Investments (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Increase (decrease) in gross AFS securities unrealized losses	$ (969)
Trading Securities, Unrealized Holding Gains (Losses)	$ (3)	$ (100)	$ 45
Number of partnerships in alternative investment portfolio | item	202	190
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	$ 171
Investment commitments	1,200
Investment commitments for limited partnerships	741
Investment commitments for mortgage loans on real estate	261
Investment commitments for private placements	$ 183
California [Member]
Largest mortgage loan concentration in geographic region	20.00%	21.00%
Texas [Member]
Largest mortgage loan concentration in geographic region	11.00%	10.00%
Federal Home Loan Mortgage Corporation
Fair value	$ 1,500	$ 1,800
Concentration risk, percentage	1.00%	2.00%
Fannie Mae
Fair value	$ 1,100	$ 1,200
Concentration risk, percentage	1.00%	1.00%
Consumer Non-Cyclical Industry [Member]
Fair value	$ 13,700	$ 12,000
Concentration risk, percentage	13.00%	12.00%
Utilities Industry [Member]
Fair value	$ 13,200	$ 12,800
Concentration risk, percentage	12.00%	13.00%
Corporate bonds [Member]
Percentage of fair value rated as investment grade	95.00%	96.00%
Amortized cost of portfolio rated below investment grade	$ 3,800	$ 3,600
Fair value of portfolio rated below investment grade	$ 3,700	$ 3,300
ABS [Member]
Percentage of fair value rated as investment grade	96.00%	96.00%
Amortized cost of portfolio rated below investment grade	$ 91	$ 107
Fair value of portfolio rated below investment grade	$ 75	$ 92
MBS [Member]
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%