Fair Value Measurements - Long-term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying Value	$ 1,218.1	$ 1,012.4
Senior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	435.0	451.9
Junior Loans [Member]
Debt Instrument [Line Items]
Carrying Value	43.2	47.1
Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Carrying Value	216.3	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	1,258.6	1,033.7
Estimated Fair Value [Member] | Level 3 [Member] | Senior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	453.8	463.9
Estimated Fair Value [Member] | Level 3 [Member] | Junior Loans [Member]
Debt Instrument [Line Items]
Estimated Fair Value	44.7	48.1
Estimated Fair Value [Member] | Level 3 [Member] | Fixed Rate Loan [Member]
Debt Instrument [Line Items]
Estimated Fair Value	221.5	0.0
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series A [Member]
Debt Instrument [Line Items]
Estimated Fair Value	436.3	416.0
Estimated Fair Value [Member] | Level 2 [Member] | Equipment Notes, Series B [Member]
Debt Instrument [Line Items]
Estimated Fair Value	$ 102.3	$ 105.7