Pension and Other Postretirement Benefits - Schedule of Fair Value of Pension Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	$ 5,348	$ 5,007
Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,060	2,010
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,848	1,825
Long/Short Equity Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	5	8
Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	207	177
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,694	2,470
Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,338	2,149
Cash and Cash Equivalent Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	354	316
Fixed Income, Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2	5
Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	594	527
Real Estate Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	557	495
Other Investments, Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	37	32
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,753	1,732
Level 1 [Member] | Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	379	612
Level 1 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	379	612
Level 1 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,090	912
Level 1 [Member] | Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	925	889
Level 1 [Member] | Cash and Cash Equivalent Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	165	23
Level 1 [Member] | Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	284	208
Level 1 [Member] | Real Estate Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	284	208
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	796	705
Level 2 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	796	705
Level 2 [Member] | Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	794	700
Level 2 [Member] | Fixed Income, Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2	5
Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,799	2,570
Net Asset Value [Member] | Equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,681	1,398
Net Asset Value [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,469	1,213
Net Asset Value [Member] | Long/Short Equity Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	5	8
Net Asset Value [Member] | Private Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	207	177
Net Asset Value [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	808	853
Net Asset Value [Member] | Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	619	560
Net Asset Value [Member] | Cash and Cash Equivalent Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	189	293
Net Asset Value [Member] | Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	310	319
Net Asset Value [Member] | Real Estate Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	273	287
Net Asset Value [Member] | Other Investments, Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	$ 37	$ 32