Contracts Accounted for as Insurance - Net Earned Premiums (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial Guarantee Insurance Premiums [Line Items]
Scheduled net earned premiums									$ 385	$ 381	$ 416
Accelerations
Refundings									269	390	294
Terminations									17	79	37
Total Accelerations									286	469	331
Accretion of discount on net premiums receivable									17	14	17
Financial guaranty insurance net earned premiums									688	864	764
Other									2	0	2
Net	$ 178	$ 186	$ 162	$ 164	$ 236	$ 231	$ 214	$ 183	690	864	766
Variable Interest Entity, Primary Beneficiary [Member]
Accelerations
Net									$ 15	$ 16	$ 21