Commitments and Contingencies (Schedule of Estimated Purchased Power Commitments) (Details) (USD $) In Millions	6 Months Ended
	Jun. 30, 2011
Coal Agreement 2011 [Member]
Long-term commitments	$ 669
Coal Agreement 2012 [Member]
Long-term commitments	1,153
Coal Agreement 2013 [Member]
Long-term commitments	801
Coal Agreement 2014 [Member]
Long-term commitments	643
Coal Agreement 2015 [Member]
Long-term commitments	634
Coal Agreement Thereafter [Member]
Long-term commitments	1,651
Purchased Power Commitments 2011 [Member]
Long-term commitments	178
Purchased Power Commitments 2012 [Member]
Long-term commitments	200
Purchased Power Commitments 2013 [Member]
Long-term commitments	314
Purchased Power Commitments 2014 [Member]
Long-term commitments	129
Purchased Power Commitments 2015 [Member]
Long-term commitments	55
Purchased Power Commitments Thereafter [Member]
Long-term commitments	$ 826