 12 months, Fair Market Value	3.8	2.2
Over 12 months, Gross Unrealized Loss	-0.3	-0.2
Total, Fair Market Value	12.3	30.8
Total, Gross Unrealized Loss	-0.8	-2.1
Number of Securities	7	18
U.S. Government [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	193.3	72.7
0-12 months, Gross Unrealized Loss	-2.6	-0.3
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	193.3	72.7
Total, Gross Unrealized Loss	-2.6	-0.3
Number of Securities	38	9
Municipal [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	2.5	1.4
0-12 months, Gross Unrealized Loss	-0.1	-0.1
Over 12 months, Fair Market Value	1.3	0
Over 12 months, Gross Unrealized Loss	-0.2	0
Total, Fair Market Value	3.8	1.4
Total, Gross Unrealized Loss	-0.3	-0.1
Number of Securities	3	1
Corporate [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	611.6	170.8
0-12 months, Gross Unrealized Loss	-15.9	-0.6
Over 12 months, Fair Market Value	7.5	0
Over 12 months, Gross Unrealized Loss	-0.8	0
Total, Fair Market Value	619.1	170.8
Total, Gross Unrealized Loss	-16.7	-0.6
Number of Securities	331	63
Non-U.S. Government-Backed Corporate [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	6.9	10.1
0-12 months, Gross Unrealized Loss	0	0
Over 12 months, Fair Market Value	4.8	2
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	11.7	12.1
Total, Gross Unrealized Loss	0	0
Number of Securities	3	3
Foreign Government [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	234.5	87.7
0-12 months, Gross Unrealized Loss	-3.1	-0.1
Over 12 months, Fair Market Value	4	4
Over 12 months, Gross Unrealized Loss	-0.1	0
Total, Fair Market Value	238.5	91.7
Total, Gross Unrealized Loss	-3.2	-0.1
Number of Securities	40	20
Asset-Backed [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	43.8	0.7
0-12 months, Gross Unrealized Loss	-0.3	0
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	43.8	0.7
Total, Gross Unrealized Loss	-0.3	0
Number of Securities	39	1
Non-Agency Commercial Mortgage-Backed [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	0	'
0-12 months, Gross Unrealized Loss	0	'
Over 12 months, Fair Market Value	0	'
Over 12 months, Gross Unrealized Loss	0	'
Total, Fair Market Value	0	'
Total, Gross Unrealized Loss	0	'
Number of Securities	0	'
Agency Mortgage-Backed [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	384.2	26.7
0-12 months, Gross Unrealized Loss	-10	-0.1
Over 12 months, Fair Market Value	0	0
Over 12 months, Gross Unrealized Loss	0	0
Total, Fair Market Value	384.2	26.7
Total, Gross Unrealized Loss	-10	-0.1
Number of Securities	85	14
U.S. Agency [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
0-12 months, Fair Market Value	67.1	'
0-12 months, Gross Unrealized Loss	-0.7	'
Over 12 months, Fair Market Value	0	'
Over 12 months, Gross Unrealized Loss	0	'
Total, Fair Market Value	67.1	'
Total, Gross Unrealized Loss	($0.70)	'
Number of Securities	14	'

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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Investments Schedule [Abstract]	'	'	'	'
(Purchases) of fixed income maturities - Available for sale	($566.80)	($351.80)	($1,748.60)	($1,057.10)
(Purchases) of fixed income maturities - Trading	-283.2	-73	-555.5	-187.7
(Purchases) of equity securities - Available for sale	0	-12	-2.5	-41.5
(Purchases) of equity securities - Trading	-38.6	0	-275.4	0
Proceeds from sales and maturities of fixed income maturities - Available for sale	578.6	240.4	1,564.40	989.7
Proceeds from sales and maturities of fixed income maturities - Trading	67.8	49.8	276.3	164.5
Proceeds from sales of equity securities - Available for sale	29.7	10.1	61.8	35.9
Proceeds from sales of equity securities - Trading	5	0	15.1	0
Net change in (payable)/receivable for securities (purchased) /sold	6.6	-22.8	5.6	19.3
Net (purchases)/sales of short-term investments	95.5	13.2	275.7	-190.4
Net (purchases)/sales for the period	($105.40)	($146.10)	($383.10)	($267.30)

