Contracts Accounted for as Insurance - Selected Information for Policies Paid In Installments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commissions payable	$ 1,378	$ 1,372	$ 1,321	$ 1,286
Financial Guarantee Policies Paid in Installments
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commissions payable	1,377	1,371
Gross deferred premium revenue	$ 1,669	$ 1,664
Weighted-average risk-free rate used to discount premiums	1.60%	1.60%
Weighted-average period of premiums receivable (in years)	12 years 8 months 12 days	12 years 9 months 18 days