Pension and Other Postretirement Benefits - Schedule of Amounts Expected to be Recognized in Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition		$ 468	$ 391	$ 489
Prior service cost (benefit) recognition		16	18	19
Pension Benefits [Member] | Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition	$ 413
Prior service cost (benefit) recognition	15
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition		17	13	35
Prior service cost (benefit) recognition		$ (37)	$ (25)	$ (18)
Other Postretirement Benefits [Member] | Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition	21
Prior service cost (benefit) recognition	$ (26)