Financial Instruments - Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Restricted cash, current	$ 25,860	$ 44,387
Restricted cash, non-current	32,416	32,397
Total restricted cash	58,276	76,784
Consolidated Entity, Excluding VIEs
Variable Interest Entity [Line Items]
Restricted cash, current	21,125	36,418
Restricted cash, non-current	4,812	5,649
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Restricted cash, current	4,735	7,969
Restricted cash, non-current	$ 27,604	$ 26,748