Real Estate Properties - Summary of Income Statement Information for Properties Disposed of (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statements of Operations:
Rental income	$ 125,459	$ 174,216	$ 457,128	$ 518,663
Tenant reimbursements and other income	33,749	42,379	118,829	130,386
Total revenues	159,208	216,595	575,957	649,049
Operating expenses	73,571	99,392	261,128	293,824
General and administrative	16,249	47,450	43,718	96,395
Total expenses	130,342	204,055	478,866	576,839
Operating income	28,866	12,540	97,091	72,210
Interest and other income	637	406	4,813	1,071
Interest expense	(25,111)	(35,245)	(82,926)	(111,079)
Income from discontinued operations	$ 0	95	$ 0	8,220
Discontinued operations, properties sold
Statements of Operations:
Rental income		(16)		14,220
Tenant reimbursements and other income		407		1,730
Total revenues		391		15,950
Operating expenses		261		7,115
General and administrative		6		9
Total expenses		267		7,124
Operating income		124		8,826
Interest and other income		2		2
Interest expense		(31)		(608)
Income from discontinued operations		$ 95		$ 8,220