Investments - Summary of Roll-Forward of Ambac's Cumulative Credit Losses on Debt Securities for Which Portion of Other-than-Temporary Impairment was Recognized in Other Comprehensive Income (Parenthetical) (Detail) (Successor Ambac [Member], USD $) In Thousands, unless otherwise specified	1 Months Ended
	Apr. 30, 2013
Successor Ambac [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Cumulative credit impairments for Successor	$ 0