Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 480,840	$ 475,360	$ 457,459	$ 442,367	$ 440,656	$ 430,525	$ 413,806	$ 400,075	$ 1,856,028	$ 1,685,061	$ 1,462,921
Income from continuing operations	155,352	206,076	172,253	208,053	142,530	241,001	172,197	103,420	741,733	659,148	57,827
Total discontinued operations	0	0	0	0	0	0	0	38,179	0	38,179	294,944
Net income	155,352	206,076	172,253	208,053	142,530	241,001	172,197	141,599	741,733	697,327	352,771
Net income attributable to common stockholders	$ 155,428	$ 206,142	$ 172,324	$ 208,144	$ 142,642	$ 241,100	$ 158,086	$ 141,739	$ 742,038	$ 683,567	$ 353,141
Net income (loss) per common share - basic (in dollars per share)	$ 1.13	$ 1.54	$ 1.30	$ 1.57	$ 1.08	$ 1.83	$ 1.22	$ 1.09	$ 5.54	$ 5.22	$ 2.78
Net income (loss) per common share - diluted (in dollars per share)	$ 1.13	$ 1.53	$ 1.29	$ 1.56	$ 1.08	$ 1.83	$ 1.21	$ 1.09	$ 5.51	$ 5.21	$ 2.78