Contracts Accounted for as Insurance - Selected Information for Policies Paid In Installments (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	$ 897	$ 904	$ 944	$ 915
Financial Guarantee Policies Paid in Installments [Member]
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	896	903
Gross deferred premium revenue	$ 1,280	$ 1,313
Weighted-average risk-free rate used to discount premiums	2.30%	2.30%
Weighted-average period of premiums receivable (in years)	9 years 1 month 6 days	9 years 1 month 6 days