Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	12 Months Ended					12 Months Ended				12 Months Ended				1 Months Ended																																		1 Months Ended														12 Months Ended												12 Months Ended
	Mar. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Real estate properties held for sale property	Dec. 31, 2012 Real estate properties held for sale	Dec. 31, 2013 Unsecured revolving credit facility, due October 2015	Dec. 31, 2012 Unsecured revolving credit facility, due October 2015	Dec. 31, 2013 Unsecured revolving credit facility, due March 2016	Dec. 31, 2012 Unsecured revolving credit facility, due March 2016	Dec. 31, 2013 Unsecured term loan, due December 2016	Dec. 31, 2012 Unsecured term loan, due December 2016	Dec. 31, 2013 Unsecured term loan, due July 2017	Dec. 31, 2012 Unsecured term loan, due July 2017	Dec. 31, 2013 Senior Notes, due 2014 at 5.75%	Oct. 31, 2013 Senior Notes, due 2014 at 5.75%	Mar. 31, 2013 Senior Notes, due 2014 at 5.75%	Dec. 31, 2012 Senior Notes, due 2014 at 5.75%	Dec. 31, 2013 Senior Notes, due 2015 at 6.40%	Mar. 31, 2013 Senior Notes, due 2015 at 6.40%	Dec. 31, 2012 Senior Notes, due 2015 at 6.40%	Dec. 31, 2013 Senior Notes, due 2015 at 5.75%	Mar. 31, 2013 Senior Notes, due 2015 at 5.75%	Dec. 31, 2012 Senior Notes, due 2015 at 5.75%	Dec. 31, 2013 Senior Notes, due 2016 at 6.25%	Mar. 31, 2013 Senior Notes, due 2016 at 6.25%	Dec. 31, 2012 Senior Notes, due 2016 at 6.25%	Dec. 31, 2013 Senior Notes, due 2017 at 6.25%	Dec. 31, 2012 Senior Notes, due 2017 at 6.25%	Dec. 31, 2013 Senior Notes, due 2018 at 6.65%	Dec. 31, 2012 Senior Notes, due 2018 at 6.65%	Dec. 31, 2013 Senior Notes, due 2019 at 7.50%	Dec. 31, 2012 Senior Notes, due 2019 at 7.50%	Dec. 31, 2013 Senior Notes, due 2020 at 5.875%	Dec. 31, 2012 Senior Notes, due 2020 at 5.875%	Dec. 31, 2013 Senior Notes, due 2042 at 5.75%	Dec. 31, 2012 Senior Notes, due 2042 at 5.75%	Dec. 31, 2013 Mortgage notes maturing from 2014 through 2026 property	Dec. 31, 2013 Mortgage Notes Payable, due 2014 at 4.95%	Dec. 31, 2012 Mortgage Notes Payable, due 2014 at 4.95%	Dec. 31, 2013 Mortgage Notes Payable, due 2015 at 5.99%	Dec. 31, 2012 Mortgage Notes Payable, due 2015 at 5.99%	Dec. 31, 2013 Mortgage Notes Payable, due 2015 at 5.78%	Dec. 31, 2012 Mortgage Notes Payable, due 2015 at 5.78%	Dec. 31, 2013 Mortgage Notes Payable, due 2016 at 5.235%	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 5.235%	Dec. 31, 2013 Mortgage Notes Payable, due 2016 at 5.689%	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 5.689%	Dec. 31, 2013 Mortgage Notes Payable, due 2016 at 5.76%	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 5.76%	Dec. 31, 2013 Mortgage Notes Payable, due 2016 at 6.03%	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 6.03%	Dec. 31, 2013 Mortgage Notes Payable, due 2016 at 6.29%	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 6.29%	Dec. 31, 2013 Mortgage Notes Payable, due 2016 at 7.36%	Dec. 31, 2012 Mortgage Notes Payable, due 2016 at 7.36%	Dec. 31, 2013 Mortgage Notes Payable, due 2017 at 5.67%	Dec. 31, 2012 Mortgage Notes Payable, due 2017 at 5.67%	Dec. 31, 2013 Mortgage Notes Payable, due 2017 at 5.68%	Dec. 31, 2012 Mortgage Notes Payable, due 2017 at 5.68%	Dec. 31, 2013 Mortgage Notes Payable, due 2017 at 5.95%	Dec. 31, 2012 Mortgage Notes Payable, due 2017 at 5.95%	Dec. 31, 2013 Mortgage Notes Payable, due 2019	Dec. 31, 2012 Mortgage Notes Payable, due 2019	Dec. 31, 2013 Mortgage Notes Payable, due 2021 at 5.69%	Dec. 31, 2012 Mortgage Notes Payable, due 2021 at 5.69%	Dec. 31, 2013 Mortgage Notes Payable, due 2021 at 5.30%	Dec. 31, 2012 Mortgage Notes Payable, due 2021 at 5.30%	Dec. 31, 2013 Mortgage Notes Payable, due 2022 at 6.75%	Dec. 31, 2012 Mortgage Notes Payable, due 2022 at 6.75%	Dec. 31, 2013 Mortgage Notes Payable, due 2023 at 6.14%	Dec. 31, 2012 Mortgage Notes Payable, due 2023 at 6.14%	Dec. 31, 2013 Mortgage Notes Payable, due 2026 at 5.71%	Dec. 31, 2012 Mortgage Notes Payable, due 2026 at 5.71%	Dec. 31, 2013 Unsecured revolving credit facility and unsecured term loan	Jun. 19, 2013 Unsecured revolving credit facility and unsecured term loan
