Outstanding and Weighted Average Common Shares - Changes in Outstanding Common Shares (Detail) - shares shares in Thousands	12 Months Ended			58 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Earnings Per Share [Abstract]
Balance at January 1	316,500	286,000	285,800
Shares issued	0	28,800	0
Activity related to restricted stock awards, net	800	1,100	800
Stock options exercised	200	200	200
PSU vested	200	400	600
Shares repurchased	(2,630)	0	(1,450)	(66,740)
Balance at December 31	315,100	316,500	286,000	315,100