Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums	$ 92	$ 97	$ 180	$ 200
Refundings	38	49	84	105
Terminations	1	10	7	12
Total Accelerations	39	59	91	117
Accretion of discount on net premiums receivable	4	5	8	8
Financial guaranty insurance net earned premiums	135	161	279	325
Non-financial guaranty net earned premiums	1	1	2	1
Net	136	162	281	326
Variable Interest Entity, Primary Beneficiary [Member]
Financial Guarantee Insurance Premiums [Line Items]
Net	$ 3	$ 4	$ 6	$ 8