Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Balance Sheet (Parenthetical) (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Apr. 30, 2013
Condensed Balance Sheet Statements, Captions [Line Items]
Fixed income securities, amortized cost	$ 4,514,878,000	$ 5,927,254,000
Preferred stock, par value	$ 0.01	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000us-gaap_PreferredStockSharesAuthorized	20,000,000us-gaap_PreferredStockSharesAuthorized	20,000,000us-gaap_PreferredStockSharesAuthorized
Preferred stock, shares issued	0us-gaap_PreferredStockSharesIssued	0us-gaap_PreferredStockSharesIssued
Preferred stock, shares outstanding	0us-gaap_PreferredStockSharesOutstanding	0us-gaap_PreferredStockSharesOutstanding
Common stock, par value	$ 0.01	$ 0.01	$ 0.01
Common stock, shares authorized	130,000,000us-gaap_CommonStockSharesAuthorized	130,000,000us-gaap_CommonStockSharesAuthorized	130,000,000us-gaap_CommonStockSharesAuthorized
Common stock, shares issued	45,005,932us-gaap_CommonStockSharesIssued	45,003,461us-gaap_CommonStockSharesIssued	45,000,000us-gaap_CommonStockSharesIssued
Common stock, shares outstanding	45,005,932us-gaap_CommonStockSharesOutstanding	45,003,461us-gaap_CommonStockSharesOutstanding
Treasury stock, shares	2,459us-gaap_TreasuryStockShares	937us-gaap_TreasuryStockShares
Successor [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, shares issued			45,000,000
Ambac Financial Group, Inc Parent Company Only [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Fixed income securities, amortized cost	$ 234,294	$ 28,149
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	130,000,000	130,000,000
Common stock, shares issued	45,005,932	45,003,461
Common stock, shares outstanding	45,005,932	45,003,461
Treasury stock, shares	2,459	937

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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