Fair_Value_Measurements_Financ

Fair Value Measurements - Financial Assets Measured on Recurring Basis (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Derivatives at fair value	$5.60	$2
Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total	6,856.80	6,642
Recurring [Member] | Foreign Exchange Contracts [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Derivatives at fair value	'	2
Recurring [Member] | Short-Term Investments - Trading, at Fair Value [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	0.2	2.4
Recurring [Member] | Liabilities Under Derivative Contracts [Member] | Foreign Exchange Contracts [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Liabilities under derivative contracts, fair value	-0.9	-6.1
Recurring [Member] | Liabilities Under Derivative Contracts [Member] | Interest Rate Swaps [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Liabilities under derivative contracts, fair value	-1.4	-1.3
Recurring [Member] | Equity Investments - Trading, at Fair Value [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	278.3	'
Recurring [Member] | Derivatives at Fair Value [Member] | Foreign Exchange Contracts [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Derivatives at fair value	5.6	'
Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | U.S. Government [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	1,021.10	1,126.30
Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | U.S. Agency [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	281.5	308.6
Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Municipal [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	29.9	39.7
Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Corporate [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	2,018.10	2,038.50
Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Non-U.S. Government-Backed Corporate [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	78.9	101.1
Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Foreign Government [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	822.2	641
Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Asset-Backed [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	116.5	53.8
Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Non-Agency Commercial Mortgage-Backed [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	64.4	71.1
Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Agency Mortgage-Backed [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	1,112.30	1,177.20
Total Fixed Income Maturities - Available For Sale [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	5,544.90	5,557.30
Short-term Investments - Available for Sale, at Fair Value [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	148.3	431.5
Equity Securities - Available for Sale, at Fair Value [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	163.2	200.1
Fixed Income Maturities - Trading [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	718.6	456.1
Fixed Income Maturities - Trading [Member] | Recurring [Member] | U.S. Government [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	31.1	9.4
Fixed Income Maturities - Trading [Member] | Recurring [Member] | U.S. Agency [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	0.2	0.2
Fixed Income Maturities - Trading [Member] | Recurring [Member] | Municipal [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	0.6	2.9
Fixed Income Maturities - Trading [Member] | Recurring [Member] | Corporate [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	471	414.4
Fixed Income Maturities - Trading [Member] | Recurring [Member] | Foreign Government [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	136.3	26.3
Fixed Income Maturities - Trading [Member] | Recurring [Member] | Asset-Backed [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	6.6	2.9
Fixed Income Maturities - Trading [Member] | Recurring [Member] | Bank Loans [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	72.8	'
Level 1 [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total	2,267.20	2,255.80
Level 1 [Member] | Recurring [Member] | Short-Term Investments - Trading, at Fair Value [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	0	2.4
Level 1 [Member] | Recurring [Member] | Equity Investments - Trading, at Fair Value [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	278.3	'
Level 1 [Member] | Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | U.S. Government [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	1,021.10	1,126.30
Level 1 [Member] | Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Foreign Government [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	612.6	487.2
Level 1 [Member] | Total Fixed Income Maturities - Available For Sale [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	1,633.70	1,613.50
Level 1 [Member] | Short-term Investments - Available for Sale, at Fair Value [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	118	426.2
Level 1 [Member] | Equity Securities - Available for Sale, at Fair Value [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	163.2	200.1
Level 1 [Member] | Fixed Income Maturities - Trading [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	74	13.6
Level 1 [Member] | Fixed Income Maturities - Trading [Member] | Recurring [Member] | U.S. Government [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	31.1	9.4
Level 1 [Member] | Fixed Income Maturities - Trading [Member] | Recurring [Member] | Foreign Government [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	42.9	4.2
Level 2 [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total	4,589.60	4,386.20
Level 2 [Member] | Recurring [Member] | Foreign Exchange Contracts [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Derivatives at fair value	'	2
Level 2 [Member] | Recurring [Member] | Short-Term Investments - Trading, at Fair Value [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	0.2	'
Level 2 [Member] | Recurring [Member] | Liabilities Under Derivative Contracts [Member] | Foreign Exchange Contracts [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Liabilities under derivative contracts, fair value	-0.9	-6.1
Level 2 [Member] | Recurring [Member] | Liabilities Under Derivative Contracts [Member] | Interest Rate Swaps [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Liabilities under derivative contracts, fair value	-1.4	-1.3
Level 2 [Member] | Recurring [Member] | Derivatives at Fair Value [Member] | Foreign Exchange Contracts [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Derivatives at fair value	5.6	'
Level 2 [Member] | Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | U.S. Agency [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	281.5	308.6
Level 2 [Member] | Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Municipal [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	29.9	39.7
Level 2 [Member] | Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Corporate [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	2,018.10	2,038.50
Level 2 [Member] | Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Non-U.S. Government-Backed Corporate [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	78.9	101.1
Level 2 [Member] | Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Foreign Government [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	209.6	153.8
Level 2 [Member] | Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Asset-Backed [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	116.5	53.8
Level 2 [Member] | Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Non-Agency Commercial Mortgage-Backed [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	64.4	71.1
Level 2 [Member] | Fixed Income Maturities - Available for Sale [Member] | Recurring [Member] | Agency Mortgage-Backed [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	1,112.30	1,177.20
Level 2 [Member] | Total Fixed Income Maturities - Available For Sale [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	3,911.20	3,943.80
Level 2 [Member] | Short-term Investments - Available for Sale, at Fair Value [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	30.3	5.3
Level 2 [Member] | Fixed Income Maturities - Trading [Member] | Recurring [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	644.6	442.5
Level 2 [Member] | Fixed Income Maturities - Trading [Member] | Recurring [Member] | U.S. Agency [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	0.2	0.2
Level 2 [Member] | Fixed Income Maturities - Trading [Member] | Recurring [Member] | Municipal [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	0.6	2.9
Level 2 [Member] | Fixed Income Maturities - Trading [Member] | Recurring [Member] | Corporate [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	471	414.4
Level 2 [Member] | Fixed Income Maturities - Trading [Member] | Recurring [Member] | Foreign Government [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	93.4	22.1
Level 2 [Member] | Fixed Income Maturities - Trading [Member] | Recurring [Member] | Asset-Backed [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	6.6	2.9
Level 2 [Member] | Fixed Income Maturities - Trading [Member] | Recurring [Member] | Bank Loans [Member]	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Investments, fair value	$72.80	'

