Contracts Accounted for as Insurance - Deferred Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Beginning of period	$ 106	$ 114	$ 121
DAC adjustments related to Radian Asset Acquisition on April 1, 2015	(2)	0	1
Costs deferred during the period:
Commissions on assumed and ceded business	0	(2)	(1)
Premium taxes	5	4	2
Compensation and other acquisition costs	11	9	11
Total	16	11	12
Costs amortized during the period	(19)	(19)	(20)
End of period	$ 101	$ 106	$ 114