Retirement Plans - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Information on postretirement medical plan			Additionally, we maintain an unfunded postretirement medical plan that provides health benefits to certain qualified retirees from ages 55 through 65.
Settlement loss			$ 93	$ 4	$ 19
Budgeted contributions to funded pension plans for next year			45
Expense for defined contribution plans			$ 20	19	22
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Current target allocations for plan assets			45.00%
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Current target allocations for plan assets			35.00%
Other Types of Investments
Defined Benefit Plan Disclosure [Line Items]
Current target allocations for plan assets			20.00%
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Settlement loss	$ 88		$ 93	4	19
Single premium annuity contract cost	250		250
Net increase in post retirement plan liabilities	$ 239		239
Service costs			44	42	49
Interest costs			89	91	102
Amortization of unrecognized net actuarial losses			52	39	58
Expected return on plan assets			$ (180)	$ (194)	$ (168)
Scenario Forecast
Defined Benefit Plan Disclosure [Line Items]
Service costs		$ 55
Net non-service pension costs		55
Interest costs		75
Amortization of unrecognized net actuarial losses		50
Expected return on plan assets		$ (180)