Employee Benefits - Fair Values of Postretirement Plan Assets by Asset Category (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	$ 942	$ 918
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	51	57	61
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | U.S. equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	26	36
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | International equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	8	8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Convertible securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	4	4
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	4	8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	2
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value		1
Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual funds [Member] | U.S. equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	7
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	270	614
Level 1 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 1 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | U.S. equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 1 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | International equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 1 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Convertible securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 1 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 1 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 1 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 1 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual funds [Member] | U.S. equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	604	288
Level 2 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	51	57
Level 2 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | U.S. equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	26	36
Level 2 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | International equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	8	8
Level 2 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Convertible securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	4	4
Level 2 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	4	8
Level 2 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	2
Level 2 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value		1
Level 2 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual funds [Member] | U.S. equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	7
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value	68	16
Level 3 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 3 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | U.S. equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 3 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | International equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 3 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Convertible securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 3 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 3 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 3 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Common trust funds [Member] | Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value
Level 3 [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual funds [Member] | U.S. equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Assets at fair value