Debt and Other Obligations - Interest Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Debt Instrument [Line Items]
Amortization of deferred financing costs	$ 2,177	$ 1,655	$ 4,369	$ 3,146
Interest Expense, Other	46	53	119	133
Total	25,266	20,498	50,237	38,347
Senior Loans [Member]
Debt Instrument [Line Items]
Interest expense, term loans	3,878	4,257	7,823	8,573
Junior Loans [Member]
Debt Instrument [Line Items]
Interest expense, term loans	489	636	1,012	1,303
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Interest expense, term loans	6,432	4,881	12,634	9,792
Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	3,709	4,015	7,547	8,157
Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	841	930	1,721	1,942
Equipment Notes, Series C 2015-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	1,280	789	2,615	789
Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	1,997	1,576	3,986	2,257
Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	720	568	1,437	814
Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	737	654	1,491	936
Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Interest expense, term loans	1,092	466	2,171	466
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest expense, term loans	1,559	0	2,973	0
Finance Lease Obligations [Member]
Debt Instrument [Line Items]
Interest expense, term loans	$ 309	$ 18	$ 339	$ 39