Fair Value Accounting (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Assets:
Cash equivalents	$ 155.4	$ 286.0
Short-term marketable securities	42.0	158.0
Restricted cash	205.3	207.1
Fuel derivative instruments	0.4	0.1
Total assets	403.1	651.2
Fair Value, Inputs, Level 1 [Member]
Assets:
Cash equivalents	155.4	286.0
Short-term marketable securities	42.0	158.0
Restricted cash	205.3	207.1
Fuel derivative instruments	0	0
Total assets	402.7	651.1
Fair Value, Inputs, Level 2 [Member]
Assets:
Cash equivalents	0	0
Short-term marketable securities	0	0
Restricted cash	0	0
Fuel derivative instruments	0.4	0.1
Total assets	0.4	0.1
Fair Value, Inputs, Level 3 [Member]
Assets:
Cash equivalents	0	0
Short-term marketable securities	0	0
Restricted cash	0	0
Fuel derivative instruments	0	0
Total assets	$ 0	$ 0