Other Postretirement Benefits - Schedule of Fair Values of Plan Assets (Detail) (Other Postretirement Benefits [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Total investments	$ 743	$ 687
Funds for retiree health benefits	372	353
Investments (including funds for retiree health benefits)	1,115	1,040
Pending activities	(2)	7
Total fair value of plan net assets	1,113	1,047	947	942
Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	450
U.S. Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments		311
International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments		124
Other Fixed Income Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	286	229
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	7	23
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments		127
Funds for retiree health benefits	185	185
Investments (including funds for retiree health benefits)	185	312
Level 1 [Member] | Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments
Level 1 [Member] | U.S. Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments		127
Level 1 [Member] | International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments
Level 1 [Member] | Other Fixed Income Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments
Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	743	560
Funds for retiree health benefits	145	137
Investments (including funds for retiree health benefits)	888	697
Level 2 [Member] | Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	450
Level 2 [Member] | U.S. Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments		184
Level 2 [Member] | International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments		124
Level 2 [Member] | Other Fixed Income Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	286	229
Level 2 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments	7	23
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments
Funds for retiree health benefits	42	31
Investments (including funds for retiree health benefits)	42	31
Total fair value of plan net assets
Level 3 [Member] | Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments
Level 3 [Member] | U.S. Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments
Level 3 [Member] | International Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments
Level 3 [Member] | Other Fixed Income Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments
Level 3 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total investments