Dividends - Additional Information (Detail)		3 Months Ended		9 Months Ended
		Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Dividends [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent				35.00%
Target percentage used to determine annual variable dividend		33.30%		33.30%
Gainshare factor used to determine annual variable dividend range, minimum		0		0
Minimum Payments of Ordinary Dividends, Common Stock	[1]			$ 0
Gainshare factor used to determine annual variable dividend range, maximum		2		2
Gainshare factor used to determine annual variable dividend		1.54		1.54
Comprehensive income		$ 287,000,000	$ 145,500,000	$ 893,500,000	$ 683,300,000
After-tax underwriting income				$ 428,600,000

[1] 1Represents the final amount of dividends declared based on shares outstanding as of the record date.