Investments in real estate, net Acquired leases (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Values of acquired leases
Value of intangible assets, net	$ (11,405,000)	$ (14,721,000)
Minimum lease payments to be received under the terms of the operating lease agreements, excluding expense reimbursements
2014	443,942,000
2015	466,037,000
2016	444,082,000
2017	408,935,000
2018	362,370,000
Thereafter	2,161,810,000
Total minimum lease payments to be received	4,287,176,000
Above and Below Market Leases
Real Estate Properties [Line Items]
Amortization of intangible assets	3,300,000	3,200,000	9,300,000
Weighted average amortization period	2 years 4 months 12 days
Values of acquired leases
Value of intangible assets, gross	55,599,000	55,599,000
Accumulated amortization	(44,194,000)	(40,878,000)
Value of intangible assets, net	11,405,000	14,721,000
Estimated annual amortization
2014	3,223,000
2015	3,011,000
2016	2,641,000
2017	2,038,000
2018	492,000
Thereafter	0
Leases, Acquired-in-Place
Real Estate Properties [Line Items]
Amortization of intangible assets	2,400,000	2,700,000	3,400,000
Weighted average amortization period	8 years 7 months 12 days
Values of acquired leases
Value of intangible assets, gross	48,189,000	45,225,000
Accumulated amortization	(28,927,000)	(26,600,000)
Value of intangible assets, net	19,262,000	18,625,000
Estimated annual amortization
2014	2,633,000
2015	2,506,000
2016	2,299,000
2017	2,124,000
2018	1,949,000
Thereafter	$ 7,751,000