Earnings per Share (Computation of Basic and Diluted Net Income per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings per Share [Abstract]
Net income available to common stockholders - basic	$ 4,806	$ 5,730	$ 5,100	$ 5,889	$ 4,603	$ 7,009	$ 4,632	$ 4,082	$ 21,525	$ 20,326	$ 15,072
Effect of net income attributable to noncontrolling interests									3,160	3,803	4,031
Net income available to common stockholders - diluted									$ 24,685	$ 24,129	$ 19,103
Weighted average shares outstanding-basic									46,205,937	37,568,109	29,054,576
Effect of Class A and Class RS partnership units									6,783,000	7,029,000	7,770,000
Effect of Class O units and options to purchase Class A common stock on an "as if" converted basis									973,000	756,000	309,000
Weighted average shares outstanding-diluted									53,962,234	45,353,170	37,133,584
Net income per share attributable to common stockholders - basic	$ 0.10	$ 0.12	$ 0.11	$ 0.14	$ 0.11	$ 0.17	$ 0.13	$ 0.13	$ 0.47	$ 0.54	$ 0.52
Net income per share attributable to common stockholders - diluted	$ 0.10	$ 0.12	$ 0.10	$ 0.14	$ 0.11	$ 0.17	$ 0.12	$ 0.13	$ 0.46	$ 0.53	$ 0.51