Contracts Accounted for as Insurance - Selected Information for Policies Paid In Installments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	$ 915	$ 576	$ 693	$ 729
Financial Guarantee Policies Paid in Installments [Member]
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	914	576
Gross deferred premium revenue	$ 1,205	$ 1,041
Weighted-average risk-free rate used to discount premiums (as a percent)	2.30%	3.00%
Weighted-average period of premiums receivable (in years)	9 years 2 months 24 days	9 years 1 month 24 days