Fuel - Derivative Narrative (Details) (USD $) In Millions, unless otherwise specified			6 Months Ended
Jun. 30, 2012	Dec. 31, 2011 gal	Jun. 30, 2012 Cost of Fuel [Member] Operating Expense [Member]	Jun. 30, 2011 Cost of Fuel [Member] Operating Expense [Member]
Concentration Risk [Line Items]
Fuel costs percent of total operating costs			30.00%	29.00%
Number of gallons of fuel covered by fuel-derivative agreements		36,000,000
Net asset exposure to counterparty credit risk	$ 0	$ 24

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13531-108611

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