Stockholders' Equity - Shares of Common Stock (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance (in shares)	228,605,485	229,293,783	233,390,705
Incentive Plan and Awards (in shares)	1,320,129	1,209,124	1,033,664
Stock Options Exercised (in shares)	1,997,362	4,156,728	1,721,282
Treasury Stock Purchased (in shares)	(5,796,302)	(6,054,150)	(6,851,868)
Ending balance (in shares)	226,126,674	228,605,485	229,293,783