Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 940.3	$ 659.3
Senior Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	468.2	484.2
Junior Loans [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	50.7	54.3
Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	333.3	95.8
Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	88.1	25.0
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Class A equipment trust certificates	950.7	652.4
Estimated Fair Value [Member] | Level 3 [Member] | Senior Loans [Member]
Debt Instrument [Line Items]
Class A equipment trust certificates	470.7	477.8
Estimated Fair Value [Member] | Level 3 [Member] | Junior Loans [Member]
Debt Instrument [Line Items]
Class A equipment trust certificates	51.5	54.6
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Class A equipment trust certificates	340.9	94.8
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Class A equipment trust certificates	$ 87.6	$ 25.2