Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Carrying Value	$ 1,748.1	$ 1,539.6
Senior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	409.1	417.9
Junior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	37.3	39.3
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Carrying Value	510.1	518.0
Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Carrying Value	408.6	408.6
Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Carrying Value	92.0	92.0
Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	171.0	37.5
Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	57.0	12.5
Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	63.0	13.8
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Estimated Fair Value	1,740.2	1,583.2
Estimated Fair Value [Member] | Level 3 [Member] | Senior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	414.0	435.3
Estimated Fair Value [Member] | Level 3 [Member] | Junior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	37.8	40.4
Estimated Fair Value [Member] | Level 3 [Member] | Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Estimated Fair Value	502.9	528.6
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	409.8	420.9
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	92.7	94.2
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	166.4	37.4
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	55.2	12.6
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	$ 61.4	$ 13.8