Quarterly Financial Information (Unaudited) (Summarized Unaudited Consolidated Quarterly Information) (Details) (USD $) In Thousands, except Per 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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 732,856	[1]	$ 710,924	[1]	$ 683,764	[1]	$ 682,509	[1]	$ 655,730	[2]	$ 637,218	[2]	$ 610,188	[2]	$ 564,597	[2]	$ 2,810,053	$ 2,467,733	$ 1,731,704
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
Basic:
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:
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
Dividends declared per common share (in dollars per share)	$ 0.725		$ 0.67		$ 0.67		$ 0.67		$ 0.62		$ 0.62		$ 0.62		$ 0.62

[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.