Significant Accounting Policies - Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accrued Liabilities
Accrued interest	$ 20,738	$ 12,109
Partner prepayments	10,937	4,061
Other accrued liabilities	5,192	5,434
Total accrued liabilities	$ 36,867	$ 21,604