Real Estate Properties - Summary of Income Statement Information for Properties Disposed of (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statements of Operations:
Rental income	$ 163,697	$ 172,407	$ 331,669	$ 344,447
Tenant reimbursements and other income	39,997	42,787	85,080	88,007
Total revenues	203,694	215,194	416,749	432,454
Operating expenses	89,686	92,701	187,557	194,432
General and administrative	10,911	24,097	27,469	48,945
Total expenses	169,492	199,312	348,524	372,784
Operating income	34,202	15,882	68,225	59,670
Interest expense	(27,973)	(37,899)	(57,815)	(75,834)
Income from discontinued operations	$ 0	4,114	$ 0	8,125
Discontinued operations, properties sold
Statements of Operations:
Rental income		6,962		14,236
Tenant reimbursements and other income		801		1,323
Total revenues		7,763		15,559
Operating expenses		3,375		6,854
General and administrative		0		3
Total expenses		3,375		6,857
Operating income		4,388		8,702
Interest expense		(274)		(577)
Income from discontinued operations		$ 4,114		$ 8,125