Stockholder's Equity - Shares of Common Stock Issued During 2016 Under Employee Stock Purchase Plan (Detail) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares available, Beginning balance		171,069	171,069
Shares issued during period (shares)	5,171	5,534
Shares available, Ending Balance	160,364		160,364
Shares issued during period (per share price)	$ 58.48	$ 58.19