Ambac Financial (AMBC) 10-Q 9 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - $ / shares	3 Months Ended
	Mar. 31, 2019	May 06, 2019	Dec. 31, 2018	Apr. 30, 2013
Entity Information [Line Items]
Entity Incorporation, State Country Name	Delaware
Entity Tax Identification Number	133621676
Local Phone Number	212-658-7470
Entity Address, Address Line One	One State Street Plaza
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10004
Common Stock, Par or Stated Value Per Share	$ 0.01		$ 0.01	$ 0.01
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Trading Symbol	AMBC
Entity Registrant Name	AMBAC FINANCIAL GROUP INC
Entity Central Index Key	0000874501
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		45,520,541

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	$ 3,611,947
Short-term investments, at fair value (amortized cost of $908,209 and $430,405)	908,235	$ 430,331
Other investments	428,556	391,217
Total investments	4,040,503	3,937,223
Cash and cash equivalents	21,840	63,089
Restricted Cash	0	19,405
Premium receivables	487,397	495,391
Reinsurance recoverable on paid and unpaid losses	26,788	23,133
Deferred ceded premium	58,868	61,134
Subrogation recoverable	1,916,117	1,932,960
Derivative assets	76,400	59,468
Income Taxes Receivable	42,830	47,040
Insurance intangible asset	689,255	718,931
Other assets	90,977	137,628
Total assets	15,022,899	14,588,711
Liabilities:
Unearned premiums	591,397	629,971
Loss and loss expense reserves	1,694,163	1,826,078
Ceded premiums payable	31,745	32,913
Long-term debt	2,929,227	2,928,929
Accrued interest payable	391,335	375,808
Derivative Liability	86,534	76,699
Other liabilities	71,402	63,792
Total liabilities	13,357,011	12,955,564
Stockholders’ equity:
Preferred stock, par value $0.01 per share; 20,000,000 shares authorized; issued and outstanding shares—none	0	0
Common Stock, Value, Issued	456	454
Additional paid-in capital	223,545	219,429
Accumulated other comprehensive income	22,542	(48,715)
Retained earnings	1,376,244	1,421,302
Treasury stock, shares at cost: 0 and 8,202	(813)	(473)
Total Ambac Financial Group, Inc. stockholders’ equity	1,621,974	1,591,997
Noncontrolling interest	43,914	41,150
Total stockholders’ equity	1,665,888	1,633,147
Total liabilities and stockholders’ equity	15,022,899	14,588,711
Deferred Income Tax Liabilities, Net	39,201	40,130
Variable Interest Entity, Primary Beneficiary [Member]
Assets:
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	3,128,995	2,737,286
Restricted Cash	3,254	999
Derivative assets	59,228	66,302
Other assets	4,686	1,058
Total assets	7,571,924	7,093,309
Liabilities:
Long-term debt	5,737,263	5,268,596
Accrued interest payable	2,785	556
Derivative Liability	1,781,903	1,712,062
Other liabilities	56	30
Total liabilities	7,522,007	6,981,244
Stockholders’ equity:
Loans Receivable, Net	$ 4,375,761	$ 4,287,664

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale, Amortized Cost	$ 3,472,642	$ 3,451,149
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Preferred Stock, Shares Outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	130,000,000	130,000,000
Common stock, shares issued	45,560,960	45,365,170
Common stock, shares outstanding	45,520,541	45,336,278
Treasury stock, shares	40,419	28,892
Variable Interest Entity, Primary Beneficiary [Member]
Long-term debt	$ 5,401,992	$ 5,268,596
Equity investments in pooled funds [Member]
Other Investment Not Readily Marketable, Fair Value	$ 387,045	$ 351,049

Consolidated Statements of Tota

Consolidated Statements of Total Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenues:
Net premiums earned	$ 27,758	$ 30,883
Net investment income:
Securities available-for-sale and short-term	46,552	110,551
Other Investment Income	8,290	(311)
Total net investment income	54,842	110,240
Other-than-temporary impairment losses:
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities	(29)	(341)
Portion of other-than-temporary impairment recognized in other comprehensive income	0	42
Net other-than-temporary impairments	(29)	(299)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments	17,233	4,862
Change in fair value of credit derivatives:
Unrealized gains (losses)	382	(452)
Derivative, Gain (Loss) on Derivative, Net	(16,159)	25,191
Net realized gains (losses) on extinguishment of debt	0	3,115
Other Operating Income (Expense), Net	802	(509)
Income (loss) on variable interest entities	15,921	574
Total revenues	100,368	174,057
Expenses:
Losses and loss expenses (benefit)	12,407	(247,395)
Insurance intangible amortization	36,278	28,636
Operating expenses	24,915	36,434
Interest expense	67,978	48,073
Total expenses	141,578	(134,252)
Pre-tax income (loss)	(41,210)	308,309
Provision for income taxes	1,991	2,605
Net income (loss)	(43,201)	305,704
Net income (loss) attributable to common stockholders	(43,201)	305,704
Other comprehensive income (loss), after tax:
Net income (loss)	(43,201)	305,704
Unrealized gains (losses) on securities, net of income tax provision (benefit) of $(598) and $(1,708)	56,163	122,304
Gains (losses) on foreign currency translation, net of income tax provision (benefit) of $0 and $0	14,571	32,056
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, after Tax	156
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, after Tax and Reclassification Adjustment, Attributable to Parent		1,114
Changes to postretirement benefit, net of income tax provision (benefit) of $0 and $0	367	(859)
Total other comprehensive income, net of income tax	71,257	154,615
Total comprehensive income (loss)	28,056	460,319
Less: comprehensive (gain) loss attributable to the noncontrolling interest:
Total comprehensive income attributable to common stockholders	$ 28,056	$ 460,319
Earnings Per Share, Basic	$ (0.94)	$ 6.72
Earnings Per Share, Diluted	$ (0.94)	$ 6.70
Weighted Average Number of Shares Outstanding, Basic	45,832,297	45,471,083
Weighted Average Number of Shares Outstanding, Diluted	45,832,297	45,653,471

Consolidated Statements of To_2

Consolidated Statements of Total Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Unrealized (loss) gain on securities, taxes	$ (598)	$ (1,708)
Gain (loss) on foreign currency translation, taxes	0	0
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, Tax	32	230
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Plan Amendments, Tax Effect	$ 0	$ 0

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Noncontrolling Interest [Member]	Parent Company [Member]
Beginning balance at Dec. 31, 2017	$ 1,645,258	$ 1,233,845	$ (52,239)	$ 0	$ 453	$ 199,560	$ (471)	$ 264,110
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income	460,319	305,704	154,615	0	0	0	0	0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification									$ 0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | Accounting Standards Update 2016-01 [Member]		2,900	(2,900)
Stock-based compensation	4,612	0	0	0	0	4,612	0	0
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures	(1,025)	(985)	0	0	0	0	(40)	0
Stock Issued During Period, Value, New Issues	1	0	0	0	1	0	0	0
Ending balance at Mar. 31, 2018	2,109,165	1,541,464	99,476	0	454	204,172	(511)	264,110
Beginning balance at Dec. 31, 2017	1,645,258	1,233,845	(52,239)	0	453	199,560	(471)	264,110
Increase (Decrease) in Stockholders' Equity [Roll Forward]
AMPS Impact on Stockholders Equity	2,764
Ending balance at Dec. 31, 2018	1,633,147	1,421,302	(48,715)	0	454	219,429	(473)	41,150
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Total comprehensive income	28,056	(43,201)	71,257	0	0	0	0	0
Stock-based compensation	4,116	0	0	0	0	4,116	0	0
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures	(2,197)	(1,857)	0	0	0	0	(340)	0
Stock Issued During Period, Value, New Issues	2	0	0	0	2	0	0	0
Ending balance at Mar. 31, 2019	$ 1,665,888	$ 1,376,244	$ 22,542	$ 0	$ 456	$ 223,545	$ (813)	$ 43,914

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Cash Flows [Abstract]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 25,094	$ 39,619
Cash flows from operating activities:
Net income attributable to common shareholders	(43,201)	305,704
Net income (loss)	(43,201)	305,704
Adjustments to reconcile net income to net cash used in operating activities:
Depreciation and amortization	159	181
Amortization of bond premium and discount	(11,846)	(77,427)
Share-based compensation	4,116	4,612
Deferred income taxes	(935)	1,145
Current income taxes	4,232	(8,439)
Unearned premiums, net	(36,289)	(18,329)
Losses and loss expenses, net	(118,801)	(1,371,840)
Ceded premiums payable	(1,168)	(1,425)
Premium receivables	7,979	5,601
Accrued interest payable	28,971	(63,126)
Amortization of insurance intangible assets	36,278	28,636
Net mark-to-market (gains) losses	(382)	452
Net realized investment gains	(17,233)	(4,862)
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Portion Attributable to Parent, Available-for-sale Securities	29	299
(Gain) loss on extinguishment of debt	0	(3,115)
Variable interest entity activities	(15,921)	(574)
Increase (Decrease) in Derivative Assets and Liabilities	12,305	438
Other, net	56,579	10,507
Net cash used in operating activities	(95,128)	(1,191,562)
Cash flows from investing activities:
Proceeds from sales of bonds	641,032	296,078
Proceeds from matured bonds	34,681	103,995
Purchases of bonds	(182,556)	(77,469)
Proceeds from sales of other invested assets	1,749	31,327
Purchases of other invested assets	(28,559)	(11,758)
Change in short-term investments	(477,910)	236,262
Change in cash collateral receivable	56,771	(979)
Proceeds From Financial Guaranty Variable Interest Entity Assets	67,172	79,917
Other, net	(225)	(377)
Net cash provided by investing activities	112,155	656,996
Cash flows from financing activities:
Proceeds From Issuance of Tier II Notes	0	240,000
Paydown of Ambac Note	(13,146)	0
Paydowns of a secured borrowing	0	(8,797)
Paydown of Financial Guaranty Variable Interest Entity Liabilities	(63,022)	(79,917)
Payments for extinguishment of surplus notes	0	(191,258)
Payments of Debt Issuance Costs	0	(9,221)
AMPS Proceeds	2,764
Payments Related to Tax Withholding for Share-based Compensation	(2,198)	(1,025)
Net cash used in financing activities	(75,602)	(50,218)
Effect of Exchange Rate on Cash and Cash Equivalents	176	(278)
Net cash flow	(58,399)	(585,062)
Cash, cash equivalents, and restricted cash at end of period	$ 21,840	$ 38,485

Background and Business Descrip

Background and Business Description	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Background and Business Description	1. BACKGROUND AND BUSINESS DESCRIPTION The following description provides an update of Note 1. Background and Business Description in the Notes to Consolidated Financial Statements included Part II, Item 8 in the Company’s Annual Report on Form 10-K for the year ended December 2018, and should be read in conjunction with the complete descriptions provided in the Form 10-K. Capitalized terms used but not defined herein and in the other footnotes to the Consolidated Financial Statements included in this Quarterly Report on Form 10-Q shall have the meanings ascribed thereto in the Company’s Annual Report on Form 10-K for the year ended December 2018. Ambac Financial Group, Inc. (“Ambac” or the “Company”), headquartered in New York City, is a financial services holding company incorporated in the state of Delaware on April 29, 1991 . Ambac provides financial guarantee insurance policies through its principal operating subsidiary, Ambac Assurance Corporation (“Ambac Assurance" or "AAC") and its wholly owned subsidiary, Ambac Assurance UK Limited (“Ambac UK”). Insurance policies issued by Ambac Assurance and Ambac UK generally guarantee payment when due of the principal and interest on the obligations guaranteed. Ambac Assurance also has another wholly-owned subsidiary, Everspan Financial Guarantee Corp., which has been in runoff since its acquisition in 1997. Management reviews financial information, allocates resources and measures financial performance on a consolidated basis. As a result, the Company has a single reportable segment. Strategies to Enhance Shareholder Value Ambac’s primary goal is to maximize shareholder value through executing the following key strategies: • Active runoff of Ambac Assurance and its subsidiaries through transaction terminations, policy commutations, reinsurance, settlements and restructurings, with a focus on our watch list credits and known and potential future adversely classified credits, that we believe will improve our risk profile, and maximizing the risk-adjusted return on invested assets; • Ongoing rationalization of Ambac's and its subsidiaries' capital and liability structures; • Loss recovery through active litigation management and exercise of contractual and legal rights; • Ongoing review of the effectiveness and efficiency of Ambac's operating platform; and • Evaluation of opportunities in certain business sectors that meet acceptable criteria that will generate long-term stockholder value with attractive risk-adjusted returns. With respect to our new business strategy, we have identified certain business sectors adjacent to Ambac's core business in which opportunities are being evaluated. We are actively evaluating strategic opportunities in credit, insurance, asset management and other financial services that we believe are synergistic to Ambac and would leverage our core competencies. We are being measured and disciplined in our approach as we consider and pursue opportunities to deploy our capital with the goal of creating sustainable long-term shareholder value. Although we are exploring new business opportunities for Ambac, no assurance can be given that we will be able to identify or execute a suitable transaction and/or obtain the financial and other resources that may be required to finance the acquisition or development of any new businesses or assets.

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies (Notes)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block]	2. BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES The Company has disclosed its significant accounting policies in Note 2. Basis of Presentation and Significant Accounting Policies in the Notes to Consolidated Financial Statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 . The following significant accounting policies provide an update to those included in the Company’s Annual Report on Form 10-K. Consolidation: The consolidated financial statements include the accounts of Ambac and all other entities in which Ambac (directly or through its subsidiaries) has a controlling financial interest, including variable interest entities (“VIEs”) for which Ambac or an Ambac subsidiary is deemed the primary beneficiary in accordance with the Consolidation Topic of the Accounting Standards Codification ("ASC"). All significant intercompany balances have been eliminated. The usual condition for a controlling financial interest is ownership of a majority of the voting interests of an entity. However, a controlling financial interest may also exist in entities, such as VIEs, through arrangements that do not involve controlling voting interests. A VIE is an entity: a) that lacks enough equity investment at risk to permit the entity to finance its activities without additional subordinated financial support from other parties; or b) where the group of equity holders does not have: (1) the power, through voting rights or similar rights, to direct the activities of an entity that most significantly impact the entity’s economic performance; (2) the obligation to absorb the entity’s expected losses; or (3) the right to receive the entity’s expected residual returns. The determination of whether a variable interest holder is the primary beneficiary involves performing a qualitative analysis of the VIE that includes, among other factors, its capital structure, contractual terms including the rights of each variable interest holder, the activities of the VIE, whether the variable interest holder has the power to direct the activities of a VIE that most significantly impact the VIE’s economic performance, whether the variable interest holder has the obligation to absorb losses of the VIE that could potentially be significant to the VIE or the right to receive benefits from the VIE that could potentially be significant to the VIE, related party relationships and the design of the VIE. An entity that is deemed the primary beneficiary of a VIE is required to consolidate the VIE. See Note 3. Variable Interest Entities , for a detailed discussion of Ambac’s involvement in VIEs, Ambac’s methodology for determining whether Ambac is required to consolidate a VIE and the effects of VIEs being consolidated and deconsolidated. Basis of Presentation The accompanying unaudited consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles ("GAAP") for interim financial reporting and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and disclosures required by GAAP for annual periods. These consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2018 . The accompanying consolidated financial statements have not been audited by an independent registered public accounting firm in accordance with the standards of the Public Company Accounting Oversight Board (U.S.), but in the opinion of management such financial statements include all adjustments necessary for the fair presentation of the Company’s consolidated financial position and results of operations. The results of operations for the three months ended March 31, 2019 may not be indicative of the results that may be expected for the year ending December 31, 2019 . The December 31, 2018 consolidated balance sheet was derived from audited financial statements. The election to use the fair value option is made on an instrument by instrument based under ASC 825-10. At December 31, 2018, all VIE invested assets and long-term debt were reported under the fair value option as disclosed in the notes to the consolidated financial statements included in the Annual Report on Form 10-K. During the three months ended March 31, 2019, Ambac consolidated a new VIE that holds investments in fixed income securities that management has classified as available-for-sale under the Investments - Debt Securities Topic of the ASC. Long-term debt of the newly consolidated VIE is carried at par value less unamortized discount. See Note 3. Variable Interest Entities , for further discussion of the accounting elections used and the presentation of VIEs in these unaudited consolidated financial statements. The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. As additional information becomes available or actual amounts become determinable, the recorded estimates are revised and reflected in operating results. Certain amounts in the condensed consolidated financial statements and associated notes may not add due to rounding. Foreign Currency: Financial statement accounts expressed in foreign currencies are translated into U.S. dollars in accordance with the Foreign Currency Matters Topic of the ASC. The functional currencies of Ambac's subsidiaries are the local currencies of the country where the respective subsidiaries are based, which are also the primary operating environments in which the subsidiaries operate. Foreign currency translation : Functional currency assets and liabilities of Ambac’s foreign subsidiaries are translated into U.S. dollars using exchange rates in effect at the balance sheet dates and the related translation adjustments, net of deferred taxes, are included as a component of Accumulated Other Comprehensive Income in Stockholders' Equity. Functional currency operating results of foreign subsidiaries are translated using average exchange rates. Foreign currency transactions : The impact of non-functional currency transactions and the remeasurement of non-functional currency assets and liabilities into the respective subsidiaries' functional currency (collectively "foreign currency transactions gains/(losses)") are $1,960 and $5,014 for the three months ended March 31, 2019 and 2018 , of which $5,583 and $11,012 relate to the remeasurement of loss reserves, classified in Loss and loss expenses, respectively. Foreign currency transactions gains/(losses) are primarily the result of remeasuring Ambac UK's assets and liabilities denominated in currencies other than its functional currency, primarily the U.S. dollar and the Euro. Supplemental Disclosure of Cash Flow Information Three Months Ended March 31, 2019 2018 Cash paid during the period for: Income taxes $ 852 $ 9,718 Interest on long-term debt 37,850 100,958 Non-cash financing activities: Exchange of investments in Puerto Rico COFINA bonds for new bonds issued in the Plan of Adjustment $ 510,215 $ — Rehabilitation exit transaction discharge of all Deferred Amounts and cancellation of certain General Account Surplus Notes — 1,918,561 Reconciliation of cash, cash equivalents, and restricted cash reported within the Consolidated Balance Sheets to the Consolidated Statements of Cash Flows: Cash and cash equivalents $ 21,840 $ 38,485 Variable Interest Entity Restricted cash 3,254 1,134 Total cash, cash equivalents, and restricted cash shown on the Consolidated Statements of Cash Flows $ 25,094 $ 39,619 Reclassifications: Reclassifications have been made to prior years' amounts to conform to the current year's presentation. Recently Adopted Accounting Standards: Effective January 1, 2019 , Ambac adopted the following accounting standards: Equity-linked Instruments with Down Round Features In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260) and Derivatives and Hedging (Topic 815) - Accounting for Certain Financial Instruments with Down Round Features . Equity-linked instruments, such as warrants and convertible instruments may contain down round features that result in the strike price being reduced on the basis of the pricing of future equity offerings. Under the ASU, a down round feature will no longer require a freestanding equity-linked instrument (or embedded conversion option) to be classified as a liability that is remeasured at fair value through the income statement (i.e. marked-to-market). However, other features of the equity-linked instrument (or embedded conversion option) must still be evaluated to determine whether liability or equity classification is appropriate. Equity classified instruments are not marked-to-market. For earnings per share ("EPS") reporting, the ASU requires companies to recognize the effect of the down round feature only when it is triggered by treating it as a dividend and as a reduction of income available to common stockholders in basic EPS. Adoption of this ASU did not impact Ambac's financial statements. Premium Amortization on Callable Debt Securities In March 2017, the FASB issued ASU 2017-08, Receivables-Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities . The ASU shortens the amortization period for the premium on callable debt securities to the earliest call date. Under previous GAAP, a reporting entity generally amortized the premium as a yield adjustment over the contractual life (i.e. maturity) of the debt security and if that debt security is called, the entity would record a loss equal to the unamortized premium. The ASU does not change the accounting for callable debt securities held at a discount, which will continue to be amortized to maturity. Adoption of this ASU did not have a consequential impact on Ambac's financial statements. Leases In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) . This ASU was subsequently amended by ASU 2018-01, Land Easement Practical Expedient; ASU 2018-10, Codification Improvements to Topic 842; ASU 2018-11 , Targeted Improvements; ASU 2018-20, Narrow-Scope Improvements for Lessors; and ASU 2019-01 , Leases (Topic 842): Codification Improvements (collectively the "New Lease Standard") . The primary difference between previous U.S. GAAP and the New Lease Standard is the recognition of lease assets and lease liabilities for those leases classified as operating leases with a term longer than 12 months. For those operating leases, a lessee is required to: 1) recognize a right-of-use asset ("ROU") and a lease liability, initially measured at the present value of the lease payments, on the balance sheet, 2) recognize a single lease cost, calculated so that the cost is allocated over the lease term generally on a straight-line basis and 3) classify all cash payment within operating activities in the statement of cash flows. For leases classified as finance leases under the New Lease Standard, the balance sheet presentation and expense recognition pattern is similar to capital leases under current GAAP. Under the transition guidance, a reporting entity must use a modified retrospective approach and may choose to initially apply the New Lease Standard either at (1) the beginning of the earliest comparative period presented, which is January 1, 2017 or (2) its effective date, which is January 1, 2019 . If a reporting entity chooses the first option it must recast its comparative period financial statements and provide disclosures for those comparative periods. Ambac chose the second option and initially applied the New Lease Standard on January 1, 2019. Consequently financial information and disclosures were not provided for dates and periods prior to January 1, 2019. There are a number of optional practical expedients that were elected at transition. We elected the ‘package of practical expedients’, which permitted us not to reassess under the new standard our prior conclusions about lease identification, lease classification and initial direct costs. We also elected the hindsight practical expedient allowing us to use the benefit of hindsight in determining the probability of exercising any lessee options to extend or terminate the lease, or purchase the underlying asset. We did not use the practical expedient pertaining to land easements as it was not applicable to Ambac. The new New Lease Standard did not have a material effect on our financial statements. The most significant effects related to (1) the recognition of new ROU assets and lease liabilities on our balance sheet for our office and equipment operating leases of approximately $15,000 at transition and (2) providing significant new disclosures about our leasing activities. See Note 11. Leases for further information. Future Application of Accounting Standards: VIE Related Party Guidance In October 2018, the FASB issued ASU 2018-17, Consolidation (Topic 810) - Targeted Improvements to Related Party Guidance for Variable Interest Entities. To determine whether a decision-making fee is a variable interest, under the new guidance a reporting entity must consider indirect interests held through related parties under common control on a proportional basis rather than as a direct interest in its entirety (as currently required in GAAP). These amendments create alignment between determining whether a decision making fee is a variable interest and determining whether a reporting entity within a related party group is the primary beneficiary of a VIE. ASU 2018-17 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. Ambac will adopt this ASU on January 1, 2020. The ASU is not expected to have a consequential impact on Ambac's financial statements. Cloud Computing Arrangement Service Contracts In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other— Internal-Use Software (Subtopic 350-40) - Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. The new guidance requires a customer in a cloud computing arrangement that is a service contract to capitalize certain implementation costs as if the arrangement was an internal-use software project. The internal-use software guidance requires the capitalization of certain costs incurred only during the application development stage (e.g., costs of integration with on-premises software, coding, configuration, customization). That guidance also requires entities to expense costs during the preliminary project and post-implementation stages (e.g., costs of project planning, training, maintenance after implementation, data conversion) as they are incurred. ASU 2018-15 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. The ASU may be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. Ambac will adopt this ASU on January 1, 2020. The ASU is not expected to have a consequential impact on Ambac's financial statements. Defined Benefit and Other Postretirement Plans Disclosures In August 2018, the FASB issued ASU 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans - General (Subtopic 715-20) - Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans . The ASU modifies various disclosure requirements for employers that sponsor defined benefit pension or other postretirement plans. Relevant disclosures that will be removed are: i) amounts in accumulated other comprehensive income expected to be recognized as net periodic benefit cost over the next fiscal year, and ii) the effects of a one percentage point change in assumed health care cost trend rates on the (a) aggregate of the service and interest cost components of the net periodic pension cost and (b) benefit obligation for postretirement healthcare benefits. Relevant disclosures that will be added are an explanation of the reasons for significant gains and losses related to changes in the benefit obligations for the period. ASU 2018-14 is effective for fiscal years ending after December 15, 2020, with early adoption permitted. The modified disclosures must be applied on a retrospective basis for all periods presented. Ambac has not determined whether it will early adopt this ASU. The ASU is not expected to have a consequential impact on Ambac's financial statements. Fair Value Measurement Disclosures In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820) - Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement . The ASU modifies various disclosure requirements on fair value measurements. Relevant disclosures that will be removed, modified and added are as follows: • Removals: 1) Amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, 2) Policy for timing of transfers between levels, and 3) Valuation processes for Level 3 fair value measurements. • Modifications: 1) For investments in certain entities that calculate net asset value, disclosures are only required for the timing of liquidation of an investee's assets and the date when restrictions from redemption might lapse, only if the investee has communicated the timing to the reporting entity or publicly announced it, and 2) Clarification that the measurement uncertainty disclosure is to communicate information about the uncertainty in measurement as of the reporting date and not possible future changes. • Additions : 1) Changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period and 2) Range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. Alternatively, an entity may disclose other quantitative information (such as the median or arithmetic average) if it determines that it is a more reasonable and rational method to reflect the distribution of unobservable inputs used. ASU 2018-13 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. Disclosure amendments related to changes in unrealized gains and losses included in other comprehensive income for Level 3 instruments, the range and weighted average of significant unobservable inputs, and the narrative description of measurement uncertainty should be applied prospectively only for the most recent interim or annual period presented. All other disclosure amendments should be applied retrospectively to all periods presented. Ambac has not determined whether it will early adopt this ASU. The ASU is not expected to have a consequential impact on Ambac's financial statements. Measurement of Credit Losses on Financial Instruments In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326) - Measurement of Credit Losses on Financial Instruments, subsequently amended by ASU 2018-19 , Codification Improvements to Topic 326, Financial Instruments - Credit Losses

Special Purpose Entities, Inclu

Special Purpose Entities, Including Variable Interest Entities	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Special Purpose Entities, Including Variable Interest Entities	3. VARIABLE INTEREST ENTITIES A mbac, with its subsidiaries, has engaged in transactions with variable interest entities ("VIEs,") in various capacities. • Ambac most commonly provides financial guarantees, including credit derivative contracts, for various debt obligations issued by special purpose entities, including VIEs ("FG VIEs"); • Ambac sponsors special purpose entities that issued notes to investors for various purposes; and • Ambac is an investor in collateralized debt obligations, mortgage-backed and other asset-backed securities issued by VIEs and its ownership interest is generally insignificant to the VIE and/or Ambac does not have rights that direct the activities that are most significant to such VIE. FG VIEs: Ambac’s subsidiaries provide financial guarantees in respect of assets held or debt obligations of VIEs. Ambac’s primary variable interest exists through this financial guarantee insurance or credit derivative contract. The transaction structures provide certain financial protection to Ambac. Generally, upon deterioration in the performance of a transaction or upon an event of default as specified in the transaction legal documents, Ambac will obtain certain control rights that enable Ambac to remediate losses. These rights may enable Ambac to direct the activities of the entity that most significantly impact the entity’s economic performance. • We determined that Ambac’s subsidiaries generally have the obligation to absorb a FG VIE's expected losses given that they have issued financial guarantees supporting certain liabilities (and in some cases certain assets). As further described below, Ambac consolidates certain FG VIEs in cases where we also have the power to direct the activities that most significantly impact the VIE’s economic performance due to one or more of the following: (i) the transaction experiencing deterioration and breaching performance triggers, giving Ambac the ability to exercise certain control rights, (ii) Ambac being involved in the design of the VIE and receiving control rights from its inception, such as may occur from loss remediation activities, or (iii) the transaction not experiencing deterioration, however due to the passive nature of the VIE, Ambac's contingent control rights upon a future breach of performance triggers is considered to be the power over the most significant activity. FG VIEs which are consolidated may include non-recourse assets or liabilities. FG VIEs' liabilities (and in some cases assets) that are insured by the Company are with recourse, because the Company guarantees the payment of principal and interest . FG VIEs' assets and liabilities that are not insured by the Company are without recourse, because Ambac has not issued a financial guarantee and is under no obligation for the payment of principal and interest of these instruments. The Company’s exposure to consolidated FG VIEs is limited to the financial guarantees issued for recourse assets and liabilities and any additional variable interests held by Ambac. • A VIE is deconsolidated in the period that Ambac no longer has such control rights, which could occur in connection with the execution of remediation activities on the transaction or amortization of insured exposure, either of which may reduce the degree of Ambac’s control over a VIE. • Assets and liabilities of FG VIEs that are consolidated are reported within Variable interest entity assets or Variable interest entity liabilities on the Consolidated Balance Sheets. • The election to use the fair value option is made on an instrument by instrument basis. Ambac has elected the fair value option for consolidated FG VIE financial assets and financial liabilities, except in cases where Ambac was involved in the design of the VIE and was granted control rights at its inception. ◦ When the fair value option is elected, changes in the fair value of the FG VIE's financial assets and liabilities are reported within Income (loss) on variable interest entities in the Consolidated Statements of Total Comprehensive Income (Loss), except for the portion of the total change in fair value of financial liabilities caused by changes in the instrument-specific credit risk which is presented separately in Other comprehensive income (loss). ◦ In cases where the fair value option has not been elected, the FG VIE's invested assets are fixed income securities and are considered available-for-sale as defined by the Investments - Debt Securities Topic of the ASC. These assets are reported in the financial statements at fair value with unrealized gains and losses reflected in Accumulated Other Comprehensive Income in Stockholders' Equity. The financial liabilities of these FG VIEs consist of long term debt obligations and are carried at par less unamortized discount. Income from the FG VIE's available-for-sale securities (including investment income, realized gains and losses and other-than -temporary impairments as applicable) and interest expense on long term debt are reported within Income (loss) on variable interest entities in the Consolidated Statements of Total Comprehensive Income (Loss). • Upon initial consolidation of a FG VIE, Ambac recognizes a gain or loss in earnings for the difference between: (i) the fair value of the consideration paid, the fair value of any non-controlling interests and the reported amount of any previously held interests and (ii) the net amount, as measured on a fair value basis, of the assets and liabilities consolidated. Upon deconsolidation of a FG VIE, we recognize a gain or loss for the difference between: (i) the fair value of any consideration received, the fair value of any retained non-controlling investment in the VIE and the carrying amount of any non-controlling interest in the VIE and (ii) the carrying amount of the VIE’s assets and liabilities. Gains or losses from consolidation and deconsolidation that are reported in earnings are reported within Income (loss) on variable interest entities on the Consolidated Statements of Total Comprehensive Income (Loss). • The impact of consolidating such FG VIEs on Ambac’s balance sheet is the elimination of transactions between the consolidated FG VIEs and Ambac’s operating subsidiaries and the inclusion of the FG VIE’s third party assets and liabilities. For a financial guarantee insurance policy issued to a consolidated VIE, Ambac does not reflect the financial guarantee insurance policy in accordance with the related insurance accounting rules under the Financial Services – Insurance Topic of the ASC. Consequently, upon consolidation, Ambac eliminates the insurance assets and liabilities associated with the policy from the Consolidated Balance Sheets. Such insurance assets and liabilities may include premium receivables, reinsurance recoverable, deferred ceded premium, subrogation recoverable, unearned premiums, loss and loss expense reserves, ceded premiums payable and insurance intangible assets. For investment securities owned by Ambac that are debt instruments issued by the VIE, the investment securities balance is eliminated upon consolidation. In connection with the exit from rehabilitation of the Segregated Account, as further described in the Notes to Consolidated Financial Statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 , Ambac evaluated the consolidation of certain VIEs. Under the Stipulation and Order, the OCI retained the authority requiring Ambac Assurance to obtain their approval with respect to the exercise of certain control rights in connection with policies that had previously been allocated to the Segregated Account. Accordingly, Ambac did not consolidate any additional VIEs as a result of the Segregated Account's exit from rehabilitation. Ambac is not primarily liable for, and generally does not guarantee all of the debt obligations issued by the VIEs. Ambac would only be required to make payments on the VIE debt obligations in the event that the issuer of such debt obligations defaults on any principal or interest due and such obligation is guaranteed by Ambac. Additionally, Ambac’s general creditors, other than those specific policy holders which own the VIE debt obligations, do not have rights with regard to the assets of the VIEs. Ambac evaluates the net income effects and earnings per share effects to determine attributions between Ambac and non-controlling interests as a result of consolidating a VIE. Ambac has determined that the net income and earnings per share effect of consolidated FG VIEs are attributable to Ambac’s interests through financial guarantee premium and loss payments with the VIE. The following table summarizes the carrying values of assets and liabilities, along with other supplemental information related to VIEs that are consolidated as a result of financial guarantees of Ambac UK and Ambac Assurance: March 31, 2019 December 31, 2018 Ambac UK Ambac Assurance Total VIEs Ambac UK Ambac Assurance Total VIEs Fixed income securities, at fair value: Corporate obligations, fair value option $ 2,858,404 $ — $ 2,858,404 $ 2,737,286 $ — $ 2,737,286 Municipal obligations, available-for-sale (1) — 270,591 270,591 — — — Total FG VIE fixed income securities, at fair value 2,858,404 270,591 3,128,995 2,737,286 — 2,737,286 Restricted cash 1,056 2,198 3,254 999 — 999 Loans, at fair value (2) 4,375,761 — 4,375,761 4,287,664 — 4,287,664 Derivative assets 59,228 — 59,228 66,302 — 66,302 Other assets 3,320 1,366 4,686 1,058 — 1,058 Total FG VIE assets $ 7,297,769 $ 274,155 $ 7,571,924 $ 7,093,309 $ — $ 7,093,309 Accrued interest payable $ 2,785 $ — $ 2,785 $ 556 $ — $ 556 Long-term debt: Long-term debt, at fair value (3) 5,401,992 — 5,401,992 5,268,596 — 5,268,596 Long-term debt, at par less unamortized discount — 335,271 335,271 — — — Total long-term debt 5,401,992 335,271 5,737,263 5,268,596 — 5,268,596 Derivative liabilities 1,781,903 — 1,781,903 1,712,062 — 1,712,062 Other liabilities 56 — 56 30 — 30 Total FG VIE liabilities $ 7,186,736 $ 335,271 $ 7,522,007 $ 6,981,244 $ — $ 6,981,244 Number of FG VIEs consolidated 7 1 8 7 — 7 (1) Available-for-sale FG VIE fixed income securities consist of municipal obligations with an amortized cost basis of $259,474 and aggregate gross unrealized gains and (losses) of $11,117 and $(0) , respectively at March 31, 2019 . All such securities had contractual maturities due after ten years as of March 31, 2019 . (2) The unpaid principal balances of loan assets carried at fair value were $3,432,240 as of March 31, 2019 and $3,402,413 as of December 31, 2018 . (3) The unpaid principal balances of long-term debt carried at fair value were $4,605,291 as of March 31, 2019 and $4,552,643 as of December 31, 2018 . The following schedule details the components of Income (loss) on variable interest entities for the affected periods: Three Months Ended March 31, 2019 2018 Net change in fair value of VIE assets and liabilities reported under the fair value option $ 1,836 $ 1,918 Less: Credit risk changes of fair value option long-term debt reported through other comprehensive income (171 ) (1,344 ) Net change in fair value of VIE assets and liabilities reported in earnings 1,665 574 Investment income on available-for-sale securities 1,859 — Interest expense on long-term debt carried at par less unamortized cost (2,356 ) — Other expenses (111 ) — Gain (loss) from consolidating FG VIEs 14,864 — Income (loss) on Variable Interest Entities $ 15,921 $ 574 As discussed in Note 3. Variable Interest Entities , effective February 12, 2019, Ambac was required to consolidate one VIE which resulted in a gain of $14,864 . The 2019 balance sheet impact of this consolidation was an increase to total consolidated assets and liabilities by $292,003 and $363,628 in 2019, respectively. Ambac deconsolidated no VIEs for the three months ended March 31, 2019 and 2018 . The following table displays the carrying amount of the assets, liabilities and maximum exposure to loss of Ambac’s variable interests in non-consolidated VIEs resulting from financial guarantee and derivative contracts by major underlying asset classes, as of March 31, 2019 and December 31, 2018 : Carrying Value of Assets and Liabilities Maximum (1) Insurance (2) Insurance (3) Net Derivative (4) March 31, 2019: Global structured finance: Collateralized debt obligations $ 2,437 $ — $ — $ — Mortgage-backed—residential 6,418,261 1,842,283 492,435 — Other consumer asset-backed 1,615,109 14,754 234,639 — Other commercial asset-backed 481,633 9,247 6,774 — Other 2,083,055 53,206 299,939 7,021 Total global structured finance 10,600,495 1,919,490 1,033,787 7,021 Global public finance 23,927,824 315,807 343,946 (1,077 ) Total $ 34,528,319 $ 2,235,297 $ 1,377,733 $ 5,944 December 31, 2018: Global structured finance: Collateralized debt obligations $ 9,787 $ — $ — $ (2 ) Mortgage-backed—residential 6,713,437 1,859,121 546,682 — Other consumer asset-backed 1,700,984 15,435 238,234 — Other commercial asset-backed 873,343 20,735 12,264 — Other 2,122,648 53,462 301,260 7,170 Total global structured finance 11,420,199 1,948,753 1,098,440 7,168 Global public finance 24,145,956 309,071 335,437 (1,457 ) Total $ 35,566,155 $ 2,257,824 $ 1,433,877 $ 5,711 (1) Maximum exposure to loss represents the maximum future payments of principal and interest on insured obligations and derivative contracts. Ambac’s maximum exposure to loss does not include the benefit of any financial instruments (such as reinsurance or hedge contracts) that Ambac may utilize to mitigate the risks associated with these variable interests. (2) Insurance assets represent the amount included in “Premium receivables” and “Subrogation recoverable” for financial guarantee insurance contracts on Ambac’s Consolidated Balance Sheets. (3) Insurance liabilities represent the amount included in “Loss and loss expense reserves” and “Unearned premiums” for financial guarantee insurance contracts on Ambac’s Consolidated Balance Sheets. (4) Net derivative assets (liabilities) represent the fair value recognized on credit derivative contracts and interest rate swaps on Ambac’s Consolidated Balance Sheets. Ambac Sponsored VIEs: Non-consolidated VIEs: A subsidiary of Ambac transferred financial assets to a VIE. The business purpose of this entity was to provide certain financial guarantee clients with funding for their debt obligations. This VIE was established as a separate legal entity, demonstrably distinct from Ambac and that Ambac, its affiliates or its agents could not unilaterally dissolve. The permitted activities of this entity are contractually limited to purchasing assets from Ambac, issuing medium-term notes ("MTNs") to fund such purchases, executing derivative hedges and obtaining financial guarantee policies with respect to indebtedness incurred. Ambac does not consolidate this entity because Ambac Assurance’s policies issued to this entity were allocated to the Segregated Account and, as discussed above, the exercise of related control rights in such policies remain subject to OCI approval under the Stipulation and Order. Ambac elected to account for its equity interest in this entity at fair value under the fair value option in accordance with the Financial Instruments Topic of the ASC. We believe that the fair value of the investments in this entity provides for greater transparency for recording profit or loss as compared to the equity method under the Investments – Equity Method and Joint Ventures Topic of the ASC. Refer to Note 7. Fair Value Measurements for further information on the valuation technique and inputs used to measure the fair value of Ambac’s equity interest in this entity. At March 31, 2019 and December 31, 2018 the fair value of this entity was $4,148 and $4,516 , respectively, and is reported within Other assets on the Consolidated Balance Sheets. • Total principal amount of debt outstanding was $400,600 and $393,010 at March 31, 2019 and December 31, 2018 , respectively. In each case, Ambac sold assets to this entity, which are composed of utility obligations with a weighted average rating of BBB+ at March 31, 2019 and weighted average life of 1.9 years . The purchase by this entity of financial assets was financed through the issuance of MTNs, which are cross-collateralized by the purchased assets. The MTNs have the same expected weighted average life as the purchased assets. Derivative contracts (interest rate swaps) are used within the entity for economic hedging purposes only. Derivative positions were established at the time MTNs were issued to purchase financial assets. As of March 31, 2019 Ambac Assurance had financial guarantee insurance policies issued for all assets, MTNs and derivative contracts owned and outstanding by the entity. • Insurance premiums paid to Ambac Assurance by this entity are earned in a manner consistent with other insurance policies, over the risk period. Additionally, any losses incurred on such insurance policies are included in Ambac’s Consolidated Statements of Total Comprehensive Income (Loss). Under the terms of an Administrative Agency Agreement, Ambac provides certain administrative duties, primarily collecting amounts due on the obligations and making interest payments on the MTNs. On August 28, 2014, Ambac monetized its ownership of the junior surplus note issued to it by the Segregated Account by depositing the junior surplus note into a newly formed VIE trust in exchange for cash and an owner trust certificate, which represents Ambac's right to residual cash flows from the junior surplus note. Ambac does not consolidate the VIE since it does not have a variable interest in the trust. Ambac reports its owner trust certificate as an equity investment within Other investments on the Consolidated Balance Sheets with associated results from operations included within Net investment income: Other investments on the Consolidated Statements of Total Comprehensive Income (Loss). The equity investment had a carrying value of $41,511 and $40,168 as of March 31, 2019 and December 31, 2018 , respectively. On February 12, 2018, Ambac formed a VIE, Ambac LSNI, to issue Secured Notes in connection with the Rehabilitation Exit Transactions. Ambac does not consolidate the VIE since it does not have a variable interest in the trust. Ambac reports its holdings of Secured Notes within Fixed Income Securities in the Consolidated Balance Sheets. The carrying value of Secured Notes held by Ambac was $583,550 and $656,473 at March 31, 2019 and December 31, 2018, respectively. Ambac's debt obligation to the VIE (the Ambac Note) had a carrying value of $1,927,144 and $1,940,289

