Fair Value Accounting (Details) - Fair value measurements, recurring - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Assets:
Cash equivalents	$ 114.7	$ 174.0
Restricted cash	183.9	195.8
Equity instruments	3.4
Total assets	302.0	369.8
Liabilities:
Total liabilities	1.6	2.4
Accrued liabilities | Fuel derivative instruments
Liabilities:
Derivative liabilities		0.8
Accrued liabilities | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	1.6	1.6
Level 1
Assets:
Cash equivalents	114.7	174.0
Restricted cash	183.9	195.8
Equity instruments	0.0
Total assets	298.6	369.8
Liabilities:
Total liabilities	0.0	0.0
Level 1 | Accrued liabilities | Fuel derivative instruments
Liabilities:
Derivative liabilities		0.0
Level 1 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	0.0	0.0
Level 2
Assets:
Cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Equity instruments	3.4
Total assets	3.4	0.0
Liabilities:
Total liabilities	1.6	2.4
Level 2 | Accrued liabilities | Fuel derivative instruments
Liabilities:
Derivative liabilities		0.8
Level 2 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	1.6	1.6
Level 3
Assets:
Cash equivalents	0.0	0.0
Restricted cash	0.0	0.0
Equity instruments	0.0
Total assets	0.0	0.0
Liabilities:
Total liabilities	0.0	0.0
Level 3 | Accrued liabilities | Fuel derivative instruments
Liabilities:
Derivative liabilities		0.0
Level 3 | Accrued liabilities | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	$ 0.0	$ 0.0