Reinsurance and Other Monoline Exposures - Reinsurance and Other Exposures to Monolines (Details) $ in Millions	Jun. 30, 2017 USD ($)
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	$ 9,041
Assumed Par Outstanding	11,064
Second-to- Pay Insured Par Outstanding	9,421
Other [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	30
Assumed Par Outstanding	114
Second-to- Pay Insured Par Outstanding	372
Non-affiliated Reinsurers [Member]
Ceded Credit Risk [Line Items]
Funds held under reinsurance agreements	312
Investment Grade Reinsurer [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	3,200
Assumed Par Outstanding	3,737
Second-to- Pay Insured Par Outstanding	3,758
Investment Grade Reinsurer [Member] | Tokio Marine & Nichido Fire Insurance Co., Ltd. (“Tokio”) [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	3,200
Assumed Par Outstanding	0
Second-to- Pay Insured Par Outstanding	0
Investment Grade Reinsurer [Member] | National [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0
Assumed Par Outstanding	3,737
Second-to- Pay Insured Par Outstanding	3,758
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	5,811
Assumed Par Outstanding	7,213
Second-to- Pay Insured Par Outstanding	5,291
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | American Overseas Reinsurance Company Limited [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	3,167
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Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Syncora Guarantee Inc [Member]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	2,027
Assumed Par Outstanding	675
Second-to- Pay Insured Par Outstanding	1,122
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Aca Financial Guaranty Corp [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	505
Assumed Par Outstanding	0
Second-to- Pay Insured Par Outstanding	12
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	112
Assumed Par Outstanding	5,437
Second-to- Pay Insured Par Outstanding	2,339
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | MBIA [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0
Assumed Par Outstanding	148
Second-to- Pay Insured Par Outstanding	811
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Ceded Par Outstanding	0
Assumed Par Outstanding	377
Second-to- Pay Insured Par Outstanding	954
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	576
Second-to- Pay Insured Par Outstanding	53
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	347
Second-to- Pay Insured Par Outstanding	$ 746