Common and Preferred Stock Issuance - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Feb. 29, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class Of Stock [Line Items]
Shares of stock issued		28,750,000	0	0
Cumulative mandatory convertible series		8.00%
Preferred stock, par value		$ 1.00
Preferred stock, liquidation preference per share		$ 1,000
Net proceeds from issuance of common stock and Mandatory convertible preferred stock, after deducting underwriting discounts, commissions, and offering expenses	$ 1,600,000,000
Series A Mandatory Convertible Preferred Stock
Class Of Stock [Line Items]
Shares of stock issued	575,000
Cumulative mandatory convertible series	8.00%
Preferred stock, par value	$ 1
Preferred stock, liquidation preference per share	$ 1,000
Series A Mandatory Convertible Preferred Stock | Final Average Price, $45.83 or less
Class Of Stock [Line Items]
Value of capped call transaction	$ 0
Series A Mandatory Convertible Preferred Stock | Final Average Price, $53.625 or higher
Class Of Stock [Line Items]
Value of capped call transaction	$ 98,000,000
Series A Mandatory Convertible Preferred Stock | Final Average Price between $45.83 and $53.625
Class Of Stock [Line Items]
Number of shares multiplied by difference between the final average price	12,550,000
Series A Mandatory Convertible Preferred Stock | Minimum
Class Of Stock [Line Items]
Number of shares issued for each share of convertible preferred stock that is converted	21.822
Series A Mandatory Convertible Preferred Stock | Maximum
Class Of Stock [Line Items]
Number of shares issued for each share of convertible preferred stock that is converted	25.642
Common Stock
Class Of Stock [Line Items]
Shares of stock issued	28,750,000
Capital in Excess of Par | Series A Mandatory Convertible Preferred Stock
Class Of Stock [Line Items]
Premium paid for capped call transaction	$ 37,000,000