Investments (Details) - Investment Information, Debt Securities - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Investment disclosure details
Fixed maturities, amortized cost	$ 73,614			$ 68,830
Allowance for expected credit losses	2			2
Gross unrealized gains	3,694			5,186
Gross unrealized losses	266			11
Fixed maturities, at fair value	77,040			74,003
Proceeds from sales of fixed maturities classified as available for sale	2,864	$ 1,994
Gross realized gains	56	52
Gross realized losses	5	3
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	3,908			2,111
Allowance for expected credit losses	0			0
Gross unrealized gains	27			38
Gross unrealized losses	20			0
Fixed maturities, at fair value	3,915			2,149
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	33,904			33,763
Allowance for expected credit losses	0			0
Gross unrealized gains	1,966			2,599
Gross unrealized losses	111			2
Fixed maturities, at fair value	35,759			36,360
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	18,064			17,289
Allowance for expected credit losses	0			0
Gross unrealized gains	1,022			1,370
Gross unrealized losses	72			2
Fixed maturities, at fair value	19,014			18,657
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	10,785			11,806
Allowance for expected credit losses	0			0
Gross unrealized gains	660			909
Gross unrealized losses	36			0
Fixed maturities, at fair value	11,409			12,715
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,170			1,343
Allowance for expected credit losses	0			0
Gross unrealized gains	66			101
Gross unrealized losses	3			0
Fixed maturities, at fair value	1,233			1,444
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	3,885			3,325
Allowance for expected credit losses	0			0
Gross unrealized gains	218			219
Gross unrealized losses	0			0
Fixed maturities, at fair value	4,103			3,544
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,037			1,028
Allowance for expected credit losses	0			0
Gross unrealized gains	12			26
Gross unrealized losses	5			0
Fixed maturities, at fair value	1,044			1,054
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,727			2,222
Allowance for expected credit losses	0			0
Gross unrealized gains	89			139
Gross unrealized losses	1			0
Fixed maturities, at fair value	1,815			2,361
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	33,026			29,683
Allowance for expected credit losses	2	$ 4	$ 2	2	$ 8	$ 0
Gross unrealized gains	1,598			2,382
Gross unrealized losses	129			9
Fixed maturities, at fair value	34,493			32,054
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	12			23
Allowance for expected credit losses	0			0
Gross unrealized gains	2			2
Gross unrealized losses	0			0
Fixed maturities, at fair value	$ 14			$ 25