Quarterly Financial Information (Unaudited) (Summarized Unaudited Consolidated Quarterly 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]
Revenues	$ 841,274	$ 827,606	$ 811,920	$ 805,598	$ 732,090	$ 704,932	$ 674,529	$ 665,262	$ 3,286,398	$ 2,776,813	$ 2,516,617
Income from continuing operations attributable to common stockholders, including real estate dispositions			131,578	102,868	86,481	90,961	107,617	90,973	406,740	376,032	374,338
Discontinued operations	(2,331)	(22,383)	18,243	17,574	20,709	18,171	30,781	30,074	11,103	99,735	79,171
Net income attributable to common stockholders	$ 124,728	$ 22,852	$ 149,821	$ 120,442	$ 107,190	$ 109,132	$ 138,398	$ 121,047	$ 417,843	$ 475,767	$ 453,509
Basic:
Income from continuing operations attributable to common stockholders (in dollars per share)					$ 0.29	$ 0.28	$ 0.37	$ 0.31	$ 1.23	$ 1.28	$ 1.28
Discontinued operations (in dollars per share)					0.07	0.05	0.10	0.10	0.03	0.34	0.27
Net income attributable to common stockholders (in dollars per share)					0.36	0.33	0.47	0.41	1.26	1.62	1.55
Diluted:
Income from continuing operations attributable to common stockholders (in dollars per share)					0.29	0.27	0.37	0.31	1.22	1.26	1.27
Discontinued operations (in dollars per share)					0.07	0.05	0.10	0.10	0.03	0.34	0.27
Net income attributable to common stockholders (in dollars per share)					0.36	0.32	0.47	0.41	$ 1.25	$ 1.60	$ 1.54
Dividends declared per common share (in dollars per share)	$ 0.730	$ 0.730	$ 0.790	$ 0.790	$ 0.79	$ 0.725	$ 0.725	$ 0.725