Contracts Accounted for as Insurance - Selected Information for Policies Paid In Installments (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	$ 944	$ 915	$ 876	$ 576
Financial Guarantee Policies Paid in Installments [Member]
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	943	914
Gross deferred premium revenue	$ 1,188	$ 1,205
Weighted-average risk-free rate used to discount premiums	2.20%	2.30%
Weighted-average period of premiums receivable (in years)	9 years 2 months	9 years 2 months