Selected Quarterly Financial Data (Unaudited) Selected Quarterly Financial Data (Unaudited) (Quarterly Financial Information - 2014) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Data [Abstract]
Net revenues	$ 73,995	$ 74,915	$ 72,956	$ 72,746	$ 71,083	$ 71,352	$ 69,836	$ 66,388	$ 294,612	$ 278,659	$ 248,460
Net income attributable to common stockholders	$ 37,083	$ 40,979	$ 37,839	$ 38,432	$ 37,798	$ 39,092	$ 38,245	$ 44,230	$ 154,333	$ 159,365	$ 151,540
Weighted average common shares outstanding:
Basic	42,351,443	42,187,077	41,704,819	41,532,154	41,532,130	41,108,699	40,982,244	39,953,804	41,943,873	40,894,219	39,013,412
Diluted	42,568,720	42,434,499	41,786,829	41,576,876	41,803,615	41,448,263	41,245,173	40,108,762	42,091,731	41,151,453	39,155,380
Basic:
Net income per common share attributable to common stockholders	$ 0.88	$ 0.97	$ 0.91	$ 0.93	$ 0.91	$ 0.95	$ 0.93	$ 1.11	$ 3.68	$ 3.90	$ 3.88
Diluted:
Net income per common share attributable to common stockholders	$ 0.87	$ 0.97	$ 0.91	$ 0.92	$ 0.90	$ 0.94	$ 0.93	$ 1.10	$ 3.67	$ 3.87	$ 3.87