Share-Based Compensation - Stock Option Activity under Stock Option Plans (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Number of Shares
Beginning balance (in shares) | shares	152,652
Exercised (in shares) | shares	(23,557)
Cancelled (in shares) | shares	(200)
Ending balance (in shares) | shares	128,895
Vested and Expected to Vest (in shares) | shares	128,895
Exercisable (in shares) | shares	128,895
Weighted-Average Exercise Price
Beginning balance (in dollars per share) | $ / shares	$ 16.71
Exercised (in dollars per share) | $ / shares	22.71
Cancelled (in dollars per share) | $ / shares	28.25
Ending balance (in dollars per share) | $ / shares	15.59
Vested and Expected to Vest (in dollars per share) | $ / shares	15.59
Exercisable (in dollars per share) | $ / shares	$ 15.59
Weighted Average Remaining Contractual Term (in Years)
Outstanding (term)	2 years 5 months 12 days
Vested and Expected to Vest (term)	2 years 5 months 12 days
Exercisable (term)	2 years 5 months 12 days
Aggregate Intrinsic Value
Outstanding | $	$ 3,365,525
Vested and Expected to Vest | $	3,365,525
Exercisable | $	$ 3,365,525