Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share
Net income attributable to common shares	$ 18,600	$ 15,758	$ 58,291	$ 47,693
Weighted-average common shares outstanding - basic	35,512,091	33,878,881	34,504,790	33,758,971
Effect of potentially dilutive common shares:
Stock options	45,453	59,201	49,349	98,660
Unvested awards	163,934	176,087	139,696	176,211
Weighted-average common shares outstanding - diluted	35,721,478	34,114,169	34,693,835	34,033,842
Net income per share attributable to common shares
Basic (in dollars per share)	$ 0.52	$ 0.47	$ 1.69	$ 1.41
Diluted (in dollars per share)	$ 0.52	$ 0.46	$ 1.68	$ 1.40
Weighted-average potentially dilutive securities excluded in calculation of basic and diluted income (loss) per share	1,003	1,205	121,373	109,953