Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 555,292	$ 550,500	$ 530,512	$ 522,326	$ 518,240	$ 516,211	$ 502,307	$ 508,498	$ 2,158,628	$ 2,045,255	$ 1,856,028
Net income	237,486	238,199	165,194	235,781	242,183	356,329	197,319	237,877	876,660	1,033,708	741,733
Net income attributable to common stockholders	$ 237,573	$ 238,248	$ 165,225	$ 235,875	$ 242,235	$ 356,392	$ 197,444	$ 237,931	$ 876,921	$ 1,034,002	$ 742,038
Net income per common share - basic (in dollars per share)	$ 1.72	$ 1.73	$ 1.20	$ 1.72	$ 1.76	$ 2.60	$ 1.44	$ 1.73	$ 6.36	$ 7.53	$ 5.54
Net income per common share - diluted (in dollars per share)	$ 1.72	$ 1.72	$ 1.20	$ 1.72	$ 1.76	$ 2.59	$ 1.44	$ 1.73	$ 6.35	$ 7.52	$ 5.51