Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying Value	$ 1,117.6	$ 1,012.4
Senior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	443.4	451.9
Junior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	45.2	47.1
FixedRateLoan [Domain]
Debt Instrument [Line Items]
Carrying Value	77.0
Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Carrying Value	440.2	409.8
Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Carrying Value	111.8	103.6
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Estimated Fair Value	1,152.5	1,033.7
Estimated Fair Value [Member] | Level 3 [Member] | Senior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	461.6	463.9
Estimated Fair Value [Member] | Level 3 [Member] | Junior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	48.3	48.1
Estimated Fair Value [Member] | Level 3 [Member] | FixedRateLoan [Domain]
Debt Instrument [Line Items]
Carrying Value	77.4	0.0
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Estimated Fair Value	451.2	416.0
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Estimated Fair Value	$ 114.0	$ 105.7