Fair Value Measurements - Information about Valuation Inputs for Fixed Income Securities Classified as Level 3 (Detail)	6 Months Ended			12 Months Ended
	Jun. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member]	Jun. 30, 2013 Successor Ambac [Member] Collateralized Debt Obligations [Member]	Jun. 30, 2013 Successor Ambac [Member] Other Commercial Asset-Backed [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Corporate Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Collateralized Debt Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Other Commercial Asset-Backed [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Coupon rate	0.345%	0.88%	0.67%	0.345%	1.05%	0.71%
Maturity	20 years 7 months 21 days	1 month 2 days	7 years 5 months 16 days	21 years 1 month 21 days	1 year 6 months 4 days	7 years 10 months 10 days
Yield	6.75%	1.43%	4.25%	6.08%	5.92%	7.50%