Lloyd's Syndicate 1221 - Additional Information (Detail) (Lloyd's, Syndicate 1221) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013 USD ($)	Mar. 31, 2013 GBP (£)	Mar. 31, 2012 USD ($)	Mar. 31, 2012 GBP (£)
Supplementary Insurance Information, by Segment [Line Items]
Syndicate 1221's stamp capacity	$ 296	£ 195	$ 300	£ 184
Percentage control of stamp capacity	100.00%	100.00%	100.00%	100.00%
Letters of credit	145.9
Cash collateral posted	$ 0.8

Carrying amount as of the balance sheet date of cash collateral held for borrowed securities, for which the cash is restricted as to withdrawal or usage.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.1)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.(a),2)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 2

-Article 7

Name:	us-gaap_CashCollateralForBorrowedSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant