Investments (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)
Investment [Line Items]
Increase (decrease) in gross AFS securities unrealized losses	$ 703
Unrealized gain (loss), trading securities	$ 7	$ (3)	$ (100)
Number of partnerships in alternative investment portfolio | item	224	202
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	$ 175
Securities that have been re-pledged	174
Investment commitments	1,300
Investment commitments for limited partnerships	752
Investment commitments for mortgage loans on real estate	320
Investment commitments for private placements	196
Federal Home Loan Mortgage Corporation [Member] | Invested Assets [Member]
Investment [Line Items]
Fair value	$ 1,300	$ 1,500
Concentration risk, percentage	1.00%	1.00%
Federal National Mortgage Association [Member] | Invested Assets [Member]
Investment [Line Items]
Fair value	$ 1,000	$ 1,100
Concentration risk, percentage	1.00%	1.00%
Consumer Non-Cyclical Industry [Member]
Investment [Line Items]
Fair value	$ 15,000	$ 13,700
Consumer Non-Cyclical Industry [Member] | Invested Assets [Member]
Investment [Line Items]
Concentration risk, percentage	13.00%	13.00%
Utilities Industry [Member] | Invested Assets [Member]
Investment [Line Items]
Fair value	$ 14,300	$ 13,200
Concentration risk, percentage	13.00%	12.00%
Mortgage Loans On Real Estate [Member] | Geographic Concentration Risk [Member] | California [Member]
Investment [Line Items]
Concentration risk, percentage	21.00%	20.00%
Mortgage Loans On Real Estate [Member] | Geographic Concentration Risk [Member] | Texas [Member]
Investment [Line Items]
Concentration risk, percentage	12.00%	11.00%
Corporate Bonds [Member]
Investment [Line Items]
Percentage of fair value rated as investment grade	96.00%	95.00%
Amortized cost of portfolio rated below investment grade	$ 3,500	$ 3,800
Fair value of portfolio rated below investment grade	$ 3,500	$ 3,700
ABS [Member]
Investment [Line Items]
Percentage of fair value rated as investment grade	98.00%	96.00%
Amortized cost of portfolio rated below investment grade	$ 46	$ 91
Fair value of portfolio rated below investment grade	$ 43	$ 75
MBS [Member]
Investment [Line Items]
Projected default rate on existing delinquencies on MBS (low end of range)	10.00%	10.00%
Projected default rate on existing delinquencies on MBS (high end of range)	100.00%	100.00%
Severity of second lien loans	100.00%
Severity of first lien loans	30.00%