Investments in Real Estate and Real Estate Under Construction - Summary of development arrangements outstanding (Details) - Jun. 30, 2015 ft² in Thousands, $ in Thousands	USD ($) ft²
Investments in Real Estate and Real Estate Under Construction [Line Items]
Square Feet | ft²	2,483
Expected Maximum Commitment/Contribution	$ 368,922
Oak Creek Wisconsin [Member] | Industrial property [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Square Feet | ft²	164
Expected Maximum Commitment/Contribution	$ 22,609
Lease Term (Years)	20 years
Richmond, Virginia [Member] | Office Building [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Square Feet | ft²	330
Expected Maximum Commitment/Contribution	$ 110,137
Lease Term (Years)	15 years
Anderson, South Carolina [Member] | Industrial property [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Square Feet | ft²	1,325
Expected Maximum Commitment/Contribution	$ 70,012
Lease Term (Years)	20 years
Lake Jackson, Texas [Member] | Office Building [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Square Feet | ft²	664
Expected Maximum Commitment/Contribution	$ 166,164
Lease Term (Years)	20 years