Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Debt	$ 698,443
ODAST II Agreement | Secured Debt
Debt Instrument [Line Items]
Weighted Average Interest Rate at December 31, 2017	4.70%
Debt	$ 250,000	$ 250,000
ODART Agreement | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at December 31, 2017	4.10%
Debt	$ 102,058	133,767
RAOD Agreement | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at December 31, 2017	4.90%
Debt	$ 86,478	99,985
ODAF Agreement | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at December 31, 2017	8.60%
Debt	$ 75,000	100,000
ODAC Agreement | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at December 31, 2017	8.70%
Debt	$ 62,350	65,486
PORT II Agreement | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at December 31, 2017	3.90%
Debt	$ 63,851	52,397
Other Agreements | Revolving Credit Facility
Debt Instrument [Line Items]
Debt	50,706	30,887
Funding debt
Debt Instrument [Line Items]
Debt	690,443	732,522
Deferred Debt Issuance Cost	(6,174)	(5,883)
Total	684,269	726,639
Corporate debt
Debt Instrument [Line Items]
Total	$ 7,985	27,966
Corporate debt | Revolving Credit Facility
Debt Instrument [Line Items]
Weighted Average Interest Rate at December 31, 2017	5.50%
Debt	$ 8,000	28,000
Deferred Debt Issuance Cost	$ (15)	$ (34)