Derivative Financial Instruments - CDS notional amount by credit rating and fair value of protection sold (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Term of credit default swaps (in years)	5 years
Derivative [Line Items]
Notional amount	$ 5,312	$ 5,191
Credit default contracts
Derivative [Line Items]
Notional amount	755	754
Fair value	14	13
Single name \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	5	4
Fair value	0	0
Index \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	750	750
Fair value	14	13
AAA \| Credit default contracts
Derivative [Line Items]
Notional amount	6	6
AAA \| Single name \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	0	0
AAA \| Index \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	6	6
AA \| Credit default contracts
Derivative [Line Items]
Notional amount	12	12
AA \| Single name \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	0	0
AA \| Index \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	12	12
A \| Credit default contracts
Derivative [Line Items]
Notional amount	150	156
A \| Single name \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	0	0
A \| Index \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	150	156
BBB \| Credit default contracts
Derivative [Line Items]
Notional amount	492	492
BBB \| Single name \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	0	0
BBB \| Index \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	492	492
BB and lower \| Credit default contracts
Derivative [Line Items]
Notional amount	95	88
BB and lower \| Single name \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	5	4
BB and lower \| Index \| Corporate \| Credit default contracts
Derivative [Line Items]
Notional amount	$ 90	$ 84

Term of the credit risk derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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-Topic 815
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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