Investments in Real Estate and Real Estate Under Construction Summary of development arrangements outstanding (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014 sqft
Investments in Real Estate and Real Estate Under Construction [Line Items]
Square Feet	1,366,000
Expected Maximum Commitment/Contribution	$ 309,155
Oak Creek Wisconsin [Member] | Industrial property [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Square Feet	164,000
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	330,000
Expected Maximum Commitment/Contribution	110,137
Lease Term (Years)	15 years
Lake Jackson, Texas [Member] | Office and R and D Property [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Square Feet	664,000
Expected Maximum Commitment/Contribution	166,164
Lease Term (Years)	20 years
Thomson, Georgia [Member] | Industrial property [Member]
Investments in Real Estate and Real Estate Under Construction [Line Items]
Square Feet	208,000
Expected Maximum Commitment/Contribution	$ 10,245
Lease Term (Years)	15 years