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	Dec. 31, 2012	Dec. 31, 2011
Weighted Average Underlying Rating, B Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 1,415,276
Weighted Average Underlying Rating, AA- Rating [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	763,914
Weighted Average Underlying Rating, A Rating [Member] | Assured Guaranty Municipal Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	622,409	640,445
Weighted Average Underlying Rating, BBB- Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	2,859,999
Weighted Average Underlying Rating, BBB- Rating [Member] | MBIA Insurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	21,876	22,851
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	21,781
Weighted Average Underlying Rating, D Rating [Member] | Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	14,743
Weighted Average Underlying Rating, B+ Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		1,195,916
Weighted Average Underlying Rating, A+ Rating [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		899,075
Weighted Average Underlying Rating, BBB+ Rating [Member] | Financial Guarantee Insurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		25,074
Weighted Average Underlying Rating, CCC+ Rating [Member] | Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		38,424
Weighted Average Underlying Rating, BBB Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		2,821,785
Municipal obligations [Member] | Weighted Average Underlying Rating, B Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	66,270
Municipal obligations [Member] | Weighted Average Underlying Rating, AA- Rating [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	686,702
Municipal obligations [Member] | Weighted Average Underlying Rating, A Rating [Member] | Assured Guaranty Municipal Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	448,241	446,978
Municipal obligations [Member] | Weighted Average Underlying Rating, BBB- Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1,218,812
Municipal obligations [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	17,599
Municipal obligations [Member] | Weighted Average Underlying Rating, B+ Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		59,881
Municipal obligations [Member] | Weighted Average Underlying Rating, A+ Rating [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		816,683
Municipal obligations [Member] | Weighted Average Underlying Rating, BBB+ Rating [Member] | Financial Guarantee Insurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		17,350
Municipal obligations [Member] | Weighted Average Underlying Rating, BBB Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		1,340,892
Corporate obligations [Member] | Weighted Average Underlying Rating, B Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	4,717
Corporate obligations [Member] | Weighted Average Underlying Rating, AA- Rating [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	77,212
Corporate obligations [Member] | Weighted Average Underlying Rating, A Rating [Member] | Assured Guaranty Municipal Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	169,245	160,204
Corporate obligations [Member] | Weighted Average Underlying Rating, BBB- Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	270,907
Corporate obligations [Member] | Weighted Average Underlying Rating, BBB- Rating [Member] | MBIA Insurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	19,733	19,082
Corporate obligations [Member] | Weighted Average Underlying Rating, B+ Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		4,624
Corporate obligations [Member] | Weighted Average Underlying Rating, A+ Rating [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		82,392
Corporate obligations [Member] | Weighted Average Underlying Rating, BBB Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		266,302
Mortgage and asset-backed securities [Member] | Weighted Average Underlying Rating, B Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1,344,289
Mortgage and asset-backed securities [Member] | Weighted Average Underlying Rating, A Rating [Member] | Assured Guaranty Municipal Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	4,923	9,263
Mortgage and asset-backed securities [Member] | Weighted Average Underlying Rating, BBB- Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1,370,280
Mortgage and asset-backed securities [Member] | Weighted Average Underlying Rating, BBB- Rating [Member] | MBIA Insurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	2,143	3,769
Mortgage and asset-backed securities [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	4,182
Mortgage and asset-backed securities [Member] | Weighted Average Underlying Rating, D Rating [Member] | Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	14,743
Mortgage and asset-backed securities [Member] | Weighted Average Underlying Rating, B+ Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		1,131,411
Mortgage and asset-backed securities [Member] | Weighted Average Underlying Rating, BBB+ Rating [Member] | Financial Guarantee Insurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		7,724
Mortgage and asset-backed securities [Member] | Weighted Average Underlying Rating, CCC+ Rating [Member] | Assured Guaranty Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		38,424
Mortgage and asset-backed securities [Member] | Weighted Average Underlying Rating, BBB Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		1,190,591
Short-term [Member] | Weighted Average Underlying Rating, A Rating [Member] | Assured Guaranty Municipal Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		24,000
Short-term [Member] | Weighted Average Underlying Rating, BBB Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		$ 24,000