Fair Value Accounting (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Fuel derivative instruments
Assets:
Derivative asset	$ 0.1	$ 0.3
Fair value measurements, recurring
Assets:
Cash equivalents	114.7	188.7
Restricted cash	183.8	178.2
Equity instruments	3.5	3.1
Total assets	302.1	370.3
Liabilities:
Total liabilities	0.7	0.9
Fair value measurements, recurring | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	0.7	0.9
Fair value measurements, recurring | Other Assets [Member] | Fuel derivative instruments
Assets:
Derivative asset	0.1	0.3
Fair value measurements, recurring | Level 1
Assets:
Cash equivalents	114.7	188.7
Restricted cash	183.8	178.2
Equity instruments	0.0	0.0
Total assets	298.5	366.9
Liabilities:
Total liabilities	0.0	0.0
Fair value measurements, recurring | Level 1 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	0.0	0.0
Fair value measurements, recurring | Level 1 | Other Assets [Member] | Fuel derivative instruments
Assets:
Derivative asset	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	3.5	3.1
Total assets	3.6	3.4
Liabilities:
Total liabilities	0.7	0.9
Fair value measurements, recurring | Level 2 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	0.7	0.9
Fair value measurements, recurring | Level 2 | Other Assets [Member] | Fuel derivative instruments
Assets:
Derivative asset	0.1	0.3
Fair value measurements, recurring | Level 3
Assets:
Cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Equity instruments	0.0	0.0
Total assets	0.0	0.0
Liabilities:
Total liabilities	0.0	0.0
Fair value measurements, recurring | Level 3 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	0.0	0.0
Fair value measurements, recurring | Level 3 | Other Assets [Member] | Fuel derivative instruments
Assets:
Derivative asset	$ 0.0	$ 0.0