Share-Based Compensation - Stock Option Activity under Stock Option Plans (Details)	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Number of Shares
Beginning balance (in shares) | shares	152,652
Exercised (in shares) | shares	(51,243)
Cancelled (in shares) | shares	(200)
Ending balance (in shares) | shares	101,209
Vested and Expected to Vest (in shares) | shares	101,209
Exercisable (in shares) | shares	101,209
Weighted-Average Exercise Price
Beginning balance (in dollars per share) | $ / shares	$ 16.71
Exercised (in dollars per share) | $ / shares	21.33
Cancelled (in dollars per share) | $ / shares	28.25
Ending balance (in dollars per share) | $ / shares	14.34
Vested and Expected to Vest (in dollars per share) | $ / shares	14.34
Exercisable (in dollars per share) | $ / shares	$ 14.34
Weighted Average Remaining Contractual Term (in Years)
Outstanding (term)	2 years 4 months 28 days
Vested and Expected to Vest (term)	2 years 4 months 28 days
Exercisable (term)	2 years 4 months 28 days
Aggregate Intrinsic Value
Outstanding | $	$ 3,101,835
Vested and Expected to Vest | $	3,101,835
Exercisable | $	$ 3,101,835