Leases - Capital Leasers (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Capital Leased Assets [Line Items]
Capital lease obligation	$ 70,611	$ 70,129
991 Madison Avenue New York, NY
Capital Leased Assets [Line Items]
Capital lease term	49 years
Capital lease obligation	$ 76,600
991 Madison Avenue New York, NY | Capital Lease Obligations
Capital Leased Assets [Line Items]
Repayments capital lease obligations	$ 2,500	$ 1,300