Dividends - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended		1 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Feb. 28, 2014 Annual variable dividend	Feb. 28, 2013 Annual variable dividend	Feb. 28, 2014 Special dividend
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	1.35		1.35
Comprehensive income	$ 381.0	$ 169.1	$ 701.9	$ 576.4
After-tax underwriting income			$ 404.4
Dividend paid per share (USD per share)					$ 0.4929	$ 0.2845	$ 1.00