Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Carrying Value	$ 1,919.4	$ 1,539.6
Senior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	400.3	417.9
Junior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	35.3	39.3
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Carrying Value	502.0	518.0
Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Carrying Value	393.6	408.6
Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Carrying Value	84.0	92.0
Equipment Notes, Series C 2015-1 [Member]
Debt Instrument [Line Items]
Carrying Value	115.2	0.0
Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	190.0	37.5
Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	63.3	12.5
Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	70.0	13.8
Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	65.7	0.0
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Estimated Fair Value	1,900.1	1,583.2
Estimated Fair Value [Member] | Level 3 [Member] | Senior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	400.9	435.3
Estimated Fair Value [Member] | Level 3 [Member] | Junior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	35.6	40.4
Estimated Fair Value [Member] | Level 3 [Member] | Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Estimated Fair Value	489.5	528.6
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	394.8	420.9
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	84.6	94.2
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series C 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	114.9	0.0
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	184.8	37.4
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	61.3	12.6
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	68.2	13.8
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	$ 65.5	$ 0.0