Employee Benefit Plans (Narrative) (Details) (USD $) Share data in Millions, unless otherwise specified	12 Months Ended	6 Months Ended			6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2012 Pension Benefits [Member]	Jun. 30, 2012 Pension Benefits [Member]	Jun. 30, 2011 Pension Benefits [Member]	Dec. 31, 2012 Postretirement Benefits [Member]	Jun. 30, 2012 Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Minimum years of service for employees hired before January 1, 2009 required for participation in a defined benefit pension plan (in years)	5 years
Percentage of non-matching employer's contribution to eligible participants (as a percent)	1.00%
Employer's Common stock held under 401 (k) plan (in shares)	15.0
Market value of employer's Common stock held under 401 (k) plan	$ 443,000,000
Cash dividend received on employer's Common stock held under 401 (k) plan	11,000,000
Accumulated other comprehensive income, Unrecognized transition obligation			2,000,000
Accumulated other comprehensive income, Unrecognized prior service (credit) cost			12,000,000			16,000,000
Accumulated other comprehensive income, Unrecognized actuarial gains (losses)			1,200,000,000			63,000,000
Prior service (cost) credit, expected to be recognized in net periodic benefit cost in next fiscal year		2,000,000			2,000,000
Actuarial loss, expected to be recognized in net periodic benefit cost in next fiscal year		42,000,000			2,000,000
Projected benefit obligation for plans with projected benefit obligation in excess of plan assets			2,800,000,000	2,100,000,000
Accumulated benefit obligation for plans with projected benefit obligation in excess of plan assets			2,500,000,000	1,900,000,000
Fair value of plan assets for plans with projected benefit obligations in excess of plan assets			1,900,000,000	1,700,000,000
Projected benefit obligation for plans with accumulated benefit obligation in excess of plan assets			2,700,000,000	671,000,000
Accumulated benefit obligation for plans with accumulated benefit obligation in excess of plan assets			2,500,000,000	657,000,000
Fair value of plan assets for plans with accumulated benefit obligation in excess of plan assets			1,800,000,000	425,000,000
Accumulated benefit obligation			2,700,000,000	2,300,000,000
Annual rate of increase in per capita cost of covered health care benefits (as a percent)						7.50%
Annual rate of decrease in per capita cost of covered health care benefits assumed for 2022 and thereafter (as a percent)						5.00%
Contributions and expected future benefit payments		$ 26,000,000			$ 5,000,000