Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) Entity	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Entity	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Entity
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities	$ 2,126	$ 7	$ 6,493	$ 1,158
Variable Interest Entities Deconsolidation Gain Loss	(1,998)	1,824	(1,998)	1,824
Realized Investment Gains (Losses)			71,564	82,211
Equity Method Investments	44,287		44,287		$ 40,168
Deconsolidated VIE Assets	292,003		292,003
Deconsolidated VIE Liabilities	363,628		363,628
Fair value of special purpose entities	3,388		3,388		4,516
variable interest entities deconsolidation gain loss	0	0	14,864	0
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	11,244	1,831	30,459	2,982
Other investments	462,427		462,427		391,217
Long-term debt	2,954,929		2,954,929		2,928,929
Deconsolidated VIE Assets	1,233,132		1,233,132
Deconsolidated VIE Liabilities	1,230,473		$ 1,230,473
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life			1 year 4 months 24 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	379,360		$ 379,360		393,010
Variable Interest Entities [Member]
Variable Interest Entities [Line Items]
VIE Change in FV of Assets Liabilities, Net of Credit Risk Change	128	1,831	4,495	2,982
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Interest Expense, Long-term Debt	(2,840)	0	(9,285)	0
Other Expenses	(119)	$ 0	(938)	$ 0
Long-term debt	$ 4,353,171		$ 4,353,171		$ 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	7		7		7
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Number of DeConsolidated Variable Interest Entities			1	3
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 4,151,433		$ 4,151,433		$ 5,268,596
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	6		6		7
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Carry Value Secured Notes from LSNI	$ 578,831		$ 578,831		$ 656,473
Fixed Income Investments [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Gross Investment Income, Operating	2,511	$ 0	7,896	$ 0
Realized Investment Gains (Losses)	11,564	$ 0	13,427	$ 0
Ambac Note Post Exit From Rehabilitation [Member] | Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 1,911,560		$ 1,911,560		$ 1,940,289