Borrowings Under Credit Facilities and Commercial Paper Program - Narrative (Details)	1 Months Ended	3 Months Ended
	Jan. 31, 2019 USD ($)	Mar. 31, 2019 USD ($) term bank	Dec. 31, 2018 USD ($)	[1]	Mar. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Number of banks in consortium | bank		31
Borrowings outstanding		$ 419,293,000	$ 1,147,000,000		$ 865,000,000
Available to borrow in alternate currencies		$ 1,000,000,000
Applicable margin		0.825%
Facility fee		0.15%
Totals		$ 12,410,495,000
Accordion Feature
Line of Credit Facility [Line Items]
Unsecured line of credit arrangement		1,000,000,000
Unsecured Revolving Credit Facility
Line of Credit Facility [Line Items]
Unsecured line of credit arrangement		3,000,000,000
Borrowings outstanding		$ 0
Number of successive terms | term		2
Extended expiration period		6 months
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
Term Credit Facility, CAD Denominated | Accordion Feature
Line of Credit Facility [Line Items]
Unsecured line of credit arrangement		250,000,000
Unsecured Credit Facility In 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
Totals		419,293,000
Principal outstanding		419,700,000
Unamortized discount		$ 407,000
Weighted average interest rate		2.84%
Commercial Paper Note Program | Maximum
Line of Credit Facility [Line Items]
Debt instrument, term	397 days
Commercial Paper Note Program | Weighted Average
Line of Credit Facility [Line Items]
Debt instrument, term		12 days

[1] The consolidated balance sheet at December 31, 2018 has been derived from the audited financial statements at that date but does not include all of the information and footnotes required by U.S. generally accepted accounting principles for complete financial statements.