Lloyd's Syndicate 1221 - Additional Information (Detail) In Millions, unless otherwise specified	1 Months Ended	9 Months Ended
Mar. 31, 2011	Sep. 30, 2011 Lloyd's Operations Syndicate 1221 USD ($)	Sep. 30, 2011 Lloyd's Operations Syndicate 1221 GBP (£)	Sep. 30, 2010 Lloyd's Operations Syndicate 1221 USD ($)	Sep. 30, 2010 Lloyd's Operations Syndicate 1221 GBP (£)
Ceded Credit Risk [Line Items]
Syndicate 1221's stamp capacity		$ 273	£ 175	$ 264	£ 168
Percentage control of stamp capacity		100.00%	100.00%	100.00%	100.00%
Letters of credit		$ 132.6
Issuance of new letters of credit, expiry date	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2011
Credit facility expiry date	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2012

Date when the debt instrument is scheduled to be fully repaid, which may be presented in a variety of ways (year, month and year, day, month and year, quarter, etc.).

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(2))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Namespace Prefix:	us-gaap
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration