Real Estate Properties (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013 property	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013 property	Dec. 31, 2012	Dec. 31, 2011
Real Estate Properties
Number of properties sold									2
Number of properties held for sale	3								3
Balance Sheet:
Real estate properties	$ 1,380,927				$ 1,311,202				$ 1,380,927	$ 1,311,202
Acquired real estate leases, net	142,266				144,402				142,266	144,402
Rents receivable, net	33,350				28,882				33,350	28,882
Other assets, net	25,031				10,360				25,031	10,360
Total assets	1,632,452				1,562,134				1,632,452	1,562,134
Other liabilities	23,216				18,910				23,216	18,910
Total liabilities	642,777				534,683				642,777	534,683
Statement of Operations:
Rental income	58,271	56,401	55,934	56,304	54,629	52,426	48,584	48,061	226,910	203,700	168,074
Real estate taxes									(25,710)	(22,485)	(18,426)
Utility expenses									(17,116)	(15,767)	(13,918)
Other operating expenses									(41,134)	(37,074)	(29,773)
Depreciation and amortization									(55,699)	(49,070)	(37,776)
General and administrative									(12,710)	(11,924)	(10,469)
Loss on asset impairment									(10,142)	(494)
Income (loss) from discontinued operations									(889)	900	3,806
Discontinued operations
Balance Sheet:
Real estate properties	25,574				46,784				25,574	46,784
Acquired real estate leases, net					82					82
Rents receivable, net	381				217				381	217
Other assets, net	42				59				42	59
Total assets	25,997				47,142				25,997	47,142
Other liabilities	276				298				276	298
Total liabilities	276				298				276	298
Statement of Operations:
Rental income									4,580	7,376	10,876
Real estate taxes									(678)	(928)	(919)
Utility expenses									(539)	(1,043)	(1,398)
Other operating expenses									(966)	(1,484)	(2,011)
Depreciation and amortization									(1,025)	(2,096)	(2,313)
General and administrative									(287)	(431)	(429)
Loss on asset impairment									(10,142)	(494)
Net gain on sale of properties									8,168
Income (loss) from discontinued operations									$ (889)	$ 900	$ 3,806