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Detail)	Sep. 30, 2013	Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract]	'	'
Quotes per fixed income investment	2.6	2.9
Quotes per equity investment	4.9	4.9

Fair_Value_Measurements_Pricin

Fair Value Measurements - Pricing Sources Used in Pricing Fixed Income Investments (Detail)	Sep. 30, 2013	Dec. 31, 2012
Fair Value Disclosures [Abstract]	'	'
Index providers	87.00%	89.00%
Pricing services	10.00%	9.00%
Broker-dealers	3.00%	2.00%
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, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	$5,422.10	$5,360.90
% of Total Fair Value by Security Type	87.00%	89.00%
Fixed Income Maturities - Available for Sale [Member] | U.S. Government [Member]	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	1,013.90	1,034.50
% of Total Fair Value by Security Type	96.00%	91.00%
Fixed Income Maturities - Available for Sale [Member] | U.S. Agency [Member]	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	267.7	291.9
% of Total Fair Value by Security Type	95.00%	95.00%
Fixed Income Maturities - Available for Sale [Member] | Municipal [Member]	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	13.3	20.5
% of Total Fair Value by Security Type	43.00%	48.00%
Fixed Income Maturities - Available for Sale [Member] | Corporate [Member]	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	2,358.30	2,339.80
% of Total Fair Value by Security Type	95.00%	95.00%
Fixed Income Maturities - Available for Sale [Member] | Non-U.S. Government-Backed Corporate [Member]	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	66.4	84.5
% of Total Fair Value by Security Type	0.00%	84.00%
Fixed Income Maturities - Available for Sale [Member] | Foreign Government [Member]	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	685.5	516.9
% of Total Fair Value by Security Type	72.00%	77.00%
Fixed Income Maturities - Available for Sale [Member] | Asset-Backed [Member]	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	117.8	44.6
% of Total Fair Value by Security Type	96.00%	79.00%
Fixed Income Maturities - Available for Sale [Member] | Non-Agency Commercial Mortgage-Backed [Member]	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	62.7	70.4
% of Total Fair Value by Security Type	97.00%	99.00%
Fixed Income Maturities - Available for Sale [Member] | Agency Mortgage-Backed [Member]	'	'
Fair Value [Line Items]	'	'
Fair Market Value Determined using Prices from Index Providers	$836.50	$957.80
% of Total Fair Value by Security Type	75.00%	81.00%

Reinsurance_Additional_Informa

Reinsurance - Additional Information (Detail)	Sep. 30, 2013	Dec. 31, 2012
Lloyd's [Member]	'	'
Reinsurance [Line Items]	'	'
Reinsurance recoverables	28.10%	20.20%
Munich Re [Member]	'	'
Reinsurance [Line Items]	'	'
Reinsurance recoverables	9.40%	10.50%

