Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 Successor Ambac [Member] Weighted Average Underlying Rating, CCC+ Rating [Member] Ambac Assurance Corporation [Member]	Sep. 30, 2013 Successor Ambac [Member] Weighted Average Underlying Rating, AA- Rating [Member] Financial Guarantee [Member]	Sep. 30, 2013 Successor Ambac [Member] Weighted Average Underlying Rating, A Rating [Member] Assured Guaranty Municipal Corporation [Member]	Sep. 30, 2013 Successor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member] MBIA Insurance Corporation [Member]	Sep. 30, 2013 Successor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Assured Guaranty Corporation [Member]	Sep. 30, 2013 Successor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Financial Guarantee Insurance Corporation [Member]	Sep. 30, 2013 Successor Ambac [Member] Weighted Average Underlying Rating, BB Rating [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Weighted Average Underlying Rating, AA- Rating [Member] Financial Guarantee [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Weighted Average Underlying Rating, A Rating [Member] Assured Guaranty Municipal Corporation [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member] MBIA Insurance Corporation [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Assured Guaranty Corporation [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Weighted Average Underlying Rating, B Rating [Member] Ambac Assurance Corporation [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Weighted Average Underlying Rating, A- Rating [Member] Financial Guarantee Insurance Corporation [Member]	Sep. 30, 2013 Municipal Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, CCC+ Rating [Member] Ambac Assurance Corporation [Member]	Sep. 30, 2013 Municipal Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, AA- Rating [Member] Financial Guarantee [Member]	Sep. 30, 2013 Municipal Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, A Rating [Member] Assured Guaranty Municipal Corporation [Member]	Sep. 30, 2013 Municipal Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member] MBIA Insurance Corporation [Member]	Sep. 30, 2013 Municipal Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Assured Guaranty Corporation [Member]	Sep. 30, 2013 Municipal Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Financial Guarantee Insurance Corporation [Member]	Sep. 30, 2013 Municipal Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, BB Rating [Member]	Dec. 31, 2012 Municipal Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, AA- Rating [Member] Financial Guarantee [Member]	Dec. 31, 2012 Municipal Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, A Rating [Member] Assured Guaranty Municipal Corporation [Member]	Dec. 31, 2012 Municipal Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member]	Dec. 31, 2012 Municipal Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member] MBIA Insurance Corporation [Member]	Dec. 31, 2012 Municipal Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Assured Guaranty Corporation [Member]	Dec. 31, 2012 Municipal Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, B Rating [Member] Ambac Assurance Corporation [Member]	Dec. 31, 2012 Municipal Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, A- Rating [Member] Financial Guarantee Insurance Corporation [Member]	Sep. 30, 2013 Corporate Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, CCC+ Rating [Member] Ambac Assurance Corporation [Member]	Sep. 30, 2013 Corporate Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, AA- Rating [Member] Financial Guarantee [Member]	Sep. 30, 2013 Corporate Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, A Rating [Member] Assured Guaranty Municipal Corporation [Member]	Sep. 30, 2013 Corporate Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member] MBIA Insurance Corporation [Member]	Sep. 30, 2013 Corporate Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Assured Guaranty Corporation [Member]	Sep. 30, 2013 Corporate Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Financial Guarantee Insurance Corporation [Member]	Sep. 30, 2013 Corporate Obligations [Member] Successor Ambac [Member] Weighted Average Underlying Rating, BB Rating [Member]	Dec. 31, 2012 Corporate Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, AA- Rating [Member] Financial Guarantee [Member]	Dec. 31, 2012 Corporate Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, A Rating [Member] Assured Guaranty Municipal Corporation [Member]	Dec. 31, 2012 Corporate Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member]	Dec. 31, 2012 Corporate Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member] MBIA Insurance Corporation [Member]	Dec. 31, 2012 Corporate Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Assured Guaranty Corporation [Member]	Dec. 31, 2012 