Background and Basis of Presentation - Surplus Note Exchange - Ambac Assurance Corporation [Member]	3 Months Ended
	Dec. 31, 2020 USD ($)	Feb. 11, 2021 USD ($)	Jan. 19, 2021 USD ($)	Dec. 31, 2019 USD ($)
Insurance [Line Items]
Long-term Debt, Gross	$ 2,884,000,000			$ 2,980,000,000
5.1 % Surplus Notes Due 2020 [Member]
Insurance [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.10%			5.10%
5.1% Junior Surplus Notes Due 2020 [Member]
Insurance [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.10%			5.10%
Corolla Notes | 5.1 % Surplus Notes Due 2020 [Member]
Insurance [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			5.10%
Lease Reducing Notes | 5.1% Junior Surplus Notes Due 2020 [Member]
Insurance [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.10%
5.1% Junior Surplus Notes, General Account Due 2020 [Member]
Insurance [Line Items]
Long-term Debt, Gross	$ 365,000,000			$ 365,000,000
5.1% Junior Surplus Notes, General Account Due 2020 [Member] | Corolla Notes
Insurance [Line Items]
One Dollar Outstanding			$ 1.00
Payment Amount in Exchange			0.9125
Note Holders	1
5.1% Junior Surplus Notes, General Account Due 2020 [Member] | Corolla Certificate
Insurance [Line Items]
One Dollar Outstanding			1.00
Payment Amount in Exchange			0.64
5.1% Junior Surplus Notes, General Account Due 2020 [Member] | Lease Reducing Notes
Insurance [Line Items]
One Dollar Outstanding		$ 1.00
Payment Amount in Exchange		0.8581
5.1% Junior Surplus Notes, General Account Due 2020 [Member] | Lease Reducing Notes | Lease Reducing Notes
Insurance [Line Items]
Long-term Debt Reduced in Exchange		15,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member]
Insurance [Line Items]
Long-term Debt, Gross	$ 531,000,000			$ 531,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member] | Corolla Notes
Insurance [Line Items]
Long-term Debt Issued in Exchange Transaction			267,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member] | Lease Reducing Notes
Insurance [Line Items]
Long-term Debt Issued in Exchange Transaction			$ 13,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member] | Corolla Notes & Junior Surplus Notes
Insurance [Line Items]
Long-term Debt, Gross		853,000,000
Long-term Debt, Gross and Accrued and Unpaid Interest		1,414,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member] | Corolla Notes & Junior Surplus Notes | Parent Company [Member]
Insurance [Line Items]
Long-term Debt, Gross		$ 83,000,000