Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Jul. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) Transactions Entity	Dec. 31, 2015 USD ($) Entity	Dec. 31, 2014 USD ($)
Variable Interest Entities [Line Items]
Secured Debt		$ 102,403,000	$ 130,571,000
Consolidated VIE assets		13,367,834,000	14,288,497,000
Consolidated VIE liabilities		13,235,425,000	14,259,776,000
Number of RMBS Sold	17
Cash Received From Delaware Trust	$ 146,000,000
Par Value of Securities Placed in Trust			331,035,000
Fair Value of Securities Placed in Trust			360,759,000
Securities Issued by Delaware Trust	$ 146,000,000
Equity Method Investments		30,003,000	25,339,000
Par Value of Private Placement				$ 299,175,000
Private Placement Par Value Proceeds				80.00%
Private Placement Maturity Date				Aug. 28, 2039
Interest Rate on Private Placement Obligations				5.10%
Loans, Unpaid principal balance		4,873,000	6,205,000
Successor [Member]
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities		(14,093,000)	30,997,000	$ (32,212,000)
Available-for-sale Securities		6,049,908,000	5,334,120,000
variable interest entities deconsolidation gain loss		0	572,000	0
Variable Interest Entity, Measure of Activity, Income or Loss before Tax		(14,093,000)	31,569,000	(32,212,000)
Fair value of special purpose entities		7,382,000	8,696,000
Proceeds from the sale of Junior Surplus Notes of the Segregated Account		$ 0	0	$ 224,262,000
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life		4 years 10 months 23 days
Average rating of assets held by sponsored special purpose entities		BBB
Sponsored Variable Interest Entities [Member]
Variable Interest Entities [Line Items]
Number of individual transactions with special purpose entities | Transactions		15
Number of transactions outstanding with special purpose entities | Transactions		3
Total principal amount of debt outstanding		$ 388,950,000	454,290,000
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Secured Debt			0
Variable Interest Entities [Member] | Successor [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities		2,622,566,000	2,588,556,000
Loans, Estimated fair value		10,658,963,000	11,690,324,000
Loans, Unpaid principal balance		7,641,756,000	9,182,284,000
Long-term Debt, Fair Value		11,155,936,000	12,327,960,000
Long-term debt, Unpaid principal balance		$ 8,854,530,000	$ 11,069,070,000
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of DeConsolidated Variable Interest Entities		1	2	1
Number of consolidated Variable Interest Entities | Entity		12	13
Ambac UK [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity		8	9
Consolidated VIE assets		$ 12,950,009,000	$ 13,769,985,000
Consolidated VIE liabilities		$ 12,833,466,000	$ 13,636,628,000
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity		4	4
London Interbank Offered Rate (LIBOR) [Member]
Variable Interest Entities [Line Items]
Interest Rate on Securities Issued by Delaware Trust	2.80%
Corporate Debt Securities [Member] | Successor [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities		$ 2,622,566,000	$ 2,588,556,000