Comprehensive Income - Schedule of Amounts Reclassed Out of Each Component of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	4 Months Ended						3 Months Ended		5 Months Ended	9 Months Ended	3 Months Ended		5 Months Ended	9 Months Ended	3 Months Ended		5 Months Ended	9 Months Ended	3 Months Ended		5 Months Ended	9 Months Ended	3 Months Ended		5 Months Ended	9 Months Ended	3 Months Ended		5 Months Ended	9 Months Ended
	Apr. 30, 2013 Predecessor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Unrealized Gains (Losses) on Available-for-Sale Securities [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Unrealized Gains (Losses) on Available-for-Sale Securities [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]	Sep. 30, 2014 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2014 Successor Ambac [Member]	Sep. 30, 2014 Successor Ambac [Member] Unrealized Gains (Losses) on Available-for-Sale Securities [Member]	Sep. 30, 2013 Successor Ambac [Member] Unrealized Gains (Losses) on Available-for-Sale Securities [Member]	Sep. 30, 2013 Successor Ambac [Member] Unrealized Gains (Losses) on Available-for-Sale Securities [Member]	Sep. 30, 2014 Successor Ambac [Member] Unrealized Gains (Losses) on Available-for-Sale Securities [Member]	Sep. 30, 2014 Successor Ambac [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]	Sep. 30, 2013 Successor Ambac [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]	Sep. 30, 2013 Successor Ambac [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]	Sep. 30, 2014 Successor Ambac [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]	Sep. 30, 2014 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member]	Sep. 30, 2013 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member]	Sep. 30, 2013 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member]	Sep. 30, 2014 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member]	Sep. 30, 2014 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Unrealized Gains (Losses) on Available-for-Sale Securities [Member]	Sep. 30, 2013 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Unrealized Gains (Losses) on Available-for-Sale Securities [Member]	Sep. 30, 2013 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Unrealized Gains (Losses) on Available-for-Sale Securities [Member]	Sep. 30, 2014 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Unrealized Gains (Losses) on Available-for-Sale Securities [Member]	Sep. 30, 2014 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]	Sep. 30, 2013 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]	Sep. 30, 2013 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]	Sep. 30, 2014 Successor Ambac [Member] Reclassification out of Accumulated Other Comprehensive Income [Member] Other Postretirement Benefit Plan, Defined Benefit [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains		$ (13,349)									$ (5,003)	$ 49,561	$ 33,089	$ (5,219)
Tax (expense) benefit	755				0	(704)	2,344	594	1,107	3,414
Net of tax and noncontrolling interest		(13,349)		(13,164)							(5,003)	49,561	33,089	(5,219)					(5,207)	49,561	33,089	(5,831)
Prior service cost						1,616																					(166)	0	0	(498)
Actuarial gains (losses)						(727)																					(38)	0	0	(114)
Total before tax						889																					(204)	0	0	(612)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	$ (13,164)	$ (13,349)	$ 185			$ (185)	$ (5,207)	$ 49,561	$ 33,089	$ (5,831)	$ (5,003)	$ 49,561	$ 33,089	$ (5,219)	$ (204)	$ 0	$ 0	$ (612)									$ 204	$ 0	$ 0	$ 612