Derivative_Contracts_Fair_Valu

Derivative Contracts - Fair Value of Derivative Instruments (Detail) (Not Designated as Hedging Instrument [Member], USD $)	Sep. 30, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Interest Rate Swaps [Member] | Liabilities under Derivative Contracts [Member]	'		'
Derivatives, Fair Value [Line Items]	'		'
Notional Amount	$1,000		$1,000
Fair Value	-1.4	[1]	-1.3	[1]
Foreign Exchange Contracts [Member] | Liabilities under Derivative Contracts [Member]	'		'
Derivatives, Fair Value [Line Items]	'		'
Notional Amount	38.6		88.6
Fair Value	-0.9		-6.1
Foreign Exchange Contracts [Member] | Derivative at Fair Value [Member]	'		'
Derivatives, Fair Value [Line Items]	'		'
Notional Amount	275.9		335.4
Fair Value	$5.60		$2
[1]	Net of $34.1 million of cash collateral provided to counterparties as security for the Company’s net liability position (December 31, 2012 — $52.0 million).

Derivative_Contracts_Fair_Valu1

Derivative Contracts - Fair Value of Derivative Instruments (Parenthetical) (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Derivative Instruments and Hedging Activities Disclosure [Abstract]	'	'
Collateral Already Posted, Aggregate Fair Value	$34.10	$52

Derivative_Contracts_GainLoss_

Derivative Contracts - Gain/(Loss) Recognized in Income on Derivative (Detail) (Change in Fair Value of Derivatives [Member], USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Foreign Exchange Contracts [Member]	'	'	'	'
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'
Amount of Income/(Loss) Recognized in the Statement of Operations	$10.80	$3.20	($3.70)	($1.10)
Interest Rate Swaps [Member]	'	'	'	'
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'
Amount of Income/(Loss) Recognized in the Statement of Operations	($4.20)	($8.10)	$3.20	($22.90)

Derivative_Contracts_Additiona

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

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

Sep. 30, 2012

Sep. 30, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

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

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

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

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

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

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

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

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

Foreign Exchange Contracts [Member]

Foreign Exchange Contracts [Member]

Foreign Exchange Contracts [Member]

Foreign Exchange Contracts [Member]

Interest Rate Swaps [Member]

Interest Rate Swaps [Member]

Interest Rate Swaps [Member]

Interest Rate Swaps [Member]

Derivative Instruments, Gain (Loss) [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

Collateral Already Posted, Aggregate Fair Value

'

'

$34,100,000

$52,000,000

'

'

'

'

'

'

'

'

Purchase of US and foreign exchange

'

'

314,500,000

'

'

'

'

'

'

'

'

'

Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net

'

'

'

'

10,800,000

3,200,000

-3,700,000

-1,100,000

-4,200,000

-8,100,000

3,200,000

-22,900,000

Impact of foreign exchange contract on net income

3,200,000

-1,100,000

'

'

'

'

'

'

'

'

'

'

Notional amount of interest rate swaps

'

'

1,000,000,000

1,000,000,000

'

'

'

'

'

'

'

'

Gain (charge) in interest rate swaps

-8,100,000

-22,900,000

'

'

'

'

'

'

'

'

'

'

Pledged assets

'

'

$0

$0

'

'

'

'

'

'

'

'

Deferred_Policy_Acquisition_Co2

Deferred Policy Acquisition Costs (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]	'	'	'	'
Balance at the beginning of the period	$264.40	$233.20	$223	$184.50
Acquisition costs deferred	108.2	101.9	361.4	348.7
Amortization of deferred policy acquisition costs	-110.5	-103.1	-322.3	-301.2
Balance at the end of the period	$262.10	$232	$262.10	$232

Reserves_for_Losses_and_Adjust2

Reserves for Losses and 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, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Insurance [Abstract]	'	'	'	'
Provision for losses and LAE at the start of the year	$4,779.70	$4,525.20	$4,525.20	'
Less reinsurance recoverable	'	'	-499	-426.6
Net loss and LAE at the start of the year	4,280.70	4,098.60	4,098.60	'
Net loss and LAE expenses (disposed)	-34.5	'	-9	'
Provision for losses and LAE for claims incurred:	'	'	'	'
Current year	979.5	'	1,375.90	'
Prior years	-87.2	-95.4	-137.4	'
Total incurred	892.3	'	1,238.50	'
Losses and LAE payments for claims incurred:	'	'	'	'
Current year	-156.8	'	-244.3	'
Prior years	-671	'	-835.7	'
Total paid	-827.8	'	-1,080	'
Foreign exchange losses	-37.3	'	32.6	'
Net losses and LAE reserves at period end	4,273.40	'	4,280.70	'
Plus reinsurance recoverable on unpaid losses at period end	442.2	'	499	'
Provision for losses and LAE end of period	$4,715.60	'	$4,779.70	'

