Investments (Continuous Gross Unrealized Losses for Fixed Maturity and Equity Securities Available-For-Sale) (Details) $ in Millions	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale [Line Items]
Total number of securities in an unrealized loss position less than 12 months	6,790	2,598
Total number of securities in an unrealized loss position equal to or greater than 12 months	2,159	1,955
Fixed Maturity Securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Estimated Fair Value	$ 98,579	$ 41,943
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,943	435
Equal to or Greater than 12 Months Estimated Fair Value	25,309	23,375
Equal to or Greater than 12 Months Gross Unrealized Loss	2,272	1,468
U.S. government and agency
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Estimated Fair Value	20,235	18,273
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	252	93
Equal to or Greater than 12 Months Estimated Fair Value	5,982	3,560
Equal to or Greater than 12 Months Gross Unrealized Loss	512	186
Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Estimated Fair Value	9,325	4,234
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	305	83
Equal to or Greater than 12 Months Estimated Fair Value	3,054	3,251
Equal to or Greater than 12 Months Gross Unrealized Loss	259	229
Foreign corporate
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Estimated Fair Value	16,098	4,422
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	784	99
Equal to or Greater than 12 Months Estimated Fair Value	4,998	6,802
Equal to or Greater than 12 Months Gross Unrealized Loss	602	577
Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Estimated Fair Value	12,322	6,359
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	356	50
Equal to or Greater than 12 Months Estimated Fair Value	4,633	4,159
Equal to or Greater than 12 Months Gross Unrealized Loss	294	141
State and political subdivision
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Estimated Fair Value	1,900	182
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	68	2
Equal to or Greater than 12 Months Estimated Fair Value	334	346
Equal to or Greater than 12 Months Gross Unrealized Loss	27	12
ABS
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Estimated Fair Value	5,851	1,695
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	23	7
Equal to or Greater than 12 Months Estimated Fair Value	409	729
Equal to or Greater than 12 Months Gross Unrealized Loss	16	31
CMBS
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Estimated Fair Value	4,532	1,174
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	86	9
Equal to or Greater than 12 Months Estimated Fair Value	682	413
Equal to or Greater than 12 Months Gross Unrealized Loss	55	33
U.S. corporate
Debt Securities, Available-for-sale [Line Items]
Less than 12 months Estimated Fair Value	28,316	5,604
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,069	92
Equal to or Greater than 12 Months Estimated Fair Value	5,217	4,115
Equal to or Greater than 12 Months Gross Unrealized Loss	$ 507	$ 259