Share-Based Payments - Summary of Unvested RSUs Activity (Detail) - Restricted stock units	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested shares beginning of period (in shares) | shares	77,050
Granted (in shares) | shares	57,100
Vested (in shares) | shares	(7,500)
Forfeited/expired (in shares) | shares	(7,300)
Unvested shares end of period (in shares) | shares	119,350
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Weighted-average grant date fair value, Unvested shares beginning of period (in usd per share) | $ / shares	$ 49.60
Weighted-average grant date fair value, Granted (in usd per share) | $ / shares	50.40
Weighted-average grant date fair value, Vested (in usd per share) | $ / shares	122.22
Weighted-average grant date fair value, Forfeited/expired (in usd per share) | $ / shares	39.76
Weighted-average grant date fair value, Unvested shares end of period (in usd per share) | $ / shares	$ 46.02