Fair Value Accounting (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Assets:
Cash equivalents	$ 162.0	$ 286.0
Short-term marketable securities		158.0
Restricted cash	229.4	207.1
Equity call agreement with TRIP Holdings equity investor	0.7
Fuel Commodities, Fair Value Disclosure		0.1
Total assets	392.1	651.2
Interest rate hedges
Equity put agreement with TRIP Holdings equity investor	3.1
Total liabilities	60.2	94.0
TRIP Holdings [Member] \| Level 1 [Member]
Interest rate hedges
Interest rate hedges	0	0
Level 1 [Member]
Assets:
Cash equivalents	162.0	286.0
Short-term marketable securities		158.0
Restricted cash	229.4	207.1
Equity call agreement with TRIP Holdings equity investor	0
Fuel Commodities, Fair Value Disclosure		0
Total assets	391.4	651.1
Interest rate hedges
Equity put agreement with TRIP Holdings equity investor	0
Total liabilities	0	0
Level 1 [Member] \| Wholly Owned Subsidiaries [Member]
Interest rate hedges
Interest rate hedges	0	0
TRIP Holdings [Member] \| Level 2 [Member]
Interest rate hedges
Interest rate hedges	4.7	48.3
Level 2 [Member]
Assets:
Cash equivalents	0	0
Short-term marketable securities		0
Restricted cash	0	0
Equity call agreement with TRIP Holdings equity investor	0
Fuel Commodities, Fair Value Disclosure		0.1
Total assets	0	0.1
Interest rate hedges
Equity put agreement with TRIP Holdings equity investor	0
Total liabilities	57.1	94.0
Level 2 [Member] \| Wholly Owned Subsidiaries [Member]
Interest rate hedges
Interest rate hedges	52.4	45.7
TRIP Holdings [Member] \| Level 3 [Member]
Interest rate hedges
Interest rate hedges	0	0
Level 3 [Member]
Assets:
Cash equivalents	0	0
Short-term marketable securities		0
Restricted cash	0	0
Equity call agreement with TRIP Holdings equity investor	0.7
Fuel Commodities, Fair Value Disclosure		0
Total assets	0.7	0
Interest rate hedges
Equity put agreement with TRIP Holdings equity investor	3.1
Total liabilities	3.1	0
Level 3 [Member] \| Wholly Owned Subsidiaries [Member]
Interest rate hedges
Interest rate hedges	0	0
TRIP Holdings [Member]
Interest rate hedges
Interest rate hedges	4.7	48.3
Wholly Owned Subsidiaries [Member]
Interest rate hedges
Interest rate hedges	$ 52.4	$ 45.7

Investments held in fuel commodities.

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