Investments (Continuous Gross Unrealized Losses for Fixed Maturity and Equity Securities Available-For-Sale) (Details) $ in Millions	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
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	2,939	903
Total number of securities in an unrealized loss position equal to or greater than 12 months	787	619
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	$ 24,383	$ 10,561
Less than 12 Months Gross Unrealized Loss	709	89
Equal to or Greater than 12 Months Estimated Fair Value	6,297	5,403
Equal to or Greater than 12 Months Gross Unrealized Loss	553	260
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	9,367	1,762
Less than 12 Months Gross Unrealized Loss	306	21
Equal to or Greater than 12 Months Estimated Fair Value	1,706	1,413
Equal to or Greater than 12 Months Gross Unrealized Loss	146	68
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	2,367	4,764
Less than 12 Months Gross Unrealized Loss	50	36
Equal to or Greater than 12 Months Estimated Fair Value	1,713	1,573
Equal to or Greater than 12 Months Gross Unrealized Loss	159	80
RMBS
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	3,778	2,308
Less than 12 Months Gross Unrealized Loss	101	13
Equal to or Greater than 12 Months Estimated Fair Value	1,541	1,292
Equal to or Greater than 12 Months Gross Unrealized Loss	116	41
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	3,348	636
Less than 12 Months Gross Unrealized Loss	128	8
Equal to or Greater than 12 Months Estimated Fair Value	529	559
Equal to or Greater than 12 Months Gross Unrealized Loss	81	54
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	1,074	171
Less than 12 Months Gross Unrealized Loss	34	3
Equal to or Greater than 12 Months Estimated Fair Value	152	106
Equal to or Greater than 12 Months Gross Unrealized Loss	13	4
CMBS
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	3,007	603
Less than 12 Months Gross Unrealized Loss	66	6
Equal to or Greater than 12 Months Estimated Fair Value	537	335
Equal to or Greater than 12 Months Gross Unrealized Loss	32	10
ABS
Continuous Gross Unrealized Loss and OTTI Loss for Fixed Maturity and Equity Securities Available-for-Sale
Less than 12 months Estimated Fair Value	994	165
Less than 12 Months Gross Unrealized Loss	7	0
Equal to or Greater than 12 Months Estimated Fair Value	27	75
Equal to or Greater than 12 Months Gross Unrealized Loss	1	2
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	448	152
Less than 12 Months Gross Unrealized Loss	17	2
Equal to or Greater than 12 Months Estimated Fair Value	92	50
Equal to or Greater than 12 Months Gross Unrealized Loss	$ 5	$ 1