Quarterly Financial Information (Unaudited) (Summarized Unaudited Consolidated Quarterly Information) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 803,987	$ 779,035	$ 751,254	$ 741,470	$ 733,185	$ 711,249	$ 684,109	$ 682,909	$ 3,075,746	$ 2,811,452	$ 2,469,615
Income from continuing operations attributable to common stockholders	107,601	109,391	138,653	118,016	108,555	127,470	133,139	120,624	473,661	489,788	308,814
Discontinued operations	(411)	(259)	(255)	3,031	(115)	(9,174)	(18,559)	(8,431)	2,106	(36,279)	53,986
Net income attributable to common stockholders	$ 107,190	$ 109,132	$ 138,398	$ 121,047	$ 108,440	$ 118,296	$ 114,580	$ 112,193	$ 475,767	$ 453,509	$ 362,800
Basic:
Income from continuing operations attributable to common stockholders (in dollars per share)	$ 0.36	$ 0.37	$ 0.47	$ 0.40	$ 0.37	$ 0.43	$ 0.45	$ 0.41	$ 1.61	$ 1.67	$ 1.06
Discontinued operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.01	$ 0.00	$ (0.03)	$ (0.06)	$ (0.03)	$ 0.01	$ (0.12)	$ 0.18
Net income attributable to common stockholders (in dollars per share)	$ 0.36	$ 0.37	$ 0.47	$ 0.41	$ 0.37	$ 0.40	$ 0.39	$ 0.38	$ 1.62	$ 1.55	$ 1.24
Diluted:
Income from continuing operations attributable to common stockholders (in dollars per share)	$ 0.36	$ 0.37	$ 0.47	$ 0.40	$ 0.37	$ 0.43	$ 0.45	$ 0.41	$ 1.59	$ 1.66	$ 1.05
Discontinued operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.01	$ 0.00	$ (0.03)	$ (0.06)	$ (0.03)	$ 0.01	$ (0.12)	$ 0.18
Net income attributable to common stockholders (in dollars per share)	$ 0.36	$ 0.37	$ 0.47	$ 0.41	$ 0.37	$ 0.40	$ 0.39	$ 0.38	$ 1.60	$ 1.54	$ 1.23
Dividends declared per common share (in dollars per share)	$ 0.79	$ 0.725	$ 0.725	$ 0.725	$ 0.725	$ 0.67	$ 0.67	$ 0.67