Debt and Other Obligations - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt Instrument [Line Items]
Commitment fees	$ 51	$ 28	$ 103	$ 58
Amortization of debt discounts	1,655	1,290	3,146	2,521
Total	20,478	13,727	38,278	26,112
Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	466	0	466	0
Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	654	0	936	0
Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	568	0	814	0
Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	1,576	0	2,257	0
Equipment Notes, Series C 2015-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	789	0	789	0
Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	930	1,108	1,942	2,292
Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	4,015	4,321	8,157	8,629
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Interest expense, term loans	4,881	1,586	9,792	1,744
Junior Loans [Member]
Debt Instrument [Line Items]
Interest expense, term loans	636	775	1,303	1,578
Senior Loans [Member]
Debt Instrument [Line Items]
Interest expense, term loans	$ 4,257	$ 4,619	$ 8,573	$ 9,290