Common and Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended												12 Months Ended
	Feb. 29, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class Of Stock [Line Items]
Shares of stock issued														0	0	28,800,000
Cumulative mandatory convertible series														8.00%	8.00%
Preferred stock, par value		$ 1.00				$ 1.00								$ 1.00	$ 1.00
Preferred stock, liquidation preference per share		$ 1,000				1,000								$ 1,000	$ 1,000
Net proceeds from issuance of common stock and Mandatory convertible preferred stock, after deducting underwriting discounts, commissions, and offering expenses	$ 1,600															$ 1,600
Common stock repurchased, share repurchase program														$ 1,380	$ 120
Common stock repurchased, share repurchase program, share														25,200,000	2,600,000	0
Additional amount of common stock that may be repurchased		$ 650												$ 650
Average cost per share														$ 54.85	$ 45.67
Cash dividends on common stock		$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 1.00	$ 1.00	$ 1.00
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, Exceeds $45.83
Class Of Stock [Line Items]
Shares covered by cap call	12,550,000
Series A Mandatory Convertible Preferred Stock | Final Average Price between $45.83 and $53.625
Class Of Stock [Line Items]
Shares covered by cap call	12,550,000
Series A Mandatory Convertible Preferred Stock | Final Average Price, $45.83
Class Of Stock [Line Items]
Shares covered by cap call	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
Common stock repurchased, share repurchase program														$ 25	$ 3
Capital in Excess of Par
Class Of Stock [Line Items]
Common stock repurchased, share repurchase program														$ 541	$ 43
Capital in Excess of Par | Series A Mandatory Convertible Preferred Stock
Class Of Stock [Line Items]
Premium paid for capped call transaction	$ 37															$ 37