Reserves_for_Losses_and_Adjust3

Reserves for Losses and Adjustment Expenses - Additional Information (Detail) (USD $)	9 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Insurance [Abstract]	'	'	'
Reserve releases	($87.20)	($95.40)	($137.40)
Liability For Unpaid Claims And Claims Adjustment Expense Amount Disposed Of	-34.5	'	-9
Loss portfolio transfers (disposed)	'	$8.80	'

Capital_Structure_Summary_of_A

Capital Structure - Summary of Authorized and Issued Share Capital (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2006	Dec. 31, 2005
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	65,700,665	70,753,723	'	'
Issued ordinary shares of 0.15144558¢ per share	100	107	'	'
Total issued share capital	135	132	'	'
5.625% Preference Shares [Member]	'	'	'	'
Issued share capital:	'	'	'	'
Number of issued preference shares of 0.15144558¢ each with a liquidation preference	0	4,600,000	4,600,000	4,600,000
Issued preference shares of 0.15144558¢ each with a liquidation preference	'	7	'	'
7.401% Preference Shares [Member]	'	'	'	'
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 [Member]	'	'	'	'
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 [Member]	'	'	'	'
Issued share capital:	'	'	'	'
Number of issued preference shares of 0.15144558¢ each with a liquidation preference	11,000,000	0	'	'
Issued preference shares of 0.15144558¢ each with a liquidation preference	17	'	'	'

Capital_Structure_Summary_of_A1

Capital Structure - Summary of Authorized and Issued Share Capital (Parenthetical) (Detail) (USD $)	Sep. 30, 2013	Apr. 25, 2013	Dec. 31, 2012
Class of Stock [Line Items]	'	'	'
Ordinary shares, par value	$0.15	'	$0.15
Non-voting shares, par value	$0.15	'	$0.15
Preference shares, par value	$0.15	'	$0.15
Redemption price per share	$25	'	'
5.625% Preference Shares [Member]	'	'	'
Class of Stock [Line Items]	'	'	'
Preference shares, par value	$0.15	'	$0.15
Redemption price per share	'	$50	$50
Preference shares, rate	5.63%	'	5.63%
7.401% Preference Shares [Member]	'	'	'
Class of Stock [Line Items]	'	'	'
Preference shares, par value	$0.15	'	$0.15
Redemption price per share	$25	'	$25
Preference shares, rate	7.40%	'	7.40%
7.250% Preference Shares [Member]	'	'	'
Class of Stock [Line Items]	'	'	'
Preference shares, par value	$0.15	'	$0.15
Redemption price per share	$25	'	$25
Preference shares, rate	7.25%	'	7.25%
5.950% Preference Shares [Member]	'	'	'
Class of Stock [Line Items]	'	'	'
Preference shares, par value	$0.15	'	$0.15
Preference shares, rate	5.95%	'	0.00%

Capital_Structure_Additional_I

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

3 Months Ended

9 Months Ended

12 Months Ended

9 Months Ended

1 Months Ended

12 Months Ended

9 Months Ended

0 Months Ended

1 Months Ended

0 Months Ended

1 Months Ended

9 Months Ended

1 Months Ended

3 Months Ended

9 Months Ended

In Millions, except Share data, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2013

Apr. 25, 2013

Dec. 31, 2006

Dec. 31, 2005

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

2-May-13

2-May-13

Feb. 07, 2013

Feb. 07, 2013

Aug. 29, 2013

Mar. 31, 2013

Sep. 30, 2013

Mar. 21, 2013

Sep. 30, 2013

Sep. 30, 2013

Minimum [Member]

Reclassification from Additional Paid in Capital to Retained Earnings [Member]

5.625% Preference Shares [Member]

5.625% Preference Shares [Member]

5.625% Preference Shares [Member]

5.625% Preference Shares [Member]

5.625% Preference Shares [Member]

Preference Shares [Member]

Perpetual Non-Cumulative Preference Shares [Member]

Perpetual Non-Cumulative Preference Shares [Member]

Existing Share Repurchase Program [Member]

New Repurchase Program [Member]

Accelerated Share Repurchase Agreement [Member]

Accelerated Share Repurchase Agreement [Member]

