Contracts Accounted for as Insurance - Selected Information for Policies Paid In Installments (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	$ 1,233	$ 1,286	$ 897	$ 904
Financial Guarantee Policies Paid in Installments [Member]
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	1,232	1,284
Gross deferred premium revenue	$ 1,635	$ 1,637
Weighted-average risk-free rate used to discount premiums	1.70%	1.70%
Weighted-average period of premiums receivable (in years)	13 years 2 months 12 days	13 years 3 months 18 days