Retirement Plans - Fair Value of Financial Assets of Funded Pension Plans (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	$ 3,043	$ 2,732
Cash and Short-term Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	44	57
U.S. Equities (domestic)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	749	638
International Equities (non-U.S.)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	489	419
Global Equities (domestic and non-U.S.)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	225	204
Treasury and Government Related - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	399	428
Mortgage-backed Securities - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	186	171
Corporate - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	443	375
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	73	65
Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	251	191
Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	184	184
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	746	802
Level 1 | Cash and Short-term Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	44	57
Level 1 | U.S. Equities (domestic)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	585	638
Level 1 | International Equities (non-U.S.)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	94	80
Level 1 | Global Equities (domestic and non-U.S.)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 1 | Treasury and Government Related - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 1 | Mortgage-backed Securities - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 1 | Corporate - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 1 | Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 1 | Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 1 | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	23	27
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	898	851
Level 2 | Cash and Short-term Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 2 | U.S. Equities (domestic)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 2 | International Equities (non-U.S.)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	43	37
Level 2 | Global Equities (domestic and non-U.S.)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	8	8
Level 2 | Treasury and Government Related - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	350	372
Level 2 | Mortgage-backed Securities - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	116	141
Level 2 | Corporate - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	381	293
Level 2 | Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 2 | Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 2 | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 3 | Cash and Short-term Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 3 | U.S. Equities (domestic)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 3 | International Equities (non-U.S.)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 3 | Global Equities (domestic and non-U.S.)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 3 | Treasury and Government Related - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 3 | Mortgage-backed Securities - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 3 | Corporate - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 3 | Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 3 | Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Level 3 | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	1,399	1,079
Net Asset Value | Cash and Short-term Investment Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	0	0
Net Asset Value | U.S. Equities (domestic)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	164	0
Net Asset Value | International Equities (non-U.S.)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	352	302
Net Asset Value | Global Equities (domestic and non-U.S.)
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	217	196
Net Asset Value | Treasury and Government Related - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	49	56
Net Asset Value | Mortgage-backed Securities - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	70	30
Net Asset Value | Corporate - Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	62	82
Net Asset Value | Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	73	65
Net Asset Value | Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	251	191
Net Asset Value | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of financial assets of funded pension plans	$ 161	$ 157