Pension Plans and Other Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Corresponding decrease in other comprehensive income related to the net liability increase	$ 157	$ 0	$ 347	$ 0
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net liability increase for the period			327
Corresponding decrease in other comprehensive income related to the net liability increase			327
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net liability increase for the period			24
Corresponding decrease in other comprehensive income related to the net liability increase			24
Unfunded Pension Plans [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected future employer contributions					$ 25
Unfunded Pension Plans [Member] | Pension Plan [Member] | Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected future employer contributions			23
Expected employer contributions in 2016			$ 82