Lloyd's Syndicate 1221 - Additional Information (Detail) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Mar. 31, 2011	Dec. 31, 2011 Lloyd's Operations Syndicate 1221 USD ($)	Dec. 31, 2011 Lloyd's Operations Syndicate 1221 GBP (£)	Dec. 31, 2010 Lloyd's Operations Syndicate 1221 USD ($)	Dec. 31, 2010 Lloyd's Operations Syndicate 1221 GBP (£)	Dec. 31, 2009 Lloyd's Operations Syndicate 1221
Ceded Credit Risk [Line Items]
Syndicate 1221's stamp capacity		$ 271	£ 175	$ 264	£ 168
Percentage control of stamp capacity		100.00%	100.00%	100.00%	100.00%	100.00%
Letters of credit		$ 149.6
Issuance of new letters of credit, expiry date	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2011