Employee Benefit Plans - Plan's Assets Carried at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	$ 582	$ 568
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	108	157
Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	253	227
Other Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	221	184
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	516	505
Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	108	157
Fair Value, Inputs, Level 2 [Member] | Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	253	227
Fair Value, Inputs, Level 2 [Member] | Other Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	155	121
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	66	63	$ 55
Fair Value, Inputs, Level 3 [Member] | Other Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fair Value Measurements	$ 66	$ 63