Short-Term and Long-Term Debt (Credit Facilities and Letters of Credit) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Credit Facilities	Total
Maximum Available	$ 4,846
LOCs issued	$ 3,310
Five-year revolving credit facility [Member]
Line of Credit Facility [Line Items]
Credit Facilities	Five-year revolving credit facility
Expiration Date	Jun. 30, 2021
Maximum Available	$ 2,500
LOCs issued	$ 1,001
LOC facility due December 2019 [Member]
Line of Credit Facility [Line Items]
Credit Facilities	LOC facility	[1]
Expiration Date	Dec. 06, 2019	[1]
Maximum Available	$ 350	[1]
LOCs issued	$ 350	[1]
LOC facility due August 2031 [Member]
Line of Credit Facility [Line Items]
Credit Facilities	LOC facility	[1]
Expiration Date	Aug. 26, 2031	[1]
Maximum Available	$ 990	[1]
LOCs issued	$ 953	[1]
LOC facility due October 2031 [Member]
Line of Credit Facility [Line Items]
Credit Facilities	LOC facility	[1]
Expiration Date	Oct. 01, 2031	[1]
Maximum Available	$ 1,006	[1]
LOCs issued	$ 1,006	[1]

[1] Our wholly-owned subsidiaries entered into irrevocable LOC facility agreements with third-party lenders supporting inter-company reinsurance agreements.