Plan Assets Related to Pension and OPB Benefits by Level within the Fair Value Hierarchy (Detail) - USD ($) $ in Millions		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Defined Benefit Plan, Plan Assets, Amount		$ 3,017		$ 3,432
Defined Benefit Plan Accrued Income Threshold for Inclusion, Percent of Total Plan Assets		0.90%		700000.00%
Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Plan Assets, Amount		$ 971	[1]	$ 1,743	[2]
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Plan Assets, Amount		2,046	[1]	1,689	[2]
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1]	0	[2]
Money Market Funds [Member]
Defined Benefit Plan, Plan Assets, Amount		112	[3]	138	[4]
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Plan Assets, Amount		112	[1],[3]	138	[2],[4]
Money Market Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1],[3]	0	[2],[4]
Money Market Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1],[3]	0	[2],[4]
Pension Plan [Member]
Defined Benefit Plan, Plan Assets, Amount	[5]	2,885		3,288		$ 3,016
Pension Plan [Member] | Equity Securities [Member]
Defined Benefit Plan, Plan Assets, Amount		382		936
Pension Plan [Member] | Equity Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Plan Assets, Amount		382	[1]	936	[2]
Pension Plan [Member] | Equity Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1]	0	[2]
Pension Plan [Member] | Equity Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1]	0	[2]
Pension Plan [Member] | Derivative [Member]
Defined Benefit Plan, Plan Assets, Amount		2		(5)
Pension Plan [Member] | Derivative [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1]	(5)	[2]
Pension Plan [Member] | Derivative [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Plan Assets, Amount		2	[1]	0	[2]
Pension Plan [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1]	0	[2]
Pension Plan [Member] | Fixed Income Securities [Member]
Defined Benefit Plan, Plan Assets, Amount		2,377		2,201
Pension Plan [Member] | Fixed Income Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Plan Assets, Amount		333	[1]	512	[2]
Pension Plan [Member] | Fixed Income Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Plan Assets, Amount		2,044	[1]	1,689	[2]
Pension Plan [Member] | Fixed Income Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1]	0	[2]
Pension Plan [Member] | Other Assets [Member]
Defined Benefit Plan, Plan Assets, Amount				9
Pension Plan [Member] | Other Assets [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Plan Assets, Amount	[2]			9
Pension Plan [Member] | Other Assets [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Plan Assets, Amount	[2]			0
Pension Plan [Member] | Other Assets [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Plan Assets, Amount	[2]			0
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Plan Assets, Amount		155		164		$ 157
Other Postretirement Benefits Plan [Member] | Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Plan Assets, Amount		11		11
Other Postretirement Benefits Plan [Member] | Equity Funds [Member]
Defined Benefit Plan, Plan Assets, Amount		46	[6]	56	[7]
Other Postretirement Benefits Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Plan Assets, Amount		46	[1],[6]	56	[2],[7]
Other Postretirement Benefits Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1],[6]	0	[2],[7]
Other Postretirement Benefits Plan [Member] | Equity Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1],[6]	0	[2],[7]
Other Postretirement Benefits Plan [Member] | Nontaxable Municipal Bonds [Member]
Defined Benefit Plan, Plan Assets, Amount		86	[6]	85	[7]
Other Postretirement Benefits Plan [Member] | Nontaxable Municipal Bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Plan Assets, Amount		86	[1],[6]	85	[2],[7]
Other Postretirement Benefits Plan [Member] | Nontaxable Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1],[6]	0	[2],[7]
Other Postretirement Benefits Plan [Member] | Nontaxable Municipal Bonds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1],[6]	0	[2],[7]
Other Postretirement Benefits Plan [Member] | Taxable Fixed Income Index Funds [Member]
Defined Benefit Plan, Plan Assets, Amount		12	[6]	12	[7]
Other Postretirement Benefits Plan [Member] | Taxable Fixed Income Index Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan, Plan Assets, Amount		12	[1],[6]	12	[2],[7]
Other Postretirement Benefits Plan [Member] | Taxable Fixed Income Index Funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan, Plan Assets, Amount		0	[1],[6]	0	[2],[7]
Other Postretirement Benefits Plan [Member] | Taxable Fixed Income Index Funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan, Plan Assets, Amount		$ 0	[1],[6]	$ 0	[2],[7]

[1] Fair value measurements do not include pension benefits accrued income amounting to less than 0.9% of total plan assets.

[2] Fair value measurements do not include pension benefits accrued income amounting to less than 0.7% of total plan assets.

[3] Includes $11 million for other postretirement benefit plans.

[4] Includes $11 million for other postretirement benefit plans.

[5] Employer contributions represent the benefits that were paid to nonqualified plan participants. Unfunded nonqualified supplemental pension plans are not funded through plan assets.

[6] Relates exclusively to other postretirement benefit plans.

[7] Relates exclusively to other postretirement benefit plans.