Fair Value Accounting (Details) (Fair value measurements, recurring, USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Assets:
Cash equivalents	$ 415.2		$ 230.6
Restricted cash	234.7		260.7
Total assets	649.9		491.3
Liabilities:
Total liabilities	10.5		23.8
Fuel derivative instruments
Liabilities:
Derivative liabilities	2.1	[1]
Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Derivative liabilities	6.4	[1]	21.7	[1]
Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Derivative liabilities	2.0	[1]	2.1	[1]
Level 1
Assets:
Cash equivalents	415.2		230.6
Restricted cash	234.7		260.7
Total assets	649.9		491.3
Liabilities:
Total liabilities	0		0
Level 1 | Fuel derivative instruments
Liabilities:
Derivative liabilities	0	[1]
Level 1 | Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Derivative liabilities	0	[1]	0	[1]
Level 1 | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Derivative liabilities	0	[1]	0	[1]
Level 2
Assets:
Cash equivalents	0		0
Restricted cash	0		0
Total assets	0		0
Liabilities:
Total liabilities	10.5		23.8
Level 2 | Fuel derivative instruments
Liabilities:
Derivative liabilities	2.1	[1]
Level 2 | Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Derivative liabilities	6.4	[1]	21.7	[1]
Level 2 | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Derivative liabilities	2.0	[1]	2.1	[1]
Level 3
Assets:
Cash equivalents	0		0
Restricted cash	0		0
Total assets	0		0
Liabilities:
Total liabilities	0		0
Level 3 | Fuel derivative instruments
Liabilities:
Derivative liabilities	0	[1]
Level 3 | Wholly-owned subsidiaries | Interest rate hedges
Liabilities:
Derivative liabilities	0	[1]	0	[1]
Level 3 | Partially-owned subsidiaries | Interest rate hedges
Liabilities:
Derivative liabilities	$ 0	[1]	$ 0	[1]

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