Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Carrying Value	$ 2,256.4	$ 2,234.5
Senior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	373.2	382.4
Junior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	29.0	31.1
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Carrying Value	649.5	625.1
Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Carrying Value	378.6	378.6
Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Carrying Value	80.0	80.0
Equipment Notes, Series C 2015-1 [Member]
Debt Instrument [Line Items]
Carrying Value	109.5	109.5
Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	235.4	242.5
Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	78.5	80.8
Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	77.2	83.7
Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Carrying Value	85.5	85.5
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Carrying Value	160.0	135.3
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Estimated Fair Value	2,286.1	2,169.6
Estimated Fair Value [Member] | Level 3 [Member] | Senior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	384.0	373.6
Estimated Fair Value [Member] | Level 3 [Member] | Junior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	29.5	31.1
Estimated Fair Value [Member] | Level 3 [Member] | Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Estimated Fair Value	665.6	600.1
Estimated Fair Value [Member] | Level 3 [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Estimated Fair Value	160.0	135.3
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	386.2	374.8
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	80.8	78.1
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series C 2015-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	110.3	107.9
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series AA 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	231.3	228.8
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	76.3	76.6
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	75.8	79.1
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series C 2017-1 [Member]
Debt Instrument [Line Items]
Estimated Fair Value	$ 86.3	$ 84.2