Stockholders' Equity - Additional Information (Detail) - USD ($)		3 Months Ended	6 Months Ended	12 Months Ended
	Apr. 21, 2015	Jun. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Stockholders Equity Note [Line Items]
Preferred stock, shares authorized		10,000,000	10,000,000	10,000,000
Preferred stock, without par value		$ 0	$ 0	$ 0
Preferred stock		$ 388,500,000	$ 388,500,000	$ 388,500,000
Stock repurchase, shares		1,295,263	2,851,133
Average price paid per share for common stock repurchased		$ 74.64	$ 71.52
Stock repurchase, aggregate cost		$ 96,700,000	$ 203,900,000
Stock repurchase authorized, remaining shares		13,704,737	13,704,737
Upper Limit
Stockholders Equity Note [Line Items]
Stock repurchase authorized shares		15,000,000	15,000,000
After June 30 2015 through June 2016 | Upper Limit
Stockholders Equity Note [Line Items]
Stock Repurchase Authorized Amount		$ 578,300,000	$ 578,300,000
Series C Preferred Stock
Stockholders Equity Note [Line Items]
Stock issued during period			16,000,000	16,000,000
Per share ownership interest, percentage		0.10%	0.10%	0.10%
Preferred stock, without par value		$ 0	$ 0	$ 0
Preferred stock, liquidation preference		$ 25,000	$ 25,000	$ 25,000
Depository shares, liquidation preference per share		$ 25	$ 25	$ 25
Preferred stock		$ 388,500,000	$ 388,500,000	$ 388,500,000
Preferred stock, annual dividend rate			5.85%
Preferred Stock, Dividends Per Share, Declared	$ 365.625
Dividends Payable, Date to be Paid	Jul. 01, 2015
Dividends Payable, Date of Record	Jun. 15, 2015