Summary of PSO Activities (Detail) - Performance Stock Options (PSOs) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Shares Subject to Outstanding PSOs
Options granted	0	0
2013 Equity Incentive Plan
Shares Subject to Outstanding PSOs
Beginning balance	164,783
Options exercised	(17,448)
Options canceled	(4,000)
Ending balance	143,335	164,783
Weighted-Average Exercise Price Per Share
Beginning balance	$ 23.76
Options exercised	23.76
Options canceled	23.76
Ending balance	$ 23.76	$ 23.76