Income Per Share - (Narrative) (Details) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Mar. 31, 2016
Shares outstanding	30,479,969
Outstanding stock options to purchase common stock	1,516,044	1,719,701	1,598,318
Outstanding stock options to purchase common stock, per share	$ 21.89	$ 20.56	$ 20.62
Warrants granted, exercise price per share		$ 20.00	$ 20.00	$ 20.00
Restricted shares outstanding	418,638	490,528
Warrant [Member]
Dilutive Convertible Securities		33,783