Summary of Accounting Policies (Policies)	9 Months Ended
Sep. 30, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation—The consolidated financial statements and accompanying notes have been prepared in accordance with generally accepted accounting principles in the United States of America (“GAAP”). The financial information as of December 31, 2019 that is included in this quarterly report is derived from the audited consolidated financial statements and notes for the year ended December 31, 2019 included in Item 8 in our annual report for the year ended December 31, 2019. Such financial information should be read in conjunction with the notes and management’s discussion and analysis of the consolidated financial statements included in our annual report. The unaudited consolidated interim financial statements, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, necessary to present fairly our financial position as of September 30, 2020, the statements of comprehensive income (loss) and statements of changes in mezzanine equity and stockholders’ equity for the three and nine months ended September 30, 2020 and 2019, and the statement of cash flows for the nine months ended September 30, 2020 and 2019. The results for the three and nine months ended September 30, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020 or for any interim period or for any other future year.
Principles of Consolidation	Principles of Consolidation—The unaudited consolidated interim financial statements include the accounts of Redfin and its wholly owned subsidiaries, including those entities in which we have a variable interest and of which we are the primary beneficiary. Intercompany transactions and balances have been eliminated.
Use of Estimates	Use of Estimates—The preparation of consolidated financial statements, in conformity with GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and results of operations during the respective periods. We evaluate our estimates on an ongoing basis. In January 2020 we adopted ASU 2016-13, Financial Instruments—Credit Losses (Topic 326), which modifies the measurement of credit losses on financial instruments. As part of the adoption we estimated the current expected credit losses for certain classes of relevant assets. The amounts realized from the affected assets will depend on, among other factors, general business conditions, including the impacts from COVID-19, and could differ in the near term from carrying amounts reflected in the consolidated financial statements. Further description of the impact of this pronouncement is included in "—Recently Adopted Accounting Pronouncements."
Accounts Receivable and Allowance for Credit Losses	Accounts Receivable and Allowance for Credit Losses—We have two material classes of receivables: (i) real estate services receivables and (ii) receivables from the sale of homes through our properties business. Accounts receivable related to these classes represent closed transactions for which cash has not yet been received. We establish an allowance for expected credit losses based on historical experience of collectibility, current external economic conditions that may affect collectibility, and current or expected changes to the regulatory environment in which we operate our real estate services and properties businesses. As the majority of our transactions are processed through escrow, collectibility is not a significant risk, and we have determined the nature of our receivables to have similar credit quality indicators. We evaluate for changes in credit quality indicators on an annual basis or in the event of a material economic event or material change in the regulatory environment in which we operate, with the most recent assessment being performed in June 2020.
Investments	Investments—We have two types of investments: (i) available-for-sale investments that are available to support our operational needs, which are reported on the balance sheets as short-term and long-term investments, and (ii) long-term equity investments accounted for under the cost method, which are reported in other non-current assets. Available-for-sale Our short-term and long-term investments consist primarily of U.S. treasury securities, including inflation protected securities, and other federal or local government issued securities, all of which are classified as available-for-sale. Available-for-sale debt securities are recorded at fair value, and unrealized holding gains and losses are recorded as a component of accumulated other comprehensive income. Available-for-sale securities with maturities of one year or less and those identified by management at the time of purchase to be used to fund operations within one year are classified as short-term. All other available-for-sale securities are classified as long-term. We evaluate our available-for-sale securities, both ones classified as cash equivalents and as investments, for expected credit losses on a quarterly basis. An expected credit loss reserve is charged against the fair value of an available-for-sale debt security when it is identified, with a credit loss charged against net earnings. We review factors to determine whether an expected credit loss exists based on credit quality indicators, such as the extent to which the fair value as of the reporting date is less than the amortized cost basis, present value of cash flows expected to be collected, the financial condition and prospects of the issuer, adverse conditions specifically related to the security, and any changes to the credit rating of the security by a rating agency. Realized gains and losses are accounted for using the specific identification method. Purchases and sales are recorded on a trade date basis. Cost Method Investments We have purchased equity interests in privately held companies, which are classified as long-term. The investments are equity securities without readily determinable fair values that are accounted for at cost minus any impairment, plus or minus changes resulting from observable price changes in orderly transactions for identical or similar investments of the same issuer. We perform a qualitative assessment considering impairment indicators to evaluate whether the investments are impaired as of the end of each reporting period.
Advertising and Advertising Production Costs	Advertising and Advertising Production Costs—We expense advertising costs as they are incurred and advertising production costs as of the first date the advertisement takes place.
Recently Adopted and Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements—In January 2020, we adopted ASU 2016-13, Financial Instruments—Credit Losses (Topic 326), using a modified-retrospective approach. The adoption of this guidance requires a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is effective. The pronouncement, along with the related subsequent pronouncements that include clarifications, modifies the measurement of credit losses on financial instruments. This guidance requires the use of an expected loss impairment model for instruments measured at amortized cost based on relevant information about past events, including historical experience, current conditions, and reasonable and supportable forecasts that affect the collectibility of the reported amount. The adoption of this pronouncement did not have a material impact on our consolidated financial statements. See "—Accounts Receivable and Allowance for Credit Losses" for specific accounting policies for accounts receivable and available-for-sale debt securities, and see Note 2 and Note 3 for additional impacts from the adoption. Recently Issued Accounting Pronouncements—In August 2020, the Financial Accounting Standards Board issued authoritative guidance under ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity's Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity's Own Equity, which simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity's own equity. This guidance removes the liability and equity separation models for convertible instruments with a cash conversion feature or beneficial conversion feature. As a result, after adoption, entities will not separately present in equity an embedded conversion feature in such debt. Similarly, the embedded conversion feature will no longer be amortized into income as interest expense over the life of the instrument. Instead, entities will account for a convertible debt instrument wholly as debt unless (1) a convertible instrument contains features that require bifurcation as a derivative under ASC Topic 815, Derivatives and Hedging, or (2) a convertible debt instrument was issued at a substantial premium. Among other potential impacts, this change is expected to reduce reported interest expense, increase reported net income, and result in a reclassification of certain conversion features from stockholders' equity to liabilities as it relates to certain convertible debt instruments. Additionally, this guidance requires the application of the if-converted method to calculate the impact of convertible instruments on diluted earnings per share. The ASU is effective for fiscal years beginning after December 15, 2021, and interim periods within those fiscal years. Early adoption is permitted for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years. We have not yet completed our assessment of the impact of the new standard on our consolidated financial statements.
Stockholders' Equity Note, Redeemable Preferred Stock, Issue, Policy	Mezzanine Equity—We have issued convertible preferred stock that we have determined is a financial instrument with both equity and debt characteristics and are such classified as mezzanine equity in our consolidated financial statements. The instrument is initially recognized at fair value net of issuance costs. We reassess whether the instrument is currently redeemable or probable to become redeemable in the future as of each reporting date, in which, if the instrument meets either criteria, we will accrete the carrying value to the redemption value based on the effective interest method over the remaining term. To assess classification, we review all features of the instrument, including mandatory redemption features and conversion features that may be substantive. All financial instruments that are classified as mezzanine equity are evaluated for embedded derivative features by evaluating each feature against the nature of the host instrument (e.g. more equity-like or debt-like). Features identified as embedded derivatives that are material are recognized separately as a derivative asset or liability in the consolidated financial statements. We have evaluated our convertible preferred stock and determined that its nature is that of an equity host and no material embedded derivatives exist that would require bifurcation on our balance sheet. See Note 11 for more information.

