Other Postretirement Benefits - Fair Value of Plan Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Other Fixed Income [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total fair value of plan net assets		$ 29
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	687	606
Funds for retiree health benefits	353	336
Investments (including funds for retiree health benefits)	1,040	942
Pending activities	7	5
Total fair value of plan net assets	1,047	947	942	866
Other Postretirement Benefits [Member] | U.S. Equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	311	277
Other Postretirement Benefits [Member] | International Equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	124	104
Other Postretirement Benefits [Member] | Other Fixed Income [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	229	207
Other Postretirement Benefits [Member] | Cash and Cash Equivalents [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	23	18
Level 1 [Member] | Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	127	115
Funds for retiree health benefits	185	174
Investments (including funds for retiree health benefits)	312	289
Pending activities
Level 1 [Member] | Other Postretirement Benefits [Member] | U.S. Equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	127	115
Level 1 [Member] | Other Postretirement Benefits [Member] | International Equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments
Level 1 [Member] | Other Postretirement Benefits [Member] | Other Fixed Income [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments
Level 1 [Member] | Other Postretirement Benefits [Member] | Cash and Cash Equivalents [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments
Level 2 [Member] | Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	560	491
Funds for retiree health benefits	137	134
Investments (including funds for retiree health benefits)	697	625
Pending activities
Level 2 [Member] | Other Postretirement Benefits [Member] | U.S. Equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	184	162
Level 2 [Member] | Other Postretirement Benefits [Member] | International Equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	124	104
Level 2 [Member] | Other Postretirement Benefits [Member] | Other Fixed Income [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	229	207
Level 2 [Member] | Other Postretirement Benefits [Member] | Cash and Cash Equivalents [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments	23	18
Level 3 [Member] | Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments
Funds for retiree health benefits	31	28
Investments (including funds for retiree health benefits)	31	28
Pending activities
Total fair value of plan net assets	31	28	219
Level 3 [Member] | Other Postretirement Benefits [Member] | U.S. Equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments
Level 3 [Member] | Other Postretirement Benefits [Member] | International Equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments
Level 3 [Member] | Other Postretirement Benefits [Member] | Other Fixed Income [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments
Total fair value of plan net assets			189
Level 3 [Member] | Other Postretirement Benefits [Member] | Cash and Cash Equivalents [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total investments