Real Estate Properties (Details 3) (USD $) In Thousands, 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
Statements of Operations:
Rental income									$ 707,380	$ 709,390	$ 601,632
Tenant reimbursements and other income									178,156	172,515	139,712
Total revenues	198,419	198,283	246,181	242,653	231,957	222,632	217,346	209,970	885,536	881,905	741,344
Operating expenses									370,869	356,604	297,408
Depreciation and amortization									218,854	209,759	171,662
General and administrative									77,209	46,057	38,221
Acquisition related costs									318	5,648	9,731
Total expenses									667,250	618,068	520,058
Operating income									218,286	263,837	221,286
Interest and other income									1,229	1,404	1,662
Interest expense									(173,011)	(201,840)	(191,876)
(Loss) income from discontinued operations									15,867	9,783	22,307
Discontinued operations, properties sold or held for sale
Statements of Operations:
Rental income									110,955	134,962	163,159
Tenant reimbursements and other income									18,453	22,128	27,905
Total revenues									129,408	157,090	191,064
Operating expenses									76,203	87,774	104,449
Depreciation and amortization									28,098	48,533	51,493
General and administrative									7,563	8,675	9,254
Acquisition related costs											471
Total expenses									111,864	144,982	165,667
Operating income									17,544	12,108	25,397
Interest and other income									65	79	58
Interest expense									(1,742)	(2,404)	(3,148)
(Loss) income from discontinued operations									$ 15,867	$ 9,783	$ 22,307