Summary of Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Basis of Presentation—The consolidated financial statements and accompanying notes have been prepared in accordance with generally accepted accounting principles in the United States of America (“GAAP”).

The financial information as of December 31, 2019 that is included in this quarterly report is derived from the audited consolidated financial statements and notes for the year ended December 31, 2019 included in Item 8 in our annual report for the year ended December 31, 2019. Such financial information should be read in conjunction with the notes and management’s discussion and analysis of the consolidated financial statements included in our annual report.

The unaudited consolidated interim financial statements, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, necessary to present fairly our financial position as of September 30, 2020, the statements of comprehensive income (loss) and statements of changes in mezzanine equity and stockholders’ equity for the three and nine months ended September 30, 2020 and 2019, and the statement of cash flows for the nine months ended September 30, 2020 and 2019. The results for the three and nine months ended September 30, 2020 are not necessarily indicative of the results to be expected for the year ending December 31, 2020 or for any interim period or for any other future year.

Principles of Consolidation

Principles of Consolidation—The unaudited consolidated interim financial statements include the accounts of Redfin and its wholly owned subsidiaries, including those entities in which we have a variable interest and of which we are the primary beneficiary. Intercompany transactions and balances have been eliminated.

Use of Estimates

Use of Estimates—The preparation of consolidated financial statements, in conformity with GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and results of operations during the respective periods. We evaluate our estimates on an ongoing basis. In January 2020 we adopted ASU 2016-13, Financial Instruments—Credit Losses (Topic 326), which modifies the measurement of credit losses on financial instruments. As part of the adoption we estimated the current expected credit losses for certain classes of relevant assets. The amounts realized from the affected assets will depend on, among other factors, general business conditions, including the impacts from COVID-19, and could differ in the near term from carrying amounts reflected in the consolidated financial statements. Further description of the impact of this pronouncement is included in "—Recently Adopted Accounting Pronouncements."

Accounts Receivable and Allowance for Credit Losses

Accounts Receivable and Allowance for Credit Losses—We have two material classes of receivables: (i) real estate services receivables and (ii) receivables from the sale of homes through our properties business. Accounts receivable related to these classes represent closed transactions for which cash has not yet been received. We establish an allowance for expected credit losses based on historical experience of collectibility, current external economic conditions that may affect collectibility, and current or expected changes to the regulatory environment in which we operate our real estate services and properties businesses. As the majority of our transactions are processed through escrow, collectibility is not a significant risk, and we have determined the nature of our receivables to have similar credit quality indicators. We evaluate for changes in credit quality indicators on an annual basis or in the event of a material economic event or material change in the regulatory environment in which we operate, with the most recent assessment being performed in June 2020.

