Fair Value Measurement - Quantitative Information - Assets (Details) - Valuation, Income Approach - Level 3 - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Obligations of state and political subdivisions
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets carried at fair value	$ 110	$ 101
Obligations of state and political subdivisions | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.20%	6.40%
Obligations of state and political subdivisions | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	49.50%	33.40%
Obligations of state and political subdivisions | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	5.90%	12.80%
RMBS
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets carried at fair value	$ 238	$ 255
RMBS | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	3.40%	3.70%
RMBS | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	6.00%	5.90%
RMBS | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.60%	4.50%
RMBS | CPR | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	0.00%	0.40%
RMBS | CPR | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	22.20%	30.00%
RMBS | CPR | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	9.60%	7.10%
RMBS | CDR | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	1.40%	1.50%
RMBS | CDR | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	12.00%	9.90%
RMBS | CDR | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	6.00%	6.00%
RMBS | Loss severity | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	50.00%	45.00%
RMBS | Loss severity | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	125.00%	125.00%
RMBS | Loss severity | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	84.60%	83.60%
Life insurance transactions
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets carried at fair value	$ 382	$ 367
Yield (as a percent)	4.60%	5.20%
CLOs
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets carried at fair value	$ 543	$ 532
CLOs | Discount margin | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	0.80%	0.10%
CLOs | Discount margin | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	3.40%	3.10%
CLOs | Discount margin | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	1.80%	1.90%
Others
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets carried at fair value	$ 43	$ 41
Others | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	2.80%	2.60%
Others | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	7.30%	9.00%
Others | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	7.30%	9.00%
FG VIEs and other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets carried at fair value	$ 271	$ 296
FG VIEs and other | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	1.50%	1.90%
FG VIEs and other | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	7.90%	6.00%
FG VIEs and other | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	4.80%	4.80%
FG VIEs and other | CPR | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	0.90%	0.90%
FG VIEs and other | CPR | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	21.70%	19.00%
FG VIEs and other | CPR | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	12.20%	9.40%
FG VIEs and other | CDR | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	1.40%	1.90%
FG VIEs and other | CDR | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	26.90%	26.60%
FG VIEs and other | CDR | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	7.90%	6.00%
FG VIEs and other | Loss severity | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	45.00%	45.00%
FG VIEs and other | Loss severity | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	100.00%	100.00%
FG VIEs and other | Loss severity | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Conditional prepayment rate	81.60%	81.50%
Equity securities | Assets of Consolidated Investment Vehicles
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets carried at fair value	$ 34	$ 2
Yield (as a percent)		9.70%
Other
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets carried at fair value	$ 23	$ 52
Term (years)	10 years	10 years
Other | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	2.70%	3.40%
Other | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	3.30%	4.20%
Other | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Yield (as a percent)	3.00%	3.80%
Corporate securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Total assets carried at fair value		$ 30
Yield (as a percent)		42.00%