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 ($)	Jun. 30, 2017 USD ($) Entity	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Entity	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Entity
Variable Interest Entities [Line Items]
Equity Method Investments		$ 32,418		$ 32,418		$ 30,003
Number of RMBS Sold	17
Cash Received From Delaware Trust	$ 146,000
Secured Debt		85,981		85,981		102,403
Par Value of Securities Placed in Trust		311,528		311,528
Fair Value of Securities Placed in Trust		352,446		352,446		360,759
Securities Issued by Delaware Trust	$ 146,000
Consolidated VIE assets		14,095,210		14,095,210		13,367,834
Consolidated VIE liabilities		13,967,628		13,967,628		13,235,425
Successor [Member]
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities		(1,219)	$ 8,987	2,482	$ (18,176)
Fair value of special purpose entities		6,691		6,691		7,382
variable interest entities deconsolidation gain loss		0	0	0	0
Variable Interest Entity, Measure of Activity, Income or Loss before Tax		(1,219)	$ 8,987	$ 2,482	$ (18,176)
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life				4 years 10 months 15 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		$ 409,830		$ 409,830		$ 388,950
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of Newly Consolidated Variable Interest Entities		0	0	0	0
Number of DeConsolidated Variable Interest Entities		0	0	0	0
Number of consolidated Variable Interest Entities | Entity		12		12		12
Ambac UK [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity		8		8		8
Consolidated VIE assets		$ 13,700,687		$ 13,700,687		$ 12,950,009
Consolidated VIE liabilities		$ 13,583,319		$ 13,583,319		$ 12,833,466
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity		4		4		4
London Interbank Offered Rate (LIBOR) [Member]
Variable Interest Entities [Line Items]
Interest Rate on Securities Issued by Delaware Trust	2.80%