Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 723.5	$ 659.3
Senior Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	476.2	484.2
Junior Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	52.6	54.3
Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	154.2	95.8
Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	40.5	25.0
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Class A equipment trust certificates	722.0	652.4
Estimated Fair Value [Member] | Level 3 [Member] | Senior Loans [Member]
Debt Instrument [Line Items]
Class A equipment trust certificates	476.0	477.8
Estimated Fair Value [Member] | Level 3 [Member] | Junior Loans [Member]
Debt Instrument [Line Items]
Class A equipment trust certificates	53.2	54.6
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Class A equipment trust certificates	154.2	94.8
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Class A equipment trust certificates	$ 38.6	$ 25.2