Lloyd's Syndicate 1221 - Additional Information (Detail) (Lloyd's Operations, Syndicate 1221) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2012 USD ($)	Sep. 30, 2012 GBP (£)	Sep. 30, 2011 USD ($)	Sep. 30, 2011 GBP (£)
Supplementary Insurance Information, by Segment [Line Items]
Syndicate 1221's stamp capacity	$ 296	£ 184	$ 280	£ 175
Percentage control of stamp capacity	100.00%	100.00%	100.00%	100.00%
Letters of credit	$ 150.0

Amount borrowed under the credit facility as of the balance sheet date.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

Name:	us-gaap_LineOfCreditFacilityAmountOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant