Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Fair Value Of Securities Deposited With Governmental Authorities	$ 6	$ 6
Market Value of Secured Note Collateral	$ 197	$ 210

Fair value of securities deposited with governmental authorities.

No definition available.

Name:	ambc_FairValueOfSecuritiesDepositedWithGovernmentalAuthorities
Namespace Prefix:	ambc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant