Debt and Other Obligations - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Commitment fees	$ 50	$ 29	$ 153	$ 87
Amortization of debt discounts	1,784	1,362	4,930	3,883
Total	21,910	14,996	60,189	41,107
Senior Loans [Member]
Debt Instrument [Line Items]
Interest expense, term loans	4,195	4,564	12,769	13,854
Junior Loans [Member]
Debt Instrument [Line Items]
Interest expense, term loans	605	746	1,908	2,323
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Interest expense, term loans	4,844	2,811	14,636	4,555
Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	4,057	4,366	12,214	12,995
Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	939	1,118	2,881	3,410
Equipment Notes, Series C 2015-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	1,420	0	2,209	0
Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	1,751	0	4,008	0
Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	631	0	1,445	0
Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	700	0	1,636	0
Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	$ 934	$ 0	$ 1,400	$ 0