Real Estate Properties - Schedule of Disposal Groups, Including Discontinued Operations, Income Statement and Additional Disclosures (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statements of Operations:
Rental income									$ 691,699	$ 763,262	$ 774,600
Tenant reimbursements and other income									170,158	189,767	186,487
Total revenues	212,808	216,595	215,194	217,260	214,028	214,201	262,188	262,612	861,857	953,029	961,087
Operating expenses									387,982	410,045	397,673
Depreciation and amortization									227,532	234,402	230,284
General and administrative									113,155	80,504	49,312
Total expenses									913,741	849,522	682,917
Operating (loss) income									(51,884)	103,507	278,170
Interest and other income									1,561	1,229	1,410
Interest expense									(143,230)	(173,011)	(202,055)
(Loss) income from discontinued operations									8,389	6,393	(4,341)
Discontinued operations, properties sold or held for sale
Statements of Operations:
Rental income									14,243	55,073	69,752
Tenant reimbursements and other income									1,900	6,842	8,156
Total revenues									16,143	61,915	77,908
Operating expenses									7,138	37,027	46,705
Depreciation and amortization									0	12,550	28,008
General and administrative									10	4,268	5,420
Total expenses									7,148	53,845	80,133
Operating (loss) income									8,995	8,070	(2,225)
Interest and other income									2	65	73
Interest expense									(608)	(1,742)	(2,189)
(Loss) income from discontinued operations									$ 8,389	$ 6,393	$ (4,341)