Stockholder's Equity - Shares of Common Stock Issued During 2016 Under Employee Stock Purchase Plan (Detail) - $ / shares	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares available, Beginning balance			189,611	189,611
Shares issued during period (shares)	4,775	4,152	5,891
Shares available, Ending Balance	174,793			174,793
Shares issued during period (per share price)	$ 64.49	$ 51.21	$ 43.42