Earnings Per Common Share - Computation of basic and diluted EPS - (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic earnings per common share:
Net income	$ 35,046	$ 28,095	$ 85,133	$ 77,105
Weighted Average Number of Shares Outstanding, Basic	29,115	24,016	29,023	23,989
Basic earnings per common share (in dollars per share)	$ 1.20	$ 1.17	$ 2.92	$ 3.21
Diluted earnings per share:
Net income	$ 35,046	$ 28,095	$ 85,133	$ 77,105
Weighted Average Number of Shares Outstanding, Basic	29,115	24,016	29,023	23,989
Effect of dilutive securities (in shares)	270	262	268	240
Weighted-average dilutive shares	29,385	24,278	29,291	24,229
Diluted earnings per common share (in dollars per share)	$ 1.19	$ 1.16	$ 2.90	$ 3.18