Reinsurance and Other Monoline Exposures - Amounts Due (To) From All Reinsurers (Details) $ in Millions	Sep. 30, 2017 USD ($)
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	$ 58
Ceded Premium, net of Commissions	(25)
Assumed Expected Loss to be Paid	(63)
Ceded Expected Loss to be Paid	25
Other Reinsurer [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	0
Ceded Premium, net of Commissions	(3)
Assumed Expected Loss to be Paid	0
Ceded Expected Loss to be Paid	0
External Credit Rating, Investment Grade [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	5
Ceded Premium, net of Commissions	0
Assumed Expected Loss to be Paid	1
Ceded Expected Loss to be Paid	0
External Credit Rating, Non Investment Grade [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	53
Ceded Premium, net of Commissions	(22)
Assumed Expected Loss to be Paid	(64)
Ceded Expected Loss to be Paid	25
External Credit Rating, Non Investment Grade [Member] | American Overseas Reinsurance Company Limited [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	0
Ceded Premium, net of Commissions	(4)
Assumed Expected Loss to be Paid	0
Ceded Expected Loss to be Paid	36
External Credit Rating, Non Investment Grade [Member] | Syncora Guarantee Inc [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	13
Ceded Premium, net of Commissions	(18)
Assumed Expected Loss to be Paid	0
Ceded Expected Loss to be Paid	(11)
External Credit Rating, Non Investment Grade [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	31
Ceded Premium, net of Commissions	0
Assumed Expected Loss to be Paid	1
Ceded Expected Loss to be Paid	0
External Credit Rating, Non Investment Grade [Member] | MBIA [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	0
Ceded Premium, net of Commissions	0
Assumed Expected Loss to be Paid	(3)
Ceded Expected Loss to be Paid	0
External Credit Rating, Non Investment Grade [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	3
Ceded Premium, net of Commissions	0
Assumed Expected Loss to be Paid	(16)
Ceded Expected Loss to be Paid	0
External Credit Rating, Non Investment Grade [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	6
Ceded Premium, net of Commissions	0
Assumed Expected Loss to be Paid	(46)
Ceded Expected Loss to be Paid	$ 0