Postretirement Benefits (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Jun. 30, 2013	Jun. 30, 2012
Pension Benefits
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service Cost	$ 76	$ 63	$ 150	$ 130
Interest Cost	141	152	281	309
Expected Return on Plan Assets	(148)	(142)	(296)	(288)
Amortization of Deferred Amounts	6	5	9	11
Recognized Net Actuarial Loss	53	26	106	52
Defined Benefit Plan Curtailments And Settlements	2	0	2	0
Gross Benefit Cost (Credit)	130	104	252	214
Dividends on ESOP Preferred Stock	0	0	0	0
Net Periodic Benefit Cost (Credit)	130	104	252	214
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Expected Long-term Return on Assets					7.30%	7.40%
Other Retiree Benefits
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]
Service Cost	48	35	95	71
Interest Cost	66	69	130	138
Expected Return on Plan Assets	(96)	(108)	(191)	(216)
Amortization of Deferred Amounts	(5)	(5)	(10)	(10)
Recognized Net Actuarial Loss	50	24	100	49
Defined Benefit Plan Curtailments And Settlements	0	0	0	0
Gross Benefit Cost (Credit)	63	15	124	32
Dividends on ESOP Preferred Stock	(18)	(18)	(35)	(37)
Net Periodic Benefit Cost (Credit)	$ 45	$ (3)	$ 89	$ (5)
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Expected Long-term Return on Assets					8.30%	9.20%