Investments (Narrative) (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($) item
Investment [Line Items]
Increase (decrease) in gross AFS securities unrealized losses	$ (548)
Number of partnerships in alternative investment portfolio | item	206	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	$ 177
Securities that have been re-pledged	80
Investment commitments	1,700
Investment commitments for limited partnerships	794
Investment commitments for mortgage loans on real estate	443
Investment commitments for private placements	$ 466
California [Member]
Investment [Line Items]
Largest mortgage loan concentration in geographic region	20.00%	20.00%
Texas [Member]
Investment [Line Items]
Largest mortgage loan concentration in geographic region	11.00%	11.00%
Federal Home Loan Mortgage Corporation [Member]
Investment [Line Items]
Fair value	$ 1,400	$ 1,500
Federal Home Loan Mortgage Corporation [Member] | Invested Assets [Member]
Investment [Line Items]
Concentration risk, percentage	1.00%	1.00%
Fannie Mae [Member]
Investment [Line Items]
Fair value	$ 1,000	$ 1,100
Fannie Mae [Member] | Invested Assets [Member]
Investment [Line Items]
Concentration risk, percentage	1.00%	1.00%
Consumer Non-Cyclical Industry [Member]
Investment [Line Items]
Fair value	$ 14,700	$ 13,700
Consumer Non-Cyclical Industry [Member] | Invested Assets [Member]
Investment [Line Items]
Concentration risk, percentage	13.00%	13.00%
Utilities Industry [Member]
Investment [Line Items]
Fair value	$ 13,800	$ 13,200
Utilities Industry [Member] | Invested Assets [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%	95.00%
Amortized cost of portfolio rated below investment grade	$ 3,500	$ 3,800
Fair value of portfolio rated below investment grade	$ 3,400	$ 3,700
ABS [Member]
Investment [Line Items]
Percentage of fair value rated as investment grade	96.00%	96.00%
Amortized cost of portfolio rated below investment grade	$ 86	$ 91
Fair value of portfolio rated below investment grade	$ 73	$ 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%	100.00%
Severity of first lien loans	30.00%	30.00%