Selected Quarterly Information (Unaudited) (Details) (USD $)	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
Selected Quarterly Information (Unaudited)
Total revenues	$ 302,883,000	$ 293,365,000	$ 289,686,000	$ 287,871,000	$ 272,562,000	$ 280,341,000	$ 291,195,000	$ 278,435,000	$ 1,173,805,000	$ 1,122,532,000	$ 978,266,000
Income from continuing operations	52,350,000	52,116,000	41,225,000	46,837,000	39,371,000	39,371,000	49,094,000	44,369,000	192,528,000	172,205,000	153,428,000
Income (loss) from discontinued operations	971,000	(43,590,000)	493,000	550,000	651,000	463,000	876,000	1,264,000	(41,576,000)	3,254,000	3,531,000
Net income	53,321,000	8,526,000	41,718,000	47,387,000	40,022,000	39,834,000	49,970,000	45,633,000	150,952,000	175,459,000	156,959,000
Net income per share, basic and diluted:
Income from continuing operations (in dollars per share)	$ 0.61	$ 0.61	$ 0.48	$ 0.55	$ 0.46	$ 0.46	$ 0.57	$ 0.52	$ 2.25	$ 2.01	$ 1.79
Income (loss) from discontinued operations (in dollars per share)	$ 0.01	$ (0.51)			$ 0.01		$ 0.01	$ 0.01	$ (0.49)	$ 0.04	$ 0.04
Net income (in dollars per share)	$ 0.62	$ 0.10	$ 0.48	$ 0.55	$ 0.47	$ 0.46	$ 0.58	$ 0.53	$ 1.76	$ 2.05	$ 1.83
Non-cash impairment charge to the sale of Legend	$ 42,400,000								$ 42,400,000