Employee Benefit Plans (Fair Values of the Company's Pension Plan Assets by Asset Category) (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	$ 3,216	$ 2,965
Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	2,506	2,252
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	338	324
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	1,252	1,093
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	916	835	$ 795
Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	710	713
Defined Benefit Plan, Equity Securities, US, Large Cap | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	159	138
Defined Benefit Plan, Equity Securities, US, Large Cap | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	65	58
Defined Benefit Plan, Equity Securities, US, Large Cap | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	94	80
Defined Benefit Plan, Equity Securities, US, Large Cap | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Defined Benefit Plan, Equity Securities, US, Small Cap | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	42	41
Defined Benefit Plan, Equity Securities, US, Small Cap | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	42	41
Defined Benefit Plan, Equity Securities, US, Small Cap | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Defined Benefit Plan, Equity Securities, US, Small Cap | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Defined Benefit Plan, Equity Securities, Non-US | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	224	227
Defined Benefit Plan, Equity Securities, Non-US | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	144	143
Defined Benefit Plan, Equity Securities, Non-US | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	80	84
Defined Benefit Plan, Equity Securities, Non-US | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Defined Benefit Plan, Cash and Cash Equivalents | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	15	26
Defined Benefit Plan, Cash and Cash Equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	5	10
Defined Benefit Plan, Cash and Cash Equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	10	16
Defined Benefit Plan, Cash and Cash Equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Debt Security, Corporate, US | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	464	414
Debt Security, Corporate, US | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Debt Security, Corporate, US | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	386	337
Debt Security, Corporate, US | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	78	77
Debt Security, Developed Markets And Emerging Economies | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	2	2
Debt Security, Developed Markets And Emerging Economies | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Debt Security, Developed Markets And Emerging Economies | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	2	2
Debt Security, Developed Markets And Emerging Economies | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Debt Security, Diverse Portfolio | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	126	237
Debt Security, Diverse Portfolio | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Debt Security, Diverse Portfolio | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	126	237
Debt Security, Diverse Portfolio | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
US Government Agencies Debt Securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	102	81
US Government Agencies Debt Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	82	72
US Government Agencies Debt Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	20	9
US Government Agencies Debt Securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Developed Markets, Government Agencies Debt Securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	19	7
Developed Markets, Government Agencies Debt Securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Developed Markets, Government Agencies Debt Securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	19	7
Developed Markets, Government Agencies Debt Securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Other | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	421	395
Other | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Other | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	18
Other | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	421	377
Defined Benefit Plan, Real Estate, Hedge Funds, And Other [Member] | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	932	684
Defined Benefit Plan, Real Estate, Hedge Funds, And Other [Member] | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	0	0
Defined Benefit Plan, Real Estate, Hedge Funds, And Other [Member] | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	515	303
Defined Benefit Plan, Real Estate, Hedge Funds, And Other [Member] | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair values of pension plan assets	$ 417	$ 381