Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator for basic and diluted earnings per share:
Income from continuing operations attributable to common stockholders	$ 108,338	[1]	$ 127,268	[1]	$ 132,895	[1]	$ 120,429	[1]	$ 101,439	[2]	$ 115,737	[2]	$ 43,413	[2]	$ 47,246	[2]	$ 488,930	$ 307,835	$ 362,308
Discontinued operations	102	[1]	(8,972)	[1]	(18,315)	[1]	(8,236)	[1]	(15,172)	[2]	(3,855)	[2]	30,612	[2]	43,380	[2]	(35,421)	54,965	2,185
Net income attributable to common stockholders	$ 108,440		$ 118,296		$ 114,580		$ 112,193		$ 86,267		$ 111,882		$ 74,025		$ 90,626		$ 453,509	$ 362,800	$ 364,493
Denominator:
Denominator for basic earnings per share - weighted average shares																	292,654,000	292,064,000	228,453,000
Effect of dilutive securities:
Stock options (in shares)																	534,000	496,000	449,000
Restricted stock awards (in shares)																	99,000	92,000	53,000
OP units (in shares)																	1,823,000	1,836,000	942,000
Convertible notes (in shares)																	0	0	893,000
Denominator for diluted earnings per share - adjusted weighted average shares																	295,110,000	294,488,000	230,790,000
Basic earnings per share:
Income from continuing operations attributable to common stockholders (in dollars per share)	$ 0.37		$ 0.43		$ 0.45		$ 0.41		$ 0.35		$ 0.39		$ 0.15		$ 0.16		$ 1.67	$ 1.05	$ 1.59
Discontinued operations (in dollars per share)	$ 0.00		$ (0.03)		$ (0.06)		$ (0.03)		$ (0.05)		$ (0.01)		$ 0.11		$ 0.15		$ (0.12)	$ 0.19	$ 0.01
Net income attributable to common stockholders (in dollars per share)	$ 0.37		$ 0.40		$ 0.39		$ 0.38		$ 0.30		$ 0.38		$ 0.26		$ 0.31		$ 1.55	$ 1.24	$ 1.60
Diluted earnings per share:
Income from continuing operations attributable to common stockholders (in dollars per share)	$ 0.37		$ 0.43		$ 0.45		$ 0.41		$ 0.35		$ 0.39		$ 0.15		$ 0.16		$ 1.66	$ 1.04	$ 1.57
Discontinued operations (in dollars per share)	$ 0.00		$ (0.03)		$ (0.06)		$ (0.03)		$ (0.05)		$ (0.01)		$ 0.10		$ 0.15		$ (0.12)	$ 0.19	$ 0.01
Net income attributable to common stockholders (in dollars per share)	$ 0.37		$ 0.40		$ 0.39		$ 0.38		$ 0.30		$ 0.38		$ 0.25		$ 0.31		$ 1.54	$ 1.23	$ 1.58
Anti-dilutive options outstanding (in shares)																	504,815	372,440	309,650

[1] The amounts presented for the three months ended March 31, 2013, June 30, 2013 and September 30, 2013 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 2013 or classified as held for sale as of December 31, 2013.

[2] The amounts presented for the three months ended March 31, 2012, June 30, 2012, September 30, 2012 and December 31, 2012 differ from the amounts previously reported in our Annual Report on Form 10-K for the year ended December 31, 2012 as a result of discontinued operations consisting of properties sold in 2013 or classified as held for sale as of December 31, 2013.