Retirement Plans - Schedule of Fair Value of 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 value of plan assets	$ 189	$ 173	$ 150
Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	5
Common Stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	6
Other large-cap U.S. financial services companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	4
Other large-cap U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	60	54
Other equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
U.S. large-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
U.S. small-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	3
U.S. mid-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	5
Non-U.S. equities growth fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	9
U.S. mid-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	12
U.S. small-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	4
Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	5
U.S. government agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45	51
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
U.S. investment-grade fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24	12
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	144	121
Level 1 | Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	5
Level 1 | Common Stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	6
Level 1 | Other large-cap U.S. financial services companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	4
Level 1 | Other large-cap U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	60	54
Level 1 | Other equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Level 1 | U.S. large-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Level 1 | U.S. small-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	3
Level 1 | U.S. mid-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	5
Level 1 | Non-U.S. equities growth fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	9
Level 1 | U.S. mid-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	12
Level 1 | U.S. small-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	4
Level 1 | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	5
Level 1 | U.S. government agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 | U.S. investment-grade fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24	12
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45	52
Level 2 | Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Common Stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Other large-cap U.S. financial services companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Other large-cap U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Other equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | U.S. large-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | U.S. small-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | U.S. mid-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Non-U.S. equities growth fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | U.S. mid-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | U.S. small-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | U.S. government agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45	51
Level 2 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Level 2 | U.S. investment-grade fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Common Stock
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Other large-cap U.S. financial services companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Other large-cap U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Other equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. large-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. small-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. mid-cap equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Non-U.S. equities growth fund
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. mid-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. small-cap
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. government agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | U.S. investment-grade fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0