Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying Value	$ 1,354.7	$ 1,012.4
Senior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	426.4	451.9
Junior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	41.3	47.1
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Carrying Value	363.4	0.0
Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Carrying Value	423.6	409.8
Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Carrying Value	100.0	103.6
Estimated Fair Value [Member]
Debt Instrument [Line Items]
Estimated Fair Value	1,402.9	1,033.7
Estimated Fair Value [Member] | Level 3 [Member] | Senior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	446.2	463.9
Estimated Fair Value [Member] | Level 3 [Member] | Junior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	42.7	48.1
Estimated Fair Value [Member] | Level 3 [Member] | Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Estimated Fair Value	370.1	0.0
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Estimated Fair Value	440.6	416.0
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Estimated Fair Value	$ 103.3	$ 105.7