Fair Value Accounting (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Assets:
Cash equivalents	$ 246.6	$ 286.0
Short-term marketable securities		158.0
Restricted cash	240.3	207.1
Equity call agreement with TRIP Holdings equity investor	0.7
Fuel derivative instruments		0.1
Total assets	487.6	651.2
Interest rate hedges
Equity put agreement with TRIP Holdings equity investor	3.1
Fuel derivative instruments	0.1
Total liabilities	56.9	94.0
Wholly Owned Subsidiaries [Member]
Interest rate hedges
Wholly-owned subsidiary	48.9	45.7
TRIP Holdings [Member]
Interest rate hedges
Wholly-owned subsidiary	4.8	48.3
Level 1 [Member]
Assets:
Cash equivalents	246.6	286.0
Short-term marketable securities		158.0
Restricted cash	240.3	207.1
Total assets	486.9	651.1
Level 2 [Member]
Assets:
Fuel derivative instruments		0.1
Total assets		0.1
Interest rate hedges
Fuel derivative instruments	0.1
Total liabilities	53.8	94.0
Level 2 [Member] | Wholly Owned Subsidiaries [Member]
Interest rate hedges
Wholly-owned subsidiary	48.9	45.7
Level 2 [Member] | TRIP Holdings [Member]
Interest rate hedges
Wholly-owned subsidiary	4.8	48.3
Level 3 [Member]
Assets:
Equity call agreement with TRIP Holdings equity investor	0.7
Total assets	0.7
Interest rate hedges
Equity put agreement with TRIP Holdings equity investor	3.1
Total liabilities	$ 3.1