Comprehensive Income

Comprehensive Income	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Comprehensive Income	4. COMPREHENSIVE INCOME The following tables detail the changes in the balances of each component of accumulated other comprehensive income for the affected periods: Unrealized Gains (1) Amortization of (1) Gain (Loss) on (1) Credit Risk Changes of Fair Value Option Liabilities (1) (2) Total Three Months Ended March 31, 2019: Beginning Balance $ 85,903 $ 8,874 $ (141,527 ) $ (1,965 ) $ (48,715 ) Other comprehensive income (loss) before reclassifications 73,367 689 14,571 — 88,627 Amounts reclassified from accumulated other comprehensive income (loss) (17,204 ) (322 ) — 156 (17,370 ) Net current period other comprehensive income (loss) 56,163 367 14,571 156 71,257 Balance at March 31, 2019 $ 142,066 $ 9,241 $ (126,956 ) $ (1,809 ) $ 22,542 Three Months Ended March 31, 2018: Beginning Balance $ 30,755 $ 10,640 $ (93,634 ) $ — $ (52,239 ) Adjustments to opening balance, net of taxes (3) — — — (2,900 ) (2,900 ) Adjusted balance, beginning of period 30,755 10,640 (93,634 ) (2,900 ) (55,139 ) Other comprehensive income (loss) before reclassifications 125,159 (556 ) 32,056 — 156,659 Amounts reclassified from accumulated other comprehensive income (loss) (2,855 ) (303 ) — 1,114 (2,044 ) Net current period other comprehensive income (loss) 122,304 (859 ) 32,056 1,114 154,615 Balance at March 31, 2018 $ 153,059 $ 9,781 $ (61,578 ) $ (1,786 ) $ 99,476 (1) All amounts are net of tax and noncontrolling interest. Amounts in parentheses indicate reductions to Accumulated Other Comprehensive Income. (2) Represents the changes in fair value attributable to instrument-specific credit risk of liabilities for which the fair value option is elected. (3) Beginning in 2018, credit risk changes of fair value option liabilities are reflected as a component of Accumulated Other Comprehensive Income pursuant to the adoption of ASU 2016-01. See Note 2. Basis of Presentation and Significant Accounting Policies to the Consolidated Financial Statements included in this Form 10-Q for further information regarding this change. The following table details the significant amounts reclassified from each component of accumulated other comprehensive income, shown in the above rollforward tables, for the affected periods: Details about Accumulated Other Comprehensive Income Components Amount Reclassified from Accumulated Other Comprehensive Income (1) Affected Line Item in the Three Months Ended March 31, 2019 2018 Unrealized Gains (Losses) on Available-for-Sale Securities $ (16,693 ) $ (4,563 ) Net realized investment gains (losses) and other-than-temporary impairment losses (511 ) 1,708 Provision for income taxes $ (17,204 ) $ (2,855 ) Net of tax and noncontrolling interest Amortization of Postretirement Benefit Prior service cost $ (241 ) $ (241 ) Other income (2) Actuarial (losses) (81 ) (62 ) Other income (2) (322 ) (303 ) Total before tax — — Provision for income taxes $ (322 ) $ (303 ) Net of tax and noncontrolling interest Credit risk changes of fair value option liabilities $ 188 $ 1,344 Credit Risk Changes of Fair Value Option Liabilities (32 ) (230 ) Provision for income taxes $ 156 $ 1,114 Net of tax and noncontrolling interest Total reclassifications for the period $ (17,370 ) $ (2,044 ) Net of tax and noncontrolling interest (1) Amounts in parentheses indicate reductions to Accumulated Other Comprehensive Income with corresponding increases to the affected line items in the Consolidated Statement of Total Comprehensive Income. (2)

Net Income Per Share

Net Income Per Share	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Net Income Per Share	5. NET INCOME PER SHARE As of March 31, 2019 , 45,520,541 shares of Ambac's common stock (par value $0.01 ) and warrants entitling holders to acquire up to 4,877,783 shares of new common stock at an exercise price of $16.67 per share were issued and outstanding. Common shares increased during the three months ended March 31, 2019 due to settlements of employee restricted stock units. Basic net income per share is computed by dividing net income attributable to common stockholders by the weighted-average number of common shares outstanding and vested restricted stock units. Diluted net income per share is computed by dividing net income attributable to common stockholders by the weighted-average number of common shares used for basic earnings per share plus all potential dilutive common shares outstanding during the period. All potential dilutive common shares outstanding consider common stock deliverable pursuant to warrants issued under Ambac's Chapter 11 Reorganization Plan, vested and unvested options, unvested restricted stock units and performance stock units granted under employee and director compensation plans. The following table provides a reconciliation of the common shares used for basic net income per share to the diluted shares used for diluted net income per share: Three Months Ended March 31, 2019 2018 Basic weighted average shares outstanding 45,832,297 45,471,083 Effect of potential dilutive shares (1) : Stock options — — Warrants — — Restricted stock units — 45,713 Performance stock units (2) — 136,675 Diluted weighted average shares outstanding 45,832,297 45,653,471 Anti-dilutive shares excluded from the above reconciliation: Stock options 16,667 126,667 Warrants 4,877,783 4,053,476 Restricted stock units 271,763 — Performance stock units (2) 478,739 — (1) For the three months ended March 31, 2019 , Ambac had a net loss and accordingly excluded all potentially dilutive securities from the determination of diluted loss per share as their impact was anti-dilutive. (2)

Financial Guarantee Insurance C

Financial Guarantee Insurance Contracts	3 Months Ended
	Mar. 31, 2019
Insurance [Abstract]
Financial Guarantee Insurance Contracts	6. FINANCIAL GUARANTEE INSURANCE CONTRACTS Amounts presented in this Note relate only to Ambac’s non-derivative insurance business for insurance policies issued to beneficiaries, including VIEs, for which we do not consolidate the VIE. Net Premiums Earned: Gross premiums are received either upfront or in installments. For premiums received upfront, an unearned premium revenue (“UPR”) liability is established, which is initially recorded as the cash amount received. For installment premium transactions, a premium receivable asset and offsetting UPR liability is initially established in an amount equal to: (i) the present value of future contractual premiums due (the “contractual” method) or (ii) if the assets underlying the insured obligation are homogenous pools which are contractually prepayable, the present value of premiums to be collected over the expected life of the transaction (the “expected” method). An appropriate risk-free rate corresponding to the weighted average life of each policy and currency is used to discount the future premiums contractually due or expected to be collected. For example, U.S. dollar exposures are discounted using U.S. Treasury rates while exposures denominated in a foreign currency are discounted using the appropriate risk-free rate for the respective currency. The weighted average risk-free rate at March 31, 2019 and December 31, 2018 , was 2.6% and 2.7% , respectively, and the weighted average period of future premiums used to estimate the premium receivable at March 31, 2019 and December 31, 2018 , was 8.8 years and 8.7 years , respectively. In evaluating the credit quality of the premium receivables, management evaluates the obligor's ability to pay. For structured finance transactions, this evaluation will include a review of the priority for the payment of financial guarantee premiums to Ambac, as required by bond indentures, in the transaction's waterfall structure. The financial guarantee premium is generally senior in the waterfall. Uncollectable premiums are determined on a policy basis and utilize a combination of historical premium collection data in addition to cash flow analysis to determine if an impairment in the related policy's premium receivables exist. At March 31, 2019 and December 31, 2018 , $7,467 and $7,136 respectively, of premium receivables were deemed uncollectable. As of March 31, 2019 and December 31, 2018 , approximately 19% and 20% of the premium receivables, net of uncollectable receivables, related to transactions with non-investment grade internal ratings, mainly structured finance transactions. Past due premiums on policies insuring non-investment grade obligations amounted to less than $100 at March 31, 2019 . Below is the gross premium receivable roll-forward for the affected periods: Three Months Ended March 31, 2019 2018 Beginning premium receivable $ 495,391 $ 586,312 Premium receipts (13,204 ) (15,381 ) Adjustments for changes in expected and contractual cash flows (1) 470 (1,289 ) Accretion of premium receivable discount 3,228 3,846 Changes to uncollectable premiums (352 ) 604 Other adjustments (including foreign exchange) 1,864 6,615 Ending premium receivable (2) $ 487,397 $ 580,707 (1) Adjustments for changes in expected and contractual cash flows primarily due to reductions in insured exposure as a result of early policy terminations and unscheduled principal paydowns. (2) Premium receivable includes premiums to be received in foreign denominated currencies most notably in British Pounds and Euros. At March 31, 2019 and 2018 , premium receivables include British Pounds of $141,527 ( £108,825 ) and $163,926 ( £116,815 ), respectively, and Euros of $30,062 ( €26,807 ) and $36,679 ( €29,767 ), respectively. When a bond issue insured by Ambac Assurance has been retired early, typically due to an issuer call, any remaining UPR is recognized at that time to the extent the financial guarantee contract is legally extinguished, causing accelerated premium revenue. For installment premium paying transactions, we offset the recognition of any remaining UPR by the reduction of the related premium receivable to zero (as it will not be collected as a result of the retirement), which may cause negative accelerated premium revenue. Ambac’s accelerated premium revenue for retired obligations for the three months ended March 31, 2019 and 2018 was $12,223 and $9,392 , respectively. The effect of reinsurance on premiums written and earned for the respective periods was as follows: Three Months Ended March 31, 2019 2018 Written Earned Written Earned Direct $ 3,346 $ 29,433 $ 4,261 $ 32,609 Assumed — 20 — 19 Ceded (501 ) 1,695 (819 ) 1,745 Net premiums $ 3,847 $ 27,758 $ 5,080 $ 30,883 The following table summarizes net premiums earned by location of risk for the respective periods: Three Months Ended March 31, 2019 2018 United States $ 28,244 $ 24,718 United Kingdom 4,190 4,856 Other international (4,676 ) 1,309 Total $ 27,758 $ 30,883 The table below summarizes the future gross undiscounted premiums to be collected and future premiums earned, net of reinsurance at March 31, 2019 : Future Premiums Collected (1) Future (1) Three months ended: June 30, 2019 $ 12,906 $ 12,178 September 30, 2019 11,923 12,015 December 31, 2019 11,852 11,875 Twelve months ended: December 31, 2020 46,963 46,762 December 31, 2021 41,005 42,504 December 31, 2022 39,159 39,760 December 31, 2023 37,641 37,075 Five years ended: December 31, 2028 166,827 152,126 December 31, 2033 128,064 101,459 December 31, 2038 67,195 51,134 December 31, 2043 26,556 16,644 December 31, 2048 11,831 7,130 December 31, 2053 2,252 1,817 December 31, 2058 31 50 Total $ 604,205 $ 532,529 (1) Future premiums to be collected are undiscounted and are used to derive the discounted premium receivable asset recorded on Ambac's balance sheet. Future premiums to be earned, net of reinsurance relate to the unearned premiums liability and deferred ceded premium asset recorded on Ambac’s balance sheet. The use of contractual lives for many bond types which do not have homogeneous pools of underlying collateral is required in the calculation of the premium receivable, as further described in Note 2. Basis of Presentation and Significant Accounting Policies in the Notes to Consolidated Financial Statements included in Ambac's Annual Report on Form 10-K for the year ended December 31, 2018 . This results in a different premium receivable balance than if expected lives were considered. If installment paying policies are retired or prepay early, premiums reflected in the premium receivable asset and amounts reported in the above table for such policies may not be collected. Future premiums to be earned also considers the use of contractual lives for many bond types which do not have homogeneous pools of underlying collateral, which may result in different unearned premium than if expected lives were considered. If those bonds types are retired early, premium earnings may be negative in the period of call or refinancing. Loss and Loss Expense Reserves: The loss and loss expense reserve (“loss reserve”) policy for financial guarantee insurance relates only to Ambac’s non-derivative insurance business for insurance policies issued to beneficiaries, including VIEs, for which we do not consolidate the VIE. Ambac’s loss reserves are based on management’s on-going review of the financial guarantee credit portfolio. Below are the components of the Loss and loss expense reserves liability and the Subrogation recoverable asset at March 31, 2019 and December 31, 2018 : Present Value of Expected Unearned Gross Loss and Balance Sheet Line Item Claims and Recoveries March 31, 2019: Loss and loss expense reserves $ 2,045,886 $ (274,846 ) $ (76,877 ) $ 1,694,163 Subrogation recoverable 175,114 (2,091,231 ) — (1,916,117 ) Totals $ 2,221,000 $ (2,366,077 ) $ (76,877 ) $ (221,954 ) December 31, 2018: Loss and loss expense reserves $ 2,246,335 $ (313,595 ) $ (106,662 ) $ 1,826,078 Subrogation recoverable 175,694 (2,108,654 ) — (1,932,960 ) Totals $ 2,422,029 $ (2,422,249 ) $ (106,662 ) $ (106,882 ) Below is the loss and loss expense reserve roll-forward, net of subrogation recoverable and reinsurance, for the affected periods: Three Months Ended March 31, 2019 2018 Beginning gross loss and loss expense reserves $ (106,882 ) $ 4,113,802 Reinsurance recoverable 22,623 40,658 Beginning balance of net loss and loss expense reserves (129,505 ) 4,073,144 Losses and loss expenses (benefit): Current year 681 778 Prior years 11,726 (248,173 ) Total (1) (2) (3) 12,407 (247,395 ) Loss and loss expenses paid (recovered): Current year 28 — Prior years (3) 64,409 3,631,177 Total 64,437 3,631,177 Foreign exchange effect 5,502 11,016 Ending net loss and loss expense reserves (176,033 ) 205,588 Impact of VIE consolidation (72,159 ) — Reinsurance recoverable (4) 26,238 38,735 Ending gross loss and loss expense reserves $ (221,954 ) $ 244,323 (1) Total losses and loss expenses (benefit) includes $(4,996) and $1,354 for the three months ended March 31, 2019 and 2018 , respectively, related to ceded reinsurance. (2) Ambac records the impact of estimated recoveries related to securitized loans in RMBS transactions that breached certain representations and warranties ("R&W"s) by transaction sponsors within losses and loss expenses (benefit). The losses and loss expense (benefit) incurred associated with changes in estimated representation and warranties for the three months ended March 31, 2019 and 2018 was $4,407 and $800 , respectively. (3) On February 12, 2018, Deferred Amounts and Interest Accrued on Deferred Amounts in the amount of $3,000,158 and $856,834 , respectively were settled in connection with the Rehabilitation Exit Transactions. 2018 includes a $288,204 loss and loss expense benefit on these settled Deferred Amounts. (4) Represents reinsurance recoverable on future loss and loss expenses. Additionally, the Balance Sheet line "Reinsurance recoverable on paid and unpaid losses" includes reinsurance recoverables (payables) of $550 and $90 as of March 31, 2019 and 2018 , respectively, related to previously presented loss and loss expenses and subrogation. For 2019 , the net negative development in prior years was primarily a result of deterioration in Public Finance credits, partially offset by positive development in the RMBS and Ambac UK portfolios. For 2018, the net positive development in prior years was primarily a result of the discount recorded on the Rehabilitation Exit Transactions partially offset by negative development in the RMBS portfolio and interest accrued on Deferred Amounts prior to the Rehabilitation Exit Transactions. The tables below summarize information related to policies currently included in Ambac’s loss and loss expense reserves or subrogation recoverable at March 31, 2019 and December 31, 2018 . Gross par exposures include capital appreciation bonds which are reported at the par amount at the time of issuance of the insurance policy as opposed to the current accreted value of the bond. The weighted average risk-free rate used to discount loss reserves at March 31, 2019 and December 31, 2018 was 2.5% and 2.8% , respectively. Surveillance Categories as of March 31, 2019 I IA II III IV V Total Number of policies 33 28 12 16 147 3 239 Remaining weighted-average contract period (in years) (1) 9 18 8 18 14 3 15 Gross insured contractual payments outstanding: Principal $ 934,336 $ 754,502 $ 177,251 $ 909,382 $ 5,299,368 $ 43,140 $ 8,117,979 Interest 514,643 646,737 87,602 443,526 2,144,310 13,401 3,850,219 Total $ 1,448,979 $ 1,401,239 $ 264,853 $ 1,352,908 $ 7,443,678 $ 56,541 $ 11,968,198 Gross undiscounted claim liability $ 6,344 $ 56,257 $ 25,466 $ 591,366 $ 2,182,417 $ 56,510 $ 2,918,360 Discount, gross claim liability (620 ) (7,729 ) (1,897 ) (202,850 ) (564,236 ) (3,455 ) (780,787 ) Gross claim liability before all subrogation and before reinsurance 5,724 48,528 23,569 388,516 1,618,181 53,055 2,137,573 Less: Gross RMBS subrogation (2) — — — — (1,803,302 ) — (1,803,302 ) Discount, RMBS subrogation — — — — 37,280 — 37,280 Discounted RMBS subrogation, before reinsurance — — — — (1,766,022 ) — (1,766,022 ) Less: Gross other subrogation (3) — — — (46,914 ) (598,954 ) (12,880 ) (658,748 ) Discount, other subrogation — — — 6,295 48,954 3,444 58,693 Discounted other subrogation, before reinsurance — — — (40,619 ) (550,000 ) (9,436 ) (600,055 ) Gross claim liability, net of all subrogation and discounts, before reinsurance 5,724 48,528 23,569 347,897 (697,841 ) 43,619 (228,504 ) Less: Unearned premium revenue (3,791 ) (9,639 ) (1,640 ) (7,219 ) (54,395 ) (193 ) (76,877 ) Plus: Loss expense reserves 1,809 5,330 902 2,362 73,024 — 83,427 Gross loss and loss expense reserves $ 3,742 $ 44,219 $ 22,831 $ 343,040 $ (679,212 ) $ 43,426 $ (221,954 ) Reinsurance recoverable reported on Balance Sheet (4) $ 216 $ 8,901 $ 3,758 $ 27,138 $ (13,225 ) $ — $ 26,788 (1) Remaining weighted-average contract period is weighted based on projected gross claims over the lives of the respective policies. (2) RMBS subrogation represents Ambac’s estimate of subrogation recoveries from RMBS transaction sponsors for representation and warranty ("R&W") breaches. (3) Other subrogation represents subrogation related to excess spread and other contractual cash flows on public finance and structured finance transactions, including RMBS. (4) Reinsurance recoverable reported on the Balance Sheet includes reinsurance recoverables of $26,238 related to future loss and loss expenses and $550 related to presented loss and loss expenses and subrogation. Surveillance Categories as of December 31, 2018 I IA II III IV V Total Number of policies 21 28 18 16 145 3 231 Remaining weighted-average contract period (in years) (1) 9 19 9 22 14 3 16 Gross insured contractual payments outstanding: Principal $ 916,530 $ 708,249 $ 622,820 $ 1,705,464 $ 5,407,202 $ 43,140 $ 9,403,405 Interest 487,702 631,708 293,293 6,979,130 2,177,539 13,401 10,582,773 Total $ 1,404,232 $ 1,339,957 $ 916,113 $ 8,684,594 $ 7,584,741 $ 56,541 $ 19,986,178 Gross undiscounted claim liability $ 4,019 $ 63,712 $ 36,000 $ 992,019 $ 2,295,968 $ 56,510 $ 3,448,228 Discount, gross claim liability (481 ) (13,008 ) (3,069 ) (433,709 ) (637,548 ) (4,143 ) (1,091,958 ) Gross claim liability before all subrogation and before reinsurance 3,538 50,704 32,931 558,310 1,658,420 52,367 2,356,270 Less: Gross RMBS subrogation (2) — — — — (1,809,937 ) — (1,809,937 ) Discount, RMBS subrogation — — — — 39,391 — 39,391 Discounted RMBS subrogation, before reinsurance — — — — (1,770,546 ) — (1,770,546 ) Less: Gross other subrogation (3) — (10,816 ) — (136,541 ) (624,654 ) (12,880 ) (784,891 ) Discount, other subrogation — 7,318 — 67,008 55,088 3,774 133,188 Discounted other subrogation, before reinsurance — (3,498 ) — (69,533 ) (569,566 ) (9,106 ) (651,703 ) Gross claim liability, net of all subrogation and discounts, before reinsurance 3,538 47,206 32,931 488,777 (681,692 ) 43,261 (65,979 ) Less: Unearned premium revenue (943 ) (10,073 ) (5,085 ) (36,365 ) (53,987 ) (209 ) (106,662 ) Plus: Loss expense reserves 1,369 4,253 2,564 (5,926 ) 63,499 — 65,759 Gross loss and loss expense reserves $ 3,964 $ 41,386 $ 30,410 $ 446,486 $ (672,180 ) $ 43,052 $ (106,882 ) Reinsurance recoverable reported on Balance Sheet (4) $ 367 $ 7,285 $ 4,223 $ 26,096 $ (14,838 ) $ — $ 23,133 (1) Remaining weighted-average contract period is weighted based on projected gross claims over the lives of the respective policies. (2) RMBS subrogation represents Ambac’s estimate of subrogation recoveries from RMBS transaction sponsors for R&W breaches. (3) Other subrogation represents subrogation related to excess spread and other contractual cash flows on public finance and structured finance transactions, including RMBS. (4) Reinsurance recoverable reported on Balance Sheet includes reinsurance recoverables of $22,623 related to future loss and loss expenses and $510 related to presented loss and loss expenses and subrogation. Puerto Rico: Ambac has exposure to the Commonwealth of Puerto Rico (the "Commonwealth") and its instrumentalities across several different issuing entities. Each has its own credit risk profile attributable to discrete revenue sources, direct general obligation pledges or general obligation guarantees. The Commonwealth of Puerto Rico and certain of its instrumentalities have defaulted and may continue to default on debt service payments, including payments owed on bonds insured by Ambac Assurance. Ambac Assurance may be required to make significant amounts of policy payments over the next several years, the recoverability of which is subject to great uncertainty, which may lead to a material increase in permanent losses causing a material adverse impact on our results of operations and financial condition. Our exposure to Puerto Rico is impacted by the amount of monies available for debt service, which is in turn affected by a number of factors including demographic trends, economic growth, tax policy and revenues, impact of reforms, fiscal plans, government actions, budgetary performance and flexibility, weather events, litigation outcomes, as well as federal funding of Commonwealth needs. In the near term, the financial and economic outlook for Puerto Rico is dependent upon a still fragile infrastructure, heightening its vulnerability to additional weather events. The longer term recovery of the Commonwealth economy and its essential infrastructure will likely be dependent on, amongst other factors, the management, usage and efficacy of federal resources. Also important to Puerto Rico's economic growth, government reform and creditor outcomes is the revised Fiscal Plan for the Commonwealth of Puerto Rico ("Revised Commonwealth Fiscal Plan"), certified by the Financial Oversight and Management Board for Puerto Rico ("Oversight Board") on October 23, 2018. The Revised Commonwealth Fiscal Plan outlines a series of reforms, projects the fiscal and economic impact of those reforms, and provides forecasts of resulting budgetary surpluses over a fiscal year series. However, as was the case with prior Commonwealth Fiscal Plans, the Revised Commonwealth Fiscal Plan lacks a high degree of transparency regarding the underlying data, assumptions and rationales supporting those assumptions, making reconciliation and due diligence difficult. As a result, it is difficult to predict the long-term capacity and willingness of the Puerto Rico government and its instrumentalities to pay debt service on bonded debt and how their debt burden and financial flexibility might affect Ambac Assurance's claim potential, risk profile and long-term financial strength. Substantial uncertainty exists with respect to the ultimate outcome for creditors in Puerto Rico, such as Ambac Assurance, due to, amongst other matters, legislation enacted by the Commonwealth and the federal government, including PROMESA, as well as actions taken pursuant to such laws, including Title III filings. Ambac Assurance is involved in multiple litigations relating to such actions and other issues and may not be successful in pursuing claims or protecting its interests. As a result of litigation or other aspects of the restructuring processes, the difference among the credits insured by Ambac Assurance may not be respected. Ambac Assurance is participating in a mediation process with respect to potential debt restructurings. Mediation may not be productive or may not resolve Ambac Assurance's claims in a manner that avoids significant losses. It is possible that certain restructuring process solutions, together with associated legislation, budgetary, and/or public policy proposals could be adopted and could significantly or further impair our exposures causing losses that could have a material adverse impact on our results of operations and financial condition. While our reserving scenarios reflect a wide range of possible outcomes, reflecting the significant uncertainty regarding future developments and outcomes, given our exposure to Puerto Rico and the economic, fiscal, legal and political uncertainties associated therewith as well as the uncertainties emanating from the damage caused by hurricanes Maria and Irma, our loss reserves may ultimately prove to be insufficient to cover our losses, potentially by a material amount, and may be subject to material volatility. Ambac has considered these developments and other factors in evaluating its Puerto Rico loss reserves. During the three months ended March 31, 2019 , Ambac had incurred losses associated with its Domestic Public Finance insured portfolio of $69,304 , which was primarily impacted by the continued uncertainty and volatility of the situation in Puerto Rico. While management believes its reserves are adequate to cover losses in its Public Finance insured portfolio, there can be no assurance that Ambac may not incur additional losses in the future, particularly given the developing economic, political, and legal circumstances in Puerto Rico. Such additional losses may have a material adverse effect on Ambac’s results of operations and financial condition. For public finance credits, including Puerto Rico, as well as other issuers, for which Ambac has an estimate of expected loss at March 31, 2019 , the possible increase in loss reserves under stress or other adverse conditions and circumstances was estimated to be approximately $950,000 . This possible increase in loss reserves under stress or other adverse conditions is significant and if we were to experience such incremental losses, our stockholders’ equity as of March 31, 2019 would decrease from $1,665,888 to $715,888 . However, there can be no assurance that losses may not exceed such amount. COFINA Debt Restructuring On January 16-17, 2019, the hearings for the confirmation of the COFINA Plan of Adjustment ("POA") and the Commonwealth 9019 motion were held. On February 4, 2019, the COFINA POA was confirmed and the Commonwealth 9019 motion was approved by the U.S. District Court for the District of Puerto Rico. On February 12, 2019, the COFINA POA went effective. Pursuant to the POA, all existing COFINA senior and subordinate bonds were discharged and exchanged for cash and new COFINA current interest and capital appreciation bonds ("new COFINA bonds"). The cash and new COFINA bonds allocated to COFINA senior bondholders equaled approximately 93% (considering the new COFINA bonds at par) of such senior bondholders’ allowed claim, in the amount of the COFINA senior bond accreted value, as of, but not including, May 5, 2017 (the COFINA Title III Petition Date). Pursuant to the POA, each holder of Ambac Assurance-insured senior COFINA bonds had the option to elect by January 11, 2019 to either (i) commute their rights in respect of the Ambac Assurance insurance policy associated with the existing senior COFINA bonds, which bonds would be discharged and Ambac Assurance policy obligations with respect thereto would be released, in exchange for new COFINA bonds, cash amounts to be paid by COFINA plus additional cash consideration provided by Ambac Assurance equal to 5.25% of the accreted value of the Ambac Assurance-insured senior COFINA bonds as of the COFINA Petition Date or (ii) agree to deposit their Ambac Assurance-insured senior COFINA bonds into a a trust in exchange for units issued by the trust (the "COFINA Class 2 Trust"), which trust would receive the new COFINA bonds and the cash amounts to be paid by COFINA that such bondholders would have otherwise received to the extent they had elected the recovery under clause (i) above (thereby entitling the COFINA Class 2 Trust to receive debt service payments from COFINA with respect to the new COFINA bonds deposited into the trust), plus any accelerated policy payments (made solely at Ambac Assurance's own discretion) or claim payments due under the existing Ambac Assurance insurance policy for the deficiency relating to the existing senior COFINA bonds at the relevant scheduled payment dates (2047 through 2054). Any claims payable under the existing Ambac Assurance policy for the Ambac Assurance-insured senior COFINA bonds held in the trust will be reduced by all amounts distributed or deemed distributed from the trust to the holders of the trust units from the new COFINA bonds and cash as well as accelerated policy payments made by Ambac Assurance at its own discretion. Ambac makes no representation and can give no assurances that the new COFINA bonds or COFINA Class 2 Trust units, both of which are not insured by Ambac Assurance, will trade at par or any other price. Under the COFINA POA, Ambac Assurance-insured bondholders who did not affirmatively elect the trust option in clause (ii) above were deemed to have elected the commutation option described in clause (i) above. Approximately 75% of Ambac Assurance-insured senior COFINA bondholders, by measure of insured par, elected the commutation option or did not affirmatively elect to exchange their bonds for units of the COFINA Class 2 Trust. As a result of the POA commutation and subsequent redemptions of obligations of the COFINA Class 2 Trust, Ambac Assurance's net par outstanding was reduced to $184,997 as of March 31, 2019 . Ambac Assurance's remaining policy obligation of $184,997 net par is an asset of the COFINA Class 2 Trust, which holds a ratable distribution of new COFINA bonds, the interest and principal from which can be used to partially offset Ambac’s remaining insurance liability. As further discussed in Note 3. Variable Interest Entities , Ambac Assurance is required to consolidate the COFINA Class 2 Trust. At this time, it is unclear what impact the COFINA restructuring will have on the prospective recoveries of Ambac Assurance's other insured Puerto Rico instrumentalities. Representation and Warranty Recoveries: Ambac records estimated subrogation recoveries for breaches of R&Ws by sponsors of certain RMBS transactions. For a discussion of the approach utilized to estimate R&W subrogation recoveries, see Note 2. Basis of Presentation and Significant Accounting Policies in the Notes to Consolidated Financial Statements included Part II, Item 8 in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 . Ambac has recorded R&W subrogation recoveries of $1,766,022 ( $1,739,836 net of reinsurance) and $1,770,546 ( $1,744,243 net of reinsurance) at March 31, 2019 and December 31, 2018 , respectively. Below is the rollforward of R&W subrogation for the affected periods: Three Months Ended March 31, 2019 2018 Discounted R&W subrogation (gross of reinsurance) at beginning of period $ 1,770,546 $ 1,834,387 Impact of sponsor actions (1) — — All other changes (2) (4,524 ) (877 ) Discounted R&W subrogation (gross of reinsurance) at end of period $ 1,766,022 $ 1,833,510 (1) Sponsor actions include loan repurchases, direct payments to Ambac and other contributions from sponsors. (2) All other changes which may impact RMBS R&W subrogation recoveries include changes in actual or projected collateral performance, changes in the creditworthiness of a sponsor and/or the projected timing of recoveries. Our ability to realize R&W subrogation recoveries is subject to significant uncertainty, including risks inherent in litigation, collectability of such amounts from counterparties (and/or their respective parents and affiliates), timing of receipt of any such recoveries, intervention by OCI, which could impede our ability to take actions required to realize such recoveries, and uncertainty inherent in the assumptions used in estimating such recoveries. Failure to realize R&W subrogation recoveries for any reason or the realization of R&W subrogation recoveries materially below the amount recorded on Ambac's consolidated balance sheet would have a material adverse effect on our results of operations and financial condition and may result in adverse consequences such as impairing the ability of Ambac Assurance to honor its financial obligations; the initiation of rehabilitation proceedings against Ambac Assurance; decreased likelihood of Ambac Assurance delivering value to Ambac, through dividends or otherwise; and a significant drop in the value of securities issued or insured by Ambac or Ambac Assurance. Insurance intangible asset: The insurance intangible amortization expense is included in insurance intangible amortization on the Consolidated Statements of Total Comprehensive Income (Loss). For the three months ended March 31, 2019 and 2018 , the insurance intangible amortization expense was $36,278 and $28,636 , respectively. As of March 31, 2019 and December 31, 2018 , the gross carrying value of the insurance intangible asset was $1,561,070 and $1,551,576 , respectively. Accumulated amortization of the insurance intangible asset was $871,815 and $832,645 , as of March 31, 2019 and December 31, 2018 , respectively, resulting in a net insurance intangible asset of $689,255 and $718,931 , respectively. The estimated future amortization expense for the net insurance intangible asset is as follows: Amortization expense (1) 2019 (nine months) $ 47,410 2020 58,672 2021 53,229 2022 49,565 2023 46,241 Thereafter 434,138 (1) Future amortization considers the use of contractual lives for many bond types which do not have homogeneous pools of underlying collateral. Actual maturities will differ from contractual maturities because

