Lloyd's Syndicate 1221 - Additional Information (Detail) (Lloyd's, Syndicate 1221) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2014 GBP (£)	Dec. 31, 2013 USD ($)	Dec. 31, 2013 GBP (£)	Dec. 31, 2012
Supplementary Insurance Information, by Segment [Line Items]
Syndicate 1221's stamp capacity		$ 336.9	£ 215.0	$ 323.7	£ 195.0
Percentage control of stamp capacity		100.00%	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