Financial Instruments - Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Restricted cash, current	$ 17,931	$ 44,387
Restricted cash, non-current	30,779	32,397
Total restricted cash	48,710	76,784
Consolidated Entity, Excluding VIEs
Variable Interest Entity [Line Items]
Restricted cash, current	14,679	36,418
Restricted cash, non-current	3,001	5,649
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Restricted cash, current	3,252	7,969
Restricted cash, non-current	$ 27,778	$ 26,748