Other Postretirement Benefits - Reconciliation of Fair Value Balances for Net Assets (Detail) - Other Postretirement Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	$ 1,084	$ 1,113
FAIR VALUE OF PLAN ASSETS AT END OF YEAR	994	1,084
Level 3
Defined Benefit Plan Disclosure [Line Items]
Assets Sold During the Year – Realized Gains/(Losses)	0	0
Purchases Sales and Settlements	0
Transfers In/(Out) of Level 3	0	0
Level 3 | Funds for Retiree Health Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	43	42
Assets Still Held at Reporting Date – Unrealized Gains/ (Losses)	0	1
Assets Sold During the Year – Realized Gains/(Losses)	0	0
Purchases Sales and Settlements		0
Transfers In/(Out) of Level 3	0	0
FAIR VALUE OF PLAN ASSETS AT END OF YEAR	43	43
Level 3 | Investments (including funds for retiree health benefits) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	43	42
Assets Still Held at Reporting Date – Unrealized Gains/ (Losses)	0	1
Assets Sold During the Year – Realized Gains/(Losses)	0	0
Purchases Sales and Settlements		0
Transfers In/(Out) of Level 3	0	0
FAIR VALUE OF PLAN ASSETS AT END OF YEAR	$ 43	$ 43