Debt (Narrative) (Details)		3 Months Ended		9 Months Ended
	May 01, 2018 shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($)	Jul. 16, 2018 USD ($)	Mar. 02, 2018 USD ($)	Jan. 15, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 12, 2015 USD ($) shares
Debt Instrument [Line Items]
Gain (Loss) on extinguishment of debt		$ 1,000,000	$ 0	$ (17,000,000)	$ 0
Issuance of common stock under Equity Units stock purchase contracts, shares | shares				6,000,000
Short-term borrowings		1,270,000,000		$ 1,270,000,000					$ 1,275,000,000
Repayments of Long-term Debt				1,393,000,000	$ 930,000,000
Equity Unit Purchase Agreements [Member]
Debt Instrument [Line Items]
Equity Units Issued | shares										25,000,000
Equity Units, stated value per unit (whole dollars)										$ 50
Equity Units, ownership interest percentage in remarketable subordinated notes										5.00%
Debt instrument, face amount										$ 1,250,000,000
Equity Units Stock Purchase Contracts Settlement Rate | shares	0.2412
Issuance of common stock under Equity Units stock purchase contracts, shares | shares	6,000,000
Senior Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 3,500,000,000		3,500,000,000
Convertible Debt [Member]
Debt Instrument [Line Items]
Repurchased Face Amount				35,000,000
Gain (Loss) on extinguishment of debt				1,000,000
Repayments of Long-term Debt				$ 118,000,000
Lines of Credit [Member]
Debt Instrument [Line Items]
Number of lines of credit		2		2
Maximum borrowing capacity		$ 500,000,000		$ 500,000,000
Short-term borrowings		450,000,000		450,000,000					450,000,000
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank, advances, short-term		$ 820,000,000		$ 820,000,000					$ 825,000,000
Debt instrument interest rate		2.149%		2.149%					1.386%
Surplus Notes [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 25,000,000		$ 25,000,000					$ 25,000,000
Remarketable Subordinated Notes [Member]
Debt Instrument [Line Items]
Remarketable subordinated notes tendered, cash payment				4,000,000
Remarketable Subordinated Notes [Member] | Four Point One Zero One Percent Due Two Thousand Twenty Eight [Member]
Debt Instrument [Line Items]
Debt instrument, face amount							$ 1,250,000,000
Remarketable Subordinated Notes [Member] | Four Point Five Five Zero Percent Due Two Thousand Forty Eight [Member]
Debt Instrument [Line Items]
Debt instrument, face amount							$ 850,000,000
Remarketable Subordinated Notes [Member] | One Point Nine Zero Zero Percent Due Two Thousand Twenty Eight [Member]
Debt Instrument [Line Items]
Debt instrument, face amount										$ 1,000
Debt Instrument, Interest Rate, Stated Percentage										1.90%
Remarketable Subordinated Notes [Member] | Remarketable Subordinated Notes [Member]
Debt Instrument [Line Items]
Gain (Loss) on extinguishment of debt				(18,000,000)
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Long-term Debt		17,135,000,000		17,135,000,000					16,329,000,000
Senior Unsecured Notes [Member] | Two Point Three Zero Zero Percent Due Two Thousand Eighteen [Member]
Debt Instrument [Line Items]
Debt instruments, repurchased face amount						$ 650,000,000
Senior Unsecured Notes [Member] | One Point Eight Seven Five Percent Due Two Thousand Eighteen [Member]
Debt Instrument [Line Items]
Debt instruments, repurchased face amount								$ 625,000,000
Commercial Paper Program
Debt Instrument [Line Items]
Commercial paper authorized		2,500,000,000		2,500,000,000
Commercial paper		$ 750,000,000		$ 750,000,000					$ 804,000,000