Short-Term Debt - Schedule of Short-Term Borrowings (Detail) - USD ($)	Jun. 30, 2021	Dec. 31, 2020	Dec. 18, 2020	Oct. 30, 2020	Jul. 14, 2020
Line Of Credit Facility [Line Items]
Credit Facilities	$ 800,000,000	$ 1,250,000,000	$ 80,000,000
Borrowings Outstanding	410,000,000	775,000,000
Letters of Credit Outstanding	1,000,000	1,000,000
Credit Facilities [Member]
Line Of Credit Facility [Line Items]
Borrowings Outstanding	0
Commercial Paper [Member]
Line Of Credit Facility [Line Items]
Borrowings Outstanding	410,000,000
5-year Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	800,000,000	800,000,000
Borrowings Outstanding		345,000,000
Letters of Credit Outstanding	1,000,000	1,000,000
5-year Facility [Member] | Credit Facilities [Member]
Line Of Credit Facility [Line Items]
Borrowings Outstanding	0
5-year Facility [Member] | Commercial Paper [Member]
Line Of Credit Facility [Line Items]
Borrowings Outstanding	410,000,000
1-year Term Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	0	300,000,000
Borrowings Outstanding		300,000,000
Letters of Credit Outstanding	0	0
1-year Term Facility [Member] | Credit Facilities [Member]
Line Of Credit Facility [Line Items]
Borrowings Outstanding	0
1-year Term Facility [Member] | Commercial Paper [Member]
Line Of Credit Facility [Line Items]
Borrowings Outstanding	0
3-year Accounts Receivable Facility [Member]
Line Of Credit Facility [Line Items]
Credit Facilities	0	150,000,000		$ 150,000,000	$ 150,000,000
Borrowings Outstanding		130,000,000
Letters of Credit Outstanding	0	$ 0
3-year Accounts Receivable Facility [Member] | Credit Facilities [Member]
Line Of Credit Facility [Line Items]
Borrowings Outstanding	0
3-year Accounts Receivable Facility [Member] | Commercial Paper [Member]
Line Of Credit Facility [Line Items]
Borrowings Outstanding	$ 0