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
Net income attributable to common shares	$ 19,389	$ 15,643	$ 39,691	$ 31,935
Weighted-average common shares outstanding - basic	34,049,391	33,835,727	33,992,792	33,698,022
Effect of potentially dilutive common shares:
Stock options	48,720	76,968	51,267	117,550
Unvested awards	122,210	141,121	139,349	194,358
Weighted-average common shares outstanding - diluted	34,220,321	34,053,816	34,183,408	34,009,930
Net income per share attributable to common shares
Basic (in dollars per share)	$ 0.57	$ 0.46	$ 1.17	$ 0.95
Diluted (in dollars per share)	$ 0.57	$ 0.46	$ 1.16	$ 0.94
Weighted-average potentially dilutive securities excluded in calculation of basic and diluted income (loss) per share	908	562	105,829	92,260