Fair Value Measurements (Financial Instruments' Carrying Amounts and Fair Values) (Parenthetical) (Detail) - USD ($) $ in Millions	1 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2020	Jun. 30, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Valuation of certain fair value hedges associated with fixed rate debt		$ 3	$ 3
Supplemental line of credit facility borrowings expiration period		1 year
364-Day Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument Term	364 days	364 days	364 days