Earnings Per Share (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
Numerator for basic and diluted earnings per share:
Income from continuing operations attributable to common stockholders			$ 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
Denominator:
Denominator for basic earnings per share - weighted average shares (in shares)									330,311,000	294,175,000	292,654,000
Effect of dilutive securities:
Stock options (in shares)									360,000	495,000	534,000
Restricted stock awards (in shares)									41,000	55,000	99,000
OP units (in shares)									3,295,000	1,952,000	1,823,000
Denominator for diluted earnings per share - adjusted weighted average shares (in shares)									334,007,000	296,677,000	295,110,000
Basic earnings per share:
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 earnings per share:
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
Anti-dilutive options outstanding (in shares)									852,805	479,291	504,815