Selected Quarterly Information (Unaudited) (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected Quarterly Information (Unaudited)
Total revenues	$ 375,004,000	$ 347,089,000	$ 331,706,000	$ 316,555,000	$ 302,883,000	$ 293,365,000	$ 289,686,000	$ 287,871,000	$ 1,370,354,000	$ 1,173,805,000	$ 1,122,532,000
Income from continuing operations					52,350,000	52,116,000	41,225,000	46,837,000	252,998,000	192,528,000	172,205,000
Income (loss) from discontinued operations					971,000	(43,590,000)	493,000	550,000		(41,576,000)	3,254,000
Net income	78,759,000	68,419,000	51,957,000	53,863,000	53,321,000	8,526,000	41,718,000	47,387,000	252,998,000	150,952,000	175,459,000
Net income per share, basic and diluted:
Income from continuing operations (in dollars per share)					$ 0.61	$ 0.61	$ 0.48	$ 0.55	$ 2.96	$ 2.25	$ 2.01
Income (loss) from discontinued operations (in dollars per share)					$ 0.01	$ (0.51)				$ (0.49)	$ 0.04
Net income (in dollars per share)	$ 0.92	$ 0.80	$ 0.61	$ 0.63	$ 0.62	$ 0.10	$ 0.48	$ 0.55	$ 2.96	$ 1.76	$ 2.05
Non-cash impairment charge to the sale of Legend						$ 42,400,000