Reinsurance and Other Monoline Exposures - Effect of Reinsurance on Statement of Operations (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
Premiums Written:
Direct									$ 297	$ 165	$ 164
Assumed									10	(11)	17
Ceded									18	(17)	10
Net									325	137	191
Premiums Earned:
Direct									693	887	792
Assumed									27	27	40
Ceded									(30)	(50)	(66)
Net	$ 178	$ 186	$ 162	$ 164	$ 236	$ 231	$ 214	$ 183	690	864	766
Loss and LAE:
Direct									404	327	399
Assumed									11	0	45
Ceded									(27)	(32)	(20)
Net	$ 34	$ 223	$ 72	$ 59	$ 112	$ (9)	$ 102	$ 90	$ 388	$ 295	$ 424