Fair Value Accounting (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fuel derivative instruments
Liabilities:
Liability	$ 0.3
Fair value measurements, recurring
Assets:
Cash equivalents	188.7	$ 174.0
Restricted cash	178.2	195.8
Equity instruments	3.1
Total assets	370.3	369.8
Liabilities:
Total liabilities	0.9	2.4
Fair value measurements, recurring | Other assets | Fuel derivative instruments
Assets:
Derivative Asset	0.3
Fair value measurements, recurring | Accrued liabilities | Fuel derivative instruments
Liabilities:
Liability		0.8
Fair value measurements, recurring | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Liability	0.9	1.6
Fair value measurements, recurring | Level 1
Assets:
Cash equivalents	188.7	174.0
Restricted cash	178.2	195.8
Equity instruments	0.0
Total assets	366.9	369.8
Liabilities:
Total liabilities	0.0	0.0
Fair value measurements, recurring | Level 1 | Other assets | Fuel derivative instruments
Assets:
Derivative Asset	0.0
Fair value measurements, recurring | Level 1 | Accrued liabilities | Fuel derivative instruments
Liabilities:
Liability		0.0
Fair value measurements, recurring | Level 1 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Liability	0.0	0.0
Fair value measurements, recurring | Level 2
Assets:
Cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Equity instruments	3.1
Total assets	3.4	0.0
Liabilities:
Total liabilities	0.9	2.4
Fair value measurements, recurring | Level 2 | Other assets | Fuel derivative instruments
Assets:
Derivative Asset	0.3
Fair value measurements, recurring | Level 2 | Accrued liabilities | Fuel derivative instruments
Liabilities:
Liability		0.8
Fair value measurements, recurring | Level 2 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Liability	0.9	1.6
Fair value measurements, recurring | Level 3
Assets:
Cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Equity instruments	0.0
Total assets	0.0	0.0
Liabilities:
Total liabilities	0.0	0.0
Fair value measurements, recurring | Level 3 | Other assets | Fuel derivative instruments
Assets:
Derivative Asset	0.0
Fair value measurements, recurring | Level 3 | Accrued liabilities | Fuel derivative instruments
Liabilities:
Liability		0.0
Fair value measurements, recurring | Level 3 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Liability	$ 0.0	$ 0.0