Reinsurance and Other Monoline Exposures - Exposure by Reinsurer (Details) $ in Millions	Mar. 31, 2017 USD ($)
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	$ 9,333
Second-to- Pay Insured Par Outstanding (2)	10,514
Assumed Par Outstanding	11,992
Non-affiliated Reinsurers [Member]
Ceded Credit Risk [Line Items]
Funds held under reinsurance agreements	285
Investment Grade Reinsurer [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	3,269
Second-to- Pay Insured Par Outstanding (2)	4,057
Assumed Par Outstanding	3,962
Investment Grade Reinsurer [Member] | Tokio Marine & Nichido Fire Insurance Co., Ltd. (“Tokio”) [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	3,269
Second-to- Pay Insured Par Outstanding (2)	0
Assumed Par Outstanding	0
Investment Grade Reinsurer [Member] | National [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	0
Second-to- Pay Insured Par Outstanding (2)	4,057
Assumed Par Outstanding	3,962
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	5,993
Second-to- Pay Insured Par Outstanding (2)	5,919
Assumed Par Outstanding	7,884
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | American Overseas Reinsurance Company Limited [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	3,307
Second-to- Pay Insured Par Outstanding (2)	0
Assumed Par Outstanding	30
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Syncora Guarantee Inc [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	1,950
Second-to- Pay Insured Par Outstanding (2)	1,097
Assumed Par Outstanding	654
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	621
Second-to- Pay Insured Par Outstanding (2)	12
Assumed Par Outstanding	0
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	115
Second-to- Pay Insured Par Outstanding (2)	2,583
Assumed Par Outstanding	6,061
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | MBIA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	0
Second-to- Pay Insured Par Outstanding (2)	949
Assumed Par Outstanding	151
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	0
Second-to- Pay Insured Par Outstanding (2)	1,221
Assumed Par Outstanding	404
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	0
Second-to- Pay Insured Par Outstanding (2)	57
Assumed Par Outstanding	584
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	71
Second-to- Pay Insured Par Outstanding (2)	538
Assumed Par Outstanding	146
External Credit Rating, Non Investment Grade [Member] | Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding (1)	359
Second-to- Pay Insured Par Outstanding (2)	$ 742