Investments

Investments—We have two types of investments: (i) available-for-sale investments that are available to support our operational needs, which are reported on the balance sheets as short-term and long-term investments, and (ii) long-term equity investments accounted for under the cost method, which are reported in other non-current assets.

Available-for-sale

Our short-term and long-term investments consist primarily of U.S. treasury securities, including inflation protected securities, and other federal or local government issued securities, all of which are classified as available-for-sale. Available-for-sale debt securities are recorded at fair value, and unrealized holding gains and losses are recorded as a component of accumulated other comprehensive income. Available-for-sale securities with maturities of one year or less and those identified by management at the time of purchase to be used to fund operations within one year are classified as short-term. All other available-for-sale securities are classified as long-term. We evaluate our available-for-sale securities, both ones classified as cash equivalents and as investments, for expected credit losses on a quarterly basis. An expected credit loss reserve is charged against the fair value of an available-for-sale debt security when it is identified, with a credit loss charged against net earnings. We review factors to determine whether an expected credit loss exists based on credit quality indicators, such as the extent to which the fair value as of the reporting date is less than the amortized cost basis, present value of cash flows expected to be collected, the financial condition and prospects of the issuer, adverse conditions specifically related to the security, and any changes to the credit rating of the security by a rating agency. Realized gains and losses are accounted for using the specific identification method. Purchases and sales are recorded on a trade date basis.

Cost Method Investments

We have purchased equity interests in privately held companies, which are classified as long-term. The investments are equity securities without readily determinable fair values that are accounted for at cost minus any impairment, plus or minus changes resulting from observable price changes in orderly transactions for identical or similar investments of the same issuer. We perform a qualitative assessment considering impairment indicators to evaluate whether the investments are impaired as of the end of each reporting period.

Advertising and Advertising Production Costs

Advertising and Advertising Production Costs—We expense advertising costs as they are incurred and advertising production costs as of the first date the advertisement takes place.

Recently Adopted and Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements—In January 2020, we adopted ASU 2016-13, Financial Instruments—Credit Losses (Topic 326), using a modified-retrospective approach. The adoption of this guidance requires a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period in which the guidance is effective. The pronouncement, along with the related subsequent pronouncements that include clarifications, modifies the measurement of credit losses on financial instruments. This guidance requires the use of an expected loss impairment model for instruments measured at amortized cost based on relevant information about past events, including historical experience, current conditions, and reasonable and supportable forecasts that affect the collectibility of the reported amount. The adoption of this pronouncement did not have a material impact on our consolidated financial statements. See "—Accounts Receivable and Allowance for Credit Losses" for specific accounting policies for accounts receivable and available-for-sale debt securities, and see Note 2 and Note 3 for additional impacts from the adoption.

Recently Issued Accounting Pronouncements—In August 2020, the Financial Accounting Standards Board issued authoritative guidance under ASU 2020-06, Debt—Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity's Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity's Own Equity, which simplifies the accounting for certain financial instruments with characteristics of liabilities and equity, including convertible instruments and contracts on an entity's own equity. This guidance removes the liability and equity separation models for convertible instruments with a cash conversion feature or beneficial conversion feature. As a result, after adoption, entities will not separately present in equity an embedded conversion feature in such debt. Similarly, the embedded conversion feature will no longer be amortized into income as interest expense over the life of the instrument. Instead, entities will account for a convertible debt instrument wholly as debt unless (1) a convertible instrument contains features that require bifurcation as a derivative under ASC Topic 815, Derivatives and Hedging, or (2) a convertible debt instrument was issued at a substantial premium. Among other potential impacts, this change is expected to reduce reported interest expense, increase reported net income, and result in a reclassification of certain conversion features from stockholders' equity to liabilities as it relates to certain convertible debt instruments. Additionally, this guidance requires the application of the if-converted method to calculate the impact of convertible instruments on diluted earnings per share. The ASU is effective for fiscal years beginning after December 15, 2021, and interim periods within those fiscal years. Early adoption is permitted for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years. We have not yet completed our assessment of the impact of the new standard on our consolidated financial statements.

Stockholders' Equity Note, Redeemable Preferred Stock, Issue, Policy

Mezzanine Equity—We have issued convertible preferred stock that we have determined is a financial instrument with both equity and debt characteristics and are such classified as mezzanine equity in our consolidated financial statements. The instrument is initially recognized at fair value net of issuance costs. We reassess whether the instrument is currently redeemable or probable to become redeemable in the future as of each reporting date, in which, if the instrument meets either criteria, we will accrete the carrying value to the redemption value based on the effective interest method over the remaining term. To assess classification, we review all features of the instrument, including mandatory redemption features and conversion features that may be substantive. All financial instruments that are classified as mezzanine equity are evaluated for embedded derivative features by evaluating each feature against the nature of the host instrument (e.g. more equity-like or debt-like). Features identified as embedded derivatives that are material are recognized separately as a derivative asset or liability in the consolidated financial statements. We have evaluated our convertible preferred stock and determined that its nature is that of an equity host and no material embedded derivatives exist that would require bifurcation on our balance sheet. See Note 11 for more information.