Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2015 USD ($)	Jun. 30, 2018 USD ($) Entity	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Entity	Jun. 30, 2017 USD ($)	Feb. 12, 2018 USD ($)	Dec. 31, 2017 USD ($) Entity
Variable Interest Entities [Line Items]
Equity Method Investments		$ 37,497		$ 37,497			$ 34,941
Number of RMBS Sold	17
Cash Received From Delaware Trust	$ 146,000
Secured Debt		0		0			73,993
Fair Value of Securities Placed in Trust		0		0			346,212
Securities Issued by Delaware Trust	$ 146,000
Consolidated VIE assets		13,570,666		13,570,666			14,500,507
Consolidated VIE liabilities		13,447,562		13,447,562			14,366,434
Successor [Member]
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities		(450)	$ (1,219)	1,468	$ 2,482
Fair value of special purpose entities		5,255		5,255			5,979
variable interest entities deconsolidation gain loss		0	0	0	0
Variable Interest Entity, Measure of Activity, Income or Loss before Tax		577	(1,219)	1,151	2,482
Other investments		394,396		$ 394,396			431,630
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life				2 years 6 months 25 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		410,760		$ 410,760			$ 420,600
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		$ 1,027	$ 0	$ (317)	$ 0
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of Newly Consolidated Variable Interest Entities		0	0		0
Number of DeConsolidated Variable Interest Entities		0	0	0	0
Number of consolidated Variable Interest Entities | Entity		11		11			11
Ambac UK [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity		8		8			8
Consolidated VIE assets		$ 13,274,042		$ 13,274,042			$ 14,160,152
Consolidated VIE liabilities		13,151,223		13,151,223			$ 14,026,704
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity							3
Secured Notes Received from Ambac LSNI						$ 643,583
CarryValueSecuredNotesfromLSNI		621,849		621,849
Unsettled Amounts Secured Notes from LSNI		50,878		50,878
Afg [Member]
Variable Interest Entities [Line Items]
Secured Notes Received from Ambac LSNI						$ 124,881
CarryValueSecuredNotesfromLSNI		120,233		120,233
Unsettled Amounts Secured Notes from LSNI		$ 9,837		$ 9,837
London Interbank Offered Rate (LIBOR) [Member]
Variable Interest Entities [Line Items]
Interest Rate on Securities Issued by Delaware Trust	2.80%