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	7. FAIR VALUE MEASUREMENTS The Fair Value Measurement Topic of the ASC establishes a framework for measuring fair value and disclosures about fair value measurements. Fair Value Hierarchy: The Fair Value Measurement Topic of the ASC specifies a fair value hierarchy based on whether the inputs to valuation techniques used to measure fair value are observable or unobservable. Observable inputs reflect market data obtained from independent sources, while unobservable inputs reflect Company-based assumptions. The fair value hierarchy prioritizes model inputs into three broad levels as follows: l Level 1 Quoted prices for identical instruments in active markets. Assets and liabilities classified as Level 1 include US Treasury and other foreign government obligations traded in highly liquid and transparent markets, exchange traded futures contracts, variable rate demand obligations and money market funds. l Level 2 Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations in which all significant inputs and significant value drivers are observable in active markets. Assets and liabilities classified as Level 2 generally include investments in fixed income securities representing municipal, asset-backed and corporate obligations, certain interest rate swap contracts and most long-term debt of variable interest entities consolidated under the Consolidation Topic of the ASC. l Level 3 Model derived valuations in which one or more significant inputs or significant value drivers are unobservable. This hierarchy requires the use of observable market data when available. Assets and liabilities classified as Level 3 include credit derivative contracts, certain uncollateralized interest rate swap contracts, equity interests in Ambac sponsored special purpose entities and certain investments in fixed income securities. Additionally, Level 3 assets and liabilities generally include loan receivables, and certain long-term debt of variable interest entities consolidated under the Consolidation Topic of the ASC. The following table sets forth the carrying amount and fair value of Ambac’s financial assets and liabilities as of March 31, 2019 and December 31, 2018 , including the level within the fair value hierarchy at which fair value measurements are categorized. As required by the Fair Value Measurement Topic of the ASC, financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. Carrying Total Fair Fair Value Measurements Categorized as: March 31, 2019: Level 1 Level 2 Level 3 Financial assets: Fixed income securities: Municipal obligations $ 424,884 $ 424,884 $ — $ 424,884 $ — Corporate obligations 1,233,544 1,233,544 — 1,233,544 — Foreign obligations 31,292 31,292 30,379 913 — U.S. government obligations 100,429 100,429 100,429 — — Residential mortgage-backed securities 256,285 256,285 — 256,285 — Collateralized debt obligations 132,977 132,977 — 132,977 — Other asset-backed securities 440,400 440,400 — 368,545 71,855 Fixed income securities, pledged as collateral: U.S. government obligations 83,901 83,901 83,901 — — Short term investments 908,235 908,235 864,065 44,170 — Other investments (1) 428,556 404,775 102,257 — 17,730 Cash, cash equivalents and restricted cash 21,840 21,840 15,891 5,949 — Derivative assets: Interest rate swaps—asset position 73,218 73,218 — 19,107 54,111 Futures contracts 3,182 3,182 3,182 — — Other assets - equity in sponsored VIE 4,148 4,148 — — 4,148 Other assets-Loans 10,124 12,187 — — 12,187 Variable interest entity assets: Fixed income securities: Corporate obligations 2,858,404 2,858,404 — — 2,858,404 Fixed income securities: Municipal obligations 270,591 270,591 — 270,591 — Restricted cash 3,254 3,254 3,254 — — Loans 4,375,761 4,375,761 — — 4,375,761 Derivative assets: Currency swaps-asset position 59,228 59,228 — 59,228 — Total financial assets $ 11,720,253 $ 11,698,535 $ 1,203,358 $ 2,816,193 $ 7,394,196 Financial liabilities: Long term debt, including accrued interest $ 3,320,562 $ 3,324,313 $ — $ 2,942,229 $ 382,084 Derivative liabilities: Credit derivatives 1,077 1,077 — — 1,077 Interest rate swaps—asset position — — — — Interest rate swaps—liability position 85,457 85,457 — 85,457 — Futures contracts — — — — Liabilities for net financial guarantees (2) (839,056 ) 509,830 — — 509,830 Variable interest entity liabilities: Long-term debt (includes $5,401,992 carried at fair value) 5,737,263 5,785,582 — 5,561,135 224,447 Derivative liabilities: Interest rate swaps—liability position 1,781,903 1,781,903 — 1,781,903 — Total financial liabilities $ 10,087,206 $ 11,488,162 $ — $ 10,370,724 $ 1,117,438 Carrying Total Fair Fair Value Measurements Categorized as: December 31, 2018: Level 1 Level 2 Level 3 Financial assets: Fixed income securities: Municipal obligations $ 879,919 $ 879,919 $ — $ 879,919 $ — Corporate obligations 1,278,122 1,278,122 — 1,278,122 — Foreign obligations 30,834 30,834 29,922 912 — U.S. government obligations 94,394 94,394 94,394 — — Residential mortgage-backed securities 258,607 258,607 — 258,607 — Collateralized debt obligations 131,356 131,356 — 131,356 — Other asset-backed securities 442,443 442,443 — 370,372 72,071 Short term investments 430,331 430,331 304,880 125,451 — Other investments (1) 391,217 367,315 71,108 — 16,266 Cash and cash equivalents 82,494 82,494 52,661 29,833 — Derivative assets: Interest rate swaps—asset position 59,468 59,468 — 12,008 47,460 Other assets - equity in sponsored VIE 4,516 4,516 — — 4,516 Other assets-Loans 9,913 11,620 — — 11,620 Variable interest entity assets: Fixed income securities: Corporate obligations 2,737,286 2,737,286 — — 2,737,286 Restricted cash 999 999 999 — — Loans 4,287,664 4,287,664 — — 4,287,664 Derivative assets: Currency swaps—asset position 66,302 66,302 — 66,302 — Total financial assets $ 11,185,865 $ 11,163,670 $ 553,964 $ 3,152,882 $ 7,176,883 Financial liabilities: Long term debt, including accrued interest $ 3,304,737 $ 3,259,966 $ — $ 2,909,272 $ 350,694 Derivative liabilities: Credit derivatives 1,459 1,459 — — 1,459 Interest rate swaps—liability position 71,861 71,861 — 71,861 — Futures contracts 3,379 3,379 3,379 — Liabilities for net financial guarantees written (2) (718,388 ) 558,824 — — 558,824 Variable interest entity liabilities: Long-term debt 5,268,596 5,268,596 — 5,051,504 217,092 Derivative liabilities: Interest rate swaps—liability position 1,712,062 1,712,062 — 1,712,062 — Total financial liabilities $ 9,643,706 $ 10,876,147 $ 3,379 $ 9,744,699 $ 1,128,069 (1) Excluded from the fair value measurement categories in the table above are investment funds of $284,788 and $279,941 as of March 31, 2019 and December 31, 2018 , respectively, which are measured using NAV per share as a practical expedient. (2) The carrying value of net financial guarantees written includes the following balance sheet items: Premium receivables; Reinsurance recoverable on paid and unpaid losses; Deferred ceded premium; Subrogation recoverable; Insurance intangible asset; Unearned premiums; Loss and loss expense reserves; Ceded premiums payable, premiums taxes payable and other deferred fees recorded in Other liabilities. Determination of Fair Value: When available, Ambac uses quoted active market prices specific to the financial instrument to determine fair value, and classifies such items within Level 1. The determination of fair value for financial instruments categorized in Level 2 or 3 involves significant judgment due to the complexity of factors contributing to the valuation. Third-party sources from which we obtain independent market quotes also use assumptions, judgments and estimates in determining financial instrument values and different third parties may use different methodologies or provide different values for financial instruments. In addition, the use of internal valuation models may require assumptions about hypothetical or inactive markets. As a result of these factors, the actual trade value of a financial instrument in the market, or exit value of a financial instrument position by Ambac, may be significantly different from its recorded fair value. Ambac’s financial instruments carried at fair value are mainly comprised of investments in fixed income securities, equity interests in pooled investment funds, derivative instruments, certain variable interest entity assets and liabilities and interests in Ambac sponsored special purpose entities. Valuation of financial instruments is performed by Ambac’s finance group using methods approved by senior financial management with consultation from risk management and portfolio managers as appropriate. Preliminary valuation results are discussed with portfolio managers quarterly to assess consistency with market transactions and trends as applicable. Market transactions such as trades or negotiated settlements of similar positions, if any, are reviewed to validate fair value model results. However, many of the financial instruments valued using significant unobservable inputs have very little or no observable market activity. Methods and significant inputs and assumptions used to determine fair values across portfolios are reviewed quarterly by senior financial management. Other valuation control procedures specific to particular portfolios are described further below. We reflect Ambac’s own creditworthiness in the fair value of financial liabilities by including a credit valuation adjustment (“CVA”) in the determination of fair value. A decline (increase) in Ambac’s creditworthiness as perceived by market participants will generally result in a higher (lower) CVA, thereby lowering (increasing) the fair value of Ambac’s financial liabilities as reported. Fixed Income Securities: The fair values of fixed income investment securities are based primarily on market prices received from dealer quotes or alternative pricing sources with reasonable levels of price transparency. Because many fixed income securities do not trade on a daily basis, pricing sources apply available market information through processes such as matrix pricing to calculate fair value. Such prices generally consider a variety of factors, including recent trades of the same and similar securities. In those cases, the items are classified within Level 2. For those fixed income investments where quotes were not available or cannot be reasonably corroborated, fair values are based on internal valuation models. Key inputs to the internal valuation models generally include maturity date, coupon and yield curves for asset-type and credit rating characteristics that closely match those characteristics of the specific investment securities being valued. Items valued using valuation models are classified according to the lowest level input or value driver that is significant to the valuation. Thus, an item may be classified in Level 3 even though there may be significant inputs that are readily observable. Longer (shorter) expected maturities or higher (lower) yields used in the valuation model will, in isolation, result in decreases (increases) in fair value. Generally, lower credit ratings or longer expected maturities will be accompanied by higher yields used to value a security. At March 31, 2019 , approximately 8% , 90% and 2% of the fixed income investment portfolio (excluding variable interest entity investments) was valued using dealer quotes, alternative pricing sources with reasonable levels of price transparency and internal valuation models, respectively. At December 31, 2018 , approximately 8% , 90% and 2% of the fixed income investment portfolio (excluding variable interest entity investments) was valued using dealer quotes, alternative pricing sources with reasonable levels of price transparency and internal valuation models, respectively. Ambac performs various review and validation procedures to quoted and modeled prices for fixed income securities, including price variance analyses, missing and static price reviews, overall valuation analysis by portfolio managers and finance managers and reviews associated with our ongoing impairment analysis. Unusual prices identified through these procedures will be evaluated further against alternative third party quotes (if available) and/or internally modeled prices, and the pricing source values will be challenged as necessary. Price challenges generally result in the use of the pricing source’s quote as originally provided or as revised by the source following their internal diligence process. A price challenge may result in a determination by either the pricing source or Ambac management that the pricing source cannot provide a reasonable value for a security or cannot adequately support a quote, in which case Ambac would resort to using either other quotes or internal models. Results of price challenges are reviewed by portfolio managers and finance managers. Information about the valuation inputs for fixed income securities classified as Level 3 is included below: Other asset-backed securities: These securities are a subordinated tranche of a resecuritization collateralized by Ambac-insured military housing bonds. The fair value of such securities classified as Level 3 was $71,855 and $72,071 at March 31, 2019 and December 31, 2018 , respectively. Fair value was calculated using a discounted cash flow approach with expected future cash flows discounted using a yield consistent with the security type and rating. Significant inputs for the valuation at March 31, 2019 and December 31, 2018 include the following weighted averages: March 31, 2019: a. Coupon rate: 5.97% b. Average Life: 16.11 years c. Yield: 12.00% December 31, 2018: a. Coupon rate: 5.97% b. Maturity: 16.29 years c. Yield: 12.00% Other Investments: Other investments primarily relate to investments in pooled investment funds. The fair value of pooled investment funds is determined using dealer quotes or alternative pricing sources when such investments have readily determinable fair values. When fair value is not readily determinable, pooled investment funds are valued using the net asset value (“NAV”) per share as a practical expedient as permitted under the Fair Value Measurement Topic of the ASC. Refer to Note 8. Investments for additional information about such investments in pooled funds that are reported at fair value using NAV as a practical expedient. Other investments also includes Ambac's equity interest in a non-consolidated VIE, which is carried under the equity method. Valuation of this equity interest is internally calculated using a discounted cash flow approach and is classified as Level 3. Derivative Instruments: Ambac’s derivative instruments primarily comprise interest rate swaps, credit default swaps and exchange traded futures contracts. Fair value is determined based upon market quotes from independent sources, when available. When independent quotes are not available, fair value is determined using valuation models. These valuation models require market-driven inputs, including contractual terms, credit spreads and ratings on underlying referenced obligations, yield curves and tax-exempt interest ratios. The valuation of certain interest rate as well as all credit derivative contracts also require the use of data inputs and assumptions that are determined by management and are not readily observable in the market. Under the Fair Value Measurement Topic of the ASC, Ambac is required to consider its own credit risk when measuring the fair value of derivatives and other liabilities. Additional factors considered in estimating the amount of any Ambac CVA on such contracts include collateral posting provisions, right of set-off with the counterparty, the period of time remaining on the derivative and the pricing of recent terminations. The fair value of credit derivative liabilities was reduced by $93 and $138 at March 31, 2019 and December 31, 2018 , respectively, as a result of incorporating an Ambac CVA into the valuation model for these contracts. Interest rate swap liabilities are collateralized and are not adjusted with an Ambac CVA at March 31, 2019 and December 31, 2018 . Interest rate swaps that are not centrally cleared are valued using vendor-developed models that incorporate interest rates and yield curves that are observable and regularly quoted. These models provide the net present value of the derivatives based on contractual terms and observable market data. Generally, the need for counterparty (or Ambac) CVAs on interest rate derivatives is mitigated by the existence of collateral posting agreements under which adequate collateral has been posted. Certain of these derivative contracts entered into with financial guarantee customers are not subject to collateral posting agreements. Counterparty credit risk related to such customer derivative assets is included in our fair value adjustments. Ambac's remaining credit derivatives ("CDS") are valued using an internal model that uses traditional financial guarantee CDS pricing to calculate the fair value of the derivative contract based on the reference obligation's current pricing, remaining life and credit rating and Ambac's own credit risk. The model calculates the difference between the present value of the projected fees receivable under the CDS and our estimate of the fees a financial guarantor of comparable credit quality would charge to provide the same protection at the balance sheet date. Unobservable inputs used include Ambac's internal reference obligation credit ratings and expected life, estimates of fees that would be charged to assume the credit derivative obligation and Ambac's CVA. Ambac is party to only two remaining credit derivatives with internal credit ratings of AA or better at March 31, 2019 . Ambac has not made any significant changes to its modeling techniques or related model inputs for the periods presented. Financial Guarantees: Fair value of net financial guarantees written represents our estimate of the cost to Ambac to completely transfer its insurance obligation to another market participant of comparable credit worthiness. In theory, this amount should be the same amount that another market participant of comparable credit worthiness would hypothetically charge in the market place, on a present value basis, to provide the same protection as of the balance sheet date. This fair value estimate of financial guarantees is presented on a net basis and includes direct and assumed contracts written, net of ceded reinsurance contracts. The fair value estimate of financial guarantees is computed by utilizing cash flows calculated at the policy level. For direct and assumed contracts, net cash flows for each policy includes future: (i) installment premium receipts, (ii) gross claim payments, and (iii) subrogation receipts. For ceded reinsurance contracts, net cash flows for each policy includes future: (i) installment ceded premium payments, (ii) ceding commission receipts, (iii) ceded claim receipts, and (iv) ceded subrogation payments. For each assumed or ceded reinsurance contract, the respective undiscounted cash flow components are aggregated to determine if we are in a net asset or net liability position. U.S. GAAP requires that the nonperformance risk of a financial liability be included in the estimation of fair value, which includes considering Ambac Assurance’s own credit risk. Accordingly, for each contract in a net liability position, we estimate the fair value using internally developed discount rates and market pricing that incorporate Ambac’s own credit risk and subsequently apply a profit margin. This profit margin represents what another market participant would require to assume the financial guarantee contracts. A profit margin was developed based on discussions with the third-party institutions with valuation expertise and discussions with industry participants. The discount rates used for contracts in a net liability position are derived from the rates implicit in the fair value of Ambac guaranteed securities with future cash flows that are highly dependent upon Ambac financial guarantee payments. For each contract in a net asset position, we estimate the fair value using a discount rate that is commensurate with a hypothetical buyer’s cost of capital. This methodology is based on management’s expectations of how a market participant would estimate net cash flows. We are aware of a number of factors that may cause such fair or exit value to differ, perhaps materially. For example, since no financial guarantor with Ambac Assurance’s credit quality is writing or otherwise obtaining financial guarantee business (e.g. reinsurance or novation of policies from other insurers) we do not have access to observable pricing data points. Long-term Debt: Long-term debt includes Ambac Assurance senior surplus notes and junior surplus notes and the Ambac Note and Tier 2 Notes issued in connection with the Rehabilitation Exit Transactions. The fair values of senior surplus notes, the Ambac Note and Tier 2 Notes are classified as Level 2. The fair value of junior surplus notes are classified as Level 3. Other Financial Assets and Liabilities: The fair values of Loans and Ambac’s equity interest in Ambac sponsored VIEs are estimated based upon internal valuation models and are classified as Level 3. Variable Interest Entity Assets and Liabilities: The financial assets and liabilities of VIEs consolidated under the Consolidation Topic of the ASC consist primarily of fixed income securities, loans, derivative and debt instruments and are generally carried at fair value. These consolidated VIEs are securitization entities which have liabilities and/or assets guaranteed by Ambac Assurance or Ambac UK. The fair values of VIE debt instruments are determined using the same methodologies used to value Ambac’s fixed income securities in its investment portfolio as described above. VIE debt fair value is based on market prices received from dealer quotes or alternative pricing sources with reasonable levels of price transparency. Such quotes are considered Level 2 and generally consider a variety of factors, including recent trades of the same and similar securities. For those VIE debt instruments where quotes were not available, the debt instrument fair values are considered Level 3 and are based on internal discounted cash flow models. Comparable to the sensitivities of investments in fixed income securities described above, longer (shorter) expected maturities or higher (lower) yields used in the valuation model will, in isolation, result in decreases (increases) in fair value liability measurement for VIE debt. VIE debt instruments considered Level 3 include fixed rate notes secured by various asset types, primarily European ABS. Information about the valuation inputs for the various VIE debt categories classified as Level 3 is as follows: European ABS transactions: The fair value of such obligations classified as Level 3 was $224,447 and $217,092 at March 31, 2019 and December 31, 2018 , respectively. Fair values were calculated by using a discounted cash flow approach. The discount rates used were based on the rates implied from the third party quoted values for comparable notes from the same securitization entity. Significant inputs for the valuation at March 31, 2019 and December 31, 2018 include the following weighted averages: March 31, 2019: a. Coupon rate: 2.20% b. Maturity: 18.68 years c. Yield: 3.05% December 31, 2018: a. Coupon rate: 2.20% b. Maturity: 18.93 years c. Yield: 3.18% VIE derivative asset and liability fair values are determined using valuation models. When specific derivative contractual terms are available and may be valued primarily by reference to interest rates, foreign exchange rates and yield curves that are observable and regularly quoted, the derivatives are valued using vendor-developed models. Other derivatives within the VIEs that include significant unobservable valuation inputs are valued using internally developed models. VIE derivative liability fair value balances at March 31, 2019 and December 31, 2018 were developed using vendor-developed models and do not use significant unobservable inputs. The fair value of VIE assets are obtained from market quotes when available. Typically VIE asset fair values are not readily available from market quotes and are estimated internally. The consolidated VIEs are securitization entities in which net cash flows from assets and derivatives (after adjusting for financial guarantor cash flows and other expenses) will be paid out to note holders or equity interests. Internal valuations of VIE assets (fixed income securities or loans), therefore, are generally derived from the fair value of notes and derivatives, as described above, adjusted for the fair value of cash flows from Ambac’s financial guarantee. The fair value of financial guarantee cash flows include: (i) estimated future premiums discounted at a rate consistent with that implicit in the fair value of the VIE’s liabilities and (ii) internal estimates of future loss payments by Ambac discounted at a rate that includes Ambac’s own credit risk. Estimated future premium payments to be paid by the VIEs were discounted at a weighted average rate of 3.0% and 3.1% at March 31, 2019 and December 31, 2018 , respectively. The value of future loss payments to be paid by Ambac to the VIEs was adjusted to include an Ambac CVA appropriate for the term of expected Ambac claim payments. Additional Fair Value Information for Financial Assets and Liabilities Accounted for at Fair Value: The following tables present the changes in the Level 3 fair value category for the periods presented in 2019 and 2018 . Ambac classifies financial instruments in Level 3 of the fair value hierarchy when there is reliance on at least one significant unobservable input to the valuation model. In addition to these unobservable inputs, the valuation models for Level 3 financial instruments typically also rely on a number of inputs that are readily observable either directly or indirectly. Thus, the gains and losses presented below include changes in the fair value related to both observable and unobservable inputs. Level 3 - Financial Assets and Liabilities Accounted for at Fair Value VIE Assets and Liabilities Investments Other (1) Derivatives Investments Loans Long-term Total Three Months Ended March 31, 2019: Balance, beginning of period $ 72,071 $ 4,516 $ 46,001 $ 2,737,286 $ 4,287,664 $ (217,092 ) $ 6,930,446 Total gains/(losses) realized and unrealized: Included in earnings 383 (368 ) 8,187 66,919 88,281 (3,053 ) 160,349 Included in other comprehensive income (270 ) — — 54,199 85,061 (4,302 ) 134,688 Purchases — — — — — — — Issuances — — — — — — — Sales — — — — — — — Settlements (329 ) — (1,154 ) — (85,245 ) — (86,728 ) Balance, end of period $ 71,855 $ 4,148 $ 53,034 $ 2,858,404 $ 4,375,761 $ (224,447 ) $ 7,138,755 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ (368 ) $ 8,096 $ 66,919 $ 88,281 $ (3,053 ) $ 159,875 Three Months Ended March 31, 2018: Balance, beginning of period $ 808,557 $ 5,979 $ 60,808 $ 2,914,145 $ 11,529,384 $ (2,757,688 ) $ 12,561,185 Total gains/(losses) realized and unrealized: Included in earnings 35,184 (358 ) (9,375 ) (69,026 ) (202,588 ) 159,985 (86,178 ) Included in other comprehensive income (52,508 ) — — 110,644 423,892 (103,877 ) 378,151 Purchases — — — — — — — Issuances — — — — — — — Sales — — — — — — — Settlements (713,603 ) — (1,525 ) — (192,357 ) 5,064 (902,421 ) Balance, end of period $ 77,630 $ 5,621 $ 49,908 $ 2,955,763 $ 11,558,331 $ (2,696,516 ) $ 11,950,737 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ (358 ) $ (9,481 ) $ (69,026 ) $ (202,588 ) $ 159,985 $ (121,468 ) (1) Other assets carried at fair value and classified as Level 3 relate to an equity interest in an Ambac sponsored VIE. The tables below provide roll-forward information by class of investments and derivatives measured using significant unobservable inputs. Level 3 - Investments by Class: Three Months Ended March 31, 2019 Three Months Ended March 31, 2018 Other Asset Non-Agency RMBS Total Other Asset Non-Agency RMBS Total Balance, beginning of period $ 72,071 $ — $ 72,071 $ 72,540 $ 736,017 $ 808,557 Total gains/(losses) realized and unrealized: Included in earnings 383 — 383 457 34,727 35,184 Included in other comprehensive income (270 ) — (270 ) (370 ) (52,138 ) (52,508 ) Purchases — — — — — — Issuances — — — — — — Sales — — — — — — Settlements (329 ) — (329 ) (306 ) (713,297 ) (713,603 ) Balance, end of period $ 71,855 $ — $ 71,855 $ 72,321 $ 5,309 $ 77,630 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ — $ — $ — $ — $ — Level 3 - Derivatives by Class: Three Months Ended March 31, 2019 Three Months Ended March 31, 2018 Interest Credit Total Interest Credit Total Balance, beginning of period $ 47,460 $ (1,459 ) $ 46,001 $ 61,374 $ (566 ) $ 60,808 Total gains/(losses) realized and unrealized: Included in earnings 7,714 473 8,187 (9,029 ) (346 ) (9,375 ) Included in other comprehensive income — — — — — — Purchases — — — — — — Issuances — — — — — — Sales — — — — — — Settlements (1,063 ) (91 ) (1,154 ) (1,419 ) (106 ) (1,525 ) Balance, end of period $ 54,111 $ (1,077 ) $ 53,034 $ 50,926 $ (1,018 ) $ 49,908 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ 7,714 $ 382 $ 8,096 $ (9,029 ) $ (452 ) $ (9,481 ) Invested assets and VIE long-term debt are transferred into Level 3 when internal valuation models that include significant unobservable inputs are used to estimate fair value. All such securities that have internally modeled fair values have been classified as Level 3. Derivative instruments are transferred into Level 3 when the use of unobservable inputs becomes significant to the overall valuation. There were no transfers between Levels 1, 2 and 3 for the periods presented. All transfers between fair value hierarchy Levels 1, 2 and 3 are recognized at the beginning of each accounting period. Gains and losses (realized and unrealized) relating to Level 3 assets and liabilities included in earnings for the affected periods are reported as follows: Net Net Gains (Losses) on Derivative Contracts Income Other Three Months Ended March 31, 2019: Total gains or losses included in earnings for the period $ 383 $ 8,187 $ 152,147 $ (368 ) Changes in unrealized gains or losses relating to the assets and liabilities still held at the reporting date — 8,096 152,147 (368 ) Three Months Ended March 31, 2018: Total gains or losses included in earnings for the period $ 35,184 $ (9,375 ) $ (111,629 ) $ (358 ) Changes in unrealized gains or losses relating to the assets and liabilities still held at the reporting date — (9,481 ) (111,629 ) (358 )

Investments

Investments	3 Months Ended
	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Investments	8. INVESTMENTS Ambac’s non-VIE invested assets are primarily comprised of fixed income securities classified as available-for-sale and equity interests in pooled investment funds. Such equity interests in the form of common stock or in-substance common stock are classified as trading securities and are reported within Other investments on the Consolidated Balance Sheets. Other investments also include Ambac's equity interest in an unconsolidated trust created in connection with its sale of Segregated Account junior surplus notes on August 28, 2014. Fixed Income Securities: The amortized cost and estimated fair value of available-for-sale investments, excluding VIE investments, at March 31, 2019 and December 31, 2018 were as follows: Amortized Gross Gross Estimated Non-credit (1) March 31, 2019: Fixed income securities: Municipal obligations $ 406,315 $ 20,356 $ 1,787 $ 424,884 $ — Corporate obligations (2) 1,223,640 14,312 4,408 1,233,544 — Foreign obligations 30,719 599 26 31,292 — U.S. government obligations 99,808 1,185 564 100,429 Residential mortgage-backed securities 216,904 39,504 123 256,285 123 Collateralized debt obligations 133,736 28 787 132,977 — Other asset-backed securities 369,410 71,623 633 440,400 — 2,480,532 147,607 8,328 2,619,811 123 Short-term 908,209 43 17 908,235 — 3,388,741 147,650 8,345 3,528,046 123 Fixed income securities pledged as collateral: U.S. government obligations 83,901 — — 83,901 — Total fixed income securities pledged as collateral 83,901 — — 83,901 — Total available-for-sale investments $ 3,472,642 $ 147,650 $ 8,345 $ 3,611,947 $ 123 December 31, 2018: Fixed income securities: Municipal obligations $ 882,631 $ 14,364 $ 17,076 $ 879,919 $ 5 Corporate obligations (2) 1,288,882 6,444 17,204 1,278,122 — Foreign obligations 30,496 399 61 30,834 — U.S. government obligations 93,636 1,371 613 94,394 — Residential mortgage-backed securities 221,825 37,575 793 258,607 27 Collateralized debt obligations 133,075 8 1,727 131,356 — Other asset-backed securities 370,199 72,868 624 442,443 — 3,020,744 133,029 38,098 3,115,675 32 Short-term 430,405 23 97 430,331 — Total available-for-sale investments $ 3,451,149 $ 133,052 $ 38,195 $ 3,546,006 $ 32 (1) Represents the amount of non-credit other-than-temporary impairment losses remaining in accumulated other comprehensive income on securities that also had a credit impairment. These losses are included in gross unrealized losses as of March 31, 2019 and December 31, 2018 . (2) Includes Ambac's holdings of the secured notes issued by Ambac LSNI in connection with the Rehabilitation Exit Transactions. The amortized cost and estimated fair value of available-for-sale investments, excluding VIE investments, at March 31, 2019 , by contractual maturity, were as follows: Amortized Estimated Due in one year or less $ 1,076,361 $ 1,076,545 Due after one year through five years 999,235 1,006,812 Due after five years through ten years 316,441 323,494 Due after ten years 360,555 375,434 2,752,592 2,782,285 Residential mortgage-backed securities 216,904 256,285 Collateralized debt obligations 133,736 132,977 Other asset-backed securities 369,410 440,400 Total $ 3,472,642 $ 3,611,947 Expected maturities will differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without call or prepayment penalties. Unrealized Losses on Fixed Income Securities: The following table shows gross unrealized losses and fair values of Ambac’s available-for-sale investments, excluding VIE investments, aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position, at March 31, 2019 and December 31, 2018 : Less Than 12 Months 12 Months or More Total Fair Value Gross Fair Value Gross Fair Value Gross March 31, 2019: Fixed income securities: Municipal obligations $ 20,997 $ 951 $ 25,865 $ 836 $ 46,862 $ 1,787 Corporate obligations 39,639 550 256,406 3,858 296,045 4,408 Foreign obligations 401 4 3,980 22 4,381 26 U.S. government obligations 12,793 360 53,861 204 66,654 564 Residential mortgage-backed securities 4,375 122 243 1 4,618 123 Collateralized debt obligations 101,939 787 — — 101,939 787 Other asset-backed securities 3,840 9 83,645 624 87,485 633 183,984 2,783 424,000 5,545 607,984 8,328 Short-term 99,429 17 — — 99,429 17 Total temporarily impaired securities $ 283,413 $ 2,800 $ 424,000 $ 5,545 $ 707,413 $ 8,345 Less Than 12 Months 12 Months or More Total Fair Value Gross Fair Value Gross Fair Value Gross December 31, 2018: Fixed income securities: Municipal obligations $ 537,904 $ 15,878 $ 28,533 $ 1,198 $ 566,437 $ 17,076 Corporate obligations 306,506 8,634 190,273 8,570 496,779 17,204 Foreign obligations 1,161 1 5,163 60 6,324 61 U.S. government obligations 5,643 135 58,495 478 64,138 613 Residential mortgage-backed securities 34,852 793 — — 34,852 793 Collateralized debt obligations 123,848 1,727 — — 123,848 1,727 Other asset-backed securities 13,813 33 77,479 591 91,292 624 1,023,727 27,201 359,943 10,897 1,383,670 38,098 Short-term 115,374 97 — — 115,374 97 Total temporarily impaired securities $ 1,139,101 $ 27,298 $ 359,943 $ 10,897 $ 1,499,044 $ 38,195 Management has determined that the unrealized losses reflected in the tables above are temporary in nature as of March 31, 2019 and December 31, 2018 based upon (i) no unexpected principal and interest payment defaults on these securities; (ii) analysis of the creditworthiness of the issuer and financial guarantor, as applicable, and analysis of projected defaults on the underlying collateral; (iii) management has no intent to sell these investments in debt securities; and (iv) it is not more likely than not that Ambac will be required to sell these debt securities before the anticipated recovery of its amortized cost basis. The assessment under (iv) is based on a comparison of future available liquidity from the investment portfolio against the projected net cash outflow from operating activities and debt service. For purposes of this assessment, available liquidity from the investment portfolio is comprised of the fair value of securities for which management has asserted its intent to sell, the fair value of other securities that are available for sale and in an unrealized gain position, trading securities plus the scheduled maturities and interest payments from the remaining securities in the portfolio. To the extent that securities that management intends to sell are in an unrealized loss position, they would have already been considered other-than-temporarily impaired with the amortized cost written down to fair value. Because the above-described assessment indicates that future available liquidity exceeds projected net cash outflow, it is not more likely than not that we would be required to sell securities in an unrealized loss position before the recovery of their amortized cost basis. In the liquidity assessment described above, principal payments on securities pledged as collateral are not considered to be available for other liquidity needs until the collateralized positions are projected to be settled. As of March 31, 2019 , for securities that have indications of possible other-than-temporary impairment but which management does not intend to sell and will not more likely than not be required to sell, management compared the present value of cash flows expected to be collected to the amortized cost basis of the securities to assess whether the amortized cost will be recovered. Cash flows were discounted at the effective interest rate implicit in the security at the date of acquisition (or Fresh Start Reporting Date of April 30, 2013 for securities purchased prior to that date) or for debt securities that are beneficial interests in securitized financial assets, at a rate equal to the current yield used to accrete the beneficial interest. For floating rate securities, future cash flows and the discount rate used were both adjusted to reflect changes in the index rate applicable to each security as of the evaluation date. Of the securities that were in a gross unrealized loss position at March 31, 2019 , $121,037 of the total fair value and $2,086 of the unrealized loss related to below investment grade and non-rated securities. The remainder of gross unrealized losses as of March 31, 2019 , are primarily on investment grade fixed-rate securities purchased during periods of lower interest rates. Management believes that the timely receipt of all principal and interest on these positions is probable . Ambac’s assessment about whether a decline in value is other-than-temporary reflects management’s current judgment regarding facts and circumstances specific to a security and the factors noted above. If that judgment changes, Ambac may ultimately record a charge for other-than-temporary impairment in future periods. Realized Gains and Losses and Other-Than-Temporary Impairments: The following table details amounts included in net realized gains (losses) and other-than-temporary impairments included in earnings for the affected periods: Three Months Ended March 31, 2019 2018 Gross realized gains on securities $ 24,219 $ 11,054 Gross realized losses on securities (4,288 ) (1,388 ) Net foreign exchange (losses) gains (2,698 ) (4,804 ) Net realized gains (losses) $ 17,233 $ 4,862 Net other-than-temporary impairments (1) $ (29 ) $ (299 ) (1) Other-than-temporary impairments exclude impairment amounts recorded in other comprehensive income under ASC Paragraph 320-10-65-1, which comprise non-credit related amounts on securities that are credit impaired but which management does not intend to sell and it is not more likely than not that the company will be required to sell before recovery of the amortized cost basis. Future changes in our estimated liquidity needs could result in a determination that Ambac no longer has the ability to hold securities that are in an unrealized loss position, which could also result in additional other-than-temporary impairment charges. The following table presents a roll-forward of Ambac’s cumulative credit losses on debt securities held as of March 31, 2019 and 2018 for which a portion of an other-than-temporary impairment was recognized in other comprehensive income: Three Months Ended March 31, 2019 2018 Balance, beginning of period $ 12,454 $ 67,085 Additions for credit impairments recognized on: Securities not previously impaired — 226 Securities previously impaired — 64 Reductions for credit impairments previously recognized on: Securities that matured or were sold during the period (49 ) (23,742 ) Balance, end of period $ 12,405 $ 43,633 Counterparty Collateral, Deposits with Regulators and Other Restrictions: Ambac routinely pledges and receives collateral related to certain transactions. Cash, cash equivalents and securities held directly in Ambac’s investment portfolio with a fair value of $111,014 and $102,904 at March 31, 2019 and December 31, 2018 , respectively, were pledged to derivative counterparties. Ambac’s derivative counterparties have the right to re-pledge the investment securities and as such, these pledged securities are separately classified on the Consolidated Balance Sheets as “Fixed income securities pledged as collateral, at fair value”. There was no cash or securities received from other counterparties that were re-pledged by Ambac. Securities carried at $6,046 and $5,975 at March 31, 2019 and December 31, 2018 , respectively, were deposited with governmental authorities or designated custodian banks as required by laws affecting insurance companies. Invested assets carried at $1,403 at March 31, 2019 were deposited as security in connection with a letter of credit issued for an office lease. Securities with a fair value of $207,759 and $209,983 at March 31, 2019 and December 31, 2018, respectively, were pledged as collateral and as sources of funding to repay the Secured Notes issued by Ambac LSNI. The securities may not be transferred or repledged by Ambac LSNI. Collateral may be sold to fund redemptions of the Secured Notes. Ambac Assurance also pledged for the benefit of the holders of Secured Notes (other than Ambac Assurance) the proceeds of interest payments and partial redemptions of the Secured Notes held by Ambac Assurance. The amount of such proceeds held by Ambac Assurance was $0 and $19,405 at March 31, 2019 and December 31, 2018, respectively, and is included in Restricted cash on the Consolidated Balance Sheet. Guaranteed Securities: Ambac’s fixed income portfolio includes securities covered by guarantees issued by Ambac Assurance and other financial guarantors (“insured securities”). The published rating agency ratings on these securities reflect the higher of the financial strength rating of the financial guarantor or the rating of the underlying issuer. Rating agencies do not always publish separate underlying ratings (those ratings excluding the insurance by the financial guarantor). In the event these underlying ratings are not available from the rating agencies, Ambac will assign an internal rating. The following table represents the fair value, including the value of the financial guarantee, and weighted-average underlying rating, excluding the financial guarantee, of the insured securities at March 31, 2019 and December 31, 2018 , respectively: Municipal Corporate (3) Mortgage Total Weighted (1) March 31, 2019: Ambac Assurance Corporation (2) $ 166,770 $ 583,550 $ 599,512 $ 1,349,832 CCC- National Public Finance Guarantee Corporation 13,839 — — 13,839 BBB- Total $ 180,609 $ 583,550 $ 599,512 $ 1,363,671 CCC- December 31, 2018: Ambac Assurance Corporation (2) $ 833,241 $ 656,473 $ 599,185 $ 2,088,899 CC National Public Finance Guarantee Corporation 15,600 — — 15,600 BBB- Total $ 848,841 $ 656,473 $ 599,185 $ 2,104,499 CC (1) Ratings are based on the lower of Standard & Poor’s or Moody’s rating. If unavailable, Ambac’s internal rating is used. (2) Includes corporate obligations and asset-backed securities with a fair value of $144,530 and $144,672 at March 31, 2019 and December 31, 2018 , respectively, insured by Ambac UK. (3) Represents Ambac's holdings of secured notes issued by Ambac LSNI in connection with the Rehabilitation Exit Transactions. These secured notes are insured by Ambac Assurance. Equity Interests: Ambac's investment portfolio includes equity interests in various pooled investment funds, which are classified as trading. The fair value and additional information about such investments in pooled funds, by investment type, is summarized in the table below. Except as noted in the table, fair value as reported is determined using NAV per share as a practical expedient. There are no unfunded commitments applicable to any of these investments for the periods disclosed. Fair Value Class of Funds March 31, December 31, Redemption Frequency Redemption Notice Period Real estate properties (1) $ 16,150 $ 16,123 quarterly 10 business days Interest rate products (2) (6) 207,704 177,357 daily, weekly or monthly 0 - 30 days Illiquid investments (3) 85,323 84,297 quarterly 180 days Insurance-linked investments (4) 28,746 29,318 quarterly 90-120 days Equity market investments (5) (6) 49,122 43,954 daily 0 days Total equity investments in pooled funds $ 387,045 $ 351,049 (1) Investments consist of UK property to generate income and capital growth. (2) This class of funds includes investments in a range of instruments including leveraged loans, CLOs, asset-backed securities and floating rate notes to generate income and capital appreciation. Funds with less frequent redemption periods limit redemptions to as little as 15% per period. Funds with a same day redemption notice period are redeemable only weekly, while funds that may be redeemed any business day have notice periods of 15-30 days. (3) This class seeks to obtain high long-term total return through investments with low liquidity and defined term, resulting in expected capital distributions to subscribers in the second half of 2019 and 2023. (4) This class aims to provide returns from the insurance and reinsurance markets through investments in catastrophe bonds, life insurance and other insurance linked investments. Redemption periods are quarterly, subject to 90-day notice for January/July redemption dates and 120-day notice for April/October redemption dates. (5) Investments represent a diversified exposure to global equity market returns through holdings of various regional market index funds. (6) Interest rate products include $53,135 at March 31, 2019 and $27,154 at December 31, 2018 and equity market investments include $49,122 at March 31, 2019 and $43,954 at December 31, 2018 that have readily determinable fair values priced through pricing vendors. Ambac also holds an equity interest in an unconsolidated trust created in connection with the 2014 sale of Segregated Account junior surplus notes, which is accounted for under the equity method. Investment Income: Net investment income was comprised of the following for the affected periods: Three Months Ended March 31, 2019 2018 Fixed income securities $ 43,666 $ 109,351 Short-term investments 4,113 2,840 Loans 184 187 Investment expense (1,411 ) (1,827 ) Securities available-for-sale and short-term 46,552 110,551 Other investments 8,290 (311 ) Total net investment income $ 54,842 $ 110,240 Net investment income from Other investments primarily represents changes in fair value on securities classified as trading or under the fair value option plus income from the above noted equity interests in an unconsolidated trust. The portion of net unrealized gains (losses) related to trading securities still held at the end of each period is as follows: Three Months Ended March 31, 2019 2018 Net gains (losses) recognized during the period on trading securities $ 6,947 $ (1,579 ) Less: net gains (losses) recognized during the reporting period on trading securities sold during the period 631 1,933 Unrealized gains (losses) recognized during the reporting period on trading securities still held at the reporting date $ 6,316 $ (3,512 )

