Share-Based Compensation - Stock Option Activity under Stock Option Plans (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Number of Shares
Beginning balance (in shares) | shares	76,300
Exercised (in shares) | shares	(3,700)
Cancelled (in shares) | shares	0
Ending balance (in shares) | shares	72,600
Vested and Expected to Vest (in shares) | shares	72,600
Exercisable (in shares) | shares	72,600
Weighted-Average Exercise Price
Beginning balance (in dollars per share) | $ / shares	$ 12.40
Exercised (in dollars per share) | $ / shares	10.88
Cancelled (in dollars per share) | $ / shares	0
Ending balance (in dollars per share) | $ / shares	12.48
Vested and Expected to Vest (in dollars per share) | $ / shares	12.48
Exercisable (in dollars per share) | $ / shares	$ 12.48
Weighted Average Remaining Contractual Term (in Years)
Outstanding (term)	2 years 8 months 23 days
Vested and Expected to Vest (term)	2 years 8 months 23 days
Exercisable (term)	2 years 8 months 23 days
Aggregate Intrinsic Value
Outstanding | $	$ 1,769,628
Vested and Expected to Vest | $	1,769,628
Exercisable | $	$ 1,769,628