Earnings Per Share - Computation of basic and diluted EPS - (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic earnings per common share:
Net income	$ 40,459	$ 31,823	$ 82,785	$ 50,087
Weighted Average Number of Shares Outstanding, Basic	36,677	29,095	36,657	28,985
Basic earnings per common share (in dollars per share)	$ 1.10	$ 1.09	$ 2.25	$ 1.73
Diluted earnings per share:
Net income	$ 40,459	$ 31,823	$ 82,785	$ 50,087
Weighted Average Number of Shares Outstanding, Basic	36,677	29,095	36,657	28,985
Effect of dilutive securities (in shares)	252	270	253	267
Weighted-average dilutive shares	36,929	29,365	36,910	29,252
Diluted earnings per common share (in dollars per share)	$ 1.09	$ 1.08	$ 2.24	$ 1.71