Derivative Instruments

Derivative Instruments	3 Months Ended
	Mar. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	9. DERIVATIVE INSTRUMENTS The following tables summarize the gross fair values of individual derivative instruments and the impact of legal rights of offset as reported in the Consolidated Balance Sheets as of March 31, 2019 and December 31, 2018 : Gross Gross Net Amounts Gross Amount Net Amount March 31, 2019: Derivative Assets: Interest rate swaps $ 73,607 $ 389 $ 73,218 $ — $ 73,218 Futures contracts 3,182 — 3,182 — 3,182 Total non-VIE derivative assets $ 76,789 $ 389 $ 76,400 $ — $ 76,400 Derivative Liabilities: Credit derivatives $ 1,077 $ — $ 1,077 $ — $ 1,077 Interest rate swaps 85,846 389 85,457 73,510 11,947 Total non-VIE derivative liabilities $ 86,923 $ 389 $ 86,534 $ 73,510 $ 13,024 Variable Interest Entities Derivative Assets: Currency swaps $ 59,228 $ — $ 59,228 $ — $ 59,228 Total VIE derivative assets $ 59,228 $ — $ 59,228 $ — $ 59,228 Variable Interest Entities Derivative Liabilities: Interest rate swaps $ 1,781,903 $ — $ 1,781,903 $ — $ 1,781,903 Total VIE derivative liabilities $ 1,781,903 $ — $ 1,781,903 $ — $ 1,781,903 December 31, 2018: Derivative Assets: Interest rate swaps $ 59,768 $ 300 $ 59,468 $ — $ 59,468 Total non-VIE derivative assets $ 59,768 $ 300 $ 59,468 $ — $ 59,468 Derivative Liabilities: Credit derivatives $ 1,459 $ — $ 1,459 $ — $ 1,459 Interest rate swaps 72,161 300 71,861 67,126 4,735 Futures contracts 3,379 — 3,379 3,379 — Total non-VIE derivative liabilities $ 76,999 $ 300 $ 76,699 $ 70,505 $ 6,194 Variable Interest Entities Derivative Assets: Currency swaps $ 66,302 $ — $ 66,302 $ — $ 66,302 Total VIE derivative assets $ 66,302 $ — $ 66,302 $ — $ 66,302 Variable Interest Entities Derivative Liabilities: Interest rate swaps $ 1,712,062 $ — $ 1,712,062 $ — $ 1,712,062 Total VIE derivative liabilities $ 1,712,062 $ — $ 1,712,062 $ — $ 1,712,062 Amounts recognized for the right to reclaim cash collateral or the obligation to return cash collateral are not offset against fair value amounts recognized for derivative instruments on the Consolidated Balance Sheets. The amounts representing the right to reclaim cash collateral and posted margin, recorded in “Other assets” were $27,113 and $102,904 as of March 31, 2019 and December 31, 2018 , respectively. There were no amounts held representing an obligation to return cash collateral as of March 31, 2019 and December 31, 2018 . The following tables summarize the location and amount of gains and losses of derivative contracts in the Consolidated Statements of Total Comprehensive Income (Loss) for the three months ended March 31, 2019 and 2018 : Location of Gain or (Loss) Recognized in Consolidated Statements of Total Comprehensive Income (Loss) Amount of Gain or (Loss) Recognized in Consolidated Statement of Total Comprehensive Income (Loss) Three Months Ended March 31, 2019 2018 Non-VIE derivatives: Credit derivatives Net gains (losses) on derivative contracts $ 473 $ (346 ) Interest rate swaps Net gains (losses) on derivative contracts (2,732 ) 4,543 Futures contracts Net gains (losses) on derivative contracts (13,900 ) 20,994 Total Non-VIE derivatives $ (16,159 ) $ 25,191 Variable Interest Entities: Currency swaps Income (loss) on variable interest entities $ (7,074 ) $ (8,617 ) Interest rate swaps Income (loss) on variable interest entities (69,841 ) 49,809 Total Variable Interest Entities (76,915 ) 41,192 Total derivative contracts $ (92,601 ) $ 66,037 Credit Derivatives: Credit derivatives, which are privately negotiated contracts, provide the counterparty with credit protection against the occurrence of a specific event such as a payment default or bankruptcy relating to an underlying obligation. Credit derivatives issued are insured by Ambac Assurance. None of the outstanding credit derivative transactions at March 31, 2019 , include ratings based collateral-posting triggers or otherwise require Ambac to post collateral regardless of Ambac’s ratings or the size of the mark to market exposure to Ambac. The portfolio of our credit derivatives were written on a “pay-as-you-go” basis. Similar to an insurance policy execution, pay-as-you-go provides that Ambac pays interest shortfalls on the referenced transaction as they are incurred on each scheduled payment date, but only pays principal shortfalls upon the earlier of (i) the date on which the assets designated to fund the referenced obligation have been disposed of and (ii) the legal final maturity date of the referenced obligation. Ambac maintains internal credit ratings on its guaranteed obligations, including credit derivative contracts, solely to indicate management’s view of the underlying credit quality of the guaranteed obligations. The gross principal notional outstanding for CDS contracts was $282,587 and $295,342 as of March 31, 2019 and December 31, 2018 , respectively, all of which had internal Ambac ratings of AA in both periods. Interest Rate Derivatives: Ambac, through its subsidiary Ambac Financial Services (“AFS”), uses interest rate swaps and US Treasury futures contracts to provide an economic hedge against the effects of rising interest rates elsewhere in the Company, including on Ambac’s financial guarantee exposures. Additionally, AFS provided interest rate swaps to states, municipalities and their authorities, asset-backed issuers and other entities in connection with their financings. As of March 31, 2019 and December 31, 2018 the notional amounts of AFS’s derivatives are as follows: Notional Type of Derivative March 31, December 31, Interest rate swaps—receive-fixed/pay-variable $ 341,909 $ 493,368 Interest rate swaps—pay-fixed/receive-variable 1,269,180 1,121,532 US Treasury futures contracts—short 1,575,000 1,760,000 Derivatives of Consolidated Variable Interest Entities Certain VIEs consolidated under the Consolidation Topic of the ASC entered into derivative contracts to meet specified purposes within the securitization structure. The notional for VIE derivatives outstanding as of March 31, 2019 and December 31, 2018 are as follows: Notional Type of VIE Derivative March 31, December 31, Interest rate swaps—receive-fixed/pay-variable $ 1,427,299 $ 1,399,532 Interest rate swaps—pay-fixed/receive-variable 1,189,113 1,176,748 Currency swaps 344,714 344,992 Credit derivatives 10,457 10,254 Contingent Features in Derivatives Related to Ambac Credit Risk Ambac’s over-the-counter interest rate swaps are centrally cleared when eligible. Certain interest rate swaps remain with professional swap-dealer counterparties and direct customer counterparties. These non-cleared swaps are generally executed under standardized derivative documents including collateral support and master netting agreements. Under these agreements, Ambac is required to post collateral in the event net unrealized losses exceed predetermined threshold levels. Additionally, given that Ambac Assurance is no longer rated by an independent rating agency, counterparties have the right to terminate the swap positions. As of March 31, 2019 and December 31, 2018 , the net liability fair value of derivative instruments with contingent features linked to Ambac’s own credit risk was $74,725 and $67,071 , respectively, related to which Ambac had posted cash and securities as collateral with a fair value of $101,631 and $92,657 , respectively. All such ratings-based contingent features have been triggered as requiring maximum collateral levels to be posted by Ambac while preserving counterparties’ rights to terminate the contracts. Assuming all such contracts terminated on March 31, 2019

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	10. INCOME TAXES Ambac files a consolidated Federal income tax return with its subsidiaries. Ambac and its subsidiaries also file separate or combined income tax returns in various states, local and foreign jurisdictions. The following are the major jurisdictions in which Ambac and its subsidiaries operate and the earliest tax years subject to examination: Jurisdiction Tax Year United States 2010 New York State 2013 New York City 2014 United Kingdom 2015 Italy 2014 As of March 31, 2019 Ambac estimates it had U.S. federal ordinary net loss carryforwards totaling approximately $3,503,900 , which, if not utilized, will begin expiring in 2029 , and will fully expire in 2032 , with the exception of the tax loss generated during the three months ended March 31, 2019 of $80,197 , which if Ambac remains in a loss position at year end 2019, will expire in 2040. In accordance with the Income Tax Topic of the ASC, a valuation allowance is recognized if, based on the weight of available evidence, it is more-likely-than-not that some, or all, of the deferred tax asset will not be realized. As a result of the risks and uncertainties associated with future operating results, management believes it is more likely than not that the Company will not generate sufficient U.S. federal, state and/or local taxable income to recover the deferred tax operating assets and therefore maintains a full valuation allowance. Consolidated Pretax Income (Loss) U.S. and foreign components of pre-tax income (loss) were as follows: Three Months Ended March 31, 2019 2018 U.S. $ (63,404 ) $ 297,741 Foreign 22,194 10,568 Total $ (41,210 ) $ 308,309 Provision (Benefit) for Income Taxes The components of the provision for income taxes were as follows: Three Months Ended March 31, 2019 2018 Current taxes U. S. federal $ — $ — U.S. state and local (3,526 ) 1,037 Foreign 6,466 (39 ) Current taxes 2,940 998 Deferred taxes Foreign (949 ) 1,607 Deferred taxes (949 ) 1,607 Provision for income taxes $ 1,991 $ 2,605 NOL Usage Pursuant to the intercompany tax sharing agreement, to the extent Ambac Assurance generates taxable income after September 30, 2011, which is offset with "Allocated NOLs" of $3,650,000 , it is obligated to make payments (“Tolling Payments”), subject to certain credits, to Ambac in accordance with the following NOL usage table, where the “Applicable Percentage” is applied to the aggregate amount of federal income tax liability that would have been paid if the Allocated NOLs were not available. Pursuant to the Closing Agreement between Ambac and the Internal Revenue Service ("IRS"), the IRS will receive 12.5% of Tier C and 17.5% of Tier D payments, if made. NOL Usage Table NOL Usage Tier Allocated NOLs Applicable Percentage A The first $479,000 15% B The next $1,057,000 after Tier A 40% C The next $1,057,000 after Tier B 10% D The next $1,057,000 after Tier C 15% As a result of positive taxable income at Ambac Assurance in 2017 and 2018, Ambac has accrued approximately $44,381 in tax tolling payments. In May 2018, Ambac executed a waiver under the intercompany tax sharing agreement pursuant to which Ambac Assurance was relieved of the requirement to make the 2017 payment of approximately $27,868 by June 1, 2018. Ambac has also agreed to continue to defer the tolling payment for the 2017 use of Allocated NOLs by Ambac Assurance until such time as OCI consents to the payment. Ambac has accrued $13,885 of tolling payments based on Allocated NOLs used by Ambac Assurance in 2018, which are required to be paid by May 31, 2019. Ambac's tax positions are subject to review by the OCI, which may lead to the adoption of positions that reduce the amount of tolling payments otherwise available to Ambac. For the three months ended March 31, 2019 , Ambac Assurance generated a taxable loss of $89,326 , therefore no additional Tolling Payments have been accrued. Ambac's tax positions are subject to review by the OCI, which may lead to the adoption of positions that reduce the amount of tolling payments otherwise available to Ambac. As of March 31, 2019 , the remaining balance of the $3,650,000 NOL allocated to Ambac Assurance was $2,252,447 . As of March 31, 2019 Ambac's NOL was $1,251,453

Leases (Notes)

Leases (Notes)	3 Months Ended
	Mar. 31, 2019
Leases of Lessee Disclosure [Text Block]	11. LEASES Ambac adopted the New Lease Standard as further described in Note 2, Basis of Presentation and Significant Accounting Policies . A contract contains a lease if it conveys the right to control the use of identified property, plant, or equipment for a period of time in exchange for consideration. Ambac's evaluation of whether certain contracts contain leases may require judgment regarding what party controls the asset and whether the asset is physically distinct. The New Lease Standard provides practical expedients that affect a reporting entity’s ongoing accounting. For contracts where Ambac is the lessee, we have elected the short-term lease recognition exemption for all leases that qualify. For those leases that qualify for that exemption, we will not recognize ROU assets or lease liabilities. For all contracts where Ambac is the lessee and lessor we have also elected the practical expedient to not separate lease and non-lease components. Lessee information: Ambac is the lessee in operating leases of corporate offices, data centers and equipment. Our leases, in effect at March 31, 2019, have remaining lease terms ranging from 1 to 11 years. One data center lease has an automatic renewal of one-year unless either party elects to terminate by providing 120 days notice prior to the renewal. This renewal feature is not recognized in the lease liability or right-of-use asset as it is not reasonably certain we will elect to renew. No other leases contain extension or termination provisions. Lease costs are included in operating expenses on the consolidated statement of comprehensive income. The components of lease costs, net of sub-lessor income, is as follows: Three Months Ended March 31, 2019 Operating lease cost $ 1,526 Variable Lease Cost 179 Sublease income (65 ) Total lease cost $ 1,640 Ambac is required to make variable lease payments under certain leases which primarily related to variable costs of the lessor. Such costs include taxes, insurance, maintenance and electricity. Supplemental information related to leases is as follows: Three Months Ended March 31, 2019 Cash paid for amounts included in the measurement of operating lease liabilities $ 1,543 Right-of-use assets obtained in exchange for operating lease liabilities (non-cash) 14,360 Supplemental balance sheet information related to leases is as follows: March 31, 2019 Operating leases: Operating lease right of use assets $ 13,100 Operating lease liabilities 13,545 Weighted average remaining lease term: Operating leases 8.7 years Weighted average discount rate: Operating leases 8.0 % Operating lease right of use assets and operating lease liabilities are included in Other assets and Other liabilities, respectively, on the consolidated balance sheet. The discount rate used to initially measure the right of use assets and lease liabilities was based on Ambac's estimated secured borrowing rate. The Ambac Note, more fully described in Note 1. Background and Business Description in the Consolidated Financial Statements included Part II, Item 8 in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 , was a significant data point in estimating this rate. Future undiscounted lease payments to be made are as follows: As of March 31, Operating Leases 2019 (nine months) $ 3,580 2020 1,781 2021 1,436 2022 1,436 2023 1,436 Thereafter 9,294 Total lease payments 18,963 Less: imputed interest (5,418 ) Total $ 13,545 Excluded from the above is an operating lease, relating to office space, that has not commenced as of March 31, 2019 with total lease payments of $24,379 and imputed interest of $9,440 . This lease commenced in April 2019 with a lease term of 10.8 years. Lessor information: Ambac is the lessor in one operating sublease of corporate office space which has a remaining term of 10.8 years . There are no extension or termination provisions. Future undiscounted lease payments to be received are as follows: As of March 31, Operating Leases 2019 (nine months) $ 172 2020 1,038 2021 1,051 2022 1,076 2023 1,126 Thereafter 7,571 Total lease receipts $ 12,034
Leases of Lessee Disclosure [Text Block]	Future undiscounted lease payments to be received are as follows: As of March 31, Operating Leases 2019 (nine months) $ 172 2020 1,038 2021 1,051 2022 1,076 2023 1,126 Thereafter 7,571 Total lease receipts $ 12,034

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. COMMITMENTS AND CONTINGENCIES The following commitments and contingencies provide an update of those discussed in Note 18: Commitments and Contingencies in the Notes to Consolidated Financial Statements included Part II, Item 8 in the Company’s Annual Report on Form 10-K for the year ended December 2018 , and should be read in conjunction with the complete descriptions provided in the aforementioned Form 10-K. Litigation Against Ambac Monterey Bay Military Housing, LLC, et al. v. Ambac Assurance Corporation, et al. (United States District Court, Northern District of California, San Jose Division, Case No. 17-cv-04992-BLF, filed August 28, 2017). On February 15, 2019, Ambac Assurance and the other defendants filed a motion to dismiss the second amended complaint (filed on December 17, 2018, following the court’s dismissal of the first amended complaint without prejudice on July 17, 2018). On April 16, 2019, Plaintiffs filed a brief in opposition to the defendants’ motion to dismiss the second amended complaint. The reply briefs of Ambac Assurance and the other defendants are due May 16, 2019, and a hearing on the motion to dismiss the second amended complaint is scheduled for July 18, 2019. From time to time, Ambac is subject to allegations concerning its corporate governance that may lead to litigation, including derivative litigation, and while the monetary impacts may not be material, the matters may distract management and the Board of Directors from their principal focus on Ambac's business, strategy and objectives. Ambac Assurance’s estimates of projected losses for RMBS transactions consider, among other things, the RMBS transactions’ payment waterfall structure, including the application of interest and principal payments and recoveries, and depend in part on our interpretations of contracts and other bases of our legal rights. From time to time, bond trustees and other transaction participants have employed different contractual interpretations and have commenced, or threatened to commence, litigation to resolve these differences. It is not possible to predict whether additional disputes will arise, nor the outcomes of any potential litigation. It is possible that there could be unfavorable outcomes in this or other disputes or proceedings and that our interpretations may prove to be incorrect, which could lead to changes to our estimate of loss reserves. Ambac Assurance has periodically received various regulatory inquiries and requests for information with respect to investigations and inquiries that such regulators are conducting. Ambac Assurance has complied with all such inquiries and requests for information. The Company is involved from time to time in various routine legal proceedings, including proceedings related to litigation with present or former employees. Although the Company’s litigation with present or former employees is routine and incidental to the conduct of its business, such litigation can result in large monetary awards when a civil jury is allowed to determine compensatory and/or punitive damages for, among other things, termination of employment that is wrongful or in violation of implied contracts. It is not reasonably possible to predict whether additional suits will be filed or whether additional inquiries or requests for information will be made, and it is also not possible to predict the outcome of litigation, inquiries or requests for information. It is possible that there could be unfavorable outcomes in these or other proceedings. Legal accruals for litigation against the Company which are probable and reasonably estimable, and management's estimated range of loss for such matters, are not material to the operating results or financial position of the Company. For the litigation matters the Company is defending that do not meet the “probable and reasonably estimable” accrual threshold and where no loss estimates have been provided above, management is unable to make a meaningful estimate of the amount or range of loss that could result from unfavorable outcomes. Under some circumstances, adverse results in any such proceedings could be material to our business, operations, financial position, profitability or cash flows. The Company believes that it has substantial defenses to the claims above and, to the extent that these actions proceed, the Company intends to defend itself vigorously; however, the Company is not able to predict the outcomes of these actions. Litigation Filed or Joined by Ambac In the ordinary course of their businesses, certain of Ambac’s subsidiaries assert claims in legal proceedings against third parties to recover losses already paid and/or mitigate future losses. The amounts recovered and/or losses avoided which may result from these proceedings is uncertain, although recoveries and/or losses avoided in any one or more of these proceedings during any quarter or fiscal year could be material to Ambac’s results of operations in that quarter or fiscal year. Puerto Rico: Lex Claims, LLC et al. v. Alejandro Garcia Padilla et al. (United States District Court, District of Puerto Rico, No. 16-2374, filed July 20, 2016). Following confirmation of the COFINA Plan, several parties appealed the District Court’s confirmation order to the First Circuit Court of Appeals. On April 12, 2019, the Oversight Board and the Puerto Rico Fiscal Agency and Financial Advisory Authority ("AAFAF") moved to dismiss these appeals as equitably moot because the COFINA Plan has been consummated. Ambac Assurance Corporation v. Bank of New York Mellon (United States District Court, Southern District of New York. No. 1:17-cv-03804, filed May 2, 2017). Following confirmation of the COFINA Plan, several parties appealed the District Court’s confirmation order to the First Circuit Court of Appeals. On April 12, 2019, the Oversight Board and AAFAF moved to dismiss these appeals as equitably moot because the COFINA Plan has been consummated. Bank of New York Mellon v. COFINA, et al. (United States District Court, District of Puerto Rico, No. 1:17-ap-00133, filed May 16, 2017). Following confirmation of the COFINA Plan, several parties appealed the District Court’s confirmation order to the First Circuit Court of Appeals. On April 12, 2019, the Oversight Board and AAFAF moved to dismiss these appeals as equitably moot because the COFINA Plan has been consummated. Official Committee of Unsecured Creditors v. Whyte (United States District Court, District of Puerto Rico, No. 1:17-ap-00257, filed September 8, 2017) (the Commonwealth-COFINA Dispute). On February 21, 2019, on the joint motion of the agents for the Commonwealth and COFINA, the Oversight Board, AAFAF, and all participating interested parties, the District Court dismissed this case with prejudice. Following confirmation of the COFINA Plan, several parties appealed the District Court’s confirmation order to the First Circuit Court of Appeals. On April 12, 2019, the Oversight Board and AAFAF moved to dismiss these appeals as equitably moot because the COFINA Plan has been consummated. Financial Oversight and Management Board for Puerto Rico v. Public Buildings Authority (United States District Court, District of Puerto Rico, No. 1:18-ap-00149, filed December 21, 2018). On December 21, 2018, the Oversight Board, together with the Official Committee of Unsecured Creditors for the Commonwealth (the “ Committee ”), as Plaintiffs, together filed a complaint against the Puerto Rico Public Buildings Authority (“ PBA ”) seeking declaratory judgment that the leases between PBA and its lessees-many of whom are agencies and instrumentalities of the Commonwealth-are “disguised financings,” not true leases, and therefore should not be afforded administrative expense priority under the Bankruptcy Code. On March 12, 2019, Ambac Assurance and other interested parties were permitted to intervene in order to argue that the PBA leases are valid leases, and are entitled to administrative expense treatment under the Bankruptcy Code. Certain intervenor-defendants filed counterclaims for declarations to this effect, and a motion for judgment on the pleadings. Motion practice in connection with both is ongoing. In re Financial Oversight and Management Board for Puerto Rico (United States District Court, District of Puerto Rico, No. 1:17-bk-03283), Omnibus Objection of (I) Financial Oversight and Management Board, Acting Through its Special Claims Committee, and (II) Official Committee of Unsecured Creditors, Pursuant to Bankruptcy Code Section 502 and Bankruptcy Rule 3007, to Claims Filed or Asserted by Holders of Certain Commonwealth General Obligation Bonds (Dkt. No. 4784, filed January 14, 2019). On January 14, 2019, the Oversight Board and the Committee filed an omnibus claim objection in the Commonwealth’s Title III case challenging claims arising from certain general obligation bonds issued by the Commonwealth in 2012 and 2014 totaling approximately $6 billion , none of which are held or insured by Ambac Assurance. The court subsequently ordered certain consolidated procedures permitting parties in interest an opportunity to participate in litigation of the objection. On April 11, 2019, Ambac Assurance filed a notice of participation in support of the objection, advancing the argument, among other things, that the PBA leases are true leases, but the associated debt nonetheless should be included in the Commonwealth’s debt ceiling calculation such that the 2012 and 2014 general obligation bond issuances are null and void and claims arising therefrom should be disallowed. RMBS Litigation: In connection with Ambac Assurance’s efforts to seek redress for breaches of representations and warranties and fraud related to the information provided by both the underwriters and the sponsors of various transactions and for failure to comply with the obligation by the sponsors to repurchase ineligible loans, Ambac Assurance has filed various lawsuits: • Ambac Assurance Corporation and the Segregated Account of Ambac Assurance Corporation v. Countrywide Home Loans, Inc., Countrywide Securities Corp., Countrywide Financial Corp., and Bank of America Corp. (Supreme Court of the State of New York, County of New York, Case No. 653979/2014, filed on December 30, 2014). Discovery has been completed. The court has not yet set a schedule for summary judgment or for trial. • Ambac Assurance Corporation and The Segregated Account of Ambac Assurance Corporation v. Countrywide Securities Corp., Countrywide Financial Corp. (a.k.a. Bank of America Home Loans) and Bank of America Corp. (Supreme Court of the State of New York, County of New York, Case No. 651612/2010, filed on September 28, 2010). In August and October 2018, Defendants filed pre-trial motions seeking to (1) strike Ambac Assurance’s jury demand for its fraudulent-inducement claim; (2) strike Ambac Assurance’s jury demand for its successor-liability claim; (3) bifurcate the trials for Ambac Assurance’s primary- and successor-liability claims; (4) limit the loans for which Ambac Assurance may seek to recover damages; (5) preclude Ambac Assurance from using sampling to prove liability or damages for breach of contract; and (6) dismiss Ambac Assurance’s fraudulent-inducement claim as duplicative of its contract claim. On December 30, 2018, the court denied all six of these pre-trial motions in their entirety and Defendants appealed. On January 24, 2019, the court ordered that trial be put off until the First Department resolves Defendants’ appeals, provided that Defendants perfected the appeals for the May 2019 Term at the First Department. Defendants perfected their appeals for the May Term by filing their opening appeal briefs in the First Department on February 19, 2019 ; Ambac Assurance filed its opposition brief on March 20, 2019; and Defendants filed their reply briefs on March 29, 2019. Oral argument on Defendants’ appeals of these six pre-trial motions was held in the First Department on May 2, 2019 . • Ambac Assurance Corporation v. U.S. Bank National Association (United States District Court, Southern District of New York, Docket No. 18-cv-5182 (LGS), filed June 8, 2018 (the “SDNY Action”)); In the matter of HarborView Mortgage Loan Trust 2005-10 (Minnesota state court, Docket No. 27-TR-CV-17-32 (the “Minnesota Action”)). On February 11, 2019, Ambac Assurance filed a petition for certiorari with the United States Supreme Court for review of the denial of Ambac Assurance’s motion to dismiss the Minnesota Action. On March 21, 2019, the United States Supreme Court directed U.S. Bank to respond to this petition for certiorari; the deadline for U.S. Bank to do so is May 22, 2019. • In re application of Deutsche Bank National Trust Company as Trustee of the Harborview Mortgage Loan Trust Mortgage Loan Pass-Through Certificates, Series 2006-9

