Fair Value Accounting (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fuel derivative instruments
Liabilities:
Liability	$ 0.8
Fair value measurements, recurring
Assets:
Cash equivalents	174.0	$ 415.2
Restricted cash	195.8	234.7
Total assets	369.8	649.9
Liabilities:
Total liabilities	2.4	10.5
Fair value measurements, recurring | Accrued liabilities | Fuel derivative instruments
Liabilities:
Liability	0.8	2.1
Fair value measurements, recurring | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Liability	1.6	2.0
Fair value measurements, recurring | Accrued liabilities | Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Liability		6.4
Fair value measurements, recurring | Level 1
Assets:
Cash equivalents	174.0	415.2
Restricted cash	195.8	234.7
Total assets	369.8	649.9
Liabilities:
Total liabilities	0.0	0.0
Fair value measurements, recurring | Level 1 | Accrued liabilities | Fuel derivative instruments
Liabilities:
Liability	0.0	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 1 | Accrued liabilities | Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Liability		0.0
Fair value measurements, recurring | Level 2
Assets:
Cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Total assets	0.0	0.0
Liabilities:
Total liabilities	2.4	10.5
Fair value measurements, recurring | Level 2 | Accrued liabilities | Fuel derivative instruments
Liabilities:
Liability	0.8	2.1
Fair value measurements, recurring | Level 2 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Liability	1.6	2.0
Fair value measurements, recurring | Level 2 | Accrued liabilities | Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Liability		6.4
Fair value measurements, recurring | Level 3
Assets:
Cash equivalents	0.0	0.0
Restricted cash	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 | Fuel derivative instruments
Liabilities:
Liability	0.0	0.0
Fair value measurements, recurring | Level 3 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Liability	$ 0.0	0.0
Fair value measurements, recurring | Level 3 | Accrued liabilities | Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Liability		$ 0.0