Investments (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) item
Investment [Line Items]
Increase (decrease) in gross AFS securities unrealized losses			$ 1,200
Portion of loss recognized in other comprehensive income
Number of partnerships in alternative investment portfolio | item	238		238		224
Alternative investments as a percentage of overall invested assets			1.00%		1.00%
Fair value of collateral received that we are permitted to sell or re-pledge	$ 531		$ 531
Securities that have been re-pledged	364		364
Investment commitments	1,900		1,900
Investment commitments for limited partnerships	801		801
Investment commitments for mortgage loans on real estate	570		570
Investment commitments for private placements	501		501
Maximum [Member]
Investment [Line Items]
Portion of loss recognized in other comprehensive income	1	$ 1	1	$ 1
Federal Home Loan Mortgage Corporation [Member] | Invested Assets [Member]
Investment [Line Items]
Fair value	1,400		$ 1,400		$ 1,300
Concentration risk, percentage			1.00%		1.00%
Federal National Mortgage Association [Member] | Invested Assets [Member]
Investment [Line Items]
Fair value	1,300		$ 1,300		$ 1,000
Concentration risk, percentage			1.00%		1.00%
Consumer Non-Cyclical Industry [Member] | Invested Assets [Member]
Investment [Line Items]
Fair value	14,200		$ 14,200		$ 15,000
Concentration risk, percentage			13.00%		13.00%
Utilities Industry [Member] | Invested Assets [Member]
Investment [Line Items]
Fair value	$ 13,700		$ 13,700		$ 14,300
Concentration risk, percentage			12.00%		13.00%
Mortgage Loans On Real Estate [Member] | Geographic Concentration Risk [Member] | California [Member]
Investment [Line Items]
Concentration risk, percentage			22.00%		21.00%
Mortgage Loans On Real Estate [Member] | Geographic Concentration Risk [Member] | Texas [Member]
Investment [Line Items]
Concentration risk, percentage			12.00%		12.00%
Corporate Bonds [Member]
Investment [Line Items]
Percentage of fair value rated as investment grade	96.00%		96.00%		96.00%
Amortized cost of portfolio rated below investment grade	$ 3,400		$ 3,400		$ 3,500
Fair value of portfolio rated below investment grade	$ 3,200		$ 3,200		$ 3,500
ABS [Member]
Investment [Line Items]
Percentage of fair value rated as investment grade	98.00%		98.00%		98.00%
Amortized cost of portfolio rated below investment grade	$ 48		$ 48		$ 46
Fair value of portfolio rated below investment grade	$ 46		$ 46		$ 43
MBS [Member]
Investment [Line Items]
Projected default rate on existing delinquencies on MBS (low end of range)	10.00%		10.00%		10.00%
Projected default rate on existing delinquencies on MBS (high end of range)	100.00%		100.00%		100.00%
Severity of second lien loans	100.00%		100.00%		100.00%
Severity of first lien loans	30.00%		30.00%		30.00%