Stockholder's Equity - Shares of Common Stock Issued During 2016 Under Employee Stock Purchase Plan (Detail)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares available, Beginning balance	171,069
Shares issued during period (shares)	5,534
Shares available, Ending Balance	165,535
Shares issued during period (per share price) | $ / shares	$ 58.19