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation and Significant Accounting Policies [Text Block]	Basis of Presentation The accompanying unaudited consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles ("GAAP") for interim financial reporting and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and disclosures required by GAAP for annual periods. These consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Annual Report on Form 10-K for the year ended December 31, 2018 . The accompanying consolidated financial statements have not been audited by an independent registered public accounting firm in accordance with the standards of the Public Company Accounting Oversight Board (U.S.), but in the opinion of management such financial statements include all adjustments necessary for the fair presentation of the Company’s consolidated financial position and results of operations. The results of operations for the three months ended March 31, 2019 may not be indicative of the results that may be expected for the year ending December 31, 2019 . The December 31, 2018 consolidated balance sheet was derived from audited financial statements. The election to use the fair value option is made on an instrument by instrument based under ASC 825-10. At December 31, 2018, all VIE invested assets and long-term debt were reported under the fair value option as disclosed in the notes to the consolidated financial statements included in the Annual Report on Form 10-K. During the three months ended March 31, 2019, Ambac consolidated a new VIE that holds investments in fixed income securities that management has classified as available-for-sale under the Investments - Debt Securities Topic of the ASC. Long-term debt of the newly consolidated VIE is carried at par value less unamortized discount. See Note 3. Variable Interest Entities , for further discussion of the accounting elections used and the presentation of VIEs in these unaudited consolidated financial statements. The preparation of financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. As additional information becomes available or actual amounts become determinable, the recorded estimates are revised and reflected in operating results.
Foreign Currency Transactions and Translations Policy [Policy Text Block]	Foreign Currency: Financial statement accounts expressed in foreign currencies are translated into U.S. dollars in accordance with the Foreign Currency Matters Topic of the ASC. The functional currencies of Ambac's subsidiaries are the local currencies of the country where the respective subsidiaries are based, which are also the primary operating environments in which the subsidiaries operate. Foreign currency translation : Functional currency assets and liabilities of Ambac’s foreign subsidiaries are translated into U.S. dollars using exchange rates in effect at the balance sheet dates and the related translation adjustments, net of deferred taxes, are included as a component of Accumulated Other Comprehensive Income in Stockholders' Equity. Functional currency operating results of foreign subsidiaries are translated using average exchange rates. Foreign currency transactions : The impact of non-functional currency transactions and the remeasurement of non-functional currency assets and liabilities into the respective subsidiaries' functional currency (collectively "foreign currency transactions gains/(losses)") are $1,960 and $5,014 for the three months ended March 31, 2019 and 2018 , of which $5,583 and $11,012 relate to the remeasurement of loss reserves, classified in Loss and loss expenses, respectively. Foreign currency transactions gains/(losses) are primarily the result of remeasuring Ambac UK's assets and liabilities denominated in currencies other than its functional currency, primarily the U.S. dollar and the Euro. Supplemental Disclosure of Cash Flow Information Three Months Ended March 31, 2019 2018 Cash paid during the period for: Income taxes $ 852 $ 9,718 Interest on long-term debt 37,850 100,958 Non-cash financing activities: Exchange of investments in Puerto Rico COFINA bonds for new bonds issued in the Plan of Adjustment $ 510,215 $ — Rehabilitation exit transaction discharge of all Deferred Amounts and cancellation of certain General Account Surplus Notes — 1,918,561 Reconciliation of cash, cash equivalents, and restricted cash reported within the Consolidated Balance Sheets to the Consolidated Statements of Cash Flows: Cash and cash equivalents $ 21,840 $ 38,485 Variable Interest Entity Restricted cash 3,254 1,134 Total cash, cash equivalents, and restricted cash shown on the Consolidated Statements of Cash Flows $ 25,094 $ 39,619
Reclassifications	Reclassifications:Reclassifications have been made to prior years' amounts to conform to the current year's presentation.
New Accounting Pronouncements, Policy [Policy Text Block]	Recently Adopted Accounting Standards: Effective January 1, 2019 , Ambac adopted the following accounting standards: Equity-linked Instruments with Down Round Features In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260) and Derivatives and Hedging (Topic 815) - Accounting for Certain Financial Instruments with Down Round Features . Equity-linked instruments, such as warrants and convertible instruments may contain down round features that result in the strike price being reduced on the basis of the pricing of future equity offerings. Under the ASU, a down round feature will no longer require a freestanding equity-linked instrument (or embedded conversion option) to be classified as a liability that is remeasured at fair value through the income statement (i.e. marked-to-market). However, other features of the equity-linked instrument (or embedded conversion option) must still be evaluated to determine whether liability or equity classification is appropriate. Equity classified instruments are not marked-to-market. For earnings per share ("EPS") reporting, the ASU requires companies to recognize the effect of the down round feature only when it is triggered by treating it as a dividend and as a reduction of income available to common stockholders in basic EPS. Adoption of this ASU did not impact Ambac's financial statements. Premium Amortization on Callable Debt Securities In March 2017, the FASB issued ASU 2017-08, Receivables-Nonrefundable Fees and Other Costs (Subtopic 310-20) - Premium Amortization on Purchased Callable Debt Securities . The ASU shortens the amortization period for the premium on callable debt securities to the earliest call date. Under previous GAAP, a reporting entity generally amortized the premium as a yield adjustment over the contractual life (i.e. maturity) of the debt security and if that debt security is called, the entity would record a loss equal to the unamortized premium. The ASU does not change the accounting for callable debt securities held at a discount, which will continue to be amortized to maturity. Adoption of this ASU did not have a consequential impact on Ambac's financial statements. Leases In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) . This ASU was subsequently amended by ASU 2018-01, Land Easement Practical Expedient; ASU 2018-10, Codification Improvements to Topic 842; ASU 2018-11 , Targeted Improvements; ASU 2018-20, Narrow-Scope Improvements for Lessors; and ASU 2019-01 , Leases (Topic 842): Codification Improvements (collectively the "New Lease Standard") . The primary difference between previous U.S. GAAP and the New Lease Standard is the recognition of lease assets and lease liabilities for those leases classified as operating leases with a term longer than 12 months. For those operating leases, a lessee is required to: 1) recognize a right-of-use asset ("ROU") and a lease liability, initially measured at the present value of the lease payments, on the balance sheet, 2) recognize a single lease cost, calculated so that the cost is allocated over the lease term generally on a straight-line basis and 3) classify all cash payment within operating activities in the statement of cash flows. For leases classified as finance leases under the New Lease Standard, the balance sheet presentation and expense recognition pattern is similar to capital leases under current GAAP. Under the transition guidance, a reporting entity must use a modified retrospective approach and may choose to initially apply the New Lease Standard either at (1) the beginning of the earliest comparative period presented, which is January 1, 2017 or (2) its effective date, which is January 1, 2019 . If a reporting entity chooses the first option it must recast its comparative period financial statements and provide disclosures for those comparative periods. Ambac chose the second option and initially applied the New Lease Standard on January 1, 2019. Consequently financial information and disclosures were not provided for dates and periods prior to January 1, 2019. There are a number of optional practical expedients that were elected at transition. We elected the ‘package of practical expedients’, which permitted us not to reassess under the new standard our prior conclusions about lease identification, lease classification and initial direct costs. We also elected the hindsight practical expedient allowing us to use the benefit of hindsight in determining the probability of exercising any lessee options to extend or terminate the lease, or purchase the underlying asset. We did not use the practical expedient pertaining to land easements as it was not applicable to Ambac. The new New Lease Standard did not have a material effect on our financial statements. The most significant effects related to (1) the recognition of new ROU assets and lease liabilities on our balance sheet for our office and equipment operating leases of approximately $15,000 at transition and (2) providing significant new disclosures about our leasing activities. See Note 11. Leases for further information.
Description of New Accounting Pronouncements Not yet Adopted [Text Block]	Future Application of Accounting Standards: VIE Related Party Guidance In October 2018, the FASB issued ASU 2018-17, Consolidation (Topic 810) - Targeted Improvements to Related Party Guidance for Variable Interest Entities. To determine whether a decision-making fee is a variable interest, under the new guidance a reporting entity must consider indirect interests held through related parties under common control on a proportional basis rather than as a direct interest in its entirety (as currently required in GAAP). These amendments create alignment between determining whether a decision making fee is a variable interest and determining whether a reporting entity within a related party group is the primary beneficiary of a VIE. ASU 2018-17 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. Ambac will adopt this ASU on January 1, 2020. The ASU is not expected to have a consequential impact on Ambac's financial statements. Cloud Computing Arrangement Service Contracts In August 2018, the FASB issued ASU 2018-15, Intangibles—Goodwill and Other— Internal-Use Software (Subtopic 350-40) - Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. The new guidance requires a customer in a cloud computing arrangement that is a service contract to capitalize certain implementation costs as if the arrangement was an internal-use software project. The internal-use software guidance requires the capitalization of certain costs incurred only during the application development stage (e.g., costs of integration with on-premises software, coding, configuration, customization). That guidance also requires entities to expense costs during the preliminary project and post-implementation stages (e.g., costs of project planning, training, maintenance after implementation, data conversion) as they are incurred. ASU 2018-15 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. The ASU may be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. Ambac will adopt this ASU on January 1, 2020. The ASU is not expected to have a consequential impact on Ambac's financial statements. Defined Benefit and Other Postretirement Plans Disclosures In August 2018, the FASB issued ASU 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans - General (Subtopic 715-20) - Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans . The ASU modifies various disclosure requirements for employers that sponsor defined benefit pension or other postretirement plans. Relevant disclosures that will be removed are: i) amounts in accumulated other comprehensive income expected to be recognized as net periodic benefit cost over the next fiscal year, and ii) the effects of a one percentage point change in assumed health care cost trend rates on the (a) aggregate of the service and interest cost components of the net periodic pension cost and (b) benefit obligation for postretirement healthcare benefits. Relevant disclosures that will be added are an explanation of the reasons for significant gains and losses related to changes in the benefit obligations for the period. ASU 2018-14 is effective for fiscal years ending after December 15, 2020, with early adoption permitted. The modified disclosures must be applied on a retrospective basis for all periods presented. Ambac has not determined whether it will early adopt this ASU. The ASU is not expected to have a consequential impact on Ambac's financial statements. Fair Value Measurement Disclosures In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820) - Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement . The ASU modifies various disclosure requirements on fair value measurements. Relevant disclosures that will be removed, modified and added are as follows: • Removals: 1) Amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, 2) Policy for timing of transfers between levels, and 3) Valuation processes for Level 3 fair value measurements. • Modifications: 1) For investments in certain entities that calculate net asset value, disclosures are only required for the timing of liquidation of an investee's assets and the date when restrictions from redemption might lapse, only if the investee has communicated the timing to the reporting entity or publicly announced it, and 2) Clarification that the measurement uncertainty disclosure is to communicate information about the uncertainty in measurement as of the reporting date and not possible future changes. • Additions : 1) Changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period and 2) Range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. Alternatively, an entity may disclose other quantitative information (such as the median or arithmetic average) if it determines that it is a more reasonable and rational method to reflect the distribution of unobservable inputs used. ASU 2018-13 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. Disclosure amendments related to changes in unrealized gains and losses included in other comprehensive income for Level 3 instruments, the range and weighted average of significant unobservable inputs, and the narrative description of measurement uncertainty should be applied prospectively only for the most recent interim or annual period presented. All other disclosure amendments should be applied retrospectively to all periods presented. Ambac has not determined whether it will early adopt this ASU. The ASU is not expected to have a consequential impact on Ambac's financial statements. Measurement of Credit Losses on Financial Instruments In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326) - Measurement of Credit Losses on Financial Instruments, subsequently amended by ASU 2018-19 , Codification Improvements to Topic 326, Financial Instruments - Credit Losses

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accounting Policies Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2019
Schedule of Cash Flow Supplemental Information [Line Items]
Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]	Supplemental Disclosure of Cash Flow Information Three Months Ended March 31, 2019 2018 Cash paid during the period for: Income taxes $ 852 $ 9,718 Interest on long-term debt 37,850 100,958 Non-cash financing activities: Exchange of investments in Puerto Rico COFINA bonds for new bonds issued in the Plan of Adjustment $ 510,215 $ — Rehabilitation exit transaction discharge of all Deferred Amounts and cancellation of certain General Account Surplus Notes — 1,918,561 Reconciliation of cash, cash equivalents, and restricted cash reported within the Consolidated Balance Sheets to the Consolidated Statements of Cash Flows: Cash and cash equivalents $ 21,840 $ 38,485 Variable Interest Entity Restricted cash 3,254 1,134 Total cash, cash equivalents, and restricted cash shown on the Consolidated Statements of Cash Flows $ 25,094 $ 39,619

Special Purpose Entities, Inc_2

Special Purpose Entities, Including Variable Interest Entities (Tables)	3 Months Ended
	Mar. 31, 2019
Variable Interest Entity [Line Items]
Schedule of Variable Interest Entities [Table Text Block]	3. VARIABLE INTEREST ENTITIES A mbac, with its subsidiaries, has engaged in transactions with variable interest entities ("VIEs,") in various capacities. • Ambac most commonly provides financial guarantees, including credit derivative contracts, for various debt obligations issued by special purpose entities, including VIEs ("FG VIEs"); • Ambac sponsors special purpose entities that issued notes to investors for various purposes; and • Ambac is an investor in collateralized debt obligations, mortgage-backed and other asset-backed securities issued by VIEs and its ownership interest is generally insignificant to the VIE and/or Ambac does not have rights that direct the activities that are most significant to such VIE. FG VIEs: Ambac’s subsidiaries provide financial guarantees in respect of assets held or debt obligations of VIEs. Ambac’s primary variable interest exists through this financial guarantee insurance or credit derivative contract. The transaction structures provide certain financial protection to Ambac. Generally, upon deterioration in the performance of a transaction or upon an event of default as specified in the transaction legal documents, Ambac will obtain certain control rights that enable Ambac to remediate losses. These rights may enable Ambac to direct the activities of the entity that most significantly impact the entity’s economic performance. • We determined that Ambac’s subsidiaries generally have the obligation to absorb a FG VIE's expected losses given that they have issued financial guarantees supporting certain liabilities (and in some cases certain assets). As further described below, Ambac consolidates certain FG VIEs in cases where we also have the power to direct the activities that most significantly impact the VIE’s economic performance due to one or more of the following: (i) the transaction experiencing deterioration and breaching performance triggers, giving Ambac the ability to exercise certain control rights, (ii) Ambac being involved in the design of the VIE and receiving control rights from its inception, such as may occur from loss remediation activities, or (iii) the transaction not experiencing deterioration, however due to the passive nature of the VIE, Ambac's contingent control rights upon a future breach of performance triggers is considered to be the power over the most significant activity. FG VIEs which are consolidated may include non-recourse assets or liabilities. FG VIEs' liabilities (and in some cases assets) that are insured by the Company are with recourse, because the Company guarantees the payment of principal and interest . FG VIEs' assets and liabilities that are not insured by the Company are without recourse, because Ambac has not issued a financial guarantee and is under no obligation for the payment of principal and interest of these instruments. The Company’s exposure to consolidated FG VIEs is limited to the financial guarantees issued for recourse assets and liabilities and any additional variable interests held by Ambac. • A VIE is deconsolidated in the period that Ambac no longer has such control rights, which could occur in connection with the execution of remediation activities on the transaction or amortization of insured exposure, either of which may reduce the degree of Ambac’s control over a VIE. • Assets and liabilities of FG VIEs that are consolidated are reported within Variable interest entity assets or Variable interest entity liabilities on the Consolidated Balance Sheets. • The election to use the fair value option is made on an instrument by instrument basis. Ambac has elected the fair value option for consolidated FG VIE financial assets and financial liabilities, except in cases where Ambac was involved in the design of the VIE and was granted control rights at its inception. ◦ When the fair value option is elected, changes in the fair value of the FG VIE's financial assets and liabilities are reported within Income (loss) on variable interest entities in the Consolidated Statements of Total Comprehensive Income (Loss), except for the portion of the total change in fair value of financial liabilities caused by changes in the instrument-specific credit risk which is presented separately in Other comprehensive income (loss). ◦ In cases where the fair value option has not been elected, the FG VIE's invested assets are fixed income securities and are considered available-for-sale as defined by the Investments - Debt Securities Topic of the ASC. These assets are reported in the financial statements at fair value with unrealized gains and losses reflected in Accumulated Other Comprehensive Income in Stockholders' Equity. The financial liabilities of these FG VIEs consist of long term debt obligations and are carried at par less unamortized discount. Income from the FG VIE's available-for-sale securities (including investment income, realized gains and losses and other-than -temporary impairments as applicable) and interest expense on long term debt are reported within Income (loss) on variable interest entities in the Consolidated Statements of Total Comprehensive Income (Loss). • Upon initial consolidation of a FG VIE, Ambac recognizes a gain or loss in earnings for the difference between: (i) the fair value of the consideration paid, the fair value of any non-controlling interests and the reported amount of any previously held interests and (ii) the net amount, as measured on a fair value basis, of the assets and liabilities consolidated. Upon deconsolidation of a FG VIE, we recognize a gain or loss for the difference between: (i) the fair value of any consideration received, the fair value of any retained non-controlling investment in the VIE and the carrying amount of any non-controlling interest in the VIE and (ii) the carrying amount of the VIE’s assets and liabilities. Gains or losses from consolidation and deconsolidation that are reported in earnings are reported within Income (loss) on variable interest entities on the Consolidated Statements of Total Comprehensive Income (Loss). • The impact of consolidating such FG VIEs on Ambac’s balance sheet is the elimination of transactions between the consolidated FG VIEs and Ambac’s operating subsidiaries and the inclusion of the FG VIE’s third party assets and liabilities. For a financial guarantee insurance policy issued to a consolidated VIE, Ambac does not reflect the financial guarantee insurance policy in accordance with the related insurance accounting rules under the Financial Services – Insurance Topic of the ASC. Consequently, upon consolidation, Ambac eliminates the insurance assets and liabilities associated with the policy from the Consolidated Balance Sheets. Such insurance assets and liabilities may include premium receivables, reinsurance recoverable, deferred ceded premium, subrogation recoverable, unearned premiums, loss and loss expense reserves, ceded premiums payable and insurance intangible assets. For investment securities owned by Ambac that are debt instruments issued by the VIE, the investment securities balance is eliminated upon consolidation. In connection with the exit from rehabilitation of the Segregated Account, as further described in the Notes to Consolidated Financial Statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 , Ambac evaluated the consolidation of certain VIEs. Under the Stipulation and Order, the OCI retained the authority requiring Ambac Assurance to obtain their approval with respect to the exercise of certain control rights in connection with policies that had previously been allocated to the Segregated Account. Accordingly, Ambac did not consolidate any additional VIEs as a result of the Segregated Account's exit from rehabilitation. Ambac is not primarily liable for, and generally does not guarantee all of the debt obligations issued by the VIEs. Ambac would only be required to make payments on the VIE debt obligations in the event that the issuer of such debt obligations defaults on any principal or interest due and such obligation is guaranteed by Ambac. Additionally, Ambac’s general creditors, other than those specific policy holders which own the VIE debt obligations, do not have rights with regard to the assets of the VIEs. Ambac evaluates the net income effects and earnings per share effects to determine attributions between Ambac and non-controlling interests as a result of consolidating a VIE. Ambac has determined that the net income and earnings per share effect of consolidated FG VIEs are attributable to Ambac’s interests through financial guarantee premium and loss payments with the VIE. The following table summarizes the carrying values of assets and liabilities, along with other supplemental information related to VIEs that are consolidated as a result of financial guarantees of Ambac UK and Ambac Assurance: March 31, 2019 December 31, 2018 Ambac UK Ambac Assurance Total VIEs Ambac UK Ambac Assurance Total VIEs Fixed income securities, at fair value: Corporate obligations, fair value option $ 2,858,404 $ — $ 2,858,404 $ 2,737,286 $ — $ 2,737,286 Municipal obligations, available-for-sale (1) — 270,591 270,591 — — — Total FG VIE fixed income securities, at fair value 2,858,404 270,591 3,128,995 2,737,286 — 2,737,286 Restricted cash 1,056 2,198 3,254 999 — 999 Loans, at fair value (2) 4,375,761 — 4,375,761 4,287,664 — 4,287,664 Derivative assets 59,228 — 59,228 66,302 — 66,302 Other assets 3,320 1,366 4,686 1,058 — 1,058 Total FG VIE assets $ 7,297,769 $ 274,155 $ 7,571,924 $ 7,093,309 $ — $ 7,093,309 Accrued interest payable $ 2,785 $ — $ 2,785 $ 556 $ — $ 556 Long-term debt: Long-term debt, at fair value (3) 5,401,992 — 5,401,992 5,268,596 — 5,268,596 Long-term debt, at par less unamortized discount — 335,271 335,271 — — — Total long-term debt 5,401,992 335,271 5,737,263 5,268,596 — 5,268,596 Derivative liabilities 1,781,903 — 1,781,903 1,712,062 — 1,712,062 Other liabilities 56 — 56 30 — 30 Total FG VIE liabilities $ 7,186,736 $ 335,271 $ 7,522,007 $ 6,981,244 $ — $ 6,981,244 Number of FG VIEs consolidated 7 1 8 7 — 7 (1) Available-for-sale FG VIE fixed income securities consist of municipal obligations with an amortized cost basis of $259,474 and aggregate gross unrealized gains and (losses) of $11,117 and $(0) , respectively at March 31, 2019 . All such securities had contractual maturities due after ten years as of March 31, 2019 . (2) The unpaid principal balances of loan assets carried at fair value were $3,432,240 as of March 31, 2019 and $3,402,413 as of December 31, 2018 . (3) The unpaid principal balances of long-term debt carried at fair value were $4,605,291 as of March 31, 2019 and $4,552,643 as of December 31, 2018 . The following schedule details the components of Income (loss) on variable interest entities for the affected periods: Three Months Ended March 31, 2019 2018 Net change in fair value of VIE assets and liabilities reported under the fair value option $ 1,836 $ 1,918 Less: Credit risk changes of fair value option long-term debt reported through other comprehensive income (171 ) (1,344 ) Net change in fair value of VIE assets and liabilities reported in earnings 1,665 574 Investment income on available-for-sale securities 1,859 — Interest expense on long-term debt carried at par less unamortized cost (2,356 ) — Other expenses (111 ) — Gain (loss) from consolidating FG VIEs 14,864 — Income (loss) on Variable Interest Entities $ 15,921 $ 574 As discussed in Note 3. Variable Interest Entities , effective February 12, 2019, Ambac was required to consolidate one VIE which resulted in a gain of $14,864 . The 2019 balance sheet impact of this consolidation was an increase to total consolidated assets and liabilities by $292,003 and $363,628 in 2019, respectively. Ambac deconsolidated no VIEs for the three months ended March 31, 2019 and 2018 . The following table displays the carrying amount of the assets, liabilities and maximum exposure to loss of Ambac’s variable interests in non-consolidated VIEs resulting from financial guarantee and derivative contracts by major underlying asset classes, as of March 31, 2019 and December 31, 2018 : Carrying Value of Assets and Liabilities Maximum (1) Insurance (2) Insurance (3) Net Derivative (4) March 31, 2019: Global structured finance: Collateralized debt obligations $ 2,437 $ — $ — $ — Mortgage-backed—residential 6,418,261 1,842,283 492,435 — Other consumer asset-backed 1,615,109 14,754 234,639 — Other commercial asset-backed 481,633 9,247 6,774 — Other 2,083,055 53,206 299,939 7,021 Total global structured finance 10,600,495 1,919,490 1,033,787 7,021 Global public finance 23,927,824 315,807 343,946 (1,077 ) Total $ 34,528,319 $ 2,235,297 $ 1,377,733 $ 5,944 December 31, 2018: Global structured finance: Collateralized debt obligations $ 9,787 $ — $ — $ (2 ) Mortgage-backed—residential 6,713,437 1,859,121 546,682 — Other consumer asset-backed 1,700,984 15,435 238,234 — Other commercial asset-backed 873,343 20,735 12,264 — Other 2,122,648 53,462 301,260 7,170 Total global structured finance 11,420,199 1,948,753 1,098,440 7,168 Global public finance 24,145,956 309,071 335,437 (1,457 ) Total $ 35,566,155 $ 2,257,824 $ 1,433,877 $ 5,711 (1) Maximum exposure to loss represents the maximum future payments of principal and interest on insured obligations and derivative contracts. Ambac’s maximum exposure to loss does not include the benefit of any financial instruments (such as reinsurance or hedge contracts) that Ambac may utilize to mitigate the risks associated with these variable interests. (2) Insurance assets represent the amount included in “Premium receivables” and “Subrogation recoverable” for financial guarantee insurance contracts on Ambac’s Consolidated Balance Sheets. (3) Insurance liabilities represent the amount included in “Loss and loss expense reserves” and “Unearned premiums” for financial guarantee insurance contracts on Ambac’s Consolidated Balance Sheets. (4) Net derivative assets (liabilities) represent the fair value recognized on credit derivative contracts and interest rate swaps on Ambac’s Consolidated Balance Sheets. Ambac Sponsored VIEs: Non-consolidated VIEs: A subsidiary of Ambac transferred financial assets to a VIE. The business purpose of this entity was to provide certain financial guarantee clients with funding for their debt obligations. This VIE was established as a separate legal entity, demonstrably distinct from Ambac and that Ambac, its affiliates or its agents could not unilaterally dissolve. The permitted activities of this entity are contractually limited to purchasing assets from Ambac, issuing medium-term notes ("MTNs") to fund such purchases, executing derivative hedges and obtaining financial guarantee policies with respect to indebtedness incurred. Ambac does not consolidate this entity because Ambac Assurance’s policies issued to this entity were allocated to the Segregated Account and, as discussed above, the exercise of related control rights in such policies remain subject to OCI approval under the Stipulation and Order. Ambac elected to account for its equity interest in this entity at fair value under the fair value option in accordance with the Financial Instruments Topic of the ASC. We believe that the fair value of the investments in this entity provides for greater transparency for recording profit or loss as compared to the equity method under the Investments – Equity Method and Joint Ventures Topic of the ASC. Refer to Note 7. Fair Value Measurements for further information on the valuation technique and inputs used to measure the fair value of Ambac’s equity interest in this entity. At March 31, 2019 and December 31, 2018 the fair value of this entity was $4,148 and $4,516 , respectively, and is reported within Other assets on the Consolidated Balance Sheets. • Total principal amount of debt outstanding was $400,600 and $393,010 at March 31, 2019 and December 31, 2018 , respectively. In each case, Ambac sold assets to this entity, which are composed of utility obligations with a weighted average rating of BBB+ at March 31, 2019 and weighted average life of 1.9 years . The purchase by this entity of financial assets was financed through the issuance of MTNs, which are cross-collateralized by the purchased assets. The MTNs have the same expected weighted average life as the purchased assets. Derivative contracts (interest rate swaps) are used within the entity for economic hedging purposes only. Derivative positions were established at the time MTNs were issued to purchase financial assets. As of March 31, 2019 Ambac Assurance had financial guarantee insurance policies issued for all assets, MTNs and derivative contracts owned and outstanding by the entity. • Insurance premiums paid to Ambac Assurance by this entity are earned in a manner consistent with other insurance policies, over the risk period. Additionally, any losses incurred on such insurance policies are included in Ambac’s Consolidated Statements of Total Comprehensive Income (Loss). Under the terms of an Administrative Agency Agreement, Ambac provides certain administrative duties, primarily collecting amounts due on the obligations and making interest payments on the MTNs. On August 28, 2014, Ambac monetized its ownership of the junior surplus note issued to it by the Segregated Account by depositing the junior surplus note into a newly formed VIE trust in exchange for cash and an owner trust certificate, which represents Ambac's right to residual cash flows from the junior surplus note. Ambac does not consolidate the VIE since it does not have a variable interest in the trust. Ambac reports its owner trust certificate as an equity investment within Other investments on the Consolidated Balance Sheets with associated results from operations included within Net investment income: Other investments on the Consolidated Statements of Total Comprehensive Income (Loss). The equity investment had a carrying value of $41,511 and $40,168 as of March 31, 2019 and December 31, 2018 , respectively. On February 12, 2018, Ambac formed a VIE, Ambac LSNI, to issue Secured Notes in connection with the Rehabilitation Exit Transactions. Ambac does not consolidate the VIE since it does not have a variable interest in the trust. Ambac reports its holdings of Secured Notes within Fixed Income Securities in the Consolidated Balance Sheets. The carrying value of Secured Notes held by Ambac was $583,550 and $656,473 at March 31, 2019 and December 31, 2018, respectively. Ambac's debt obligation to the VIE (the Ambac Note) had a carrying value of $1,927,144 and $1,940,289
Components of VIE Gain (Loss) [Table Text Block]	The following schedule details the components of Income (loss) on variable interest entities for the affected periods: Three Months Ended March 31, 2019 2018 Net change in fair value of VIE assets and liabilities reported under the fair value option $ 1,836 $ 1,918 Less: Credit risk changes of fair value option long-term debt reported through other comprehensive income (171 ) (1,344 ) Net change in fair value of VIE assets and liabilities reported in earnings 1,665 574 Investment income on available-for-sale securities 1,859 — Interest expense on long-term debt carried at par less unamortized cost (2,356 ) — Other expenses (111 ) — Gain (loss) from consolidating FG VIEs 14,864 — Income (loss) on Variable Interest Entities $ 15,921 $ 574
Summary of Carrying Amount of Assets, Liabilities and Maximum Exposure to Loss of Ambac's Variable Interests in Non-Consolidated Variable Interest Entities	The following table displays the carrying amount of the assets, liabilities and maximum exposure to loss of Ambac’s variable interests in non-consolidated VIEs resulting from financial guarantee and derivative contracts by major underlying asset classes, as of March 31, 2019 and December 31, 2018 : Carrying Value of Assets and Liabilities Maximum (1) Insurance (2) Insurance (3) Net Derivative (4) March 31, 2019: Global structured finance: Collateralized debt obligations $ 2,437 $ — $ — $ — Mortgage-backed—residential 6,418,261 1,842,283 492,435 — Other consumer asset-backed 1,615,109 14,754 234,639 — Other commercial asset-backed 481,633 9,247 6,774 — Other 2,083,055 53,206 299,939 7,021 Total global structured finance 10,600,495 1,919,490 1,033,787 7,021 Global public finance 23,927,824 315,807 343,946 (1,077 ) Total $ 34,528,319 $ 2,235,297 $ 1,377,733 $ 5,944 December 31, 2018: Global structured finance: Collateralized debt obligations $ 9,787 $ — $ — $ (2 ) Mortgage-backed—residential 6,713,437 1,859,121 546,682 — Other consumer asset-backed 1,700,984 15,435 238,234 — Other commercial asset-backed 873,343 20,735 12,264 — Other 2,122,648 53,462 301,260 7,170 Total global structured finance 11,420,199 1,948,753 1,098,440 7,168 Global public finance 24,145,956 309,071 335,437 (1,457 ) Total $ 35,566,155 $ 2,257,824 $ 1,433,877 $ 5,711 (1) Maximum exposure to loss represents the maximum future payments of principal and interest on insured obligations and derivative contracts. Ambac’s maximum exposure to loss does not include the benefit of any financial instruments (such as reinsurance or hedge contracts) that Ambac may utilize to mitigate the risks associated with these variable interests. (2) Insurance assets represent the amount included in “Premium receivables” and “Subrogation recoverable” for financial guarantee insurance contracts on Ambac’s Consolidated Balance Sheets. (3) Insurance liabilities represent the amount included in “Loss and loss expense reserves” and “Unearned premiums” for financial guarantee insurance contracts on Ambac’s Consolidated Balance Sheets. (4)
Schedule of Variable Interest Entities Assets and Liabilities [Table Text Block]	The following table summarizes the carrying values of assets and liabilities, along with other supplemental information related to VIEs that are consolidated as a result of financial guarantees of Ambac UK and Ambac Assurance: March 31, 2019 December 31, 2018 Ambac UK Ambac Assurance Total VIEs Ambac UK Ambac Assurance Total VIEs Fixed income securities, at fair value: Corporate obligations, fair value option $ 2,858,404 $ — $ 2,858,404 $ 2,737,286 $ — $ 2,737,286 Municipal obligations, available-for-sale (1) — 270,591 270,591 — — — Total FG VIE fixed income securities, at fair value 2,858,404 270,591 3,128,995 2,737,286 — 2,737,286 Restricted cash 1,056 2,198 3,254 999 — 999 Loans, at fair value (2) 4,375,761 — 4,375,761 4,287,664 — 4,287,664 Derivative assets 59,228 — 59,228 66,302 — 66,302 Other assets 3,320 1,366 4,686 1,058 — 1,058 Total FG VIE assets $ 7,297,769 $ 274,155 $ 7,571,924 $ 7,093,309 $ — $ 7,093,309 Accrued interest payable $ 2,785 $ — $ 2,785 $ 556 $ — $ 556 Long-term debt: Long-term debt, at fair value (3) 5,401,992 — 5,401,992 5,268,596 — 5,268,596 Long-term debt, at par less unamortized discount — 335,271 335,271 — — — Total long-term debt 5,401,992 335,271 5,737,263 5,268,596 — 5,268,596 Derivative liabilities 1,781,903 — 1,781,903 1,712,062 — 1,712,062 Other liabilities 56 — 56 30 — 30 Total FG VIE liabilities $ 7,186,736 $ 335,271 $ 7,522,007 $ 6,981,244 $ — $ 6,981,244 Number of FG VIEs consolidated 7 1 8 7 — 7 (1) Available-for-sale FG VIE fixed income securities consist of municipal obligations with an amortized cost basis of $259,474 and aggregate gross unrealized gains and (losses) of $11,117 and $(0) , respectively at March 31, 2019 . All such securities had contractual maturities due after ten years as of March 31, 2019 . (2) The unpaid principal balances of loan assets carried at fair value were $3,432,240 as of March 31, 2019 and $3,402,413 as of December 31, 2018 . (3) The unpaid principal balances of long-term debt carried at fair value were $4,605,291 as of March 31, 2019 and $4,552,643 as of December 31, 2018

Comprehensive Income (Tables)

Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Schedule of Changes in Balances of Each Component of Accumulated Other Comprehensive Income	The following tables detail the changes in the balances of each component of accumulated other comprehensive income for the affected periods: Unrealized Gains (1) Amortization of (1) Gain (Loss) on (1) Credit Risk Changes of Fair Value Option Liabilities (1) (2) Total Three Months Ended March 31, 2019: Beginning Balance $ 85,903 $ 8,874 $ (141,527 ) $ (1,965 ) $ (48,715 ) Other comprehensive income (loss) before reclassifications 73,367 689 14,571 — 88,627 Amounts reclassified from accumulated other comprehensive income (loss) (17,204 ) (322 ) — 156 (17,370 ) Net current period other comprehensive income (loss) 56,163 367 14,571 156 71,257 Balance at March 31, 2019 $ 142,066 $ 9,241 $ (126,956 ) $ (1,809 ) $ 22,542 Three Months Ended March 31, 2018: Beginning Balance $ 30,755 $ 10,640 $ (93,634 ) $ — $ (52,239 ) Adjustments to opening balance, net of taxes (3) — — — (2,900 ) (2,900 ) Adjusted balance, beginning of period 30,755 10,640 (93,634 ) (2,900 ) (55,139 ) Other comprehensive income (loss) before reclassifications 125,159 (556 ) 32,056 — 156,659 Amounts reclassified from accumulated other comprehensive income (loss) (2,855 ) (303 ) — 1,114 (2,044 ) Net current period other comprehensive income (loss) 122,304 (859 ) 32,056 1,114 154,615 Balance at March 31, 2018 $ 153,059 $ 9,781 $ (61,578 ) $ (1,786 ) $ 99,476 (1) All amounts are net of tax and noncontrolling interest. Amounts in parentheses indicate reductions to Accumulated Other Comprehensive Income. (2) Represents the changes in fair value attributable to instrument-specific credit risk of liabilities for which the fair value option is elected. (3) Beginning in 2018, credit risk changes of fair value option liabilities are reflected as a component of Accumulated Other Comprehensive Income pursuant to the adoption of ASU 2016-01. See Note 2. Basis of Presentation and Significant Accounting Policies
Schedule of Amounts Reclassed Out of Each Component of Accumulated Other Comprehensive Income	The following table details the significant amounts reclassified from each component of accumulated other comprehensive income, shown in the above rollforward tables, for the affected periods: Details about Accumulated Other Comprehensive Income Components Amount Reclassified from Accumulated Other Comprehensive Income (1) Affected Line Item in the Three Months Ended March 31, 2019 2018 Unrealized Gains (Losses) on Available-for-Sale Securities $ (16,693 ) $ (4,563 ) Net realized investment gains (losses) and other-than-temporary impairment losses (511 ) 1,708 Provision for income taxes $ (17,204 ) $ (2,855 ) Net of tax and noncontrolling interest Amortization of Postretirement Benefit Prior service cost $ (241 ) $ (241 ) Other income (2) Actuarial (losses) (81 ) (62 ) Other income (2) (322 ) (303 ) Total before tax — — Provision for income taxes $ (322 ) $ (303 ) Net of tax and noncontrolling interest Credit risk changes of fair value option liabilities $ 188 $ 1,344 Credit Risk Changes of Fair Value Option Liabilities (32 ) (230 ) Provision for income taxes $ 156 $ 1,114 Net of tax and noncontrolling interest Total reclassifications for the period $ (17,370 ) $ (2,044 ) Net of tax and noncontrolling interest (1) Amounts in parentheses indicate reductions to Accumulated Other Comprehensive Income with corresponding increases to the affected line items in the Consolidated Statement of Total Comprehensive Income. (2)

Net Income Per Share (Tables)

Net Income Per Share (Tables)	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Reconciliation of Common Shares Used for Basic and Diluted Earnings Per Share	The following table provides a reconciliation of the common shares used for basic net income per share to the diluted shares used for diluted net income per share: Three Months Ended March 31, 2019 2018 Basic weighted average shares outstanding 45,832,297 45,471,083 Effect of potential dilutive shares (1) : Stock options — — Warrants — — Restricted stock units — 45,713 Performance stock units (2) — 136,675 Diluted weighted average shares outstanding 45,832,297 45,653,471 Anti-dilutive shares excluded from the above reconciliation: Stock options 16,667 126,667 Warrants 4,877,783 4,053,476 Restricted stock units 271,763 — Performance stock units (2) 478,739 — (1) For the three months ended March 31, 2019 , Ambac had a net loss and accordingly excluded all potentially dilutive securities from the determination of diluted loss per share as their impact was anti-dilutive. (2)

