Fair Value Accounting (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Fuel derivative instruments
Liabilities:
Derivative liabilities	$ 0.6
Fair value measurements, recurring
Assets:
Cash equivalents	137.7	$ 174.0
Restricted cash	185.6	195.8
Total assets	323.3	369.8
Liabilities:
Total liabilities	2.6	2.4
Fair value measurements, recurring | Accrued liabilities | Fuel derivative instruments
Liabilities:
Derivative liabilities	0.6	0.8
Fair value measurements, recurring | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	2.0	1.6
Fair value measurements, recurring | Level 1
Assets:
Cash equivalents	137.7	174.0
Restricted cash	185.6	195.8
Total assets	323.3	369.8
Liabilities:
Total liabilities	0.0	0.0
Fair value measurements, recurring | Level 1 | Accrued liabilities | Fuel derivative instruments
Liabilities:
Derivative 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 2
Assets:
Cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Total assets	0.0	0.0
Liabilities:
Total liabilities	2.6	2.4
Fair value measurements, recurring | Level 2 | Accrued liabilities | Fuel derivative instruments
Liabilities:
Derivative liabilities	0.6	0.8
Fair value measurements, recurring | Level 2 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	2.0	1.6
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:
Derivative 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