Earnings Per Share - Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income to shareholders	$ 10,351	$ 21,366	$ 43,342	$ 48,799
Weighted average common shares outstanding:
Basic (in shares)	29,524,243	29,101,550	29,407,762	29,030,284
Common share equivalents (in shares)	695,834	833,602	877,971	804,402
Diluted (in shares)	30,220,077	29,935,152	30,285,733	29,834,686
Earnings per share:
Basic (in dollars per share)	$ 0.35	$ 0.73	$ 1.47	$ 1.68
Common share equivalents (in dollars per share)	(0.01)	(0.02)	(0.04)	(0.04)
Diluted (in dollars per share)	$ 0.34	$ 0.71	$ 1.43	$ 1.64