Investments (Continuous Gross Unrealized Losses for Fixed Maturity and Equity Securities Available-For-Sale) (Details) $ in Millions	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Total number of securities in an unrealized loss position less than 12 months	914	1,711
Total number of securities in an unrealized loss position equal to or greater than 12 months	623	475
Fixed maturity securities
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	$ 10,561	$ 17,060
Less than 12 Months Gross Unrealized Loss	89	671
Equal to or Greater than 12 Months Estimated Fair Value	5,403	2,757
Equal to or Greater than 12 Months Gross Unrealized Loss	260	251
U.S. corporate
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	1,762	4,632
Less than 12 Months Gross Unrealized Loss	21	187
Equal to or Greater than 12 Months Estimated Fair Value	1,413	699
Equal to or Greater than 12 Months Gross Unrealized Loss	68	98
U.S. government and agency
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	4,764	4,396
Less than 12 Months Gross Unrealized Loss	36	237
Equal to or Greater than 12 Months Estimated Fair Value	1,573	0
Equal to or Greater than 12 Months Gross Unrealized Loss	80	0
RMBS
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	2,308	3,457
Less than 12 Months Gross Unrealized Loss	13	107
Equal to or Greater than 12 Months Estimated Fair Value	1,292	818
Equal to or Greater than 12 Months Gross Unrealized Loss	41	32
Foreign corporate
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	636	1,443
Less than 12 Months Gross Unrealized Loss	8	64
Equal to or Greater than 12 Months Estimated Fair Value	559	573
Equal to or Greater than 12 Months Gross Unrealized Loss	54	104
State and political subdivision
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	171	887
Less than 12 Months Gross Unrealized Loss	3	35
Equal to or Greater than 12 Months Estimated Fair Value	106	29
Equal to or Greater than 12 Months Gross Unrealized Loss	4	3
CMBS
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	603	1,553
Less than 12 Months Gross Unrealized Loss	6	26
Equal to or Greater than 12 Months Estimated Fair Value	335	171
Equal to or Greater than 12 Months Gross Unrealized Loss	10	5
ABS
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	165	450
Less than 12 Months Gross Unrealized Loss	0	5
Equal to or Greater than 12 Months Estimated Fair Value	75	461
Equal to or Greater than 12 Months Gross Unrealized Loss	2	8
Foreign government
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	152	242
Less than 12 Months Gross Unrealized Loss	2	10
Equal to or Greater than 12 Months Estimated Fair Value	50	6
Equal to or Greater than 12 Months Gross Unrealized Loss	1	1
Equity securities
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	17	57
Less than 12 Months Gross Unrealized Loss	0	2
Equal to or Greater than 12 Months Estimated Fair Value	10	40
Equal to or Greater than 12 Months Gross Unrealized Loss	$ 1	$ 7