Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	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
Numerator for basic and diluted earnings per share:
Income from continuing operations attributable to common stockholders	$ 97,992	[1]	$ 115,975	[1]	$ 43,496	[1]	$ 48,110	[1]	$ 193,553	[2]	$ 103,191	[2]	$ 18,450	[2]	$ 47,939	[2]	$ 305,573	$ 363,133	$ 213,444
Discontinued operations	(11,725)	[1]	(4,093)	[1]	30,529	[1]	42,516	[1]	(605)	[2]	(306)	[2]	1,226	[2]	1,045	[2]	57,227	1,360	32,723
Net income attributable to common stockholders	$ 86,267		$ 111,882		$ 74,025		$ 90,626		$ 192,948		$ 102,885		$ 19,676		$ 48,984		$ 362,800	$ 364,493	$ 246,167
Denominator:
Denominator for basic earnings per share - weighted average shares																	292,064,000	228,453,000	156,608,000
Effect of dilutive securities:
Stock options (in shares)																	496,000	449,000	407,000
Restricted stock awards (in shares)																	92,000	53,000	70,000
OP units (in shares)																	1,836,000	942,000	0
Convertible notes (in shares)																	0	893,000	572,000
Denominator for diluted earnings per share - adjusted weighted average shares																	294,488,000	230,790,000	157,657,000
Basic earnings per share:
Income from continuing operations attributable to common stockholders (in dollars per share)	$ 0.33		$ 0.39		$ 0.15		$ 0.16		$ 0.67		$ 0.36		$ 0.10		$ 0.30		$ 1.04	$ 1.59	$ 1.36
Discontinued operations (in dollars per share)	$ (0.04)		$ (0.01)		$ 0.11		$ 0.15		$ 0.00		$ 0.00		$ 0.01		$ 0.01		$ 0.20	$ 0.01	$ 0.21
Net income attributable to common stockholders (in dollars per share)	$ 0.29		$ 0.38		$ 0.26		$ 0.31		$ 0.67		$ 0.36		$ 0.11		$ 0.31		$ 1.24	$ 1.60	$ 1.57
Diluted earnings per share:
Income from continuing operations attributable to common stockholders (in dollars per share)	$ 0.33		$ 0.39		$ 0.15		$ 0.16		$ 0.66		$ 0.35		$ 0.10		$ 0.29		$ 1.04	$ 1.57	$ 1.35
Discontinued operations (in dollars per share)	$ (0.04)		$ (0.01)		$ 0.10		$ 0.15		$ 0.00		$ 0.00		$ 0.01		$ 0.01		$ 0.19	$ 0.01	$ 0.21
Net income attributable to common stockholders (in dollars per share)	$ 0.29		$ 0.38		$ 0.25		$ 0.31		$ 0.66		$ 0.35		$ 0.11		$ 0.30		$ 1.23	$ 1.58	$ 1.56
Anti-dilutive options outstanding (in shares)																	372,440	309,650	0

[1] The amounts presented for the three months ended March 31, 2012, June 30, 2012 and September 30, 2012 differ from the amounts previously reported in our Quarterly Reports on Form 10-Q as a result of discontinued operations consisting of properties sold in 2012 or classified as held for sale as of December 31, 2012.

[2] The amounts presented for the three months ended March 31, 2011, June 30, 2011, September 30, 2011 and December 31, 2011 differ from the amounts previously reported in our Annual Report on Form 10-K for the year ended December 31, 2011 as a result of discontinued operations consisting of properties sold in 2012 or classified as held for sale as of December 31, 2012.