Short-Term and Long-Term Debt (Credit Facilities and Letters of Credit) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Credit Facilities	Total
Maximum Available	$ 5,903
LOCs issued	$ 3,656
Five-year revolving credit facility [Member]
Credit Facilities	Five-year revolving credit facility
Expiration Date	Jun. 30, 2021
Maximum Available	$ 2,500
LOCs issued	$ 342
LOC facility due December 2019 [Member]
Credit Facilities	LOC facility (1)	[1]
Expiration Date	Dec. 06, 2019	[1]
Maximum Available	$ 350	[1]
LOCs issued	$ 350	[1]
LOC facility due March 2023 [Member]
Credit Facilities	LOC facility (2)	[2]
Expiration Date	Mar. 31, 2023	[2]
Maximum Available	$ 110	[2]
LOCs issued	$ 110	[2]
LOC facility also due March 2023 [Member]
Credit Facilities	LOC facility (1)	[1]
Expiration Date	Mar. 31, 2023	[1]
Maximum Available	$ 924	[1]
LOCs issued	$ 924	[1]
LOC facility due August 2031 [Member]
Credit Facilities	LOC facility (1)	[1]
Expiration Date	Aug. 26, 2031	[1]
Maximum Available	$ 990	[1]
LOCs issued	$ 907	[1]
LOC facility due October 2031 [Member]
Credit Facilities	LOC facility (1)	[1]
Expiration Date	Oct. 01, 2031	[1]
Maximum Available	$ 1,029	[1]
LOCs issued	$ 1,023	[1]

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

[2] We entered into an irrevocable LOC facility agreement with a third-party lender supporting certain fees owed to another third-party lender that automatically renews on an annual basis, unless not extended by the third-party upon 30 days' notice.