Stock-Based Compensation - Disclosures for 2004 Purchase Plan (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Weighted-average purchase price per share	$ 250.65	$ 217.91	$ 150.13
Weighted average grant-date fair value per share of shares purchased	$ 72.21	$ 60.49	$ 57.63
Number of shares purchased	162,076	150,044	182,175