Borrowings Under Credit Facilities and Commercial Paper Program (Narrative) (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) bank term	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Number of banks in consortium | bank		31
Borrowings outstanding		$ 0	$ 1,587,597,000
Principal outstanding		0	$ 1,588,600,000	$ 1,147,000,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Unsecured line of credit arrangement		3,000,000,000
Borrowings outstanding		$ 0
Annual facility fee for each bank based on commitment amount		0.15%
Revolving credit facility, number of successive terms | term		2
Revolving credit facility, extended term		6 months
Available to borrow in alternate currencies (up to)		$ 1,000,000,000
Revolving Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Applicable margin over index, based on certain debt ratings		0.825%
Revolving Credit Facility | Accordion Feature
Line of Credit Facility [Line Items]
Unsecured line of credit arrangement		$ 1,000,000,000
Term Credit Facility
Line of Credit Facility [Line Items]
Unsecured line of credit arrangement		500,000,000
Term Credit Facility, CAD Denominated
Line of Credit Facility [Line Items]
Unsecured line of credit arrangement		250,000,000
Revolving Credit Facility, Alternate Currencies
Line of Credit Facility [Line Items]
Borrowings outstanding		0
Commercial Paper Note Program
Line of Credit Facility [Line Items]
Unsecured line of credit arrangement	$ 1,000,000,000
Borrowings outstanding		$ 0
Commercial Paper Note Program | Maximum
Line of Credit Facility [Line Items]
Debt instrument, term	397 days