Fair Value Accounting (Details) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Fuel derivative instruments
Liabilities:
Derivative liabilities		$ 0.7
Fair value measurements, recurring
Assets:
Cash equivalents		233.2	$ 415.2
Restricted cash		211.8	234.7
Total assets		445.0	649.9
Liabilities:
Total liabilities		2.9	10.5
Fair value measurements, recurring | Level 1
Assets:
Cash equivalents		233.2	415.2
Restricted cash		211.8	234.7
Total assets		445.0	649.9
Liabilities:
Total 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		2.9	10.5
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
Accrued liabilities | Fair value measurements, recurring | Fuel derivative instruments
Liabilities:
Derivative liabilities	[1]	0.7	2.1
Accrued liabilities | Fair value measurements, recurring | Wholly-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]		6.4
Accrued liabilities | Fair value measurements, recurring | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]	2.2	2.0
Accrued liabilities | Fair value measurements, recurring | Level 1 | Fuel derivative instruments
Liabilities:
Derivative liabilities	[1]	0.0	0.0
Accrued liabilities | Fair value measurements, recurring | Level 1 | Wholly-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]		0.0
Accrued liabilities | Fair value measurements, recurring | Level 1 | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]	0.0	0.0
Accrued liabilities | Fair value measurements, recurring | Level 2 | Fuel derivative instruments
Liabilities:
Derivative liabilities	[1]	0.7	2.1
Accrued liabilities | Fair value measurements, recurring | Level 2 | Wholly-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]		6.4
Accrued liabilities | Fair value measurements, recurring | Level 2 | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]	2.2	2.0
Accrued liabilities | Fair value measurements, recurring | Level 3 | Fuel derivative instruments
Liabilities:
Derivative liabilities	[1]	0.0	0.0
Accrued liabilities | Fair value measurements, recurring | Level 3 | Wholly-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]		0.0
Accrued liabilities | Fair value measurements, recurring | Level 3 | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]	$ 0.0	$ 0.0

[1] Included in accrued liabilities on the consolidated balance sheet.