Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) Entity	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Entity	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Entity
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities	$ 2,924	$ (450)	$ 4,760
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax				$ 1,468
Realized Investment Gains (Losses)			53,093	52,010
Equity Method Investments	42,875		42,875		$ 40,168
Deconsolidated VIE Assets	292,003		292,003
Deconsolidated VIE Liabilities	363,628		363,628
Fair value of special purpose entities	3,771		3,771		4,516
variable interest entities deconsolidation gain loss	0	0	14,864	0
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	3,294	577	19,215	1,151
Other investments	469,438		469,438		391,217
Long-term debt	2,946,620		$ 2,946,620		2,928,929
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life			1 year 7 months 6 days
Average rating of assets held by sponsored special purpose entities			BBB+
Sponsored Variable Interest Entities [Member]
Variable Interest Entities [Line Items]
Total principal amount of debt outstanding	391,480		$ 391,480		393,010
Variable Interest Entities [Member]
Variable Interest Entities [Line Items]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax	(223)	1,027	(394)	(317)
VIE Change in FV of Assets Liabilities, Net of Credit Risk Change	2,701	577	4,366	1,151
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Interest Expense, Long-term Debt	(4,089)	0	(6,445)	0
Other Expenses	(708)	$ 0	(819)	$ 0
Long-term debt	$ 5,648,083		$ 5,648,083		$ 5,268,596
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of Newly Consolidated Variable Interest Entities	0	0
Number of DeConsolidated Variable Interest Entities	0	0		0
Number Of Consolidated Variable Interest Entities | Entity	8		8		7
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Number of DeConsolidated Variable Interest Entities			0
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 5,308,724		$ 5,308,724		$ 5,268,596
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	7		7		7
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Carry Value Secured Notes from LSNI	$ 575,138		$ 575,138		$ 656,473
Fixed Income Investments [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Gross Investment Income, Operating	3,527	$ 0	5,386	$ 0
Realized Investment Gains (Losses)	1,863	$ 0	1,863	$ 0
Ambac Note Post Exit From Rehabilitation [Member] | Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 1,918,357		$ 1,918,357		$ 1,940,289