Benefit Plans - Contribution and Benefit Payment Expectations (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Contribution And Benefit Payment [Line Items]
2012 expected contributions, top of range	$ 225
2012 expected contributions, bottom of range	125
2012 expected federal subsidies	4
2013 expected federal subsidies	5
2014 expected federal subsidies	5
2015 expected federal subsidies	6
2016 expected federal subsidies	7
2017-2021 expected federal subsidies	44
Pension Benefits
Contribution And Benefit Payment [Line Items]
2012 expected benefit payments	182
2013 expected benefit payments	185
2014 expected benefit payments	193
2015 expected benefit payments	198
2016 expected benefit payments	200
2017-2021 expected benefit payments	1,046
Other Postretirement Benefits
Contribution And Benefit Payment [Line Items]
2012 expected contributions	1
2012 expected benefit payments	47
2013 expected benefit payments	50
2014 expected benefit payments	53
2015 expected benefit payments	56
2016 expected benefit payments	58
2017-2021 expected benefit payments	$ 318

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Bottom end of range of the employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions.