Investments (Details) - Investment Information, Debt Securities - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Investment disclosure details
Fixed maturities, amortized cost	$ 70,286		$ 68,830
Allowance for expected credit losses	2		2	$ 0
Gross unrealized gains	3,873		5,186
Gross unrealized losses	296		11
Fixed maturities, at fair value	73,861		74,003
Proceeds from sales of fixed maturities classified as available for sale	1,238	$ 388
Gross realized gains	15	15
Gross realized losses	1	$ 1
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Fixed maturities, amortized cost	2,530		2,111
Allowance for expected credit losses	0		0
Gross unrealized gains	32		38
Gross unrealized losses	18		0
Fixed maturities, at fair value	2,544		2,149
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Fixed maturities, amortized cost	34,169		33,763
Allowance for expected credit losses	0		0
Gross unrealized gains	2,063		2,599
Gross unrealized losses	132		2
Fixed maturities, at fair value	36,100		36,360
Obligations of states, municipalities and political subdivisions, local general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	17,719		17,289
Allowance for expected credit losses	0		0
Gross unrealized gains	1,079		1,370
Gross unrealized losses	84		2
Fixed maturities, at fair value	18,714		18,657
Obligations of states, municipalities and political subdivisions, revenue [Member]
Investment disclosure details
Fixed maturities, amortized cost	11,748		11,806
Allowance for expected credit losses	0		0
Gross unrealized gains	699		909
Gross unrealized losses	44		0
Fixed maturities, at fair value	12,403		12,715
Obligations of states, municipalities and political subdivisions, state general obligation [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,337		1,343
Allowance for expected credit losses	0		0
Gross unrealized gains	81		101
Gross unrealized losses	4		0
Fixed maturities, at fair value	1,414		1,444
Obligations of states, municipalities and political subdivisions, pre-refunded [Member]
Investment disclosure details
Fixed maturities, amortized cost	3,365		3,325
Allowance for expected credit losses	0		0
Gross unrealized gains	204		219
Gross unrealized losses	0		0
Fixed maturities, at fair value	3,569		3,544
Debt securities issued by foreign governments [Member]
Investment disclosure details
Fixed maturities, amortized cost	962		1,028
Allowance for expected credit losses	0		0
Gross unrealized gains	18		26
Gross unrealized losses	4		0
Fixed maturities, at fair value	976		1,054
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Fixed maturities, amortized cost	1,939		2,222
Allowance for expected credit losses	0		0	0
Gross unrealized gains	107		139
Gross unrealized losses	1		0
Fixed maturities, at fair value	2,045		2,361
All other corporate bonds [Member]
Investment disclosure details
Fixed maturities, amortized cost	30,663		29,683
Allowance for expected credit losses	2		2	$ 0
Gross unrealized gains	1,651		2,382
Gross unrealized losses	141		9
Fixed maturities, at fair value	32,171		32,054
Redeemable preferred stock [Member]
Investment disclosure details
Fixed maturities, amortized cost	23		23
Allowance for expected credit losses	0		0
Gross unrealized gains	2		2
Gross unrealized losses	0		0
Fixed maturities, at fair value	$ 25		$ 25

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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