Pension and Other Postretirement Benefits - Plan Amendments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Pre-tax curtailment gain or settlement loss	$ 19	$ (6)
After-tax curtailment gain or settlement loss	12	(4)
Retiree Health Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Pre-tax curtailment gain or settlement loss	19
After-tax curtailment gain or settlement loss	12
Increase (decrease) to liability for defined benefit plans resulting from plan amendments	(57)
Increase (decrease) to shareholders' equity resulting from plan amendments	$ 37