Lloyd's Syndicate 1221 - Additional Information (Detail) (Lloyd's, Syndicate 1221) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 USD ($)	Mar. 31, 2014 GBP (£)	Mar. 31, 2013 USD ($)	Mar. 31, 2013 GBP (£)	Dec. 31, 2013	Dec. 31, 2012
Supplementary Insurance Information, by Segment [Line Items]
Syndicate 1221's stamp capacity	$ 359.1	£ 215.0	$ 323.7	£ 195.0
Percentage control of stamp capacity	100.00%	100.00%			100.00%	100.00%
Pre-tax gain in connection with a change in functional currency	10.0
After-tax gain in connection with a change in functional currency	6.6
Letters of credit	152.1
Cash collateral posted	$ 1.0