Pension Plans and Other Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Corresponding decrease in other comprehensive income related to the benefit obligation increase	$ 24	$ 0	$ 190	$ 0
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation increase for the period			171
Corresponding decrease in other comprehensive income related to the benefit obligation increase			171
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation increase for the period			23
Corresponding decrease in other comprehensive income related to the benefit obligation increase			23
Unfunded Plans Defined Benefit [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected future employer contributions in 2016					$ 25
Unfunded Plans Defined Benefit [Member] | Pension Plan [Member] | Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected future employer contributions in 2016			82
Expected future employer contributions in 2017			$ 23