Pension and Other Postretirement Benefits - Schedule of Benefit Payments Expected to be Paid and Expected Medicare Part D Subsidy Receipts (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2013	$ 880
2014	870
2015	880
2016	890
2017	910
2018 through 2022	4,620
Total benefit payments	9,050
Gross Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2013	285
2014	280
2015	280
2016	275
2017	270
2018 through 2022	1,200
Total benefit payments	2,590
Medicare Part D Subsidy Receipts [Member]
Defined Benefit Plan Disclosure [Line Items]
2013	30
2014	30
2015	30
2016	30
2017	35
2018 through 2022	170
Total benefit payments	325
Net Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2013	255
2014	250
2015	250
2016	245
2017	235
2018 through 2022	1,030
Total benefit payments	$ 2,265