Significant Accounting Policies (Policies)	9 Months Ended
Significant Accounting Policies (Policies)	Sep. 30, 2019
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Basis of Presentation	Basis of Presentation The accompanying unaudited consolidated financial statements have been prepared in accordance with the instructions to Form 10-Q and Article 10 of Regulation S-X and, accordingly, do not include all of the information and disclosures required by accounting principles generally accepted in the United States of America (“GAAP”) for annual periods. These 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, consisting only of normal recurring adjustments, necessary for the fair statement of the Company’s consolidated financial position and results of operations. All material intercompany balances and transactions have been eliminated. 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. The results of operations for the three and nine months ended September 30, 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, but does not include all disclosures required by GAAP for annual periods.
Loss and Loss Adjustment Expenses	Loss and Loss Adjustment Expenses The Company recognizes loss reserves on a contract-by-contract basis when the present value of expected net cash outflows to be paid under the contract discounted using a risk-free rate as of the measurement date exceeds the unearned premium revenue. A loss reserve is subsequently remeasured each reporting period for expected increases or decreases due to changes in the likelihood of default and potential recoveries. Subsequent changes to the measurement of the loss reserves are recognized as loss expense in the period of change. Measurement and recognition of loss reserves are reported gross of any reinsurance. The Company estimates the likelihood of possible claim payments and possible recoveries using probability-weighted expected cash flows based on information available as of the measurement date, including market information. Accretion of the discounts on loss reserves and recoveries is included in loss expense. The Company considers its ability to collect contractual interest on claim payments when developing its expected inflows. The inclusion of such interest may result in the Company recording recoveries in excess of its actual or expected claim payments on a policy. The Company recognizes potential recoveries on paid claims based on probability-weighted net cash inflows present valued at applicable risk-free rates as of the measurement date. Such amounts are reported within “Insurance loss recoverable” on the Company’s consolidated balance sheets. To the extent the Company had recorded potential recoveries in its loss reserves previous to a claim payment, such recoveries are reclassified to “Insurance loss recoverable” upon payment of the related claim and remeasured each reporting period. Beginning with the second quarter of 2019, the Company changed its presentation of its insurance loss recoverable and its loss and loss adjustment expense (“LAE”) reserves related to its insured first-lien RMBS exposure. The Company’s first-lien RMBS insurance loss recoverable previously represented discounted and probability-weighted estimated recoveries, net of claims expected to be paid, when the result was a net receivable, and its first-lien RMBS loss and LAE reserves previously represented discounted and probability-weighted estimated claims, net of expected recoveries to be collected, when the result was a net payable. The Company now reports its first-lien RMBS insurance loss recoverable gross of expected claim payments and all expected claim payments are reported within loss and LAE reserves on the Company’s balance sheet. This treatment is consistent with the Company’s balance sheet presentation for insurance loss recoverable and loss and LAE reserves of its other major insured exposures. Certain amounts have been reclassified in prior years’ financial statements to conform to the current presentation. This includes a reclassification of $31 million resulting in an increase to insurance loss recoverable and a corresponding increase to loss and LAE reserves on the Company’s consolidated balance sheet as of December 31, 2018. This reclassification had no impact on total revenues, total expenses, shareholders’ equity, operating cash flows, investing cash flows, or financing cash flows for all periods presented. In addition, prior period amounts included in the Company’s disclosures have been updated to reflect the new presentation. The Company’s loss reserve, insurance loss recoverable, and accruals for LAE incurred are disclosed in “Note 5: Loss and Loss Adjustment Expense Reserves.”
Recently Adopted Accounting Standards	Recently Adopted Accounting Standards Leases (Topic 842) (ASU 2016-02) In February of 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2016-02, “Leases (Topic 842)”, that amends the accounting guidance for leasing transactions. ASU 2016-02 requires a lessee to classify lease contracts as finance or operating leases, and to recognize assets and liabilities for the rights and obligations created by leasing transactions with lease terms more than twelve months. ASU 2016-02 substantially retains the criteria for classifying leasing transactions as finance or operating leases. For finance leases, a lessee recognizes a right-of-use asset and a lease liability initially measured at the present value of the lease payments, and recognizes interest expense on the lease liability separately from the amortization of the right-of-use asset. For operating leases, a lessee recognizes a right-of-use asset and a lease liability initially measured at the present value of the lease payments, and recognizes lease expense on a straight-line basis. The Company adopted ASU 2016-02 in its entirety in the first quarter of 2019, using an additional (and optional) modified retrospective transition approach. Comparative periods are presented in accordance with Accounting Stand ards Codifi cation (“ASC”) Topic 840, Leases, and do not include any retrospective adjustments to comparative periods to reflect the adoption of ASU 2016-02. The Company recorded a right-of-use asset and lease liability of $ 23 million. The gross up of the assets and liabilities does not have a cumulative effect adjustment to the opening balance of retained earnings and does not impact the Company’s statement of operations. Refer to “Note 14: Commitments and Contingencies” for information about the Company’s lease commitments. Disclosure Update and Simplification In August of 2018, the Securities and Exchange Commission (“SEC”) published Release No. 33-10532, Disclosure Update and Simplification, which amends certain disclosure requirements that were redundant, duplicative, overlapping, outdated or superseded. In addition, these amendments updated the disclosure requirements for the interim financial statement requirements to include a reconciliation of each caption of shareholders’ equity, in the notes or as a separate statement for each period for which a statement of comprehensive income is required to be included. The Company updated the presentation of its consolidated statements of changes in shareholders’ equity for all periods presented beginning in the first quarter of 2019. The Company has not adopted any other new accounting pronouncements that had a material impact on its consolidated financial statements.
New Accounting Pronouncements Not Yet Adopted	Recent Accounting Developments Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (ASU 2016-13) In June of 2016, the FASB issued ASU 2016-13, “Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” ASU 2016-13 requires financing receivables and other financial assets measured at amortized cost to be presented at the net amount expected to be collected by recording an allowance for credit losses with changes in the allowance recorded as credit loss expense or reversal of credit loss expense based on management’s current estimate of expected credit losses each period. ASU 2016-13 does not apply to credit losses on financial guarantee insurance contracts within the scope of ASC Topic 944, “Financial Services-Insurance.” ASU 2016-13 also requires impairment relating to credit losses on available-for-sale (“AFS”) debt securities to be presented through an allowance for credit losses with changes in the allowance recorded in the period of the change as credit loss expense or reversal of credit loss expense. Any impairment amount not recorded through an allowance for credit losses on AFS debt securities is recorded through other comprehensive income. ASU 2016-13 is effective for interim and annual periods beginning January 1, 2020 with early adoption permitted beginning January 1, 2019. ASU 2016-13 is applied on a modified retrospective basis except that prospective application is applied to AFS debt securities with other-than-temporary impairments (“OTTI”) recognized before the date of adoption. The Company is evaluating the impact of adopting ASU 2016-13. Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement (ASU 2018-13) In August of 2018, the FASB issued ASU 2018-13, “Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement.” ASU 2018-13 modifies the disclosure requirements on fair value measurements. ASU 2018-13 is effective for interim and annual periods beginning January 1, 2020 with early adoption permitted to remove or modify disclosures upon issuance of the standard and delay adoption of the additional disclosures until the effective date. Upon the effective date, certain amendments should be applied prospectively, while others are to be applied retrospectively to all periods presented. The Company is evaluating the impact of adopting ASU 2018-13. Since the amendments of ASU 2018-13 only impact disclosure requirements, the Company does not expect the adoption of ASU 2018-13 to have an impact on its consolidated financial statements.