Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statements of Stockholders' Equity (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	$ 274
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	0
Stock-based compensation	(12)
Issuance of common stock	(1)
AMPS Impact on Stockholders Equity	(297)
Ending balance	1,633
Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	267
Stock-based compensation	0
Issuance of common stock	(1)
AMPS Impact on Stockholders Equity	(82)
Ending balance	1,421
AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	6
Stock-based compensation	0
Issuance of common stock	0
AMPS Impact on Stockholders Equity	0
Ending balance	(49)
Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0
Stock-based compensation	0
Issuance of common stock	0
AMPS Impact on Stockholders Equity	0
Ending balance	0
Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0
Stock-based compensation	0
Issuance of common stock	0
AMPS Impact on Stockholders Equity	0
Ending balance	0
Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0
Stock-based compensation	(12)
Issuance of common stock	0
AMPS Impact on Stockholders Equity	8
Ending balance	219
Common Stock Held in Treasury, at Cost [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0
Stock-based compensation	0
Issuance of common stock	0
AMPS Impact on Stockholders Equity	0
Ending balance	0
Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	0
Stock-based compensation	0
Issuance of common stock	0
AMPS Impact on Stockholders Equity	(223)
Ending balance	41
Ambac Financial Group, Inc Parent Company Only [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	192
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	0
Stock-based compensation	(12)
Issuance of common stock	(1)
AMPS Impact on Stockholders Equity	8
Stockholders' Equity Attributable to Parent	1,592	$ 1,381
Ambac Financial Group, Inc Parent Company Only [Member] | Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	186
Issuance of common stock	(1)
Stockholders' Equity Attributable to Parent	1,421	1,234
Ambac Financial Group, Inc Parent Company Only [Member] | AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income (loss)	6
Stockholders' Equity Attributable to Parent	(49)	(52)
Ambac Financial Group, Inc Parent Company Only [Member] | Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity Attributable to Parent	0	0
Ambac Financial Group, Inc Parent Company Only [Member] | Common Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity Attributable to Parent	0	0
Ambac Financial Group, Inc Parent Company Only [Member] | Additional Paid-in Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	(12)
AMPS Impact on Stockholders Equity	8
Stockholders' Equity Attributable to Parent	219	200
Ambac Financial Group, Inc Parent Company Only [Member] | Common Stock Held in Treasury, at Cost [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock	0
Stockholders' Equity Attributable to Parent	0	$ 0
Accounting Standards Update 2016-01 [Member] | Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	3
Accounting Standards Update 2016-01 [Member] | AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(3)
Accounting Standards Update 2016-01 [Member] | Parent [Member] | Retained Earnings [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	3
Accounting Standards Update 2016-01 [Member] | Parent [Member] | AOCI Attributable to Parent [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ (3)