Fair Value Accounting - Assets and liabilities Measured at fair value on recurring basis (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fuel derivative instruments
Assets:
Derivative asset	$ 0.1	$ 0.3
Fair value measurements, recurring
Assets:
Cash equivalents	247.1	188.7
Restricted cash	176.3	178.2
Total assets	427.0	370.3
Liabilities:
Total liabilities	0.0	0.9
Fair value measurements, recurring | Other assets
Assets:
Equity instruments	1.7	3.1
Fair value measurements, recurring | Other assets | Interest rate hedge
Assets:
Derivative asset	1.8
Fair value measurements, recurring | Other assets | Fuel derivative instruments
Assets:
Derivative asset	0.1	0.3
Fair value measurements, recurring | Level 1
Assets:
Cash equivalents	247.1	188.7
Restricted cash	176.3	178.2
Total assets	423.4	366.9
Liabilities:
Total liabilities	0.0	0.0
Fair value measurements, recurring | Level 1 | Other assets
Assets:
Equity instruments	0.0	0.0
Fair value measurements, recurring | Level 1 | Other assets | Interest rate hedge
Assets:
Derivative asset	0.0
Fair value measurements, recurring | Level 1 | Other assets | 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
Total assets	3.6	3.4
Liabilities:
Total liabilities	0.0	0.9
Fair value measurements, recurring | Level 2 | Other assets
Assets:
Equity instruments	1.7	3.1
Fair value measurements, recurring | Level 2 | Other assets | Interest rate hedge
Assets:
Derivative asset	1.8
Fair value measurements, recurring | Level 2 | Other assets | 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
Total assets	0.0	0.0
Liabilities:
Total liabilities	0.0	0.0
Fair value measurements, recurring | Level 3 | Other assets
Assets:
Equity instruments	0.0	0.0
Fair value measurements, recurring | Level 3 | Other assets | Interest rate hedge
Assets:
Derivative asset	0.0
Fair value measurements, recurring | Level 3 | Other assets | Fuel derivative instruments
Assets:
Derivative asset	0.0	0.0
Partially-owned subsidiaries | Fair value measurements, recurring | Accrued liabilities | Interest rate hedge
Liabilities:
Derivative liabilities	0.0	0.9
Partially-owned subsidiaries | Fair value measurements, recurring | Level 1 | Accrued liabilities | Interest rate hedge
Liabilities:
Derivative liabilities	0.0	0.0
Partially-owned subsidiaries | Fair value measurements, recurring | Level 2 | Accrued liabilities | Interest rate hedge
Liabilities:
Derivative liabilities	0.0	0.9
Partially-owned subsidiaries | Fair value measurements, recurring | Level 3 | Accrued liabilities | Interest rate hedge
Liabilities:
Derivative liabilities	$ 0.0	$ 0.0