Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Carrying Value	$ 1,964.6	$ 1,539.6
Senior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	391.4	417.9
Junior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	33.2	39.3
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Carrying Value	493.9	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	223.3	37.5
Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	74.4	12.5
Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	76.7	13.8
Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	78.9	0.0
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Estimated Fair Value	1,940.3	1,583.2
Estimated Fair Value [Member] | Level 3 [Member] | Senior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	395.5	435.3
Estimated Fair Value [Member] | Level 3 [Member] | Junior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	33.6	40.4
Estimated Fair Value [Member] | Level 3 [Member] | Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Estimated Fair Value	487.2	528.6
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	389.8	420.9
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	83.5	94.2
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series C 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	114.4	0.0
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	213.6	37.4
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	71.1	12.6
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	73.3	13.8
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	$ 78.3	$ 0.0