Dividends - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended			3 Months Ended		9 Months Ended
	Feb. 28, 2013	Nov. 30, 2012	Feb. 29, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Dividends Payable [Line Items]
Target percentage used to determine annual variable dividend				33.33%		33.33%
Gainshare factor used to determine annual variable dividend range, minimum				0		0
Gainshare factor used to determine annual variable dividend range, maximum				2		2
Gainshare factor used to determine annual variable dividend				1.15		1.15
Comprehensive income				$ 289.1	$ 339.4	$ 865.5	$ 865.8
After-tax underwriting income						$ 554.4
Dividend paid	$ 0.2845	$ 1.00	$ 0.4072