Commitments and Contingencies (Details Textual)	6 Months Ended
	Jun. 30, 2018 USD ($) obligation	Dec. 31, 2017 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Number of outstanding credit obligations | obligation	14
Letter of credit obligation	$ 72,966,000
Outstanding construction financings for leased properties	200,569,000	$ 237,746,000
Additional financing to complete construction	369,576,000
Total contingent purchase obligations	$ 9,317,000
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	$ 1,115,108,000
Capital leases, future minimum payments due	86,352,000
Operating leases, future minimum payments due, future minimum sublease rentals	$ 75,100,000