Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information
Total revenue	$ 271,303	$ 267,151	$ 254,172	$ 246,032	$ 243,089	$ 239,568	$ 231,218	$ 222,495	$ 1,038,660	$ 936,087	$ 842,820
Income (loss) from continuing operations	68,237	84,432	58,409	53,674	48,278	42,410	33,229	28,579	264,756	152,400	92,055
Total discontinued operations	54,147	2,315	98,412	3,935	274,687	2,267	9,963	1,958	158,806	288,970	82,024
Net income attributable to common stockholders	$ 122,356	$ 86,844	$ 156,909	$ 57,758	$ 323,085	$ 44,824	$ 43,373	$ 30,341	$ 423,869	$ 441,622	$ 175,331
Net income per common share - basic	$ 1.19	$ 0.89	$ 1.64	$ 0.61	$ 3.40	$ 0.49	$ 0.50	$ 0.35	$ 4.34	$ 4.89	$ 2.08
Net income per common share - diluted	$ 1.19	$ 0.89	$ 1.63	$ 0.60	$ 3.38	$ 0.49	$ 0.49	$ 0.35	$ 4.32	$ 4.87	$ 2.07