Contracts Accounted for as Insurance - Selected Information for Policies Paid In Installments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	$ 904	$ 915	$ 576	$ 693
Financial Guarantee Policies Paid in Installments [Member]
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	903	914
Gross deferred premium revenue	$ 1,313	$ 1,205
Weighted-average risk-free rate used to discount premiums (as a percent)	2.30%	2.30%
Weighted-average period of premiums receivable (in years)	9 years 1 month 2 days	9 years 2 months 24 days