Accelerated Share Repurchase Agreement [Member]

Names' Trustee [Member]

Open Markets [Member]

Open Markets [Member]

Class of Stock [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Aggregate liquidation preferences

$568.20

'

$568.20

'

$523.20

'

'

'

'

'

'

'

'

$270.40

$270.40

'

'

'

'

'

'

'

'

Issuance costs

'

'

12.6

'

15.1

'

'

'

'

'

'

'

'

4.6

'

'

'

'

'

'

'

'

'

Existing share repurchase authorization amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

400

500

'

'

'

'

'

'

Shares repurchase and cancelled during period, shares

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

4,053,276

3,348,214

'

'

1,498,451

3,979,826

Shares repurchase and cancelled during period, value

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

150

2

54.8

143

Average price

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$37.01

'

'

'

$36.59

$35.92

ASR, commenced date

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

27-Feb-13

'

'

'

ASR, termination date

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

3-Oct-13

'

'

'

Additional ordinary shares receivable

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

705,062

'

'

'

'

'

Shares repurchased during period, shares

'

'

8,087,539

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

54,437

'

'

Preference shares, issued

'

'

'

'

'

'

'

'

4,600,000

4,600,000

0

4,600,000

'

11,000,000

11,000,000

'

'

'

'

'

'

'

'

Minimum conversion price

'

'

'

'

'

130.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Conversion price, number of trading days in period

'

'

'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

'

'

'

$50

'

$25

$25

'

'

'

'

'

'

'

'

Shares issued in respect of the redemption of the PIERS

'

'

1,835,860

'

'

'

'

1,835,860

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

PIERS redemption

0

[1]

'

[1]

7.1

[1]

0

[1]

'

'

7.1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

PIERS redeemed and cancelled

'

'

'

'

'

'

'

'

'

'

'

'

230

'

'

'

'

'

'

'

'

'

'

Annual rate of dividend

'

'

'

'

'

'

'

'

5.63%

5.63%

'

'

'

'

5.95%

'

'

'

'

'

'

'

'

Preference shares

'

'

'

'

'

'

'

'

'

'

'

'

'

$275

$275

'

'

'

'

'

'

'

'

Preference shares dividend, floating rate

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'3-month LIBOR

'

'

'

'

'

'

'

'

Preference shares dividend, variable spread

'

'

'

'

'

'

'

'

'

'

'

'

'

'

4.06%

'

'

'

'

'

'

'

'

Preference shares, callable option

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'No stated maturity but are callable at the option of the Company on or after the 10th anniversary of the date of issuance.

'

'

'

'

'

'

'

'

Preference Shares, redemption date

'

'

1-Jul-23

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

[1]

The $7.1 million deduction to net income is attributable to the reclassification from additional paid-in capital to retained earnings representing the difference between the capital raised upon issuance of the PIERS, net of the original issuance costs, and the final redemption of the PIERS in the amount of $230.0 million.

Capital_Structure_Summary_of_O

Capital Structure - Summary of Ordinary Shares (Detail)	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Shares in issue at the beginning of the year	70,753,723
Shares issued to employees under the 2003 share incentive plan and/or 2008 share purchase plan	1,188,079
Shares issued to non-employee directors	10,542
Shares repurchased from ordinary shareholders	-8,087,539
Shares issued in respect of the redemption of the PIERS	1,835,860
Shares in issue at the end of the period	65,700,665

ShareBased_Payments_Additional

Share-Based Payments - Additional Information (Detail)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

9 Months Ended

1 Months Ended

9 Months Ended

1 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

1 Months Ended

3 Months Ended

9 Months Ended

1 Months Ended

9 Months Ended

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Jun. 21, 2002

Sep. 30, 2013

Sep. 30, 2013

Feb. 28, 2013

Nov. 30, 2012

Feb. 29, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Jul. 24, 2013

Apr. 24, 2013

Feb. 02, 2012

Feb. 07, 2013

Feb. 02, 2012

Jun. 30, 2013

Apr. 30, 2013

Feb. 28, 2013

Feb. 29, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Mar. 28, 2007

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

USD ($)

USD ($)

USD ($)

USD ($)

Employee Share Purchase Plans [Member]

Employee Share Purchase Plans [Member]

Employee Share Purchase Plans [Member]

Employee Share Purchase Plans [Member]

Sharesave Scheme [Member]

2013 Share Incentive Plan [Member]

2003 Share Incentive Plan [Member]

International Employee Share Purchase Plan [Member]

Wellington [Member]

