Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Jul. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) Entity	Dec. 31, 2016 USD ($) Entity	Dec. 31, 2015 USD ($)
Variable Interest Entities [Line Items]
Secured Debt		$ 73,993,000	$ 102,403,000
Consolidated VIE assets		14,500,507,000	13,367,834,000
Consolidated VIE liabilities		14,366,434,000	13,235,425,000
Number of RMBS Sold	17
Cash Received From Delaware Trust	$ 146,000,000
Par Value of Securities Placed in Trust		293,409,000
Fair Value of Securities Placed in Trust		346,212,000
Securities Issued by Delaware Trust	$ 146,000,000
Equity Method Investments		34,941,000	30,003,000
Loans, Unpaid principal balance		20,184,000	4,873,000
Successor [Member]
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities		19,670,000	(14,093,000)	$ 30,997,000
Available-for-sale Securities		5,309,161,000	6,049,908,000
variable interest entities deconsolidation gain loss		0	0	572,000
Variable Interest Entity, Measure of Activity, Income or Loss before Tax		19,670,000	(14,093,000)	$ 31,569,000
Fair value of special purpose entities		5,979,000	7,382,000
Other Investments		$ 431,630,000	450,307,000
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life		3 years 1 month 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		$ 420,600,000	388,950,000
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		2,914,145,000	2,622,566,000
Loans, Estimated fair value		11,529,384,000	10,658,963,000
Loans, Unpaid principal balance		8,168,651,000	7,641,756,000
Long-term Debt, Fair Value		12,160,544,000	11,155,936,000
Long-term debt, Unpaid principal balance		$ 9,387,884,000	$ 8,854,530,000
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of DeConsolidated Variable Interest Entities		1	1	2
Number of consolidated Variable Interest Entities | Entity		11	12
Ambac UK [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity		8	8
Consolidated VIE assets		$ 14,160,152,000	$ 12,950,009,000
Consolidated VIE liabilities		$ 14,026,704,000	$ 12,833,466,000
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity		3	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		$ 2,914,145,000	$ 2,622,566,000
Variable Interest Entity Purchased Debt [Member]
Variable Interest Entities [Line Items]
Other Investments		$ 35,000,000