Commitments and Contingencies (Details Textuals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies (Textuals) [Abstract]
Number of outstanding letters of credit	14
Outstanding letters of credit	$ 159,151
Additional financing to complete construction	429,815
Total contingent purchase obligations	11,832
Operating lease obligations relating to certain ground leases	1,125,098
Aggregate future minimum rentals to be received	77,385
Gross assets under capital lease recorded in real property	167,324
Outstanding construction financings for leased properties	237,746	$ 506,091
Accumulated depreciation assets under capital lease recorded in real property	$ 29,303
Minimum part of economic life of leased asset to be classified as capital lease	75.00%
Minimum net present value of future minimum lease payments to be classified as capital lease	90.00%
Letter of credit expiration date maximum	Feb. 01, 2018
Letter of credit expiration date minimum	Apr. 30, 2024
Expense recognized for transferring corporate headquarters as gift	$ 40,730