Summary of PSO Activities (Detail) - Performance Stock Options (PSOs) - 2013 Equity Incentive Plan	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Shares Subject to Outstanding PSOs
Beginning balance | shares	164,783
Options exercised | shares	(1,302)
Ending balance | shares	163,481
Weighted-Average Exercise Price Per Share
Beginning balance | $ / shares	$ 23.76
Options exercised | $ / shares	23.76
Ending balance | $ / shares	$ 23.76