Expected Loss to be Paid - Net Expected Loss to be Paid (Recovered) and Net Economic Loss Development (Benefit) by Accounting Model (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid (recovered) after recoveries for R&W	$ 1,303	$ 1,198	$ 1,391
Total economic loss development (benefit)	313	139
Financial Guarantee Accounted for as Insurance Contracts [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid (recovered) after recoveries for R&W	1,226	1,083
Total economic loss development (benefit)	353	164
Financial Guarantee Variable Interest Entities And Other [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid (recovered) after recoveries for R&W	91	105
Total economic loss development (benefit)	(6)	(8)
Financial Guarantee Accounted for as Credit Derivatives [Member]
Schedule of Expected Losses to be Paid [Line Items]
Net expected loss to be paid (recovered) after recoveries for R&W	(14)	10
Total economic loss development (benefit)	$ (34)	$ (17)