Fair Value Accounting (Details) - Fair value measurements, recurring - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014
Assets:
Cash equivalents		$ 338.9	$ 415.2
Restricted cash		197.3	234.7
Total assets		536.2	649.9
Liabilities:
Total liabilities		2.5	10.5
Fuel derivative instruments
Liabilities:
Derivative liabilities	[1]	0.6	2.1
Wholly-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]		6.4
Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]	1.9	2.0
Level 1
Assets:
Cash equivalents		338.9	415.2
Restricted cash		197.3	234.7
Total assets		536.2	649.9
Liabilities:
Total liabilities		0.0	0.0
Level 1 | Fuel derivative instruments
Liabilities:
Derivative liabilities	[1]	0.0	0.0
Level 1 | Wholly-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]		0.0
Level 1 | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]	0.0	0.0
Level 2
Assets:
Cash equivalents		0.0	0.0
Restricted cash		0.0	0.0
Total assets		0.0	0.0
Liabilities:
Total liabilities		2.5	10.5
Level 2 | Fuel derivative instruments
Liabilities:
Derivative liabilities	[1]	0.6	2.1
Level 2 | Wholly-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]		6.4
Level 2 | Partially-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]	1.9	2.0
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 | Fuel derivative instruments
Liabilities:
Derivative liabilities	[1]	0.0	0.0
Level 3 | Wholly-owned subsidiaries | Interest rate hedge
Liabilities:
Derivative liabilities	[1]		0.0
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.