Stockholders' Equity - Common Stock Rollforward (Detail) - shares shares in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Shares of common stock issued
Shares at January 1		572,000		528,000	526,000
Exercise of stock options		0	[1]	1,000	1,000
Issuance of common stock	40,500	0		41,000	0
Issuance of restricted stock		2,000		2,000	1,000
Shares at December 31		574,200		572,000	528,000
Shares of common stock held in treasury
Shares at January 1		20,800		20,000	19,000
Purchase of treasury stock		22,000		0	0
Shares received for restricted stock vested and stock options exercised		0		1,000	1,000
Shares at December 31		43,400		20,800	20,000
Shares of common stock outstanding at December 31		531,000		551,000	508,000

[1] The total intrinsic value of stock options exercised was zero during 2017, $7 million during 2016, and $23 million during 2015, based on the difference between the market price at the exercise date and the exercise price.