Fair Value Accounting - Assets and liabilities measured at fair value on recurring basis - Fair value measurements, recurring - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets:
Cash equivalents	$ 113.1	$ 188.7
Restricted cash	195.2	178.2
Total assets	311.2	370.3
Liabilities:
Total liabilities	0.0	0.9
Other assets
Assets:
Equity instruments	1.3	3.1
Other assets | Interest rate hedges
Assets:
Derivative asset	1.6
Other assets | Fuel derivative instruments
Assets:
Derivative asset		0.3
Level 1
Assets:
Cash equivalents	113.1	188.7
Restricted cash	195.2	178.2
Total assets	308.3	366.9
Liabilities:
Total liabilities	0.0	0.0
Level 1 | Other assets
Assets:
Equity instruments	0.0	0.0
Level 1 | Other assets | Interest rate hedges
Assets:
Derivative asset	0.0
Level 1 | Other assets | Fuel derivative instruments
Assets:
Derivative asset		0.0
Level 2
Assets:
Cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Total assets	2.9	3.4
Liabilities:
Total liabilities	0.0	0.9
Level 2 | Other assets
Assets:
Equity instruments	1.3	3.1
Level 2 | Other assets | Interest rate hedges
Assets:
Derivative asset	1.6
Level 2 | Other assets | Fuel derivative instruments
Assets:
Derivative asset		0.3
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
Level 3 | Other assets
Assets:
Equity instruments	0.0	0.0
Level 3 | Other assets | Interest rate hedges
Assets:
Derivative asset	0.0
Level 3 | Other assets | Fuel derivative instruments
Assets:
Derivative asset		0.0
Partially-owned subsidiaries | Accrued liabilities | Interest rate hedges
Liabilities:
Liability	0.0	0.9
Partially-owned subsidiaries | Level 1 | Accrued liabilities | Interest rate hedges
Liabilities:
Liability	0.0	0.0
Partially-owned subsidiaries | Level 2 | Accrued liabilities | Interest rate hedges
Liabilities:
Liability	0.0	0.9
Partially-owned subsidiaries | Level 3 | Accrued liabilities | Interest rate hedges
Liabilities:
Liability	$ 0.0	$ 0.0