Financial Guarantee Insurance Contracts - Additional Information (Detail) £ in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2016 GBP (£)	Dec. 31, 2015 GBP (£)	Dec. 31, 2014 USD ($)
Insurance [Line Items]
Possible Increase in Loss Reserves Related to Puerto Rico									$ 1,500,000
Estimated Future Premium Payments Weighted Average Discounted Rate	2.50%				2.60%				2.50%	2.60%		2.50%	2.60%
Amount Of Insured Par Outstanding Ceded To Reinsurer	$ 3,668,000								$ 3,668,000
Ceded Principal Outstanding Major Reinsurer Percentage									5.50%
Deferred Ceded Premiums And Reinsurance Recoverables	93,192								$ 93,192
Accrued rate of interest on outstanding policy obligations (percent)									5.10%
Weighted average period of future premiums									9 years 9 months 18 days	9 years
Losses and loss expense reserves ceded to reinsurers	40,658				$ 30,767				$ 40,658	$ 30,767
Subrogation recoveries, net of reinsurance	(1,806,736)				(1,878,740)				(1,806,736)	(1,878,740)
Letters of Credit Outstanding, Amount	115,561								115,561
Financial Guarantee Outstanding Principal Ceded To Third Parties	4,424,000								4,424,000
Successor [Member]
Insurance [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	339				(349)				339	(349)	$ (60)
Financial Guarantee Insurance Contracts, Premium Receivable	586,312				661,337				586,312	661,337	831,575				$ 1,000,607
Financial Guarantee Gross Outstanding Principal Assumed From Third Parties	219,100				243,700				219,100	243,700
Reinsurance Payable	37,876				42,529				$ 37,876	$ 42,529
Transactions with non-investment grade internal ratings									22.00%	25.00%
Percentage of Premiums Receivable Related to Non Investment Grade Structured Finance Transactions									16.00%	16.00%
Uncollectable premium receivables	9,331				9,186				$ 9,331	$ 9,186
Accelerated premium revenue for retired obligations									64,494	52,416	137,400
Reinsurance recoveries of losses included in losses and loss expenses									(20,348)	5,421	47,085
Losses and loss expense reserves ceded to reinsurers	40,658				30,767				40,658	30,767	44,059				$ 100,355
Intangible amortization expense	34,168	$ 45,690	$ 33,471	$ 37,525	40,152	$ 44,553	$ 39,013	$ 50,890	150,854	174,608	169,557
Intangible Assets, Gross (Excluding Goodwill)	1,581,156				1,534,419				1,581,156	1,534,419
Insurance intangible asset	846,973				962,080				846,973	962,080
Accumulated amortization on insurance intangible asset	734,183				572,339				734,183	572,339
Premiums Past Due On Non Investment Grade Obligation									500
Successor [Member] | Insurance Intangible Asset [Member]
Insurance [Line Items]
Intangible amortization expense									$ 150,854	$ 174,608	169,557
Loss Reserves [Member] | Successor [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves									2.50%	2.70%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	$ 151,852				$ 177,878				$ 151,852	$ 177,878	$ 226,994	£ 112,342	£ 144,393	£ 154,135
Ambac UK [Member] | Successor [Member]
Insurance [Line Items]
Ballantyne Settlement with JPMIM									325,600
Ambac UK Benefit Through Loss Reduction									91,600
Previous Benefit Through Reduction of Loss and LAE									53,000
Total Benefit Related to Ballantyne Settlement									144,600
Commonwealth of Puerto Rico [Member]
Insurance [Line Items]
Domestic Public Finance Losses									$ 476,303