Other Postretirement Benefits - Schedule of Fair Values of Plan Assets (Detail) - Other Postretirement Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Total investments	$ 660	$ 725
Funds for retiree health benefits	325	358
Investments (including funds for retiree health benefits)	985	1,083
Pending activities	9	1
Total fair value of plan net assets	994	1,084	$ 1,113	$ 1,047
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Total investments	393	428
Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total investments	260	286
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total investments	7	11
Level 1
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	162	184
Investments (including funds for retiree health benefits)	162	184
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total investments	660	725
Funds for retiree health benefits	120	131
Investments (including funds for retiree health benefits)	780	856
Level 2 | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Total investments	393	428
Level 2 | Other Fixed Income Debt
Defined Benefit Plan Disclosure [Line Items]
Total investments	260	286
Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Total investments	7	11
Level 3
Defined Benefit Plan Disclosure [Line Items]
Funds for retiree health benefits	43	43
Investments (including funds for retiree health benefits)	$ 43	$ 43