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Short-Term and Long-Term Debt (Credit Facilities and Letters of Credit) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Line of Credit Facility [Line Items]  
Credit Facilities Total
Maximum Available $ 4,863
LOCs issued $ 2,596
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 $ 281
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 $ 945 [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 $ 1,023 [1]
LOCs issued $ 1,020 [1]
[1] Our wholly-owned subsidiaries entered into irrevocable LOC facility agreements with third-party lenders supporting inter-company reinsurance agreements.