Financial Guarantee Insurance_2

Financial Guarantee Insurance Contracts (Tables)	3 Months Ended
	Mar. 31, 2019
Insurance [Line Items]
Schedule of Loss And Loss Expense Reserves And Subrogation Recoverable Table [Table Text Block]	Below are the components of the Loss and loss expense reserves liability and the Subrogation recoverable asset at March 31, 2019 and December 31, 2018 : Present Value of Expected Unearned Gross Loss and Balance Sheet Line Item Claims and Recoveries March 31, 2019: Loss and loss expense reserves $ 2,045,886 $ (274,846 ) $ (76,877 ) $ 1,694,163 Subrogation recoverable 175,114 (2,091,231 ) — (1,916,117 ) Totals $ 2,221,000 $ (2,366,077 ) $ (76,877 ) $ (221,954 ) December 31, 2018: Loss and loss expense reserves $ 2,246,335 $ (313,595 ) $ (106,662 ) $ 1,826,078 Subrogation recoverable 175,694 (2,108,654 ) — (1,932,960 ) Totals $ 2,422,029 $ (2,422,249 ) $ (106,662 ) $ (106,882 )
Summary of Gross Premium Receivable Roll-Forward (Direct and Assumed Contracts)	Below is the gross premium receivable roll-forward for the affected periods: Three Months Ended March 31, 2019 2018 Beginning premium receivable $ 495,391 $ 586,312 Premium receipts (13,204 ) (15,381 ) Adjustments for changes in expected and contractual cash flows (1) 470 (1,289 ) Accretion of premium receivable discount 3,228 3,846 Changes to uncollectable premiums (352 ) 604 Other adjustments (including foreign exchange) 1,864 6,615 Ending premium receivable (2) $ 487,397 $ 580,707 (1) Adjustments for changes in expected and contractual cash flows primarily due to reductions in insured exposure as a result of early policy terminations and unscheduled principal paydowns. (2) Premium receivable includes premiums to be received in foreign denominated currencies most notably in British Pounds and Euros. At March 31, 2019 and 2018 , premium receivables include British Pounds of $141,527 ( £108,825 ) and $163,926 ( £116,815 ), respectively, and Euros of $30,062 ( €26,807 ) and $36,679 ( €29,767
Effect of Reinsurance on Premiums Written and Earned	The effect of reinsurance on premiums written and earned for the respective periods was as follows: Three Months Ended March 31, 2019 2018 Written Earned Written Earned Direct $ 3,346 $ 29,433 $ 4,261 $ 32,609 Assumed — 20 — 19 Ceded (501 ) 1,695 (819 ) 1,745 Net premiums $ 3,847 $ 27,758 $ 5,080 $ 30,883
Schedule of Revenue from External Customers Attributed to Foreign Countries by Geographic Area [Table Text Block]	The following table summarizes net premiums earned by location of risk for the respective periods: Three Months Ended March 31, 2019 2018 United States $ 28,244 $ 24,718 United Kingdom 4,190 4,856 Other international (4,676 ) 1,309 Total $ 27,758 $ 30,883
Summarized Future Gross Undiscounted Premiums Expected to be Collected and Future Expected Premiums Earned, Net of Reinsurance	The table below summarizes the future gross undiscounted premiums to be collected and future premiums earned, net of reinsurance at March 31, 2019 : Future Premiums Collected (1) Future (1) Three months ended: June 30, 2019 $ 12,906 $ 12,178 September 30, 2019 11,923 12,015 December 31, 2019 11,852 11,875 Twelve months ended: December 31, 2020 46,963 46,762 December 31, 2021 41,005 42,504 December 31, 2022 39,159 39,760 December 31, 2023 37,641 37,075 Five years ended: December 31, 2028 166,827 152,126 December 31, 2033 128,064 101,459 December 31, 2038 67,195 51,134 December 31, 2043 26,556 16,644 December 31, 2048 11,831 7,130 December 31, 2053 2,252 1,817 December 31, 2058 31 50 Total $ 604,205 $ 532,529 (1) Future premiums to be collected are undiscounted and are used to derive the discounted premium receivable asset recorded on Ambac's balance sheet. Future premiums to be earned, net of reinsurance relate to the unearned premiums liability and deferred ceded premium asset recorded on Ambac’s balance sheet. The use of contractual lives for many bond types which do not have homogeneous pools of underlying collateral is required in the calculation of the premium receivable, as further described in Note 2. Basis of Presentation and Significant Accounting Policies in the Notes to Consolidated Financial Statements included in Ambac's Annual Report on Form 10-K for the year ended December 31, 2018
Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance	Below is the loss and loss expense reserve roll-forward, net of subrogation recoverable and reinsurance, for the affected periods: Three Months Ended March 31, 2019 2018 Beginning gross loss and loss expense reserves $ (106,882 ) $ 4,113,802 Reinsurance recoverable 22,623 40,658 Beginning balance of net loss and loss expense reserves (129,505 ) 4,073,144 Losses and loss expenses (benefit): Current year 681 778 Prior years 11,726 (248,173 ) Total (1) (2) (3) 12,407 (247,395 ) Loss and loss expenses paid (recovered): Current year 28 — Prior years (3) 64,409 3,631,177 Total 64,437 3,631,177 Foreign exchange effect 5,502 11,016 Ending net loss and loss expense reserves (176,033 ) 205,588 Impact of VIE consolidation (72,159 ) — Reinsurance recoverable (4) 26,238 38,735 Ending gross loss and loss expense reserves $ (221,954 ) $ 244,323 (1) Total losses and loss expenses (benefit) includes $(4,996) and $1,354 for the three months ended March 31, 2019 and 2018 , respectively, related to ceded reinsurance. (2) Ambac records the impact of estimated recoveries related to securitized loans in RMBS transactions that breached certain representations and warranties ("R&W"s) by transaction sponsors within losses and loss expenses (benefit). The losses and loss expense (benefit) incurred associated with changes in estimated representation and warranties for the three months ended March 31, 2019 and 2018 was $4,407 and $800 , respectively. (3) On February 12, 2018, Deferred Amounts and Interest Accrued on Deferred Amounts in the amount of $3,000,158 and $856,834 , respectively were settled in connection with the Rehabilitation Exit Transactions. 2018 includes a $288,204 loss and loss expense benefit on these settled Deferred Amounts. (4) Represents reinsurance recoverable on future loss and loss expenses. Additionally, the Balance Sheet line "Reinsurance recoverable on paid and unpaid losses" includes reinsurance recoverables (payables) of $550 and $90 as of March 31, 2019 and 2018
Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable	The tables below summarize information related to policies currently included in Ambac’s loss and loss expense reserves or subrogation recoverable at March 31, 2019 and December 31, 2018 . Gross par exposures include capital appreciation bonds which are reported at the par amount at the time of issuance of the insurance policy as opposed to the current accreted value of the bond. The weighted average risk-free rate used to discount loss reserves at March 31, 2019 and December 31, 2018 was 2.5% and 2.8% , respectively. Surveillance Categories as of March 31, 2019 I IA II III IV V Total Number of policies 33 28 12 16 147 3 239 Remaining weighted-average contract period (in years) (1) 9 18 8 18 14 3 15 Gross insured contractual payments outstanding: Principal $ 934,336 $ 754,502 $ 177,251 $ 909,382 $ 5,299,368 $ 43,140 $ 8,117,979 Interest 514,643 646,737 87,602 443,526 2,144,310 13,401 3,850,219 Total $ 1,448,979 $ 1,401,239 $ 264,853 $ 1,352,908 $ 7,443,678 $ 56,541 $ 11,968,198 Gross undiscounted claim liability $ 6,344 $ 56,257 $ 25,466 $ 591,366 $ 2,182,417 $ 56,510 $ 2,918,360 Discount, gross claim liability (620 ) (7,729 ) (1,897 ) (202,850 ) (564,236 ) (3,455 ) (780,787 ) Gross claim liability before all subrogation and before reinsurance 5,724 48,528 23,569 388,516 1,618,181 53,055 2,137,573 Less: Gross RMBS subrogation (2) — — — — (1,803,302 ) — (1,803,302 ) Discount, RMBS subrogation — — — — 37,280 — 37,280 Discounted RMBS subrogation, before reinsurance — — — — (1,766,022 ) — (1,766,022 ) Less: Gross other subrogation (3) — — — (46,914 ) (598,954 ) (12,880 ) (658,748 ) Discount, other subrogation — — — 6,295 48,954 3,444 58,693 Discounted other subrogation, before reinsurance — — — (40,619 ) (550,000 ) (9,436 ) (600,055 ) Gross claim liability, net of all subrogation and discounts, before reinsurance 5,724 48,528 23,569 347,897 (697,841 ) 43,619 (228,504 ) Less: Unearned premium revenue (3,791 ) (9,639 ) (1,640 ) (7,219 ) (54,395 ) (193 ) (76,877 ) Plus: Loss expense reserves 1,809 5,330 902 2,362 73,024 — 83,427 Gross loss and loss expense reserves $ 3,742 $ 44,219 $ 22,831 $ 343,040 $ (679,212 ) $ 43,426 $ (221,954 ) Reinsurance recoverable reported on Balance Sheet (4) $ 216 $ 8,901 $ 3,758 $ 27,138 $ (13,225 ) $ — $ 26,788 (1) Remaining weighted-average contract period is weighted based on projected gross claims over the lives of the respective policies. (2) RMBS subrogation represents Ambac’s estimate of subrogation recoveries from RMBS transaction sponsors for representation and warranty ("R&W") breaches. (3) Other subrogation represents subrogation related to excess spread and other contractual cash flows on public finance and structured finance transactions, including RMBS. (4) Reinsurance recoverable reported on the Balance Sheet includes reinsurance recoverables of $26,238 related to future loss and loss expenses and $550 related to presented loss and loss expenses and subrogation. Surveillance Categories as of December 31, 2018 I IA II III IV V Total Number of policies 21 28 18 16 145 3 231 Remaining weighted-average contract period (in years) (1) 9 19 9 22 14 3 16 Gross insured contractual payments outstanding: Principal $ 916,530 $ 708,249 $ 622,820 $ 1,705,464 $ 5,407,202 $ 43,140 $ 9,403,405 Interest 487,702 631,708 293,293 6,979,130 2,177,539 13,401 10,582,773 Total $ 1,404,232 $ 1,339,957 $ 916,113 $ 8,684,594 $ 7,584,741 $ 56,541 $ 19,986,178 Gross undiscounted claim liability $ 4,019 $ 63,712 $ 36,000 $ 992,019 $ 2,295,968 $ 56,510 $ 3,448,228 Discount, gross claim liability (481 ) (13,008 ) (3,069 ) (433,709 ) (637,548 ) (4,143 ) (1,091,958 ) Gross claim liability before all subrogation and before reinsurance 3,538 50,704 32,931 558,310 1,658,420 52,367 2,356,270 Less: Gross RMBS subrogation (2) — — — — (1,809,937 ) — (1,809,937 ) Discount, RMBS subrogation — — — — 39,391 — 39,391 Discounted RMBS subrogation, before reinsurance — — — — (1,770,546 ) — (1,770,546 ) Less: Gross other subrogation (3) — (10,816 ) — (136,541 ) (624,654 ) (12,880 ) (784,891 ) Discount, other subrogation — 7,318 — 67,008 55,088 3,774 133,188 Discounted other subrogation, before reinsurance — (3,498 ) — (69,533 ) (569,566 ) (9,106 ) (651,703 ) Gross claim liability, net of all subrogation and discounts, before reinsurance 3,538 47,206 32,931 488,777 (681,692 ) 43,261 (65,979 ) Less: Unearned premium revenue (943 ) (10,073 ) (5,085 ) (36,365 ) (53,987 ) (209 ) (106,662 ) Plus: Loss expense reserves 1,369 4,253 2,564 (5,926 ) 63,499 — 65,759 Gross loss and loss expense reserves $ 3,964 $ 41,386 $ 30,410 $ 446,486 $ (672,180 ) $ 43,052 $ (106,882 ) Reinsurance recoverable reported on Balance Sheet (4) $ 367 $ 7,285 $ 4,223 $ 26,096 $ (14,838 ) $ — $ 23,133 (1) Remaining weighted-average contract period is weighted based on projected gross claims over the lives of the respective policies. (2) RMBS subrogation represents Ambac’s estimate of subrogation recoveries from RMBS transaction sponsors for R&W breaches. (3) Other subrogation represents subrogation related to excess spread and other contractual cash flows on public finance and structured finance transactions, including RMBS. (4) Reinsurance recoverable reported on Balance Sheet includes reinsurance recoverables of $22,623 related to future loss and loss expenses and $510
Summary of Rollforward of RMBS Subrogation, by Estimation Approach	Below is the rollforward of R&W subrogation for the affected periods: Three Months Ended March 31, 2019 2018 Discounted R&W subrogation (gross of reinsurance) at beginning of period $ 1,770,546 $ 1,834,387 Impact of sponsor actions (1) — — All other changes (2) (4,524 ) (877 ) Discounted R&W subrogation (gross of reinsurance) at end of period $ 1,766,022 $ 1,833,510 (1) Sponsor actions include loan repurchases, direct payments to Ambac and other contributions from sponsors. (2)
Insurance Intangible Asset [Member]
Insurance [Line Items]
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block]	The estimated future amortization expense for the net insurance intangible asset is as follows: Amortization expense (1) 2019 (nine months) $ 47,410 2020 58,672 2021 53,229 2022 49,565 2023 46,241 Thereafter 434,138 (1) Future amortization considers the use of contractual lives for many bond types which do not have homogeneous pools of underlying collateral. Actual maturities will differ from contractual maturities because

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities	The following table sets forth the carrying amount and fair value of Ambac’s financial assets and liabilities as of March 31, 2019 and December 31, 2018 , including the level within the fair value hierarchy at which fair value measurements are categorized. As required by the Fair Value Measurement Topic of the ASC, financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. Carrying Total Fair Fair Value Measurements Categorized as: March 31, 2019: Level 1 Level 2 Level 3 Financial assets: Fixed income securities: Municipal obligations $ 424,884 $ 424,884 $ — $ 424,884 $ — Corporate obligations 1,233,544 1,233,544 — 1,233,544 — Foreign obligations 31,292 31,292 30,379 913 — U.S. government obligations 100,429 100,429 100,429 — — Residential mortgage-backed securities 256,285 256,285 — 256,285 — Collateralized debt obligations 132,977 132,977 — 132,977 — Other asset-backed securities 440,400 440,400 — 368,545 71,855 Fixed income securities, pledged as collateral: U.S. government obligations 83,901 83,901 83,901 — — Short term investments 908,235 908,235 864,065 44,170 — Other investments (1) 428,556 404,775 102,257 — 17,730 Cash, cash equivalents and restricted cash 21,840 21,840 15,891 5,949 — Derivative assets: Interest rate swaps—asset position 73,218 73,218 — 19,107 54,111 Futures contracts 3,182 3,182 3,182 — — Other assets - equity in sponsored VIE 4,148 4,148 — — 4,148 Other assets-Loans 10,124 12,187 — — 12,187 Variable interest entity assets: Fixed income securities: Corporate obligations 2,858,404 2,858,404 — — 2,858,404 Fixed income securities: Municipal obligations 270,591 270,591 — 270,591 — Restricted cash 3,254 3,254 3,254 — — Loans 4,375,761 4,375,761 — — 4,375,761 Derivative assets: Currency swaps-asset position 59,228 59,228 — 59,228 — Total financial assets $ 11,720,253 $ 11,698,535 $ 1,203,358 $ 2,816,193 $ 7,394,196 Financial liabilities: Long term debt, including accrued interest $ 3,320,562 $ 3,324,313 $ — $ 2,942,229 $ 382,084 Derivative liabilities: Credit derivatives 1,077 1,077 — — 1,077 Interest rate swaps—asset position — — — — Interest rate swaps—liability position 85,457 85,457 — 85,457 — Futures contracts — — — — Liabilities for net financial guarantees (2) (839,056 ) 509,830 — — 509,830 Variable interest entity liabilities: Long-term debt (includes $5,401,992 carried at fair value) 5,737,263 5,785,582 — 5,561,135 224,447 Derivative liabilities: Interest rate swaps—liability position 1,781,903 1,781,903 — 1,781,903 — Total financial liabilities $ 10,087,206 $ 11,488,162 $ — $ 10,370,724 $ 1,117,438 Carrying Total Fair Fair Value Measurements Categorized as: December 31, 2018: Level 1 Level 2 Level 3 Financial assets: Fixed income securities: Municipal obligations $ 879,919 $ 879,919 $ — $ 879,919 $ — Corporate obligations 1,278,122 1,278,122 — 1,278,122 — Foreign obligations 30,834 30,834 29,922 912 — U.S. government obligations 94,394 94,394 94,394 — — Residential mortgage-backed securities 258,607 258,607 — 258,607 — Collateralized debt obligations 131,356 131,356 — 131,356 — Other asset-backed securities 442,443 442,443 — 370,372 72,071 Short term investments 430,331 430,331 304,880 125,451 — Other investments (1) 391,217 367,315 71,108 — 16,266 Cash and cash equivalents 82,494 82,494 52,661 29,833 — Derivative assets: Interest rate swaps—asset position 59,468 59,468 — 12,008 47,460 Other assets - equity in sponsored VIE 4,516 4,516 — — 4,516 Other assets-Loans 9,913 11,620 — — 11,620 Variable interest entity assets: Fixed income securities: Corporate obligations 2,737,286 2,737,286 — — 2,737,286 Restricted cash 999 999 999 — — Loans 4,287,664 4,287,664 — — 4,287,664 Derivative assets: Currency swaps—asset position 66,302 66,302 — 66,302 — Total financial assets $ 11,185,865 $ 11,163,670 $ 553,964 $ 3,152,882 $ 7,176,883 Financial liabilities: Long term debt, including accrued interest $ 3,304,737 $ 3,259,966 $ — $ 2,909,272 $ 350,694 Derivative liabilities: Credit derivatives 1,459 1,459 — — 1,459 Interest rate swaps—liability position 71,861 71,861 — 71,861 — Futures contracts 3,379 3,379 3,379 — Liabilities for net financial guarantees written (2) (718,388 ) 558,824 — — 558,824 Variable interest entity liabilities: Long-term debt 5,268,596 5,268,596 — 5,051,504 217,092 Derivative liabilities: Interest rate swaps—liability position 1,712,062 1,712,062 — 1,712,062 — Total financial liabilities $ 9,643,706 $ 10,876,147 $ 3,379 $ 9,744,699 $ 1,128,069 (1) Excluded from the fair value measurement categories in the table above are investment funds of $284,788 and $279,941 as of March 31, 2019 and December 31, 2018 , respectively, which are measured using NAV per share as a practical expedient. (2)
Summary of Changes in Level 3 Fair Value Category	The following tables present the changes in the Level 3 fair value category for the periods presented in 2019 and 2018 . Ambac classifies financial instruments in Level 3 of the fair value hierarchy when there is reliance on at least one significant unobservable input to the valuation model. In addition to these unobservable inputs, the valuation models for Level 3 financial instruments typically also rely on a number of inputs that are readily observable either directly or indirectly. Thus, the gains and losses presented below include changes in the fair value related to both observable and unobservable inputs. Level 3 - Financial Assets and Liabilities Accounted for at Fair Value VIE Assets and Liabilities Investments Other (1) Derivatives Investments Loans Long-term Total Three Months Ended March 31, 2019: Balance, beginning of period $ 72,071 $ 4,516 $ 46,001 $ 2,737,286 $ 4,287,664 $ (217,092 ) $ 6,930,446 Total gains/(losses) realized and unrealized: Included in earnings 383 (368 ) 8,187 66,919 88,281 (3,053 ) 160,349 Included in other comprehensive income (270 ) — — 54,199 85,061 (4,302 ) 134,688 Purchases — — — — — — — Issuances — — — — — — — Sales — — — — — — — Settlements (329 ) — (1,154 ) — (85,245 ) — (86,728 ) Balance, end of period $ 71,855 $ 4,148 $ 53,034 $ 2,858,404 $ 4,375,761 $ (224,447 ) $ 7,138,755 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ (368 ) $ 8,096 $ 66,919 $ 88,281 $ (3,053 ) $ 159,875 Three Months Ended March 31, 2018: Balance, beginning of period $ 808,557 $ 5,979 $ 60,808 $ 2,914,145 $ 11,529,384 $ (2,757,688 ) $ 12,561,185 Total gains/(losses) realized and unrealized: Included in earnings 35,184 (358 ) (9,375 ) (69,026 ) (202,588 ) 159,985 (86,178 ) Included in other comprehensive income (52,508 ) — — 110,644 423,892 (103,877 ) 378,151 Purchases — — — — — — — Issuances — — — — — — — Sales — — — — — — — Settlements (713,603 ) — (1,525 ) — (192,357 ) 5,064 (902,421 ) Balance, end of period $ 77,630 $ 5,621 $ 49,908 $ 2,955,763 $ 11,558,331 $ (2,696,516 ) $ 11,950,737 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ (358 ) $ (9,481 ) $ (69,026 ) $ (202,588 ) $ 159,985 $ (121,468 ) (1) Other assets carried at fair value and classified as Level 3 relate to an equity interest in an Ambac sponsored VIE. The tables below provide roll-forward information by class of investments and derivatives measured using significant unobservable inputs. Level 3 - Investments by Class: Three Months Ended March 31, 2019 Three Months Ended March 31, 2018 Other Asset Non-Agency RMBS Total Other Asset Non-Agency RMBS Total Balance, beginning of period $ 72,071 $ — $ 72,071 $ 72,540 $ 736,017 $ 808,557 Total gains/(losses) realized and unrealized: Included in earnings 383 — 383 457 34,727 35,184 Included in other comprehensive income (270 ) — (270 ) (370 ) (52,138 ) (52,508 ) Purchases — — — — — — Issuances — — — — — — Sales — — — — — — Settlements (329 ) — (329 ) (306 ) (713,297 ) (713,603 ) Balance, end of period $ 71,855 $ — $ 71,855 $ 72,321 $ 5,309 $ 77,630 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ — $ — $ — $ — $ — $ — Level 3 - Derivatives by Class: Three Months Ended March 31, 2019 Three Months Ended March 31, 2018 Interest Credit Total Interest Credit Total Balance, beginning of period $ 47,460 $ (1,459 ) $ 46,001 $ 61,374 $ (566 ) $ 60,808 Total gains/(losses) realized and unrealized: Included in earnings 7,714 473 8,187 (9,029 ) (346 ) (9,375 ) Included in other comprehensive income — — — — — — Purchases — — — — — — Issuances — — — — — — Sales — — — — — — Settlements (1,063 ) (91 ) (1,154 ) (1,419 ) (106 ) (1,525 ) Balance, end of period $ 54,111 $ (1,077 ) $ 53,034 $ 50,926 $ (1,018 ) $ 49,908 The amount of total gains/(losses) included in earnings attributable to the change in unrealized gains or losses relating to assets and liabilities still held at the reporting date $ 7,714 $ 382 $ 8,096 $ (9,029 ) $ (452 ) $ (9,481 )
Summary of Gains and Losses (Realized and Unrealized) Relating to Level 3 Assets and Liabilities Included in Earnings	Gains and losses (realized and unrealized) relating to Level 3 assets and liabilities included in earnings for the affected periods are reported as follows: Net Net Gains (Losses) on Derivative Contracts Income Other Three Months Ended March 31, 2019: Total gains or losses included in earnings for the period $ 383 $ 8,187 $ 152,147 $ (368 ) Changes in unrealized gains or losses relating to the assets and liabilities still held at the reporting date — 8,096 152,147 (368 ) Three Months Ended March 31, 2018: Total gains or losses included in earnings for the period $ 35,184 $ (9,375 ) $ (111,629 ) $ (358 ) Changes in unrealized gains or losses relating to the assets and liabilities still held at the reporting date — (9,481 ) (111,629 ) (358 )

Investments (Tables)

Investments (Tables)	3 Months Ended
	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments	The amortized cost and estimated fair value of available-for-sale investments, excluding VIE investments, at March 31, 2019 and December 31, 2018 were as follows: Amortized Gross Gross Estimated Non-credit (1) March 31, 2019: Fixed income securities: Municipal obligations $ 406,315 $ 20,356 $ 1,787 $ 424,884 $ — Corporate obligations (2) 1,223,640 14,312 4,408 1,233,544 — Foreign obligations 30,719 599 26 31,292 — U.S. government obligations 99,808 1,185 564 100,429 Residential mortgage-backed securities 216,904 39,504 123 256,285 123 Collateralized debt obligations 133,736 28 787 132,977 — Other asset-backed securities 369,410 71,623 633 440,400 — 2,480,532 147,607 8,328 2,619,811 123 Short-term 908,209 43 17 908,235 — 3,388,741 147,650 8,345 3,528,046 123 Fixed income securities pledged as collateral: U.S. government obligations 83,901 — — 83,901 — Total fixed income securities pledged as collateral 83,901 — — 83,901 — Total available-for-sale investments $ 3,472,642 $ 147,650 $ 8,345 $ 3,611,947 $ 123 December 31, 2018: Fixed income securities: Municipal obligations $ 882,631 $ 14,364 $ 17,076 $ 879,919 $ 5 Corporate obligations (2) 1,288,882 6,444 17,204 1,278,122 — Foreign obligations 30,496 399 61 30,834 — U.S. government obligations 93,636 1,371 613 94,394 — Residential mortgage-backed securities 221,825 37,575 793 258,607 27 Collateralized debt obligations 133,075 8 1,727 131,356 — Other asset-backed securities 370,199 72,868 624 442,443 — 3,020,744 133,029 38,098 3,115,675 32 Short-term 430,405 23 97 430,331 — Total available-for-sale investments $ 3,451,149 $ 133,052 $ 38,195 $ 3,546,006 $ 32 (1) Represents the amount of non-credit other-than-temporary impairment losses remaining in accumulated other comprehensive income on securities that also had a credit impairment. These losses are included in gross unrealized losses as of March 31, 2019 and December 31, 2018 . (2) Includes Ambac's holdings of the
Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments Held by Successor Ambac, by Contractual Maturity	The amortized cost and estimated fair value of available-for-sale investments, excluding VIE investments, at March 31, 2019 , by contractual maturity, were as follows: Amortized Estimated Due in one year or less $ 1,076,361 $ 1,076,545 Due after one year through five years 999,235 1,006,812 Due after five years through ten years 316,441 323,494 Due after ten years 360,555 375,434 2,752,592 2,782,285 Residential mortgage-backed securities 216,904 256,285 Collateralized debt obligations 133,736 132,977 Other asset-backed securities 369,410 440,400 Total $ 3,472,642 $ 3,611,947
Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments	The following table shows gross unrealized losses and fair values of Ambac’s available-for-sale investments, excluding VIE investments, aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position, at March 31, 2019 and December 31, 2018 : Less Than 12 Months 12 Months or More Total Fair Value Gross Fair Value Gross Fair Value Gross March 31, 2019: Fixed income securities: Municipal obligations $ 20,997 $ 951 $ 25,865 $ 836 $ 46,862 $ 1,787 Corporate obligations 39,639 550 256,406 3,858 296,045 4,408 Foreign obligations 401 4 3,980 22 4,381 26 U.S. government obligations 12,793 360 53,861 204 66,654 564 Residential mortgage-backed securities 4,375 122 243 1 4,618 123 Collateralized debt obligations 101,939 787 — — 101,939 787 Other asset-backed securities 3,840 9 83,645 624 87,485 633 183,984 2,783 424,000 5,545 607,984 8,328 Short-term 99,429 17 — — 99,429 17 Total temporarily impaired securities $ 283,413 $ 2,800 $ 424,000 $ 5,545 $ 707,413 $ 8,345 Less Than 12 Months 12 Months or More Total Fair Value Gross Fair Value Gross Fair Value Gross December 31, 2018: Fixed income securities: Municipal obligations $ 537,904 $ 15,878 $ 28,533 $ 1,198 $ 566,437 $ 17,076 Corporate obligations 306,506 8,634 190,273 8,570 496,779 17,204 Foreign obligations 1,161 1 5,163 60 6,324 61 U.S. government obligations 5,643 135 58,495 478 64,138 613 Residential mortgage-backed securities 34,852 793 — — 34,852 793 Collateralized debt obligations 123,848 1,727 — — 123,848 1,727 Other asset-backed securities 13,813 33 77,479 591 91,292 624 1,023,727 27,201 359,943 10,897 1,383,670 38,098 Short-term 115,374 97 — — 115,374 97 Total temporarily impaired securities $ 1,139,101 $ 27,298 $ 359,943 $ 10,897 $ 1,499,044 $ 38,195
Summary of Amounts Included in Net Realized (Losses) Gains and Other-Than-Temporary Impairments	The following table details amounts included in net realized gains (losses) and other-than-temporary impairments included in earnings for the affected periods: Three Months Ended March 31, 2019 2018 Gross realized gains on securities $ 24,219 $ 11,054 Gross realized losses on securities (4,288 ) (1,388 ) Net foreign exchange (losses) gains (2,698 ) (4,804 ) Net realized gains (losses) $ 17,233 $ 4,862 Net other-than-temporary impairments (1) $ (29 ) $ (299 ) (1)
Summary of Roll-Forward of Ambac's Cumulative Credit Losses on Debt Securities for Which Portion of Other-than-Temporary Impairment was Recognized in Other Comprehensive Income	The following table presents a roll-forward of Ambac’s cumulative credit losses on debt securities held as of March 31, 2019 and 2018 for which a portion of an other-than-temporary impairment was recognized in other comprehensive income: Three Months Ended March 31, 2019 2018 Balance, beginning of period $ 12,454 $ 67,085 Additions for credit impairments recognized on: Securities not previously impaired — 226 Securities previously impaired — 64 Reductions for credit impairments previously recognized on: Securities that matured or were sold during the period (49 ) (23,742 ) Balance, end of period $ 12,405 $ 43,633
Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities	The following table represents the fair value, including the value of the financial guarantee, and weighted-average underlying rating, excluding the financial guarantee, of the insured securities at March 31, 2019 and December 31, 2018 , respectively: Municipal Corporate (3) Mortgage Total Weighted (1) March 31, 2019: Ambac Assurance Corporation (2) $ 166,770 $ 583,550 $ 599,512 $ 1,349,832 CCC- National Public Finance Guarantee Corporation 13,839 — — 13,839 BBB- Total $ 180,609 $ 583,550 $ 599,512 $ 1,363,671 CCC- December 31, 2018: Ambac Assurance Corporation (2) $ 833,241 $ 656,473 $ 599,185 $ 2,088,899 CC National Public Finance Guarantee Corporation 15,600 — — 15,600 BBB- Total $ 848,841 $ 656,473 $ 599,185 $ 2,104,499 CC (1) Ratings are based on the lower of Standard & Poor’s or Moody’s rating. If unavailable, Ambac’s internal rating is used. (2) Includes corporate obligations and asset-backed securities with a fair value of $144,530 and $144,672 at March 31, 2019 and December 31, 2018 , respectively, insured by Ambac UK. (3)
Equity Method Investments [Table Text Block]	There are no unfunded commitments applicable to any of these investments for the periods disclosed. Fair Value Class of Funds March 31, December 31, Redemption Frequency Redemption Notice Period Real estate properties (1) $ 16,150 $ 16,123 quarterly 10 business days Interest rate products (2) (6) 207,704 177,357 daily, weekly or monthly 0 - 30 days Illiquid investments (3) 85,323 84,297 quarterly 180 days Insurance-linked investments (4) 28,746 29,318 quarterly 90-120 days Equity market investments (5) (6) 49,122 43,954 daily 0 days Total equity investments in pooled funds $ 387,045 $ 351,049 (1) Investments consist of UK property to generate income and capital growth. (2) This class of funds includes investments in a range of instruments including leveraged loans, CLOs, asset-backed securities and floating rate notes to generate income and capital appreciation. Funds with less frequent redemption periods limit redemptions to as little as 15% per period. Funds with a same day redemption notice period are redeemable only weekly, while funds that may be redeemed any business day have notice periods of 15-30 days. (3) This class seeks to obtain high long-term total return through investments with low liquidity and defined term, resulting in expected capital distributions to subscribers in the second half of 2019 and 2023. (4) This class aims to provide returns from the insurance and reinsurance markets through investments in catastrophe bonds, life insurance and other insurance linked investments. Redemption periods are quarterly, subject to 90-day notice for January/July redemption dates and 120-day notice for April/October redemption dates. (5) Investments represent a diversified exposure to global equity market returns through holdings of various regional market index funds. (6) Interest rate products include $53,135 at March 31, 2019 and $27,154 at December 31, 2018 and equity market investments include $49,122 at March 31, 2019 and $43,954 at December 31, 2018
Summary of Net Investment Income	Net investment income was comprised of the following for the affected periods: Three Months Ended March 31, 2019 2018 Fixed income securities $ 43,666 $ 109,351 Short-term investments 4,113 2,840 Loans 184 187 Investment expense (1,411 ) (1,827 ) Securities available-for-sale and short-term 46,552 110,551 Other investments 8,290 (311 ) Total net investment income $ 54,842 $ 110,240 Net investment income from Other investments primarily represents changes in fair value on securities classified as trading or under the fair value option plus income from the above noted equity interests in an unconsolidated trust. The portion of net unrealized gains (losses) related to trading securities still held at the end of each period is as follows: Three Months Ended March 31, 2019 2018 Net gains (losses) recognized during the period on trading securities $ 6,947 $ (1,579 ) Less: net gains (losses) recognized during the reporting period on trading securities sold during the period 631 1,933 Unrealized gains (losses) recognized during the reporting period on trading securities still held at the reporting date $ 6,316 $ (3,512 )

Derivative Instruments (Tables)

Derivative Instruments (Tables)	3 Months Ended
	Mar. 31, 2019
Summary of Gross Fair Values of Individual Derivative Instruments	The following tables summarize the gross fair values of individual derivative instruments and the impact of legal rights of offset as reported in the Consolidated Balance Sheets as of March 31, 2019 and December 31, 2018 : Gross Gross Net Amounts Gross Amount Net Amount March 31, 2019: Derivative Assets: Interest rate swaps $ 73,607 $ 389 $ 73,218 $ — $ 73,218 Futures contracts 3,182 — 3,182 — 3,182 Total non-VIE derivative assets $ 76,789 $ 389 $ 76,400 $ — $ 76,400 Derivative Liabilities: Credit derivatives $ 1,077 $ — $ 1,077 $ — $ 1,077 Interest rate swaps 85,846 389 85,457 73,510 11,947 Total non-VIE derivative liabilities $ 86,923 $ 389 $ 86,534 $ 73,510 $ 13,024 Variable Interest Entities Derivative Assets: Currency swaps $ 59,228 $ — $ 59,228 $ — $ 59,228 Total VIE derivative assets $ 59,228 $ — $ 59,228 $ — $ 59,228 Variable Interest Entities Derivative Liabilities: Interest rate swaps $ 1,781,903 $ — $ 1,781,903 $ — $ 1,781,903 Total VIE derivative liabilities $ 1,781,903 $ — $ 1,781,903 $ — $ 1,781,903 December 31, 2018: Derivative Assets: Interest rate swaps $ 59,768 $ 300 $ 59,468 $ — $ 59,468 Total non-VIE derivative assets $ 59,768 $ 300 $ 59,468 $ — $ 59,468 Derivative Liabilities: Credit derivatives $ 1,459 $ — $ 1,459 $ — $ 1,459 Interest rate swaps 72,161 300 71,861 67,126 4,735 Futures contracts 3,379 — 3,379 3,379 — Total non-VIE derivative liabilities $ 76,999 $ 300 $ 76,699 $ 70,505 $ 6,194 Variable Interest Entities Derivative Assets: Currency swaps $ 66,302 $ — $ 66,302 $ — $ 66,302 Total VIE derivative assets $ 66,302 $ — $ 66,302 $ — $ 66,302 Variable Interest Entities Derivative Liabilities: Interest rate swaps $ 1,712,062 $ — $ 1,712,062 $ — $ 1,712,062 Total VIE derivative liabilities $ 1,712,062 $ — $ 1,712,062 $ — $ 1,712,062
Summary of Location and Amount of Gains and Losses of Derivative Contracts	The following tables summarize the location and amount of gains and losses of derivative contracts in the Consolidated Statements of Total Comprehensive Income (Loss) for the three months ended March 31, 2019 and 2018 : Location of Gain or (Loss) Recognized in Consolidated Statements of Total Comprehensive Income (Loss) Amount of Gain or (Loss) Recognized in Consolidated Statement of Total Comprehensive Income (Loss) Three Months Ended March 31, 2019 2018 Non-VIE derivatives: Credit derivatives Net gains (losses) on derivative contracts $ 473 $ (346 ) Interest rate swaps Net gains (losses) on derivative contracts (2,732 ) 4,543 Futures contracts Net gains (losses) on derivative contracts (13,900 ) 20,994 Total Non-VIE derivatives $ (16,159 ) $ 25,191 Variable Interest Entities: Currency swaps Income (loss) on variable interest entities $ (7,074 ) $ (8,617 ) Interest rate swaps Income (loss) on variable interest entities (69,841 ) 49,809 Total Variable Interest Entities (76,915 ) 41,192 Total derivative contracts $ (92,601 ) $ 66,037
Summary of Notional Amounts of AFS's Trading Derivative Products	As of March 31, 2019 and December 31, 2018 the notional amounts of AFS’s derivatives are as follows: Notional Type of Derivative March 31, December 31, Interest rate swaps—receive-fixed/pay-variable $ 341,909 $ 493,368 Interest rate swaps—pay-fixed/receive-variable 1,269,180 1,121,532 US Treasury futures contracts—short 1,575,000 1,760,000
Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure [Member]
Summary of Notional Amounts of AFS's Trading Derivative Products	The notional for VIE derivatives outstanding as of March 31, 2019 and December 31, 2018 are as follows: Notional Type of VIE Derivative March 31, December 31, Interest rate swaps—receive-fixed/pay-variable $ 1,427,299 $ 1,399,532 Interest rate swaps—pay-fixed/receive-variable 1,189,113 1,176,748 Currency swaps 344,714 344,992 Credit derivatives 10,457 10,254

