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Investments - Additional Information (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0	$ 0
Securities fair value	9	8
Ambac Assurance [Member]
Schedule of Investments [Line Items]
Unsettled Amounts Secured Notes from LSNI	0	9
Interest Rate Contract [Member]
Schedule of Investments [Line Items]
Other investments	77	78
Reported Value Measurement [Member]
Schedule of Investments [Line Items]
Other investments	115	89
Reported Value Measurement [Member] | Interest Rate Contract [Member]
Schedule of Investments [Line Items]
Other investments	0	$ 3
Commitments [Member] | Partnership Interest [Member]
Schedule of Investments [Line Items]
Other investments	$ 90

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