Commitments and Contingencies - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) ft² property renewal_option	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Property, Plant and Equipment [Line Items]
Number of Properties | property	198
Land held for development	$ 17,452	$ 17,054
Approximate square feet | ft²	14,300,000
Investment in land held for development	$ 17,500	17,100	$ 17,100
Number of real estate investments subject to office and ground leases | property	45
Number of ground leases related to real estate investments under operating lease agreements | property	94
Square feet subject to ground leases | ft²	7,600,000
Ground leases, weighted average remaining term	69 years 10 months 24 days
Operating lease expiration period maximum expiration date	2105
Approximately corporate office lease covers of rented space | ft²	36,653
Percentage on increases of annual base rent on the corporate office lease	3.25%
Rental expense relating to the operating leases	$ 5,100	4,900	4,400
Prepaid ground leases which represented rental expense	500	$ 500	$ 500
First and Second Generation Leases [Member] | Tenant Improvement Allowances [Member]
Property, Plant and Equipment [Line Items]
Anticipated amount of investment	$ 28,800
Corporate Office Lease [Member]
Property, Plant and Equipment [Line Items]
Corporate lease expiration date	Oct. 31, 2020
Tennessee [Member]
Property, Plant and Equipment [Line Items]
Number of Properties | property	2
Number of redevelopment properties under expansion | property	1
Total amount funded	$ 21,800
Land held for development	4,300
Estimated total investment	51,800
Alabama [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	6,900
Estimated Remaining Fundings (unaudited)	8,500
Estimated total investment	$ 15,400
Retail Site [Member] | Texas [Member]
Property, Plant and Equipment [Line Items]
Number of Properties | property	1
Total amount funded	$ 3,316
Estimated Remaining Fundings (unaudited)	2,259
Land held for development	1,500
Estimated total investment	$ 5,575
Approximate square feet | ft²	12,900
Construction in progress fundings	$ 3,316
Retail Site [Member] | Colorado [Member]
Property, Plant and Equipment [Line Items]
Estimated total investment	$ 26,500
Approximate square feet | ft²	98,000
Construction in progress fundings	$ 229
Minimum [Member]
Property, Plant and Equipment [Line Items]
Ground leases, initial term	50 years
Ground leases, number of renewal options | renewal_option	1
Ground leases, renewal term	75 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Ground leases, initial term	75 years
Ground leases, number of renewal options | renewal_option	2
Ground leases, renewal term	100 years