Stockholders' Equity - Additional Information (Detail) - USD ($)		12 Months Ended
	Aug. 08, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Preferred stock, shares authorized		10,000,000	10,000,000
Preferred stock, par value (in dollars per share)
Average price paid per share for common stock repurchased (in dollars per share)		$ 67.91	$ 72.52	$ 64.20
Upper Limit
Class of Stock [Line Items]
Stock repurchase authorized amount		$ 145,000,000.0
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares issued	500,000
Preferred stock, shares outstanding	500,000
Per share ownership interest, percentage	1.00%
Preferred stock, liquidation preference	$ 100,000
Depository shares, liquidation preference per share	$ 1,000
Preferred stock, annual dividend rate	4.60%
Preferred stock, variable rate	3.202%
Preferred stock, par value (in dollars per share)	$ 0
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares outstanding		16,000	16,000
Preferred stock, annual dividend rate		5.85%
Stock issued during period		16,000,000
Preferred Stock		$ 388,500,000	$ 388,500,000
Preferred Stock, redemption start date		Oct. 01, 2019
Preferred stock redemption period following regulatory capital treatment event		90 days
Series C Preferred Stock, Depositary Shares
Class of Stock [Line Items]
Preferred stock, shares outstanding		16,000,000
Per share ownership interest, percentage		0.10%	0.10%
Preferred stock, liquidation preference		$ 25,000	$ 25,000
Depository shares, liquidation preference per share		$ 25	$ 25
Preferred Stock			$ 388,500,000
Preferred stock, par value (in dollars per share)		$ 0	$ 0