Expected Net Pension and OPEB Plan Payments for Next 10 Years (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013
Pension Plans, Defined Benefit
Expected Net Pension and OPEB Plan Payments for the Next 10 Years
2014	$ 219
2015	231
2016	243
2017	261
2018	275
2019 through 2023	1,608
Total expected net benefit payments for next 10 years	2,837
Other Postretirement Benefit Plans, Defined Benefit
Expected Net Pension and OPEB Plan Payments for the Next 10 Years
2014	24
2015	27
2016	28
2017	30
2018	32
2019 through 2023	172
Total expected net benefit payments for next 10 years	$ 313