Investor Options [Member]

Investor Options [Member]

Stock Options [Member]

Performance Share Awards [Member]

Performance Share Awards [Member]

Performance Share Awards [Member]

Performance Share Awards [Member]

Performance Share Awards [Member]

Performance Share Awards [Member]

Performance Share Awards [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Restricted Share Units [Member]

Phantom Shares [Member]

Phantom Shares [Member]

Phantom Shares [Member]

Phantom Shares [Member]

Phantom Shares [Member]

Phantom Shares [Member]

Phantom Shares [Member]

Phantom Shares [Member]

Wellington Investment [Member]

Achievement One [Member]

Achievement Two [Member]

Achievement Two [Member]

Achievement Three [Member]

Achievement Three [Member]

Grants in 2007 [Member]

USD ($)

USD ($)

USD ($)

USD ($)

GBP (£)

USD ($)

GBP (£)

Agreement

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

Director [Member]

Director [Member]

Director [Member]

Director [Member]

Board of Directors Chairman [Member]

Board of Directors Chairman [Member]

USD ($)

USD ($)

USD ($)

USD ($)

2012 Performance Shares [Member]

Minimum [Member]

Maximum [Member]

Minimum [Member]

Maximum [Member]

Stock Options [Member]

2012 Performance Shares [Member]

2012 Performance Shares [Member]

2012 Performance Shares [Member]

2012 Performance Shares [Member]

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

'

'

'

'

'

'

'

'

'

'

'

'

'

'

3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Option to subscribe for ordinary shares of Aspen Holdings

'

'

'

'

'

'

'

'

'

'

'

'

'

6,787,880

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Ordinary shares issued

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

426,083

'

'

'

'

'

'

Subscription price payable under the options

'

'

'

'

'

'

'

'

'

'

'

'

£ 10

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Increase in subscription price

'

'

'

'

'

'

'

'

'

'

'

'

5.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Shares available to grant under incentive plan

'

'

'

'

'

'

'

'

'

2,845,683

595,683

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Vesting period

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'3 years

'

'

'

'

'

'3 years

'

'

'

'3 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'3 years

'

'

'

'

'

'

'

'7 years

Performance growth, percentage

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

5.00%

5.00%

10.00%

10.00%

20.00%

'

Exercise period

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'10 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Options grants in period

'

0

'

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Options exercised

204,328

1,165,201

747,023

1,221,152

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Compensation costs

$0

$0

$0

$0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$1,400,000

$3,300,000

$4,200,000

$9,900,000

$2,400,000

$1,800,000

$7,000,000

$5,800,000

'

'

'

'

'

'

'

'

'

'

$1,600,000

$500,000

$0

$1,600,000

'

'

'

'

'

'

'

Restricted share units grant to employees, time period

'

'

'3 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Granted, number of shares

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

250,066

10,006

334,125

'

'

'

'

8,571

0

307,441

345,852

'

'

'

'

'

'

542

6,521

152,541

278,143

'

'

'

'

'

'

'

'

'

'

'

Percentage of restricted share units vested and issued

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

"RSUs" for non-employee directors

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

25,533

1,449

2,110

29,071

14,188

17,705

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Instruments other than options forfeited

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

18,597

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Portion of grant eligible for vesting

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

33.33%

'

'

'

'

'

Eligible for vesting percent

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

10.00%

100.00%

100.00%

200.00%

'

Maximum shares eligible for vesting

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Employee savings under ESPP

'

'

'

'

'

'

'

'

250

'

'

500

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Employee contribution period

'

'

'

'

'

'

'

'

'3 years

'

'

'2 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Additional employee contribution period

'

'

'

'

'

'

'

'

'

'

'

'1 year

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Shares issued

1,985

200

16,339

59,184

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Purchase price of fair market value

'

'

85.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Compensation costs charged in ESPP

'

'

'

'

$100,000

$0

$200,000

$0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Intangible_Assets_Summary_of_C

Intangible Assets - Summary of Changes in Intangible Assets (Detail) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Jun. 30, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Dec. 31, 2011

Sep. 30, 2013

Jun. 30, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Dec. 31, 2011

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Mar. 31, 2012

Trade Mark [Member]

Trade Mark [Member]

Trade Mark [Member]

Trade Mark [Member]

Trade Mark [Member]

Trade Mark [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Other Intangible Assets [Member]

Other Intangible Assets [Member]

Other Intangible Assets [Member]

Other Intangible Assets [Member]

Other Intangible Assets [Member]