Corporate Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, B Rating [Member] Ambac Assurance Corporation [Member]	Dec. 31, 2012 Corporate Obligations [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, A- Rating [Member] Financial Guarantee Insurance Corporation [Member]	Sep. 30, 2013 Mortgage and Asset-Backed Securities [Member] Successor Ambac [Member] Weighted Average Underlying Rating, CCC+ Rating [Member] Ambac Assurance Corporation [Member]	Sep. 30, 2013 Mortgage and Asset-Backed Securities [Member] Successor Ambac [Member] Weighted Average Underlying Rating, AA- Rating [Member] Financial Guarantee [Member]	Sep. 30, 2013 Mortgage and Asset-Backed Securities [Member] Successor Ambac [Member] Weighted Average Underlying Rating, A Rating [Member] Assured Guaranty Municipal Corporation [Member]	Sep. 30, 2013 Mortgage and Asset-Backed Securities [Member] Successor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member] MBIA Insurance Corporation [Member]	Sep. 30, 2013 Mortgage and Asset-Backed Securities [Member] Successor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Assured Guaranty Corporation [Member]	Sep. 30, 2013 Mortgage and Asset-Backed Securities [Member] Successor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Financial Guarantee Insurance Corporation [Member]	Sep. 30, 2013 Mortgage and Asset-Backed Securities [Member] Successor Ambac [Member] Weighted Average Underlying Rating, BB Rating [Member]	Dec. 31, 2012 Mortgage and Asset-Backed Securities [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, AA- Rating [Member] Financial Guarantee [Member]	Dec. 31, 2012 Mortgage and Asset-Backed Securities [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, A Rating [Member] Assured Guaranty Municipal Corporation [Member]	Dec. 31, 2012 Mortgage and Asset-Backed Securities [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member]	Dec. 31, 2012 Mortgage and Asset-Backed Securities [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member] MBIA Insurance Corporation [Member]	Dec. 31, 2012 Mortgage and Asset-Backed Securities [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Assured Guaranty Corporation [Member]	Dec. 31, 2012 Mortgage and Asset-Backed Securities [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, B Rating [Member] Ambac Assurance Corporation [Member]	Dec. 31, 2012 Mortgage and Asset-Backed Securities [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, A- Rating [Member] Financial Guarantee Insurance Corporation [Member]	Sep. 30, 2013 Short-Term [Member] Successor Ambac [Member] Weighted Average Underlying Rating, CCC+ Rating [Member] Ambac Assurance Corporation [Member]	Sep. 30, 2013 Short-Term [Member] Successor Ambac [Member] Weighted Average Underlying Rating, AA- Rating [Member] Financial Guarantee [Member]	Sep. 30, 2013 Short-Term [Member] Successor Ambac [Member] Weighted Average Underlying Rating, A Rating [Member] Assured Guaranty Municipal Corporation [Member]	Sep. 30, 2013 Short-Term [Member] Successor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member] MBIA Insurance Corporation [Member]	Sep. 30, 2013 Short-Term [Member] Successor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Assured Guaranty Corporation [Member]	Sep. 30, 2013 Short-Term [Member] Successor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Financial Guarantee Insurance Corporation [Member]	Sep. 30, 2013 Short-Term [Member] Successor Ambac [Member] Weighted Average Underlying Rating, BB Rating [Member]	Dec. 31, 2012 Short-Term [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, AA- Rating [Member] Financial Guarantee [Member]	Dec. 31, 2012 Short-Term [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, A Rating [Member] Assured Guaranty Municipal Corporation [Member]	Dec. 31, 2012 Short-Term [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member]	Dec. 31, 2012 Short-Term [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, BBB- Rating [Member] MBIA Insurance Corporation [Member]	Dec. 31, 2012 Short-Term [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, D Rating [Member] Assured Guaranty Corporation [Member]	Dec. 31, 2012 Short-Term [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, B Rating [Member] Ambac Assurance Corporation [Member]	Dec. 31, 2012 Short-Term [Member] Predecessor Ambac [Member] Weighted Average Underlying Rating, A- Rating [Member] Financial Guarantee Insurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 1,779,902	$ 582,359	$ 574,376	$ 18,418	$ 16,775	$ 2,988	$ 2,974,818	$ 763,914	$ 622,409	$ 2,859,999	$ 21,876	$ 14,743	$ 1,415,276	$ 21,781	$ 66,324	$ 543,112	$ 406,181				$ 1,015,617	$ 720,904	$ 448,241	$ 1,253,014			$ 66,270	$ 17,599		$ 39,247	$ 168,195	$ 18,418			$ 225,860	$ 43,010	$ 169,245	$ 236,705	$ 19,733		$ 4,717		$ 1,713,578				$ 16,775	$ 2,988	$ 1,733,341		$ 4,923	$ 1,370,280	$ 2,143	$ 14,743	$ 1,344,289	$ 4,182