Commitments and Contingencies (Details Textual) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012 Number	Mar. 31, 2011	Dec. 31, 2011
Commitments and Contingencies (Textual) [Abstract]
Number of outstanding letters of credit	5
Letter of credit obligation	$ 5,515
Outstanding construction financings for leased properties	150,750		189,502
Additional financing to complete construction	240,765
Total contingent purchase obligations	50,818
Minimum part of economic life of the leased asset to be classified as capital lease	75.00%
Minimum net present value of the future minimum lease payments to be classified as capital lease.	90.00%
Operating lease obligations relating to certain ground leases		88,470
Rental expense related to company office space	290	515
Aggregated future minimum rentals to be received	$ 43,108