Share-Based Compensation - Stock Option Activity under Stock Option Plans (Details)	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Number of Shares
Beginning balance (in shares) | shares	152,652
Exercised (in shares) | shares	(44,932)
Cancelled (in shares) | shares	(200)
Ending balance (in shares) | shares	107,520
Vested and Expected to Vest (in shares) | shares	107,520
Exercisable (in shares) | shares	107,520
Weighted-Average Exercise Price
Beginning balance (in dollars per share) | $ / shares	$ 16.71
Exercised (in dollars per share) | $ / shares	21.20
Cancelled (in dollars per share) | $ / shares	28.25
Ending balance (in dollars per share) | $ / shares	14.80
Vested and Expected to Vest (in dollars per share) | $ / shares	14.80
Exercisable (in dollars per share) | $ / shares	$ 14.80
Weighted Average Remaining Contractual Term (in Years)
Outstanding (term)	2 years 6 months 15 days
Vested and Expected to Vest (term)	2 years 6 months 15 days
Exercisable (term)	2 years 6 months 15 days
Aggregate Intrinsic Value
Outstanding | $	$ 3,397,202
Vested and Expected to Vest | $	3,397,202
Exercisable | $	$ 3,397,202