Reinsurance and Other Monoline Exposures - Reinsurance and Other Monoline Exposures (Details) $ in Millions	Mar. 31, 2017 USD ($)
Ceded Credit Risk [Line Items]
Insured Financial Obligations, Ceded Securities, Amount	$ 9,333
Assumed Premium, net of Commissions	61
Ceded Premium, net of Commissions	(33)
Assumed Expected Loss and Loss Adjustment Expense Payable	(70)
Ceded Expected Loss and Loss Adjustment Expense	76
Non-affiliated Reinsurers [Member]
Ceded Credit Risk [Line Items]
Funds held under reinsurance agreements	285
External Credit Rating, Non Investment Grade [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	56
Ceded Premium, net of Commissions	(24)
Assumed Expected Loss and Loss Adjustment Expense Payable	(74)
Ceded Expected Loss and Loss Adjustment Expense	32
External Credit Rating, Non Investment Grade [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	33
Ceded Premium, net of Commissions	0
Assumed Expected Loss and Loss Adjustment Expense Payable	(2)
Ceded Expected Loss and Loss Adjustment Expense	0
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	(17)
Assumed Expected Loss and Loss Adjustment Expense Payable	0
Ceded Expected Loss and Loss Adjustment Expense	(6)
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 and Loss Adjustment Expense Payable	(48)
Ceded Expected Loss and Loss Adjustment Expense	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 and Loss Adjustment Expense Payable	(7)
Ceded Expected Loss and Loss Adjustment Expense	0
External Credit Rating, Non Investment Grade [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	4
Ceded Premium, net of Commissions	0
Assumed Expected Loss and Loss Adjustment Expense Payable	(17)
Ceded Expected Loss and Loss Adjustment Expense	0
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 and Loss Adjustment Expense Payable	0
Ceded Expected Loss and Loss Adjustment Expense	38
External Credit Rating, Non Investment Grade [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	0
Ceded Premium, net of Commissions	(3)
Assumed Expected Loss and Loss Adjustment Expense Payable	0
Ceded Expected Loss and Loss Adjustment Expense	0
External Credit Rating, Investment Grade [Member]
Ceded Credit Risk [Line Items]
Assumed Premium, net of Commissions	5
Ceded Premium, net of Commissions	(9)
Assumed Expected Loss and Loss Adjustment Expense Payable	4
Ceded Expected Loss and Loss Adjustment Expense	44
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member]
Ceded Credit Risk [Line Items]
Insured Financial Obligations, Ceded Securities, Amount	5,993
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Ambac [Member]
Ceded Credit Risk [Line Items]
Insured Financial Obligations, Ceded Securities, Amount	115
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Syncora Guarantee Inc [Member]
Ceded Credit Risk [Line Items]
Insured Financial Obligations, Ceded Securities, Amount	1,950
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Ambac Assurance Corp. Segregated account [Member]
Ceded Credit Risk [Line Items]
Insured Financial Obligations, Ceded Securities, Amount	0
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | MBIA [Member]
Ceded Credit Risk [Line Items]
Insured Financial Obligations, Ceded Securities, Amount	0
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | FGIC [Member]
Ceded Credit Risk [Line Items]
Insured Financial Obligations, Ceded Securities, Amount	0
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | American Overseas Reinsurance Company Limited [Member]
Ceded Credit Risk [Line Items]
Insured Financial Obligations, Ceded Securities, Amount	3,307
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | Other [Member]
Ceded Credit Risk [Line Items]
Insured Financial Obligations, Ceded Securities, Amount	71
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | External Credit Rating, Non Investment Grade [Member]
Ceded Credit Risk [Line Items]
Insured Financial Obligations, Ceded Securities, Amount	$ 359