Income Taxes (Tables)

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Major Jurisdictions	The following are the major jurisdictions in which Ambac and its subsidiaries operate and the earliest tax years subject to examination: Jurisdiction Tax Year United States 2010 New York State 2013 New York City 2014 United Kingdom 2015 Italy 2014
Schedule of Income before Income Tax, Domestic and Foreign [Table Text Block]	U.S. and foreign components of pre-tax income (loss) were as follows: Three Months Ended March 31, 2019 2018 U.S. $ (63,404 ) $ 297,741 Foreign 22,194 10,568 Total $ (41,210 ) $ 308,309
Schedule of Components of Income Tax Expense (Benefit)	The components of the provision for income taxes were as follows: Three Months Ended March 31, 2019 2018 Current taxes U. S. federal $ — $ — U.S. state and local (3,526 ) 1,037 Foreign 6,466 (39 ) Current taxes 2,940 998 Deferred taxes Foreign (949 ) 1,607 Deferred taxes (949 ) 1,607 Provision for income taxes $ 1,991 $ 2,605
Schedule of Net Operating Loss And Tax Credit Carryovers	Pursuant to the intercompany tax sharing agreement, to the extent Ambac Assurance generates taxable income after September 30, 2011, which is offset with "Allocated NOLs" of $3,650,000 , it is obligated to make payments (“Tolling Payments”), subject to certain credits, to Ambac in accordance with the following NOL usage table, where the “Applicable Percentage” is applied to the aggregate amount of federal income tax liability that would have been paid if the Allocated NOLs were not available. Pursuant to the Closing Agreement between Ambac and the Internal Revenue Service ("IRS"), the IRS will receive 12.5% of Tier C and 17.5% of Tier D payments, if made. NOL Usage Table NOL Usage Tier Allocated NOLs Applicable Percentage A The first $479,000 15% B The next $1,057,000 after Tier A 40% C The next $1,057,000 after Tier B 10% D The next $1,057,000 after Tier C 15%

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2019
Lessor, Operating Lease, Payments to be Received, Maturity [Table Text Block]	Future undiscounted lease payments to be received are as follows: As of March 31, Operating Leases 2019 (nine months) $ 172 2020 1,038 2021 1,051 2022 1,076 2023 1,126 Thereafter 7,571 Total lease receipts $ 12,034
Cash Flow, Supplemental Disclosures [Text Block]	Supplemental information related to leases is as follows: Three Months Ended March 31, 2019 Cash paid for amounts included in the measurement of operating lease liabilities $ 1,543 Right-of-use assets obtained in exchange for operating lease liabilities (non-cash) 14,360
Schedule of Capital Leased Assets [Table Text Block]	Supplemental balance sheet information related to leases is as follows: March 31, 2019 Operating leases: Operating lease right of use assets $ 13,100 Operating lease liabilities 13,545 Weighted average remaining lease term: Operating leases 8.7 years Weighted average discount rate: Operating leases 8.0 %
Lessee, Operating Lease, Liability, Maturity [Table Text Block]	Future undiscounted lease payments to be made are as follows: As of March 31, Operating Leases 2019 (nine months) $ 3,580 2020 1,781 2021 1,436 2022 1,436 2023 1,436 Thereafter 9,294 Total lease payments 18,963 Less: imputed interest (5,418 ) Total $ 13,545
Lease, Cost [Table Text Block]	The components of lease costs, net of sub-lessor income, is as follows: Three Months Ended March 31, 2019 Operating lease cost $ 1,526 Variable Lease Cost 179 Sublease income (65 ) Total lease cost $ 1,640

Background and Business Descr_2

Background and Business Description - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2019
Background And Basis Of Presentation [Line Items]
Entity Incorporation, Date of Incorporation	Apr. 29, 1991

Basis of Presentation and Sig_4

Basis of Presentation and Significant Accounting Policies FX gain (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Foreign Currency Transaction Gain (Loss), before Tax	$ 1,960	$ 5,014
Loss and Loss Reserves [Member]
Foreign Currency Transaction Gain (Loss), before Tax	5,583	11,012
Gain (Loss) on Investments [Member]
Foreign Currency Transaction Gain (Loss), before Tax	$ (2,698)	$ (4,804)

Basis of Presentation and Sig_5

Basis of Presentation and Significant Accounting Policies Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Operating Lease, Liability		$ 13,545	$ 15,000
Fair Value, Option, Credit Risk, Gains (Losses) on Liabilities	$ 1,114
Retained Earnings [Member] | Accounting Standards Update 2016-01 [Member]
Derivative [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	2,900
Retained Earnings [Member] | Accounting Standards Update 2016-01 [Member]
Derivative [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(2,900)
Deferred Income Tax Charge [Member] | Retained Earnings [Member] | Accounting Standards Update 2016-01 [Member]
Derivative [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 590

Basis of Presentation and Sig_6

Basis of Presentation and Significant Accounting Policies Supplemental Cash Flow information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Non Cash Exchange of Puerto Rico Bonds	$ 510,215
Income Taxes Paid	852	$ 9,718
Non Cash Impact of Rehabilitation Exit Transaction				$ 0		$ 1,918,561
Cash and cash equivalents	21,840	38,485	$ 63,089
Restricted Cash	0		19,405
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	25,094	39,619	83,493		$ 624,681
Long-term Debt [Member]
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	37,850	100,958
Variable Interest Entity, Primary Beneficiary [Member]
Restricted Cash	$ 3,254	$ 1,134	$ 999

Special Purpose Entities, Inc_3

Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Mar. 31, 2019USD ($)Entity	Mar. 31, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2018USD ($)Entity
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities	$ 1,836	$ 1,918
Equity Method Investments	41,511			$ 40,168
Deconsolidated VIE Assets	292,003
Deconsolidated VIE Liabilities	363,628
Fair value of special purpose entities	4,148			4,516
variable interest entities deconsolidation gain loss	14,864		$ 0
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	15,921	574
Other investments	428,556			391,217
Long-term debt	$ 2,929,227			2,928,929
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life	1 year 10 months 24 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	$ 400,600			393,010
Variable Interest Entities [Member]
Variable Interest Entities [Line Items]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax	(171)	(1,344)
VIE Change in FV of Assets Liabilities, Net of Credit Risk Change	1,665	574
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Interest Expense, Long-term Debt	(2,356)	0
Other Expenses	(111)	$ 0
Long-term debt	$ 5,737,263			$ 5,268,596
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of Newly Consolidated Variable Interest Entities	0	0
Number of DeConsolidated Variable Interest Entities	0	0
Number Of Consolidated Variable Interest Entities | Entity	8			7
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Number of Newly Consolidated Variable Interest Entities	1
Number of DeConsolidated Variable Interest Entities	0
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 5,401,992			$ 5,268,596
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	7			7
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Carry Value Secured Notes from LSNI	$ 583,550			$ 656,473
Fixed Income Investments [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Gross Investment Income, Operating	1,859	$ 0
Ambac Note Post Exit From Rehabilitation [Member] | Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 1,927,144			$ 1,940,289

Special Purpose Entities, Inc_4

Special Purpose Entities, Including Variable Interest Entities - Summary of Fair Value of Fixed Income Securities, by Asset-Type, Held by Consolidated Variable Interest Entities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Variable Interest Entities [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	$ 3,472,642	$ 3,451,149
Investments:
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	147,650	133,052
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,345	$ 38,195
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	259,474
Investments:
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	11,117
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 0

Special Purpose Entities, Inc_5

Special Purpose Entities, Including Variable Interest Entities - Supplemental Information about Loans Held as Assets and Long-Term Debt Associated with Consolidated Variable Interest Entities (Detail) - Variable Interest Entities [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Variable Interest Entities [Line Items]
Loans, Unpaid principal balance	$ 3,432,240	$ 3,402,413
Long-term debt, Unpaid principal balance	$ 4,605,291	$ 4,552,643

Special Purpose Entities, Inc_6

Special Purpose Entities, Including Variable Interest Entities - Summary of Carrying Amount of Assets, Liabilities and Maximum Exposure to Loss of Ambac's Variable Interests in Non-Consolidated Variable Interest Entities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Feb. 12, 2018
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 34,528,319	$ 35,566,155
Insurance Assets	2,235,297	2,257,824
Insurance Liabilities	1,377,733	1,433,877
Derivative Liabilities	5,944
satisfaction and discharge of segregated account deferred interest amounts			$ 856,834
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		5,711
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	23,927,824	24,145,956
Insurance Assets	315,807	309,071
Insurance Liabilities	343,946	335,437
Derivative Liabilities	(1,077)	(1,457)
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	10,600,495	11,420,199
Insurance Assets	1,919,490	1,948,753
Insurance Liabilities	1,033,787	1,098,440
Derivative Liabilities	7,021	7,168
Global Structured Finance [Member] | Collateralized Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,437	9,787
Insurance Assets	0	0
Insurance Liabilities	0	0
Derivative Liabilities	0	(2)
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	6,418,261	6,713,437
Insurance Assets	1,842,283	1,859,121
Insurance Liabilities	492,435	546,682
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,615,109	1,700,984
Insurance Assets	14,754	15,435
Insurance Liabilities	234,639	238,234
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	481,633	873,343
Insurance Assets	9,247	20,735
Insurance Liabilities	6,774	12,264
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,083,055	2,122,648
Insurance Assets	53,206	53,462
Insurance Liabilities	299,939	301,260
Derivative Liabilities	$ 7,021	$ 7,170

Special Purpose Entities, Inc_7

Special Purpose Entities, Including Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Thousands	Mar. 31, 2019USD ($)Entity	Dec. 31, 2018USD ($)Entity	Mar. 31, 2018USD ($)
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	$ 3,611,947
Restricted Cash	0	$ 19,405
Derivative assets	76,400	59,468
Other assets	90,977	137,628
Total assets	15,022,899	14,588,711
Interest Payable	391,335	375,808
Long-term debt	2,929,227	2,928,929
Derivative Liability	86,534	76,699
Other Liabilities	71,402	63,792
Liabilities	13,357,011	12,955,564
Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	3,128,995	2,737,286
Restricted Cash	3,254	999	$ 1,134
Loans Receivable, Net	4,375,761	4,287,664
Derivative assets	59,228	66,302
Other assets	4,686	1,058
Total assets	7,571,924	7,093,309
Interest Payable	2,785	556
Long-term debt	5,401,992	5,268,596
Long-term Debt, at par less amortized discount	335,271	0
Long-term debt	5,737,263	5,268,596
Derivative Liability	1,781,903	1,712,062
Other Liabilities	56	30
Liabilities	7,522,007	6,981,244
Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	2,858,404	2,737,286
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	270,591	0
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	2,858,404	2,737,286
Restricted Cash	1,056	999
Loans Receivable, Net	4,375,761	4,287,664
Derivative assets	59,228	66,302
Other assets	3,320	1,058
Total assets	7,297,769	7,093,309
Interest Payable	2,785	556
Long-term debt	5,401,992	5,268,596
Long-term Debt, at par less amortized discount	0	0
Long-term debt	5,401,992	5,268,596
Derivative Liability	1,781,903	1,712,062
Other Liabilities	56	30
Liabilities	7,186,736	6,981,244
Ambac UK [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	2,858,404	2,737,286
Ambac UK [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	270,591	0
Restricted Cash	2,198	0
Loans Receivable, Net	0	0
Derivative assets	0	0
Other assets	1,366	0
Total assets	274,155	0
Interest Payable	0	0
Long-term debt	0	0
Long-term Debt, at par less amortized discount	335,271	0
Long-term debt	335,271	0
Derivative Liability	0	0
Other Liabilities	0	0
Liabilities	335,271	0
Ambac Assurance [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Ambac Assurance [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	$ 270,591	$ 0
Consolidated Entities [Member]
Number Of Consolidated Variable Interest Entities | Entity	8	7
Consolidated Entities [Member] | Ambac UK [Member]
Number Of Consolidated Variable Interest Entities | Entity	7	7
Consolidated Entities [Member] | Ambac Assurance [Member]
Number Of Consolidated Variable Interest Entities | Entity	1	0

Comprehensive Income - Schedule

Comprehensive Income - Schedule of Changes in Balances of Each Component of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Restated			$ (55,139)
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			(2,900)
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	$ (48,715)	$ (52,239)
Other comprehensive income before reclassifications	(88,627)	(156,659)
Amounts reclassified from accumulated other comprehensive income	(17,370)	(2,044)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	71,257	154,615
Ending Balance	22,542	99,476
Income Tax Expense (Benefit)	1,991	2,605
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	85,903	30,755
Other comprehensive income before reclassifications	(73,367)	(125,159)
Amounts reclassified from accumulated other comprehensive income	(17,204)	(2,855)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	56,163	122,304
Ending Balance	142,066	153,059
Accumulated Translation Adjustment [Member]
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(141,527)	(93,634)
Other comprehensive income before reclassifications	(14,571)	(32,056)
Amounts reclassified from accumulated other comprehensive income	0	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	14,571	32,056
Ending Balance	(126,956)	(61,578)
Accumulated Gain (Loss), Financial Liability, Fair Value Option, Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets			$ (2,900)
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(1,965)	(2,900)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	156	1,114
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	156	1,114
Ending Balance	(1,809)	(1,786)
Other Postretirement Benefits Plan [Member]
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	8,874	10,640
Other comprehensive income before reclassifications	(689)	556
Amounts reclassified from accumulated other comprehensive income	(322)	(303)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	367	(859)
Ending Balance	9,241	9,781
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Income Tax Expense (Benefit)	(511)	1,708
Reclassification out of Accumulated Other Comprehensive Income [Member] | Other Postretirement Benefits Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax	(241)	(241)
Changes In Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income	(322)	(303)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	81	62
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	(322)	(303)
Income Tax Expense (Benefit)	$ 0	$ 0

Comprehensive Income - Schedu_2

Comprehensive Income - Schedule of Amounts Reclassed Out of Each Component of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	$ (1,991)	$ (2,605)
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, after Tax	(156)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(17,370)	(2,044)
Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized investment gains	(16,693)	(4,563)
Net of tax and noncontrolling interest	(17,204)	(2,855)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(17,204)	(2,855)
Accumulated Gain (Loss), Financial Liability, Fair Value Option, Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Unrealized Gain (Loss) Arising During Period, after Tax	156	1,114
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	156	1,114
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, before Tax, after Reclassification Adjustment	188	1,344
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax and noncontrolling interest	(17,370)	(2,044)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains (Losses) on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	511	(1,708)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss), Financial Liability, Fair Value Option, Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	32	230
Other Postretirement Benefits Plan [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(322)	(303)
Other Postretirement Benefits Plan [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	0	0
Prior service cost	(241)	(241)
Actuarial gains (losses)	(81)	(62)
Total before tax	(322)	(303)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	$ (322)	$ (303)

Net Income Per Share - Addition

Net Income Per Share - Additional Information (Detail) - $ / shares	Mar. 31, 2019	Dec. 31, 2018	May 01, 2013	Apr. 30, 2013
Schedule Of Earnings Per Share [Line Items]
Common stock, shares outstanding	45,520,541	45,336,278
Number of new common stock issued	45,560,960	45,365,170
Par value of common stock issued	$ 0.01	$ 0.01		$ 0.01
Exercise price of common stock			$ 16.67
Warrants outstanding	4,877,783

Net Income Per Share - Reconcil

Net Income Per Share - Reconciliation of Common Shares Used for Basic and Diluted Earnings Per Share (Detail) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of common shares used for basic earnings per share	45,832,297	45,471,083
Effect of potential dilutive shares:
Weighted average number of common shares and potential dilutive shares used for diluted earnings per share	45,832,297	45,653,471
Warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	4,877,783	4,053,476
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	16,667	126,667
Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	271,763	0
Performance Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	478,739	0
Warrants [Member]
Effect of potential dilutive shares:
Effect of potential dilutive shares	0	0
Employee Stock Option [Member]
Effect of potential dilutive shares:
Effect of potential dilutive shares	0	0
Restricted Stock Units (RSUs) [Member]
Effect of potential dilutive shares:
Effect of potential dilutive shares	0	45,713
Performance Shares [Member]
Effect of potential dilutive shares:
Effect of potential dilutive shares	0	136,675

Net Income Per Share Schedule o

Net Income Per Share Schedule of Basic and Dilutive Securities for EPS (Details) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Weighted Average Number of Shares Outstanding, Basic	45,832,297	45,471,083
Weighted Average Number of Shares Outstanding, Diluted	45,832,297	45,653,471
Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	271,763	0
Employee Stock Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	16,667	126,667
Warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	4,877,783	4,053,476
Performance Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	478,739	0
Performance Shares [Member]
Weighted Average Number Diluted Shares Outstanding Adjustment	0	136,675
Restricted Stock Units (RSUs) [Member]
Weighted Average Number Diluted Shares Outstanding Adjustment	0	45,713

Financial Guarantee Insurance_3

Financial Guarantee Insurance Contracts - Additional Information (Detail) € in Thousands, £ in Thousands, $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Sep. 30, 2017USD ($)	Dec. 31, 2018USD ($)	Mar. 31, 2019EUR (€)	Mar. 31, 2019GBP (£)	Feb. 12, 2019	Mar. 31, 2018EUR (€)	Mar. 31, 2018GBP (£)	Dec. 31, 2017USD ($)
Insurance [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	$ 1,766,022			$ 1,770,546
Financial Guarantee Insurance Contracts, Premium Receivable	$ 487,397	$ 580,707		$ 495,391						$ 586,312
Estimated Future Premium Payments Weighted Average Discounted Rate	2.60%			2.70%	2.60%	2.60%
Reinsurance Payable	$ 31,745			$ 32,913
Weighted average period of future premiums	8 years 9 months 18 days	8 years 8 months 12 days
Uncollectable premium receivables	$ 7,467			7,136
Past due premiums on policies insuring non-investment grade obligations amount	100
Accelerated premium revenue for retired obligations	12,223		$ 9,392
Reinsurance recoveries of losses included in losses and loss expenses	(4,996)	$ 1,354
Losses and loss expense reserves ceded to reinsurers	26,238	38,735		22,623						40,658
Subrogation recoveries, net of reinsurance	(1,739,836)			(1,744,243)
Intangible amortization expense	36,278	28,636
Intangible Assets, Gross (Excluding Goodwill)	1,561,070			1,551,576
Finite-Lived Intangible Assets, Accumulated Amortization	871,815			832,645
Insurance intangible asset	689,255			718,931
Possible Increase in Loss Reserves Related to Puerto Rico	950,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,665,888	2,109,165		1,633,147						$ 1,645,258
Stockholders Equity After Possible Increase in Domestic Public Finance Loss Reserves				$ 715,888
Percent Cash & New COFINA Bonds Allocated Under POA							93.00%
Loss Reserves [Member]
Insurance [Line Items]
Weighted average risk-free rate used to discount loss reserves	2.50%			2.80%
United Kingdom, Pounds
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	$ 141,527	163,926				£ 108,825			£ 116,815
Euro Member Countries, Euro
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Premium Receivable	30,062	$ 36,679			€ 26,807			€ 29,767
Commonwealth of Puerto Rico [Member]
Insurance [Line Items]
Domestic Public Finance Losses	$ 69,304
External Credit Rating, Non Investment Grade [Member]
Insurance [Line Items]
Transactions with non-investment grade internal ratings	19.00%			20.00%

Financial Guarantee Insurance_4

Financial Guarantee Insurance Contracts - Summary of Gross Premium Receivable Roll-Forward (Direct and Assumed Contracts) (Detail) € in Thousands, £ in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)	Mar. 31, 2019EUR (€)	Mar. 31, 2019GBP (£)	Mar. 31, 2018USD ($)	Mar. 31, 2018EUR (€)	Mar. 31, 2018GBP (£)	Sep. 30, 2017USD ($)	Dec. 31, 2018USD ($)
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount	$ 12,223						$ 9,392
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Beginning premium receivable	495,391			$ 586,312
Premium receipts	13,204			15,381
Adjustments for changes in expected and contractual cash flows	470			(1,289)
Accretion of premium receivable discount	3,228			3,846
Uncollectable premiums	(352)			604
Other adjustments (including foreign exchange)	1,864			6,615
Ending premium receivable	487,397			580,707
Premiums Receivable, Allowance for Doubtful Accounts	7,467							$ 7,136
United Kingdom, Pounds
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Ending premium receivable	141,527		£ 108,825	163,926		£ 116,815
Euro Member Countries, Euro
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Ending premium receivable	$ 30,062	€ 26,807		$ 36,679	€ 29,767

Financial Guarantee Insurance_5

Financial Guarantee Insurance Contracts - Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Written	$ 3,346	$ 4,261
Assumed Reinsurance Premiums Written	0	0
Ceded Reinsurance Premiums Written	(501)	(819)
Premiums written, net of reinsurance	(3,847)	(5,080)
Direct Premiums Earned	29,433	32,609
Assumed Reinsurance Premiums Earned	20	19
Ceded Reinsurance Premiums Earned	1,695	1,745
Reinsurance on premiums earned, Net	$ 27,758	$ 30,883

Financial Guarantee Insurance_6

Financial Guarantee Insurance Contracts - Summarized Future Gross Undiscounted Premiums Expected to be Collected, and Future Expected Premiums Earned, Net of Reinsurance (Detail) $ in Thousands	Mar. 31, 2019USD ($)
Future premiums expected to be collected, June 30. 2019	$ 12,906
Future premiums expected to be collected, September 30, 2019	11,923
Future premiums expected to be collected, December 31, 2019	11,852
Future premiums expected to be collected, December 31, 2020	46,963
Future premiums expected to be collected, December 31, 2021	41,005
Future premiums expected to be collected, December 31, 2022	39,159
Future premiums expected to be collected, December 31, 2023	37,641
Future premiums expected to be collected, December 31, 2028	166,827
Future premiums expected to be collected, December 31, 2033	128,064
Future premiums expected to be collected, December 31, 2038	67,195
Future premiums expected to be collected, December 31, 2043	26,556
Future premiums expected to be collected, December 31, 2048	11,831
Future premiums expected to be collected, December 31, 2053	2,252
Future premiums expected to be collected, December 31, 2058	31
Future premiums expected to be collected, Total	604,205
Future expected premiums to be earned, net of reinsurance, June 30, 2019	12,178
Future expected premiums to be earned, net of reinsurance, September 30, 2019	12,015
Future expected premiums to be earned, net of reinsurance, December 31, 2019	11,875
Future expected premiums to be earned, net of reinsurance, December 31, 2020	46,762
Future expected premiums to be earned, net of reinsurance, December 31, 2021	42,504
Future expected premiums to be earned, net of reinsurance, December 31, 2022	39,760
Future expected premiums to be earned, net of reinsurance, December 31, 2023	37,075
Future expected premiums to be earned, net of reinsurance, December 31, 2028	152,126
Future expected premiums to be earned, net of reinsurance, December 31, 2033	101,459
Future expected premiums to be earned, net of reinsurance, December 31, 2038	51,134
Future expected premiums to be earned, net of reinsurance, December 31, 2043	16,644
Future expected premiums to be earned, net of reinsurance, December 31, 2048	7,130
Future expected premiums to be earned, net of reinsurance, December 31, 2053	1,817
Future expected premiums to be earned, net of reinsurance, December 31, 2058	50
Future expected premiums to be earned, net of reinsurance, Total	$ 532,529

Financial Guarantee Insurance_7

Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Feb. 12, 2018	Dec. 31, 2017
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
satisfaction and discharge of segregated account deferred amounts				$ 3,000,158
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 550	$ 90	$ 510
Loss Reserves Ceded To Reinsurers	26,238	38,735	22,623		$ 40,658
Policyholder Benefits and Claims Incurred, Ceded	4,996	(1,354)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(176,033)	$ 205,588	(129,505)		$ 4,073,144
Loss and loss expense reserves	1,694,163		1,826,078
Subrogation recoverable	(1,916,117)		(1,932,960)
Claim liability reported on Balance Sheet, before reinsurance	(221,954)		(106,882)
Present Value of Expected Net Cash Flows- Claims and Loss Expenses [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	2,045,886		2,246,335
Subrogation recoverable	175,114		175,694
Claim liability reported on Balance Sheet, before reinsurance	2,221,000		2,422,029
Present Value of Expected Net Cash Flows-Recoveries [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(274,846)		(313,595)
Subrogation recoverable	(2,091,231)		(2,108,654)
Claim liability reported on Balance Sheet, before reinsurance	(2,366,077)		(2,422,249)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(76,877)		(106,662)
Subrogation recoverable	0		0
Claim liability reported on Balance Sheet, before reinsurance	$ (76,877)		$ (106,662)

Financial Guarantee Insurance_8

Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Feb. 12, 2018
Loss And Loss Adjustment Expense Reserves [Line Items]
Impact of VIE Consolidation on Gross Loss and Loss Reserves			$ (72,159)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves	$ (106,882)	$ 4,113,802
Less reinsurance on loss and loss expense reserves	22,623	40,658
Beginning balance of net loss and loss expense reserves	(129,505)	4,073,144
Current Year Claims and Claims Adjustment Expense	681	778
Prior Year Claims and Claims Adjustment Expense	11,726	(248,173)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	12,407	(247,395)
Claim and loss expense payments, net of subrogation and reinsurance	(28)	0
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(64,409)	(3,631,177)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	64,437	3,631,177
Liability for Unpaid Claims and Claims Adjustment Expense, Foreign Currency Translation Gain (Loss)	5,502	11,016
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(129,505)	4,073,144	(176,033)	$ (129,505)	$ 205,588
Add reinsurance on loss and loss expense reserves	26,238	38,735
Ending gross loss and loss expense reserves	(221,954)	244,323
Policyholder Benefits and Claims Incurred, Ceded	4,996	(1,354)
Net Incurred RMBS Subrogation Recoveries	$ (4,407)	$ (800)
satisfaction and discharge of segregated account deferred interest amounts						$ 856,834
Loss and Loss Expense Benefit on Settled Deferred Amounts						$ 288,204
Ceded Loss And Loss Expenses Paid Not Yet Recovered			$ 550	$ 510	$ 90

Financial Guarantee Insurance_9

Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019USD ($)Policies	Dec. 31, 2018USD ($)Policies	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	$ 26,238	$ 22,623	$ 38,735	$ 40,658
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 550	$ 510	$ 90
Number of policies | Policies	239	231
Remaining weighted-average contract period (in years)	15 years	16 years
Gross insured contractual payments outstanding:
Principal	$ 8,117,979	$ 9,403,405
Interest	3,850,219	10,582,773
Total	11,968,198	19,986,178
Gross undiscounted claim liability	2,918,360	3,448,228
Discount, gross claim liability	780,787	1,091,958
Gross claim liability before all subrogation and before reinsurance	2,137,573	2,356,270
Less:
Gross RMBS subrogation	(1,803,302)	(1,809,937)
Discount, RMBS subrogation	37,280	39,391
Discounted RMBS subrogation, before reinsurance	(1,766,022)	(1,770,546)
Less:
Gross other subrogation	(658,748)	(784,891)
Discount, other subrogation	58,693	133,188
Discounted other subrogation, before reinsurance	(600,055)	(651,703)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(228,504)	(65,979)
Less: Unearned premium reserves	(76,877)	(106,662)
Plus: Loss adjustment expenses reserves	83,427	65,759
Claim liability reported on Balance Sheet, before reinsurance	(221,954)	(106,882)
Reinsurance recoverable reported on Balance Sheet	$ 26,788	$ 23,133
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	33	21
Remaining weighted-average contract period (in years)	9 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 934,336	$ 916,530
Interest	514,643	487,702
Total	1,448,979	1,404,232
Gross undiscounted claim liability	6,344	4,019
Discount, gross claim liability	620	481
Gross claim liability before all subrogation and before reinsurance	5,724	3,538
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	0	0
Discount, other subrogation	0	0
Discounted other subrogation, before reinsurance	0	0
Gross claim liability, net of all subrogation and discounts, before reinsurance	5,724	3,538
Less: Unearned premium reserves	(3,791)	(943)
Plus: Loss adjustment expenses reserves	1,809	1,369
Claim liability reported on Balance Sheet, before reinsurance	3,742	3,964
Reinsurance recoverable reported on Balance Sheet	$ 216	$ 367
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	28	28
Remaining weighted-average contract period (in years)	18 years	19 years
Gross insured contractual payments outstanding:
Principal	$ 754,502	$ 708,249
Interest	646,737	631,708
Total	1,401,239	1,339,957
Gross undiscounted claim liability	56,257	63,712
Discount, gross claim liability	7,729	13,008
Gross claim liability before all subrogation and before reinsurance	48,528	50,704
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	0	(10,816)
Discount, other subrogation	0	7,318
Discounted other subrogation, before reinsurance	0	(3,498)
Gross claim liability, net of all subrogation and discounts, before reinsurance	48,528	47,206
Less: Unearned premium reserves	(9,639)	(10,073)
Plus: Loss adjustment expenses reserves	5,330	4,253
Claim liability reported on Balance Sheet, before reinsurance	44,219	41,386
Reinsurance recoverable reported on Balance Sheet	$ 8,901	$ 7,285
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	12	18
Remaining weighted-average contract period (in years)	8 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 177,251	$ 622,820
Interest	87,602	293,293
Total	264,853	916,113
Gross undiscounted claim liability	25,466	36,000
Discount, gross claim liability	1,897	3,069
Gross claim liability before all subrogation and before reinsurance	23,569	32,931
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	0	0
Discount, other subrogation	0	0
Discounted other subrogation, before reinsurance	0	0
Gross claim liability, net of all subrogation and discounts, before reinsurance	23,569	32,931
Less: Unearned premium reserves	(1,640)	(5,085)
Plus: Loss adjustment expenses reserves	902	2,564
Claim liability reported on Balance Sheet, before reinsurance	22,831	30,410
Reinsurance recoverable reported on Balance Sheet	$ 3,758	$ 4,223
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	16	16
Remaining weighted-average contract period (in years)	18 years	22 years
Gross insured contractual payments outstanding:
Principal	$ 909,382	$ 1,705,464
Interest	443,526	6,979,130
Total	1,352,908	8,684,594
Gross undiscounted claim liability	591,366	992,019
Discount, gross claim liability	202,850	433,709
Gross claim liability before all subrogation and before reinsurance	388,516	558,310
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(46,914)	(136,541)
Discount, other subrogation	6,295	67,008
Discounted other subrogation, before reinsurance	(40,619)	(69,533)
Gross claim liability, net of all subrogation and discounts, before reinsurance	347,897	488,777
Less: Unearned premium reserves	(7,219)	(36,365)
Plus: Loss adjustment expenses reserves	2,362	(5,926)
Claim liability reported on Balance Sheet, before reinsurance	343,040	446,486
Reinsurance recoverable reported on Balance Sheet	$ 27,138	$ 26,096
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	147	145
Remaining weighted-average contract period (in years)	14 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 5,299,368	$ 5,407,202
Interest	2,144,310	2,177,539
Total	7,443,678	7,584,741
Gross undiscounted claim liability	2,182,417	2,295,968
Discount, gross claim liability	564,236	637,548
Gross claim liability before all subrogation and before reinsurance	1,618,181	1,658,420
Less:
Gross RMBS subrogation	(1,803,302)	(1,809,937)
Discount, RMBS subrogation	37,280	39,391
Discounted RMBS subrogation, before reinsurance	(1,766,022)	(1,770,546)
Less:
Gross other subrogation	(598,954)	(624,654)
Discount, other subrogation	48,954	55,088
Discounted other subrogation, before reinsurance	(550,000)	(569,566)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(697,841)	(681,692)
Less: Unearned premium reserves	(54,395)	(53,987)
Plus: Loss adjustment expenses reserves	73,024	63,499
Claim liability reported on Balance Sheet, before reinsurance	(679,212)	(672,180)
Reinsurance recoverable reported on Balance Sheet	$ (13,225)	$ (14,838)
V [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	3	3
Remaining weighted-average contract period (in years)	3 years	3 years
Gross insured contractual payments outstanding:
Principal	$ 43,140	$ 43,140
Interest	13,401	13,401
Total	56,541	56,541
Gross undiscounted claim liability	56,510	56,510
Discount, gross claim liability	3,455	4,143
Gross claim liability before all subrogation and before reinsurance	53,055	52,367
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(12,880)	(12,880)
Discount, other subrogation	3,444	3,774
Discounted other subrogation, before reinsurance	(9,436)	(9,106)
Gross claim liability, net of all subrogation and discounts, before reinsurance	43,619	43,261
Less: Unearned premium reserves	(193)	(209)
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	43,426	43,052
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0

Financial Guarantee Insuranc_10

Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Phantom) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	$ 26,238	$ 22,623	$ 38,735	$ 40,658
Loss and loss expense reserves	1,694,163	1,826,078
Subrogation recoverable	(1,916,117)	(1,932,960)
Liability for Claims	(221,954)	(106,882)	244,323	$ 4,113,802
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 550	$ 510	$ 90

Financial Guarantee Insuranc_11

Financial Guarantee Insurance Contracts - Summary of Balance of RMBS Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Balance Of Rmbs Subrogation Recoveries Net Of Reinsurance	$ (1,739,836)		$ (1,744,243)
Subrogation recoveries	1,766,022		1,770,546
Random Samples [Member]
Schedule Of Balance Of Rmbs Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items]
Subrogation recoveries	1,766,022	$ 1,833,510	$ 1,770,546	$ 1,834,387
Other Changes Rmbs Subrogation	$ (4,524)	$ (877)

Financial Guarantee Insuranc_12

Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	$ 1,770,546
Changes recognized
Discounted RMBS subrogation (gross of reinsurance), ending balance	1,766,022
Random Samples [Member]
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	1,770,546	$ 1,834,387
Changes recognized
Impact of sponsor actions	0	0
Other Changes Rmbs Subrogation	(4,524)	(877)
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 1,766,022	$ 1,833,510

Financial Guarantee Insuranc_13

Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Summary of Percentage Ceded to Reinsurers and Reinsurance Recoverable and Rating Levels (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Insurance [Abstract]
Loss Reserves Ceded To Reinsurers	$ 26,238	$ 22,623	$ 38,735	$ 40,658
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 550	$ 510	$ 90

Financial Guarantee Insuranc_14

Financial Guarantee Insurance Contracts - Estimated Future Amortization Expense for Insurance Intangible Asset (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Amortization Of Intangible Assets [Line Items]
Amortization of insurance intangible assets	$ 36,278	$ 28,636
Intangible Assets, Gross (Excluding Goodwill)	1,561,070		$ 1,551,576
Finite-Lived Intangible Assets, Accumulated Amortization	871,815		832,645
2019	47,410
2020	58,672
2021	53,229
2022	49,565
2023	46,241
Thereafter	434,138
Insurance intangible asset	$ 689,255		$ 718,931

Financial Guarantee Insuranc_15

Financial Guarantee Insurance Contracts Earned Premiums by Geographic Location (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Premiums Earned, Net, Financial Guarantee Insurance Contracts	$ 27,758	$ 30,883
United States [Member] | Reportable Geographical Components [Member]
Premiums Earned, Net, Financial Guarantee Insurance Contracts	28,244	24,718
United Kingdom [Member] | Reportable Geographical Components [Member]
Premiums Earned, Net, Financial Guarantee Insurance Contracts	4,190	4,856
Other International [Member] | Reportable Geographical Components [Member]
Premiums Earned, Net, Financial Guarantee Insurance Contracts	$ (4,676)	$ 1,309

