Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 31, 2015 USD ($)	Sep. 30, 2017 USD ($) Entity	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Entity	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Entity
Variable Interest Entities [Line Items]
Equity Method Investments		$ 33,677		$ 33,677		$ 30,003
Number of RMBS Sold	17
Cash Received From Delaware Trust	$ 146,000
Secured Debt		78,319		78,319		102,403
Par Value of Securities Placed in Trust		301,130		301,130
Fair Value of Securities Placed in Trust		351,339		351,339		360,759
Securities Issued by Delaware Trust	$ 146,000
Consolidated VIE assets		14,442,384		14,442,384		13,367,834
Consolidated VIE liabilities		14,321,721		14,321,721		13,235,425
Successor [Member]
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities		(4,049)	$ 2,057	(1,567)	$ (16,119)
Fair value of special purpose entities		6,337		6,337		7,382
variable interest entities deconsolidation gain loss		0	0	0	0
Variable Interest Entity, Measure of Activity, Income or Loss before Tax		(4,049)	$ 2,057	(1,567)	$ (16,119)
Other investments		439,987		$ 439,987		450,307
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life				3 years 4 months
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		$ 421,560		$ 421,560		$ 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		$ 14,056,435		$ 14,056,435		$ 12,950,009
Consolidated VIE liabilities		$ 13,941,513		$ 13,941,513		$ 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%