Real Estate Properties - Summary of Income Statement Information for Properties Included in Discontinued Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statements of Operations:
Rental income									$ 570,382	$ 691,699	$ 763,262
Tenant reimbursements and other income									144,509	170,158	189,767
Total revenues	$ 138,934	$ 159,208	$ 203,694	$ 213,055	$ 212,808	$ 216,595	$ 215,194	$ 217,260	714,891	861,857	953,029
Operating expenses									324,948	387,982	410,045
Depreciation and amortization									194,001	227,532	234,402
General and administrative									57,457	113,155	80,504
Total expenses									593,568	913,741	849,522
Operating income (loss)									121,323	(51,884)	103,507
Interest and other income									5,989	1,561	1,229
Interest expense									(107,316)	(143,230)	(173,011)
Income from discontinued operations									$ 0	8,389	6,393
Discontinued operations, properties sold
Statements of Operations:
Rental income										14,243	55,073
Tenant reimbursements and other income										1,900	6,842
Total revenues										16,143	61,915
Operating expenses										7,138	37,027
Depreciation and amortization										0	12,550
General and administrative										10	4,268
Total expenses										7,148	53,845
Operating income (loss)										8,995	8,070
Interest and other income										2	65
Interest expense										(608)	(1,742)
Income from discontinued operations										$ 8,389	$ 6,393