Real Estate Properties (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013 property	Sep. 30, 2012	Sep. 30, 2013 property	Sep. 30, 2012	Dec. 31, 2012
Real Estate Properties
Number of properties sold			2
Number of properties held for sale	3		3
Balance Sheets:
Real estate properties	$ 1,327,318		$ 1,327,318		$ 1,311,202
Acquired real estate leases, net	131,683		131,683		144,402
Rents receivable	31,373		31,373		28,882
Other assets	18,322		18,322		10,360
Total assets	1,553,079		1,553,079		1,562,134
Other liabilities	23,250		23,250		18,910
Total liabilities	552,617		552,617		534,683
Statements of Operations:
Rental income	56,401	52,426	168,639	149,071
Real estate taxes	(6,255)	(5,498)	(19,060)	(16,520)
Utility expenses	(5,355)	(4,801)	(13,064)	(12,035)
Other operating expenses	(10,169)	(9,171)	(29,288)	(26,667)
Depreciation and amortization	(14,032)	(12,537)	(40,960)	(35,642)
General and administrative	(2,941)	(3,529)	(9,350)	(9,071)
Loss on asset impairment from discontinued operations			(10,142)
Net gain on sale of properties from discontinued operations			8,168
Income (loss) from discontinued operations	(10,055)	67	(1,121)	1,201
Discontinued operations
Balance Sheets:
Real estate properties	25,567		25,567		46,784
Acquired real estate leases, net					82
Rents receivable	517		517		217
Other assets	123		123		59
Total assets	26,207		26,207		47,142
Other liabilities	288		288		298
Total liabilities	288		288		298
Statements of Operations:
Rental income	886	1,657	3,831	5,740
Real estate taxes	(123)	(230)	(505)	(690)
Utility expenses	(112)	(339)	(447)	(810)
Other operating expenses	(255)	(394)	(775)	(1,076)
Depreciation and amortization	(241)	(519)	(1,025)	(1,639)
General and administrative	(68)	(108)	(226)	(324)
Loss on asset impairment from discontinued operations	(10,142)		(10,142)
Net gain on sale of properties from discontinued operations			8,168
Income (loss) from discontinued operations	$ (10,055)	$ 67	$ (1,121)	$ 1,201