Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Balance Sheet (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, shares issued	45,865,081	45,571,743
Treasury stock, shares	55,942	16,343
Common Stock, Shares, Outstanding	45,809,139
Assets:
Debt Securities, Available-for-sale	$ 2,949	$ 3,314
Short-term investments, at cost (approximates fair value)	2,807	3,187
Other Investments	595	478
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investments, Total	3,544	3,792
Other assets	114	95
Total assets	13,220	13,320
Taxes Payable, Current	6	0
Tolling Payment Payable to AFG		28
Liabilities:
Total liabilities	12,074	11,783
Stockholders' equity (deficit):
Preferred Stock, Value, Issued	0	0
Common stock	0	0
Additional paid-in capital	242	232
Accumulated other comprehensive income (loss)	79	42
Retained earnings	759	1,203
Common stock held in treasury at cost	(1)	0
Total Ambac Financial Group, Inc. stockholders’ equity	1,080	1,477
Total liabilities, redeemable noncontrolling interest and stockholders’ equity	13,220	13,320
Ambac Financial Group, Inc Parent Company Only [Member]
Assets:
Short-term investments, at cost (approximates fair value)	76	71
Other Investments	54	46
Debt Securities, Available-for-sale, Allowance for Credit Loss	2
Investments, Total	349	434
Cash	7	9
Investment in subsidiaries	714	993
Current taxes receivable	0	30
Other assets	13	12
Total assets	1,082	1,478
Taxes Payable, Current	0	0
Liabilities:
Accounts payable and other liabilities	2	2
Total liabilities	3	2
Stockholders' equity (deficit):
Preferred Stock, Value, Issued	0	0
Common stock	0	0
Additional paid-in capital	242	232
Accumulated other comprehensive income (loss)	79	42
Retained earnings	759	1,203
Common stock held in treasury at cost	(1)	0
Total Ambac Financial Group, Inc. stockholders’ equity	1,080	1,477
Total liabilities, redeemable noncontrolling interest and stockholders’ equity	1,082	1,478
Short-term Investments [Member]
Assets:
Debt Securities, Available-for-sale	492
Short-term investments, at cost (approximates fair value)	492	653
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Short-term Investments [Member] | Ambac Financial Group, Inc Parent Company Only [Member]
Assets:
Short-term investments, at cost (approximates fair value)	229	318
Fixed Income Securities [Member]
Assets:
Debt Securities, Available-for-sale	2,317	2,577
Short-term investments, at cost (approximates fair value)	2,175	2,450
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Investment income due and accrued	10
Fixed Income Securities [Member] | Ambac Financial Group, Inc Parent Company Only [Member]
Assets:
Debt Securities, Available-for-sale	$ 66	70
Fixed Income Securities [Member] | Short-term Investments [Member]
Assets:
Debt Securities, Available-for-sale		653
Short-term investments, at cost (approximates fair value)		$ 653