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	$ 3,611,947,000
Financial assets:
Short term investments	908,235,000	$ 430,331,000
Other investments	428,556,000	391,217,000
Derivative assets	76,789,000	59,768,000
Other assets	4,148,000	4,516,000
Long-term Debt	2,929,227,000	2,928,929,000
Financial liabilities:
Derivative liabilities	86,923,000	76,999,000
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	3,128,995,000	2,737,286,000
Financial assets:
Loans	4,375,761,000	4,287,664,000
Derivative assets	59,228,000	66,302,000
Long-term Debt	5,737,263,000	5,268,596,000
Financial liabilities:
Derivative liabilities	1,781,903,000	1,712,062,000
Long-term debt	5,401,992,000	5,268,596,000
Short-term Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	908,235,000	430,331,000
Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	2,858,404,000	2,737,286,000
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	270,591,000	0
Total Fair Value [Member]
Financial assets:
Cash	21,840,000	82,494,000
Reported Value Measurement [Member]
Financial assets:
Cash	21,840,000	82,494,000
Loans	10,124,000	9,913,000
Other assets	4,148,000	4,516,000
Total financial assets	11,720,253,000	11,185,865,000
Financial liabilities:
Liabilities for net financial guarantees written	(839,056,000)	(718,388,000)
Long-term debt	3,320,562,000	3,304,737,000
Total financial liabilities	10,087,206,000	9,643,706,000
Reported Value Measurement [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	3,254,000	999,000
Loans	4,375,761,000	4,287,664,000
Financial liabilities:
Long-term debt	5,737,263,000	5,268,596,000
Reported Value Measurement [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	908,235,000	430,331,000
Reported Value Measurement [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	428,556,000	391,217,000
Reported Value Measurement [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	2,858,404,000	2,737,286,000
Reported Value Measurement [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	270,591,000
Reported Value Measurement [Member] | Future [Member]
Financial assets:
Derivative assets	3,182,000
Financial liabilities:
Derivative liabilities	0	3,379,000
Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0
Derivative liabilities	85,457,000	71,861,000
Reported Value Measurement [Member] | Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	1,781,903,000	1,712,062,000
Reported Value Measurement [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	1,077,000	1,459,000
Other Assets [Member] | Reported Value Measurement [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	73,218,000	59,468,000
Interest Rate Swaps - Liability Position [Member] | Reported Value Measurement [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	59,228,000	66,302,000
Level 1 [Member]
Financial assets:
Cash	15,891,000	52,661,000
Total financial assets	1,203,358,000	553,964,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term debt	0	0
Total financial liabilities	0	3,379,000
Level 1 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	3,254,000	999,000
Loans	0	0
Financial liabilities:
Long-term debt	0	0
Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	864,065,000	304,880,000
Level 1 [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	0	0
Level 1 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 1 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	102,257,000	71,108,000
Level 1 [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Level 1 [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0
Level 1 [Member] | Future [Member]
Financial assets:
Derivative assets	3,182,000
Financial liabilities:
Derivative liabilities	0	3,379,000
Level 1 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0
Derivative liabilities	0	0
Level 1 [Member] | Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 1 [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 1 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	0	0
Level 1 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	0	0
Level 2 [Member]
Financial assets:
Cash	5,949,000	29,833,000
Total financial assets	2,816,193,000	3,152,882,000
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term debt	2,942,229,000	2,909,272,000
Total financial liabilities	10,370,724,000	9,744,699,000
Level 2 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	0	0
Loans	0	0
Financial liabilities:
Long-term debt	5,561,135,000	5,051,504,000
Level 2 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	44,170,000	125,451,000
Level 2 [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	0	0
Level 2 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 2 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	0	0
Level 2 [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Level 2 [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	270,591,000
Level 2 [Member] | Future [Member]
Financial assets:
Derivative assets	0
Financial liabilities:
Derivative liabilities	0	0
Level 2 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position
Derivative liabilities	85,457,000	71,861,000
Level 2 [Member] | Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	1,781,903,000	1,712,062,000
Level 2 [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 2 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	19,107,000	12,008,000
Level 2 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	59,228,000	66,302,000
Level 3 [Member]
Financial assets:
Cash	0	0
Total financial assets	7,394,196,000	7,176,883,000
Financial liabilities:
Liabilities for net financial guarantees written	509,830,000	558,824,000
Long-term debt	382,084,000	350,694,000
Total financial liabilities	1,117,438,000	1,128,069,000
Level 3 [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	0	0
Loans	4,375,761,000	4,287,664,000
Financial liabilities:
Long-term debt	224,447,000	217,092,000
Level 3 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	0	0
Level 3 [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	4,148,000	4,516,000
Level 3 [Member] | Loans Receivable [Member]
Financial assets:
Loans	12,187,000	11,620,000
Level 3 [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	17,730,000	16,266,000
Level 3 [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	2,858,404,000	2,737,286,000
Level 3 [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0
Level 3 [Member] | Future [Member]
Financial assets:
Derivative assets	0
Financial liabilities:
Derivative liabilities
Level 3 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0
Derivative liabilities	0	0
Level 3 [Member] | Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 3 [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	1,077,000	1,459,000
Level 3 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	54,111,000	47,460,000
Level 3 [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	0	0
Municipal Bonds [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	424,884,000	879,919,000
Corporate Debt Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	1,233,544,000	1,278,122,000
Debt Security, Government, Non-US [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	31,292,000	30,834,000
US Government Debt Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	100,429,000	94,394,000
Residential Mortgage-Backed Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	256,285,000	258,607,000
Collateralized Debt Obligations [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	132,977,000	131,356,000
Asset-backed Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	440,400,000	442,443,000
Fair Value, Measurements, Recurring [Member]
Financial assets:
Total financial assets	11,698,535,000	11,163,670,000
Financial liabilities:
Liabilities for net financial guarantees written	509,830,000	558,824,000
Long-term debt	3,324,313,000	3,259,966,000
Total financial liabilities	11,488,162,000	10,876,147,000
Fair Value, Measurements, Recurring [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Restricted Cash and Cash Equivalents, Current	3,254,000	999,000
Loans	4,375,761,000	4,287,664,000
Financial liabilities:
Long-term debt	5,785,582,000	5,268,596,000
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	908,235,000	430,331,000
Fair Value, Measurements, Recurring [Member] | Equity Method Investments [Member]
Financial assets:
Other assets	4,148,000	4,516,000
Fair Value, Measurements, Recurring [Member] | Loans Receivable [Member]
Financial assets:
Loans	12,187,000	11,620,000
Fair Value, Measurements, Recurring [Member] | Other Debt Obligations [Member]
Financial assets:
Other investments	404,775,000	367,315,000
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	440,400,000	442,443,000
Fair Value, Measurements, Recurring [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	132,977,000	131,356,000
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	256,285,000	258,607,000
Fair Value, Measurements, Recurring [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	100,429,000	94,394,000
Fair Value, Measurements, Recurring [Member] | Debt Security, Government, Non-US [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	31,292,000	30,834,000
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	1,233,544,000	1,278,122,000
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	2,858,404,000	2,737,286,000
Fair Value, Measurements, Recurring [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	424,884,000	879,919,000
Fair Value, Measurements, Recurring [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	270,591,000
Fair Value, Measurements, Recurring [Member] | Future [Member]
Financial assets:
Derivative assets	3,182,000
Financial liabilities:
Derivative liabilities	0	3,379,000
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities - interest rate swaps - asset position	0
Derivative liabilities	85,457,000	71,861,000
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial liabilities:
Derivative liabilities	1,781,903,000	1,712,062,000
Fair Value, Measurements, Recurring [Member] | Other Credit Derivatives [Member]
Financial liabilities:
Derivative liabilities	1,077,000	1,459,000
Fair Value, Measurements, Recurring [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	73,218,000	59,468,000
Fair Value, Measurements, Recurring [Member] | Interest Rate Swaps - Liability Position [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Financial assets:
Derivative assets	59,228,000	66,302,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	100,429,000	94,394,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Debt Security, Government, Non-US [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	30,379,000	29,922,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	368,545,000	370,372,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	132,977,000	131,356,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	256,285,000	258,607,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Debt Security, Government, Non-US [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	913,000	912,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	1,233,544,000	1,278,122,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	424,884,000	879,919,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	71,855,000	72,071,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Collateralized Debt Obligations [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Debt Security, Government, Non-US [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Municipal Bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0	$ 0
Collateral Pledged [Member] | US Government Debt Securities [Member] | Reported Value Measurement [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	83,901,000
Collateral Pledged [Member] | Fair Value, Measurements, Recurring [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	83,901,000
Collateral Pledged [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	83,901,000
Collateral Pledged [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	0
Collateral Pledged [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | US Government Debt Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	$ 93	$ 138
Other investments	428,556	391,217
Derivative liabilities	$ 86,923	$ 76,999
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.60%	2.70%
Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other investments	$ 284,788	$ 279,941
Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using dealer quotes	8.00%	8.00%
Minimum [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using internal valuation models	2.00%	2.00%
Maximum [Member] | Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using external pricing services	90.00%	90.00%
Variable Interest Entity [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	3.00%	3.10%
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative liabilities	$ 1,781,903	$ 1,712,062

Fair Value Measurements - Infor

Fair Value Measurements - Information about Valuation Inputs for Fixed Income Securities Classified as Level 3 (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	$ 7,138,755	$ 11,950,737	$ 6,930,446	$ 12,561,185
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	160,349	(86,178)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	134,688	378,151
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	86,728	902,421
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	159,875	(121,468)
Derivative [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	53,034	49,908	46,001	60,808
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	8,187	(9,375)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	1,154	1,525
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	8,096	(9,481)
Credit Derivative [Member] | Credit Derivative [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	(1,077)	(1,018)	(1,459)	(566)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	473	(346)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	91	106
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	382	(452)
Interest Rate Swap [Member] | Interest Rate Swap [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	54,111	50,926	$ 47,460	$ 61,374
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	7,714	(9,029)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	1,063	1,419
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	$ 7,714	$ (9,029)

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Information about Described Model Inputs Used to Determine Fair Value of Each Class of Credit Derivatives (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other investments	$ 428,556	$ 391,217
Fair value of derivative liabilities	73,510
Real Estate [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other investments	16,150	16,123
Interest Rate Contract [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other investments	207,704	177,357
Illiquid Investments [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other investments	$ 85,323	$ 84,297
Asset-backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Inputs Coupon Rate	5.97%	5.97%
Fair Value Inputs Maturity	16 years 1 month 9 days	16 years 3 months 14 days
Fair Value Inputs Yield	12.00%	12.00%
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair value of derivative liabilities	$ 0
Variable Interest Entity, Primary Beneficiary [Member] | European Transactions [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Inputs Coupon Rate	2.20%	2.20%
Fair Value Inputs Maturity	18 years 8 months 4 days	18 years 11 months 4 days
Fair Value Inputs Yield	3.05%	3.18%

Fair Value Measurements - Inf_2

Fair Value Measurements - Information about Valuation Inputs for Variable Interest Entity Assets and Liabilities Classified as Level 3 (Detail) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	$ 7,138,755	$ 11,950,737	$ 6,930,446	$ 12,561,185
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	160,349	(86,178)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	134,688	378,151
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(86,728)	(902,421)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	159,875	(121,468)
Investments [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	71,855	77,630	72,071	808,557
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	383	35,184
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	(270)	(52,508)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(329)	(713,603)
European ABS Transactions [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Of Consolidated Variable Interest Entity Debt Obligations Valued Using Unobservable Inputs By Asset Class	224,447		217,092
Other Assets [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	4,148	5,621	4,516	5,979
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	(368)	(358)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	(368)	(358)
Derivative Financial Instruments, Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	53,034	49,908	46,001	60,808
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	8,187	(9,375)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(1,154)	(1,525)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	8,096	(9,481)
Variable Interest Entity, Primary Beneficiary [Member] | Loan Origination Commitments [Member] | Loans Receivable [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	4,375,761	11,558,331	4,287,664	11,529,384
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	88,281	(202,588)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	85,061	423,892
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	(85,245)	(192,357)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	88,281	(202,588)
Variable Interest Entity, Primary Beneficiary [Member] | Long-term Debt [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	(224,447)	(2,696,516)	(217,092)	(2,757,688)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	(3,053)	159,985
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	(4,302)	(103,877)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	0	5,064
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	(3,053)	159,985
Variable Interest Entity, Primary Beneficiary [Member] | Investment Contracts [Member] | Corporate Debt Securities [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	2,858,404	2,955,763	$ 2,737,286	$ 2,914,145
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	66,919	(69,026)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	54,199	110,644
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	$ 66,919	$ (69,026)

Fair Value Measurements - Sum_3

Fair Value Measurements - Summary of Changes in Level 3 Fair Value Category (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 71,855	$ 77,630	$ 72,071	$ 808,557
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	383	35,184
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss)	(270)	(52,508)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(329)	(713,603)
Non-Agency RMBS [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0	5,309	0	736,017
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	0	34,727
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	(52,138)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	(713,297)
Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0
Asset-backed Securities [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	71,855	72,321	72,071	72,540
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss)	0	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings	383	457
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss)	(270)	(370)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(329)	(306)
Interest Rate Swap [Member] | Interest Rate Swap [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	54,111	50,926	47,460	61,374
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	7,714	(9,029)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	7,714	(9,029)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	1,063	1,419
Credit Derivative [Member] | Credit Derivative [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	(1,077)	(1,018)	(1,459)	(566)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	382	(452)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	473	(346)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	91	106
Derivative [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	53,034	49,908	46,001	60,808
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	8,096	(9,481)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	8,187	(9,375)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Purchases	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Issues	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Sales	0	0
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	1,154	1,525
Level 3 [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	7,138,755	11,950,737	6,930,446	12,561,185
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	159,875	(121,468)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	160,349	(86,178)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	134,688	378,151
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	86,728	902,421
Level 3 [Member] | Investments [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	71,855	77,630	72,071	808,557
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	383	35,184
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	(270)	(52,508)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	329	713,603
Level 3 [Member] | Other Assets [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	4,148	5,621	4,516	5,979
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	(368)	(358)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	(368)	(358)
Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	53,034	49,908	46,001	60,808
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	8,096	(9,481)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	8,187	(9,375)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	1,154	1,525
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Long-term Debt [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	(224,447)	(2,696,516)	(217,092)	(2,757,688)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	(3,053)	159,985
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	(3,053)	159,985
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	(4,302)	(103,877)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	0	(5,064)
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Loans Receivable [Member] | Loans [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	4,375,761	11,558,331	4,287,664	11,529,384
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	88,281	(202,588)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	88,281	(202,588)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	85,061	423,892
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Settlements	85,245	192,357
Variable Interest Entity, Primary Beneficiary [Member] | Level 3 [Member] | Corporate Debt Securities [Member] | Investments Contract [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability	2,858,404	2,955,763	$ 2,737,286	$ 2,914,145
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	66,919	(69,026)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Earnings	66,919	(69,026)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Gain Loss Included In Other Comprehensive Income Loss	$ 54,199	$ 110,644

Fair Value Measurements - Sum_4

Fair Value Measurements - Summary of Gains and Losses (Realized and Unrealized) Relating to Level 3 Assets and Liabilities Included in Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net Investment Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings for the period	$ 383	$ 35,184
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	0	0
Gain (Loss) on Derivative Instruments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings for the period	8,187	(9,375)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	8,096	(9,481)
Income Loss On Variable Interest Entities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings for the period	152,147	(111,629)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	152,147	(111,629)
Other Income or (Loss) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Total gains or losses included in earnings for the period	(368)	(358)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held	$ (368)	$ (358)

Investments - Summary of Amorti

Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 3,472,642	$ 3,451,149
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	147,650	133,052
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	283,413	1,139,101
Non - credit other - than - temporary Impairments	123	32
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,800	27,298
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	424,000	359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5,545	10,897
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,345	38,195
Available-for-sale Securities, Current	3,611,947	3,546,006
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	3,611,947
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	908,209	430,405
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	43	23
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	99,429	115,374
Non - credit other - than - temporary Impairments	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	17	97
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	17	97
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	908,235	430,331
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	2,480,532	3,020,744
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	147,607	133,029
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	183,984	1,023,727
Non - credit other - than - temporary Impairments	123	32
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,783	27,201
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	424,000	359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5,545	10,897
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,328	38,098
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	2,619,811	3,115,675
Fixed Income Securities [Member] | Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	406,315	882,631
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	20,356	14,364
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	20,997	537,904
Non - credit other - than - temporary Impairments	0	5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	951	15,878
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	25,865	28,533
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	836	1,198
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,787	17,076
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	424,884	879,919
Fixed Income Securities [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,223,640	1,288,882
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	14,312	6,444
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	39,639	306,506
Non - credit other - than - temporary Impairments	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	550	8,634
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	256,406	190,273
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,858	8,570
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,408	17,204
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	1,233,544	1,278,122
Fixed Income Securities [Member] | Foreign Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	30,719	30,496
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	599	399
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	401	1,161
Non - credit other - than - temporary Impairments	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,980	5,163
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	22	60
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	26	61
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	31,292	30,834
Fixed Income Securities [Member] | US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	99,808	93,636
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,185	1,371
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	12,793	5,643
Non - credit other - than - temporary Impairments		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	360	135
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	53,861	58,495
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	204	478
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	564	613
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	100,429	94,394
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	216,904	221,825
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	39,504	37,575
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	4,375	34,852
Non - credit other - than - temporary Impairments	123	27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	122	793
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	243	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	123	793
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	256,285	258,607
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	133,736	133,075
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	28	8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	101,939	123,848
Non - credit other - than - temporary Impairments	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	787	1,727
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	787	1,727
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	132,977	131,356
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	369,410	370,199
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	71,623	72,868
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	3,840	13,813
Non - credit other - than - temporary Impairments	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9	33
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	83,645	77,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	624	591
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	633	624
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	440,400	442,443
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	3,388,741
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	147,650
Non - credit other - than - temporary Impairments	123
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,345
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	3,528,046
Collateral Pledged [Member] | Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	83,901	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0
Non - credit other - than - temporary Impairments	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	83,901	$ 0
Collateral Pledged [Member] | Fixed Income Securities [Member] | US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	83,901
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0
Non - credit other - than - temporary Impairments	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0
Fixed income securities, at fair value (amortized cost of $2,480,532 and $3,020,744)	83,901
Asset Pledged as Collateral [Member] | Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 83,901

Investments - Summary of Amor_2

Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments Held by Successor Ambac, by Contractual Maturity (Detail) $ in Thousands	Mar. 31, 2019USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, Due in one year or less	$ 1,076,361
Amortized Cost, Due after one year through five years	999,235
Amortized Cost, Due after five years through ten years	316,441
Amortized Cost, Due after ten years	360,555
Amortized Cost, Total	2,752,592
Estimated Fair Value, Due in one year or less	1,076,545
Estimated Fair Value, Due after one year through five years	1,006,812
Estimated Fair Value, Due after five years through ten years	323,494
Estimated Fair Value, Due after ten years	375,434
Estimated Fair Value due, Total	$ 2,782,285

Investments - Summary of Gross

Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 283,413	$ 1,139,101
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,800	27,298
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	424,000	359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5,545	10,897
Debt Securities, Available-for-sale, Unrealized Loss Position	707,413	1,499,044
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,345	38,195
Below Investment Grade Securities and Non-Rated Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Fair Value	121,037
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses	2,086
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	99,429	115,374
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	17	97
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Unrealized Loss Position	99,429	115,374
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	17	97
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,345
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	183,984	1,023,727
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,783	27,201
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	424,000	359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5,545	10,897
Debt Securities, Available-for-sale, Unrealized Loss Position	607,984	1,383,670
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,328	38,098
Fixed Income Securities [Member] | Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	20,997	537,904
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	951	15,878
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	25,865	28,533
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	836	1,198
Debt Securities, Available-for-sale, Unrealized Loss Position	46,862	566,437
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,787	17,076
Fixed Income Securities [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	39,639	306,506
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	550	8,634
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	256,406	190,273
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,858	8,570
Debt Securities, Available-for-sale, Unrealized Loss Position	296,045	496,779
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,408	17,204
Fixed Income Securities [Member] | Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	401	1,161
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,980	5,163
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	22	60
Debt Securities, Available-for-sale, Unrealized Loss Position	4,381	6,324
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	26	61
Fixed Income Securities [Member] | US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	12,793	5,643
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	360	135
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	53,861	58,495
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	204	478
Debt Securities, Available-for-sale, Unrealized Loss Position	66,654	64,138
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	564	613
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	4,375	34,852
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	122	793
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	243	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	0
Debt Securities, Available-for-sale, Unrealized Loss Position	4,618	34,852
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	123	793
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	101,939	123,848
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	787	1,727
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Unrealized Loss Position	101,939	123,848
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	787	1,727
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	3,840	13,813
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9	33
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	83,645	77,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	624	591
Debt Securities, Available-for-sale, Unrealized Loss Position	87,485	91,292
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	633	$ 624
Collateral Pledged [Member] | Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0
Collateral Pledged [Member] | Fixed Income Securities [Member] | US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 0

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Other investments	$ 428,556	$ 391,217
Securities fair value	6,046	5,975
Ambac Assurance [Member]
Investment [Line Items]
Unsettled Amounts Secured Notes from LSNI	0	19,405
Ambac UK [Member]
Investment [Line Items]
Asset Backed Securities Fair Value Disclosure	144,530	144,672
Interest Rate Contract [Member]
Investment [Line Items]
Other investments	207,704	177,357
Reported Value Measurement [Member] | Interest Rate Contract [Member]
Investment [Line Items]
Other investments	$ 53,135	$ 27,154

Investments - Summary of Amount

Investments - Summary of Amounts Included in Net Realized (Losses) Gains and Other-Than-Temporary Impairments (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Investment [Line Items]
Gross realized gains on securities	$ 24,219	$ 11,054
Gross realized losses on securities	(4,288)	(1,388)
Foreign exchange (losses) gains	1,960	5,014
Net realized (losses) gains	17,233	4,862
Net other-than-temporary impairments	(29)	(299)
Gain (Loss) on Investments [Member]
Investment [Line Items]
Foreign exchange (losses) gains	$ (2,698)	$ (4,804)

Investments - Summary of Roll-F

Investments - Summary of Roll-Forward of Ambac's Cumulative Credit Losses on Debt Securities for Which Portion of Other-than-Temporary Impairment was Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning	$ 12,405	$ 43,633	$ 12,454	$ 67,085
Additions for credit impairments recognized on:
Securities not previously impaired	0	226
Securities previously impaired	0	64
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	49	23,742
Balance at end	$ 12,405	$ 43,633	$ 12,454	$ 67,085

Investments - Summary of Source

Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment [Line Items]
Fair Value Of Securities Deposited With Governmental Authorities	$ 6,046	$ 5,975
Fair Value of Securities Deposited in Connection wtih Letter of Credit	1,403
Market Value of Secured Note Collateral	207,759	209,983
Investment Portfolio [Member]
Investment [Line Items]
Fair Value of Cash and Securities Pledged to Derivative Counterparties	$ 111,014	$ 102,904

Investments - Summary of Fair V

Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Standard & Poor's, CC Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		$ 2,104,499
Standard & Poor's, CC Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 1,349,832	2,088,899
Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	13,839	15,600
Standard & Poor's, CCC Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	1,363,671
Municipal Bonds [Member] | Standard & Poor's, CC Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		848,841
Municipal Bonds [Member] | Standard & Poor's, CC Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	166,770	833,241
Municipal Bonds [Member] | Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	13,839	15,600
Municipal Bonds [Member] | Standard & Poor's, CCC Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	180,609
Corporate Debt Securities [Member] | Standard & Poor's, CC Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		656,473
Corporate Debt Securities [Member] | Standard & Poor's, CC Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	583,550	656,473
Corporate Debt Securities [Member] | Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	0	0
Corporate Debt Securities [Member] | Standard & Poor's, CCC Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	583,550
Mortgage and Asset-Backed Securities [Member] | Standard & Poor's, CC Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors		599,185
Mortgage and Asset-Backed Securities [Member] | Standard & Poor's, CC Rating [Member] | Ambac Assurance Corporation [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	599,512	$ 599,185
Mortgage and Asset-Backed Securities [Member] | Standard & Poor's, CCC Rating [Member]
Investment [Line Items]
Fair value of securities that include benefit of guarantees provided by financial guarantors	$ 599,512

Investments - Summary of Fair_2

Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Phantom) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Ambac UK [Member]
Investment [Line Items]
Corporate and Asset Backed Securities Fair Value Disclosure	$ 144,530	$ 144,672

Investments - Summary of Net In

Investments - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 6,947	$ (1,579)
Trading Securities, Realized Gain (Loss)	631	1,933
Investment expense	(1,411)	(1,827)
Securities available-for-sale and short-term	46,552	110,551
Total net investment income	54,842	110,240
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Other Investments Income	6,316	(3,512)
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	43,666	109,351
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	4,113	2,840
Loans Receivable [Member]
Investment [Line Items]
Gross Investment Income, Operating	184	187
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 8,290	$ (311)

Investments Equity Investments

Investments Equity Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Other investments	$ 428,556	$ 391,217
Real Estate [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	16,150	16,123
Interest Rate Contract [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	207,704	177,357
Illiquid Investments [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	85,323	84,297
Insurance Linked [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	28,746	29,318
Equity [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	49,122	43,954
Equity investments in pooled funds [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	387,045	351,049
Readily determinable fair value [Member] | Interest Rate Contract [Member]
Schedule of Equity Method Investments [Line Items]
Other investments	$ 53,135	$ 27,154

Derivative Instruments - Summar

Derivative Instruments - Summary of Gross Fair Values of Individual Derivative Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	$ 76,400	$ 59,468
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	0	0
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	86,534	76,699
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	73,510
Gross Amount of Collateral Received/Pledged Not Offset in the Consolidated Balance Sheet, Derivative Liabilities		70,505
Net Amount, Derivative Assets	76,400	59,468
Derivative Asset, Fair Value, Gross Asset	76,789	59,768
Derivative Asset, Fair Value, Amount Offset Against Collateral	389	300
Derivative liabilities	86,923	76,999
Derivative Liability, Fair Value, Amount Offset Against Collateral	389	300
Derivative Liabilities Net Amount	13,024	6,194
Variable Interest Entity, Primary Beneficiary [Member]
Derivatives, Fair Value [Line Items]
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	59,228	66,302
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	0	0
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	1,781,903	1,712,062
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0
Gross Amount of Collateral Received/Pledged Not Offset in the Consolidated Balance Sheet, Derivative Liabilities		0
Net Amount, Derivative Assets	59,228	66,302
Derivative Asset, Fair Value, Gross Asset	59,228	66,302
Derivative Asset, Fair Value, Amount Offset Against Collateral	0	0
Derivative liabilities	1,781,903	1,712,062
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liabilities Net Amount	1,781,903	1,712,062
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	73,218	59,468
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		0
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	85,457	71,861
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	73,510	67,126
Net Amount, Derivative Assets	73,218	59,468
Derivative Asset, Fair Value, Gross Asset	73,607	59,768
Derivative Asset, Fair Value, Amount Offset Against Collateral	389	300
Derivative liabilities	85,846	72,161
Derivative Liability, Fair Value, Amount Offset Against Collateral	389	300
Derivative Liabilities Net Amount	11,947	4,735
Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Derivatives, Fair Value [Line Items]
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	1,781,903	1,712,062
Derivative liabilities	1,781,903	1,712,062
Derivative Liabilities Net Amount	1,781,903	1,712,062
Future [Member]
Derivatives, Fair Value [Line Items]
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	3,182
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities		3,379
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		3,379
Net Amount, Derivative Assets	3,182
Derivative Asset, Fair Value, Gross Asset	3,182
Derivative Asset, Fair Value, Amount Offset Against Collateral	0
Derivative liabilities		3,379
Derivative Liability, Fair Value, Amount Offset Against Collateral		0
Derivative Liabilities Net Amount		0
Other Credit Derivatives [Member]
Derivatives, Fair Value [Line Items]
Net Amounts of Liabilities Presented in the Consolidated Balance Sheet, Derivative Liabilities	1,077	1,459
Derivative liabilities	1,077	1,459
Derivative Liability, Fair Value, Amount Offset Against Collateral	0	0
Derivative Liabilities Net Amount	1,077	1,459
Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Derivatives, Fair Value [Line Items]
Net Amounts of Assets Presented in the Consolidated Balance Sheet, Derivative Assets	59,228	66,302
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		0
Net Amount, Derivative Assets	59,228	66,302
Derivative Asset, Fair Value, Gross Asset	59,228	$ 66,302
Derivative Asset, Fair Value, Amount Offset Against Collateral	$ 0

Derivative Instruments - Additi

Derivative Instruments - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Value of right to reclaim cash collateral and posted margin, recorded in "Other assets"	$ 27,113,000		$ 102,904,000
Value of obligation to return cash collateral, recorded in "Other liabilities"	0
Gains in change in fair value of the call options	(16,159,000)	$ 25,191,000
Net liability fair value of all derivative instruments linked to Ambac's own credit risk	74,725,000		67,071,000
Fair value of posted assets as collateral	$ 101,631,000		$ 92,657,000

Derivative Instruments - Summ_2

Derivative Instruments - Summary of Location and Amount of Gains and Losses of Derivative Contracts (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses)	$ 382	$ (452)
Derivative, Gain (Loss) on Derivative, Net	(16,159)	25,191
Derivative, Gain (Loss) on Derivatives, Total Net	(92,601)	66,037
Net gains (losses) on derivative contracts [Domain] | Credit Derivatives [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net	473	(346)
Net gains (losses) on derivative contracts [Domain] | Interest Rate Swap [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net	(2,732)	4,543
Net gains (losses) on derivative contracts [Domain] | Futures Contracts [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net	(13,900)	20,994
Income Loss On Variable Interest Entities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net	(76,915)	41,192
Income Loss On Variable Interest Entities [Member] | Interest Rate Swap [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net	(69,841)	49,809
Income Loss On Variable Interest Entities [Member] | Currency Swaps [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Debt and Equity Securities, FV-NI [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ (7,074)	$ (8,617)

Derivative Instruments - Summ_3

Derivative Instruments - Summary of Notional Amounts of AFS's Trading Derivative Products (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
AFS [Member] | Interest Rate Swap [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	$ 341,909	$ 493,368
AFS [Member] | Interest Rate Swaps-Pay-Fixed/Receive-Variable [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	1,269,180	1,121,532
AFS [Member] | Futures Contracts [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	1,575,000	1,760,000
Variable Interest Entity, Primary Beneficiary [Member] | Other Credit Derivatives [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	10,457	10,254
Variable Interest Entity, Primary Beneficiary [Member] | Interest Rate Swap [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	1,427,299	1,399,532
Variable Interest Entity, Primary Beneficiary [Member] | Interest Rate Swaps-Pay-Fixed/Receive-Variable [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	$ 1,189,113	$ 1,176,748

Derivative Instruments - Summ_4

Derivative Instruments - Summary of Notional for VIE Derivatives Outstanding (Detail) - Variable Interest Entity, Primary Beneficiary [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Interest Rate Swap [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	$ 1,427,299	$ 1,399,532
Interest Rate Swaps-Pay-Fixed/Receive-Variable [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	1,189,113	1,176,748
Currency Swaps [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	344,714	344,992
Other Credit Derivatives [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	$ 10,457	$ 10,254

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Sep. 30, 2011
Tax Credit Carryforward [Line Items]
NOL allocated amount		$ 3,650,000
Afg [Member]
Tax Credit Carryforward [Line Items]
NOL allocated amount	$ 1,251,453
Minimum [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2029
Maximum [Member]
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2032

Income Taxes Income Taxes - Pro

Income Taxes Income Taxes - Provision for Income Taxes Charged To Income From Continuing Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Provision for Income Taxes [Line Items]
Current Federal Tax Expense (Benefit)	$ 0	$ 0
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(63,404)	297,741
Current Income Tax Expense (Benefit)	2,940	998
Deferred Foreign Income Tax Expense (Benefit)	(949)	1,607
Deferred Income Tax Expense (Benefit)	(949)	1,607
Income Tax Expense (Benefit)	1,991	2,605
Income (Loss) from Continuing Operations before Income Taxes, Foreign	22,194	10,568
Pre-tax income (loss)	(41,210)	308,309
Current State and Local Tax Expense (Benefit)	(3,526)	1,037
Current Foreign Tax Expense (Benefit)	$ 6,466	$ (39)

Income Taxes Income Taxes - NOL

Income Taxes Income Taxes - NOL Usage Table (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Mar. 31, 2019	Sep. 30, 2011	Dec. 31, 2018	Dec. 31, 2017
Net Income Loss Reconciliation [Line Items]
Tolling Payment Payable to AFG Generated in Prior Years	$ 44,381			$ 27,868
Tolling Payment Payable to AFG, Gross			$ 13,885
NOL allocated amount		$ 3,650,000
Tier C Tolling Payments Payable to IRS	12.50%
Tier D Tolling Payments Payable to IRS	17.50%
Current Year Loss Attributed to Tier B	$ 80,197
Ambac Assurance [Member]
Net Income Loss Reconciliation [Line Items]
AACNetTaxableIncome	89,326
NOL allocated amount	2,252,447
Afg [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount	$ 1,251,453
Tier A [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 479,000
NOL applicable percentage		15.00%
Tier B [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 1,057,000
NOL applicable percentage		40.00%
Tier C [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 1,057,000
NOL applicable percentage		10.00%
Tier D [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 1,057,000
NOL applicable percentage		15.00%
Minimum [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2029
Maximum [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2032

Income Taxes Schedule of Income

Income Taxes Schedule of Income Before Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Schedule of Income Before Income Tax, Domestic and Foreign [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic	$ (63,404)	$ 297,741
Income (Loss) from Continuing Operations before Income Taxes, Foreign	22,194	10,568
Pre-tax income (loss)	$ (41,210)	$ 308,309

Leases (Details)

Leases (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Operating Leases, Future Minimum Payments Receivable, Current	$ 172
Operating Leases, Future Minimum Payments Receivable, in Two Years	1,038
Operating Leases, Future Minimum Payments Receivable, in Three Years	1,051
Operating Leases, Future Minimum Payments Receivable, in Four Years	1,076
Operating Leases, Future Minimum Payments Receivable, in Five Years	1,126
Operating Leases, Future Minimum Payments, Due Thereafter	7,571
Unrecorded Unconditional Purchase Obligation	$ 24,379
Lessee, Operating Lease, Lease Not yet Commenced, Term of Contract	10 years 9 months 18 days
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	$ 3,580
Operating Lease, Right-of-Use Asset	13,100
Operating Lease, Payments	1,543
Operating Lease, Expense	1,526
Variable Lease, Cost	179
Operating Lease, Liability	13,545	$ 15,000
Sublease Income	(65)
Lease, Cost	$ 1,640
Operating Lease, Weighted Average Remaining Lease Term	8 years 8 months 12 days
Operating Lease, Weighted Average Discount Rate, Percent	8.00%
Lessee, Operating Lease, Liability, Payments, Due Year Two	$ 1,781
Lessee, Operating Lease, Liability, Payments, Due Year Three	1,436
Lessee, Operating Lease, Liability, Payments, Due Year Four	1,436
Lessee, Operating Lease, Liability, Payments, Due Year Five	1,436
Lessee, Operating Lease, Liability, Payments, Due after Year Five	9,294
Lessee, Operating Lease, Liability, Payments, Due	18,963
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	(5,418)
Lessor, Operating Lease, Payments to be Received	$ 12,034
Lessor, Operating Lease, Term of Contract	10 years 9 months 18 days
Right of Use Assets Obtained in Exchange for Lease Obligation, Non Cash	$ 14,360
Minimum [Member]
Lessee Operating Lease Remaining Lease Term	1 year
Maximum [Member]
Lessee Operating Lease Remaining Lease Term	11 years
Future Operating Lease Obligation
Lessee, Operating Lease, Liability, Undiscounted Excess Amount	$ 9,440

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Billions	3 Months Ended
	Mar. 31, 2019USD ($)
Loss Contingencies [Line Items]
General Obligation Bonds Issued in 2012 and 2014	$ 6

Uncategorized Items - a02-017am

Label	Element	Value
Noncontrolling Interest [Member]
AMPS Impact on Stockholders Equity	ambc_AMPSImpactonStockholdersEquity	$ 2,764,000