Earnings Per Share - Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income to shareholders	$ 16,984	$ 14,541	$ 32,617	$ 32,991
Weighted average common shares outstanding:
Basic (in shares)	29,882,988	29,406,877	29,823,982	29,348,557
Common share equivalents (in shares)	410,945	900,222	419,964	969,028
Diluted (in shares)	30,293,933	30,307,099	30,243,946	30,317,585
Earnings per share:
Basic (in dollars per share)	$ 0.57	$ 0.49	$ 1.09	$ 1.12
Common share equivalents (in dollars per share)	(0.01)	(0.01)	(0.01)	(0.03)
Diluted (in dollars per share)	$ 0.56	$ 0.48	$ 1.08	$ 1.09