Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 Successor Ambac [Member] Cash and Securities Pledged Directly from Investment Portfolio [Member]	Sep. 30, 2013 Successor Ambac [Member] Cash and Securities Pledged from Its Derivative Counterparties [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Cash and Securities Pledged Directly from Investment Portfolio [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Cash and Securities Pledged from Its Derivative Counterparties [Member]
Investment [Line Items]
Fair Value of Cash and Underlying Securities	$ 513,285		$ 646,663
Fair Value of Cash and Securities Pledged to Investment and Repurchase Agreement Counterparties	373,012		375,412
Fair Value of Cash and Securities Pledged to Derivative Counterparties	$ 140,273		$ 271,251