Investments (Past Due and Interest Accrual Status of Mortgage Loans) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Nonaccrual [Line Items]
Mortgage Loans, Gross	$ 81,393	$ 84,344
Greater than 90 Days Past Due and Still Accruing Interest	26	91
Financing Receivable, Nonaccrual	848	1,117
Financial Asset, Past Due
Financing Receivable, Nonaccrual [Line Items]
Mortgage Loans, Gross	612	818
Commercial Mortgage Loans
Financing Receivable, Nonaccrual [Line Items]
Mortgage Loans, Gross	51,336	52,434
Greater than 90 Days Past Due and Still Accruing Interest	15	7
Financing Receivable, Nonaccrual	159	317	$ 176
Commercial Mortgage Loans | Financial Asset, Past Due
Financing Receivable, Nonaccrual [Line Items]
Mortgage Loans, Gross	15	10
Residential Mortgage Loans
Financing Receivable, Nonaccrual [Line Items]
Mortgage Loans, Gross	11,704	13,782
Greater than 90 Days Past Due and Still Accruing Interest	8	64
Financing Receivable, Nonaccrual	438	534	418
Residential Mortgage Loans | Financial Asset, Past Due
Financing Receivable, Nonaccrual [Line Items]
Mortgage Loans, Gross	441	556
Agricultural Mortgage Loans
Financing Receivable, Nonaccrual [Line Items]
Mortgage Loans, Gross	18,353	18,128
Greater than 90 Days Past Due and Still Accruing Interest	3	20
Financing Receivable, Nonaccrual	251	266	$ 137
Agricultural Mortgage Loans | Financial Asset, Past Due
Financing Receivable, Nonaccrual [Line Items]
Mortgage Loans, Gross	$ 156	$ 252