Reinsurance and Other Monoline Exposures - Reinsurance and Other Exposures to Monolines (Details) $ in Millions	Sep. 30, 2017 USD ($)
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	$ 4,811
Assumed Par Outstanding	9,911
Second-to- Pay Insured Par Outstanding	7,670
Other Reinsurer [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	52
Assumed Par Outstanding	111
Second-to- Pay Insured Par Outstanding	410
Non-affiliated Reinsurers [Member]
Ceded Credit Risk [Line Items]
Funds held under reinsurance agreements	123
Investment Grade Reinsurer [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0
Assumed Par Outstanding	3,245
Second-to- Pay Insured Par Outstanding	2,730
Investment Grade Reinsurer [Member] | National [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0
Assumed Par Outstanding	3,245
Second-to- Pay Insured Par Outstanding	2,730
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	4,759
Assumed Par Outstanding	6,555
Second-to- Pay Insured Par Outstanding	4,530
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | American Overseas Reinsurance Company Limited [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	2,445
Assumed Par Outstanding	0
Second-to- Pay Insured Par Outstanding	0
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Syncora Guarantee Inc [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	1,994
Assumed Par Outstanding	674
Second-to- Pay Insured Par Outstanding	1,156
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	208
Assumed Par Outstanding	0
Second-to- Pay Insured Par Outstanding	10
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	112
Assumed Par Outstanding	4,902
Second-to- Pay Insured Par Outstanding	1,927
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | MBIA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0
Assumed Par Outstanding	137
Second-to- Pay Insured Par Outstanding	565
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0
Assumed Par Outstanding	275
Second-to- Pay Insured Par Outstanding	821
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0
Assumed Par Outstanding	567
Second-to- Pay Insured Par Outstanding	51
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | External Credit Rating, Non Investment Grade [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	305
Second-to- Pay Insured Par Outstanding	$ 774