Intangible Assets

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Beginning of the period

$18.70

$19.50

$19

$20

$1.60

$1.60

$1.60

$1.60

$1.60

$1.60

$16.60

$16.60

$16.60

$16.60

$16.60

$16.60

$0.50

$1.30

$0.80

$1.80

$1.50

Amortization

-0.1

-0.3

-0.4

-0.8

'

'

'

'

'

'

'

'

'

'

'

'

-0.1

-0.3

-0.4

-0.8

'

End of the period

$18.60

$19.20

$18.60

$19.20

$1.60

$1.60

$1.60

$1.60

$1.60

$1.60

$16.60

$16.60

$16.60

$16.60

$16.60

$16.60

$0.40

$1

$0.40

$1

$1.50

Intangible_Assets_Additional_I

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

9 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2013

Jun. 30, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Dec. 31, 2011

Dec. 31, 2010

Apr. 05, 2005

Sep. 30, 2013

Jun. 30, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Dec. 31, 2011

Sep. 30, 2013

Jun. 30, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Dec. 31, 2011

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2010

Dec. 31, 2010

Sep. 30, 2013

Jun. 30, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2011

Trade Mark [Member]

Trade Mark [Member]

Trade Mark [Member]

Trade Mark [Member]

Trade Mark [Member]

Trade Mark [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Insurance Licenses [Member]

Intangible Asset: Renewal Rights and Distribution Channels [Member]

Intangible Asset: Lock-in Period [Member]

Other Intangible Assets [Member]

Other Intangible Assets [Member]

Other Intangible Assets [Member]

Other Intangible Assets [Member]

Other Intangible Assets [Member]

Other Intangible Assets [Member]

Other Intangible Assets [Member]

AAIC [Member]

Aspen Specialty [Member]

Aspen U.K. [Member]

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

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Aggregate consideration for sale and purchase agreement

'

'

'

'

'

'

$4.80

$1.60

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Value of the asset

18.6

18.7

19

19.2

19.5

20

'

'

1.6

1.6

1.6

1.6

1.6

1.6

16.6

16.6

16.6

16.6

16.6

16.6

'

'

'

'

'

0.4

0.5

0.8

1

1.3

1.5

1.8

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_Contingencies_1

Commitments and Contingencies - Schedule of Company's Restricted Assets (Detail) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Regulatory trusts and deposits:	'		'
Total restricted assets	$3,721.40		$3,721.40
Total as percent of cash and invested assets	46.00%		45.60%
Affiliated Transactions [Member]	'		'
Regulatory trusts and deposits:	'		'
Total restricted assets	696.3		598.7
Third Party [Member]	'		'
Regulatory trusts and deposits:	'		'
Total restricted assets	2,197.90		1,933.50
Letters of Credit / Guarantees [Member]	'		'
Regulatory trusts and deposits:	'		'
Total restricted assets	$827.20	[1]	$1,189.20	[1]
[1]	As of September 30, 2013, the Company had pledged funds of $801.1 million and £16.1 million (December 31, 2012 — $1,163.2 million and £16.0 million) as collateral for the secured letters of credit.

Commitments_and_Contingencies_2

Commitments and Contingencies - Schedule of Company's Restricted Assets (Parenthetical) (Detail)	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2012
In Millions, unless otherwise specified	USD ($)	GBP (£)	USD ($)	GBP (£)
Commitments and Contingencies Disclosure [Abstract]	'	'	'	'
Securities and cash as collateral secured letters of credit	$801.10	£ 16.1	$1,163.20	£ 16

Commitments_and_Contingencies_3

Commitments and Contingencies - Additional Information (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Investment
Restricted Cash And Collateral [Line Items]	'	'
Minimum capital required	$0.60	$0.60
Collateral Already Posted, Aggregate Fair Value	34.1	52
Number of investments	2	'
Interest Rate Swaps [Member]	'	'
Restricted Cash And Collateral [Line Items]	'	'
Cash collateral	'	$52

Commitments_and_Contingencies_4

Commitments and Contingencies - Amounts Outstanding under Operating Leases (Detail) (USD $)	Sep. 30, 2013
In Millions, unless otherwise specified
Leases [Abstract]	'
Operating Lease Obligations, 2013	$5.50
Operating Lease Obligations, 2014	12.2
Operating Lease Obligations, 2015	11.6
Operating Lease Obligations, 2016	8.5
Operating Lease Obligations, 2017	6.9
Operating Lease Obligations, Later Years	11.7
Operating Lease Obligations, Total	$56.40