Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	$ 1,012.4	$ 659.3
Senior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	451.9	484.2
Junior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	47.1	54.3
Equipment Notes, Series A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	409.8	95.8
Equipment Notes, Series B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying Value	103.6	25.0
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,033.7	652.4
Estimate of Fair Value Measurement [Member] | Level 3 | Senior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	463.9	477.8
Estimate of Fair Value Measurement [Member] | Level 3 | Junior loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	48.1	54.6
Estimate of Fair Value Measurement [Member] | Level 2 | Equipment Notes, Series A
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	416.0	94.8
Estimate of Fair Value Measurement [Member] | Level 2 | Equipment Notes, Series B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 105.7	$ 25.2