Indebtedness
Outstanding indebtedness		$ 2,986,211	$ 4,324,273		$ 20,402		$ 235,000	$ 297,000		$ 95,000	$ 500,000	$ 500,000		$ 350,000				$ 244,655	$ 33,440		$ 186,000	$ 138,773		$ 250,000	$ 139,104		$ 400,000	$ 250,000	$ 250,000	$ 250,000	$ 250,000	$ 125,000	$ 125,000	$ 250,000	$ 250,000	$ 175,000	$ 175,000		$ 12,040	$ 12,356	$ 7,987	$ 8,272		$ 8,980	$ 116,000	$ 116,000		$ 7,500		$ 7,474	$ 40,334	$ 40,854	$ 144,522	$ 146,264	$ 10,804	$ 11,302	$ 41,275	$ 41,275	$ 265,000	$ 265,000		$ 18,447	$ 173,247	$ 174,870	$ 28,391	$ 28,793	$ 39,598	$ 40,185	$ 3,456	$ 3,759		$ 12,650	$ 7,240	$ 7,637
Less unamortized net premiums and discounts		19,199	25,548
Total		3,005,410	4,349,821
Interest rate stated percentage															5.75%		5.75%		6.40%	6.40%		5.75%	5.75%		6.25%	6.25%		6.25%		6.65%		7.50%		5.875%		5.75%			4.95%		5.99%		5.78%		5.235%		5.689%			5.76%	6.03%		6.29%		7.36%		5.67%		5.68%		5.95%				5.69%		5.30%		6.75%		6.14%		5.71%
Period for which interest on loan payable is fixed by a cash flow hedge																																																															7 years
Average interest rate mortgages (as a percent)																																																															5.66%
Principal	670,295																145,612			152,560			111,227			260,896
Purchase Price	726,151																148,746			164,140			121,047			292,218
Loss on early extinguishment of debt	(60,027)	(60,052)	(287)	(35)											(1,011)
Prepayment of debt																99,043																																	7,093
Maximum borrowing capacity							750,000
Maximum amount of borrowing							1,500,000
Maturity date extension period available							1 year
Increased maximum borrowing capacity available under specified circumstances											1,000,000
Variable rate basis							LIBOR		LIBOR		LIBOR		LIBOR																																																		LIBOR
Basis points on variable rate (as a percent)							1.50%	1.50%			1.85%	1.85%
Facility fee of lending commitments under our revolving credit facility (as a percent)							0.35%
Interest rate payable (as a percent)							1.70%	1.70%			2.00%	2.10%
Average interest rate (as a percent)							1.70%	1.50%			2.00%	1.80%
Amount outstanding							235,000
Available Borrowing Capacity							515,000
Percentage holding of aggregate credit exposure required by lender to accelerate payment in case of default																																																																											67.00%
Percentage holding of aggregate credit exposure required by lender to accelerate payment																																																																											51.00%
Number of Real Estate Properties Secured by Mortgage					2																																	13
Number of buildings secured by mortgage					3																																	18
Cost of properties mortgaged		5,537,165	7,829,409		536,552	164,041																																1,269,185
Mortgaged properties aggregate net book value		4,642,106	6,821,803																																			1,109,259
Mortgage notes payable, net		914,510	984,827		20,018																																	914,510
Percentage of principal amount at which the potential acquiring entity may offer to repurchase the note																																																																												51.00%
Principal payments due
2014		18,826
2015		421,821
2016		948,708
2017		560,592
2018		254,614
Thereafter		781,650
Total		$ 2,986,211	$ 4,324,273		$ 20,402		$ 235,000	$ 297,000		$ 95,000	$ 500,000	$ 500,000		$ 350,000				$ 244,655	$ 33,440		$ 186,000	$ 138,773		$ 250,000	$ 139,104		$ 400,000	$ 250,000	$ 250,000	$ 250,000	$ 250,000	$ 125,000	$ 125,000	$ 250,000	$ 250,000	$ 175,000	$ 175,000		$ 12,040	$ 12,356	$ 7,987	$ 8,272		$ 8,980	$ 116,000	$ 116,000		$ 7,500		$ 7,474	$ 40,334	$ 40,854	$ 144,522	$ 146,264	$ 10,804	$ 11,302	$ 41,275	$ 41,275	$ 265,000	$ 265,000		$ 18,447	$ 173,247	$ 174,870	$ 28,391	$ 28,793	$ 39,598	$ 40,185	$ 3,456	$ 3,759		$ 12,650	$ 7,240	$ 7,637