Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per 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
Quarterly Financial Data [Abstract]
Revenues	$ 660,711	[1]	$ 641,520	[1]	$ 614,502	[1]	$ 568,566	[1]	$ 563,145	[2]	$ 554,691	[2]	$ 359,421	[2]	$ 266,396	[2]	$ 2,485,299	$ 1,743,653	$ 1,000,634
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
Basic:
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:
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
Dividends declared per common share (in dollars per share)	$ 0.62		$ 0.62		$ 0.62		$ 0.62		$ 0.575		$ 0.4486		$ 0.7014		$ 0.575

[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.