Contracts Accounted for as Insurance - Selected Information for Policies Paid In Installments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	$ 922	$ 576	$ 608	$ 693
Financial Guarantee Policies Paid in Installments [Member]
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commission payable	922	576
Gross deferred premium revenue	$ 1,241	$ 1,041
Weighted-average risk-free rate used to discount premiums	2.40%	3.00%
Weighted-average period of premiums receivable (in years)	9 years 3 months 6 days	9 years 1 month