Summary of Significant Accounting Policies (Policies)	9 Months Ended
Sep. 30, 2019
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”). All amounts are presented in thousands, except share and per share amounts. The financial information as of December 31, 2018 that is included in this Quarterly Report is derived from the audited consolidated financial statements and notes for the year ended December 31, 2018 included in Item 8 in the Annual Report on Form 10-K (the “2018 Annual Report”) of Redfin Corporation (the "Company" or "Redfin"). Such financial information should be read in conjunction with the notes and management’s discussion and analysis of the consolidated financial statements included in the 2018 Annual Report. The unaudited condensed consolidated financial statements, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, necessary to present fairly the Company's financial position as of September 30, 2019, the statements of comprehensive income (loss) and stockholders' equity for the three and nine months ended September 30, 2019 and 2018, and the statement of cash flows for the nine months ended September 30, 2019 and 2018. The results for the three and nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any interim period or for any other future year.
Principles of Consolidation	Principles of Consolidation—The unaudited condensed consolidated interim financial statements include the accounts of Redfin and its wholly owned subsidiaries. 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. The Company evaluates its estimates on an ongoing basis. During the nine months ended September 30, 2019, the estimated useful life of capitalized software for internal use was updated from one to two years. This change in estimate was not material. In addition, with the adoption of Accounting Standards Codification Topic 842, Leases, the Company estimated its incremental borrowing rate for the determination of the present value of lease payments. Further description of the impact of this pronouncement is included in Note 6. The amounts ultimately realized from the affected assets or liabilities will depend on, among other factors, general business conditions and could differ materially in the near term from the carrying amounts reflected in the consolidated financial statements.
Cash and Cash Equivalents	Cash and Cash Equivalents—Cash equivalents consist primarily of money market instruments and U.S. treasury securities. The Company considers all highly liquid investments originally purchased by the Company with original maturities of three months or less at the date of purchase to be cash equivalents and classified as available-for-sale.
Investments	Investments—The Company has two types of investments: (i) available-for-sale investments that are available to support operational needs of the Company, which are reported on the balance sheet 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 The Company's short-term and long-term investments consist primarily of U.S. treasury 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. The Company evaluates its available-for-sale securities, both ones classified as cash equivalents and as investments, for other-than-temporary impairment on a quarterly basis. Unrealized losses are charged against net earnings when a decline in fair value is determined to be other than temporary. The Company reviews factors to determine whether a loss is other than temporary, such as the length and extent of the fair value decline, the financial condition and near-term prospects of the issuer, and whether the Company has the intent to sell or will more likely than not be required to sell before the securities' anticipated recovery, which may be at maturity. Realized gains and losses are accounted for using the specific identification method. Purchases and sales are recorded on a trade date basis. There were no other than temporary impairments during the periods presented. Cost Method Investments In December 2018, the Company purchased an equity interest in a privately held company for approximately $2,000, which is classified as long-term. The investment is an equity security without a readily determinable fair value that is 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. The Company performs a qualitative assessment considering impairment indicators to evaluate whether the investment is impaired as of the end of each reporting period.
Advertising and Advertising Production Costs	Advertising and Advertising Production Costs—The Company expenses advertising costs as they are incurred and production costs as of the first date the advertisement takes place. Advertising costs totaled $5,279 and $5,936 for the three months ended September 30, 2019 and 2018, respectively, and $59,357 and $29,167 for the nine months ended September 30, 2019 and 2018, respectively, and are included in marketing expenses. Advertising production costs totaled $17 and $7 for the three months ended September 30, 2019 and 2018, respectively, and $166 and $20 for the nine months ended September 30, 2019 and 2018, respectively, and are included in marketing expenses.
Recently Adopted and Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements—In January 2019, the Company adopted ASU 2016-02, Leases (Topic 842), using the optional alternative transition method under ASU 2018-11, Leases (Topic 842) Targeted Improvements. The optional alternative transition method applies the new leases standard at the adoption date and recognizes a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company evaluated its portfolio of leases and determined a cumulative-effect adjustment to the opening balance of retained earnings was not needed, as the portfolio of leases contains only operating leases. The Company elected the package of practical expedients permitted under the transition guidance within the standard, allowing the Company to carry forward the historical lease classification, carry forward the conclusions on whether current or expired contracts contain leases, and carry forward the accounting for initial direct costs for existing leases. Additionally, the Company elected the practical expedient for use of hindsight to determine the lease term for existing leases whereby it evaluated the performance of existing leases in relation to our leasing strategy and determined that most renewal options would not be reasonably certain to be exercised. This resulted in the shortening of lease terms for the existing leases. Adoption of the standard resulted in the recording of right of use assets and corresponding lease liabilities of $33,953 and $49,395, respectively, as of January 1, 2019, the difference of which is due to lease incentives. Further description of the impact of this pronouncement is included in Note 6. In January 2019, the Company adopted the guidance in the U.S. Securities and Exchange Commission (the "SEC") final rule under SEC Release No. 33-10532, Disclosure Update and Simplification. In August 2018, the SEC issued the final rule amending certain disclosure requirements that were redundant, duplicative, overlapping, outdated, or superseded. In addition, the amendments expanded the disclosure requirements on the analysis of stockholders' equity for interim financial statements. Under the amendments, an analysis of changes in each caption of stockholders' equity presented in the balance sheet must be provided in a note or separate statement. The analysis should present a reconciliation of the beginning balance to the ending balance of each period for which a statement of comprehensive income is required to be filed. In August 2018, the Financial Accounting Standards Board ("FASB") issued authoritative guidance under ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software: Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract. The ASU requires implementation costs incurred by customers in cloud computing arrangements (i.e., hosting arrangements) to be capitalized under the same premises of authoritative guidance for internal-use software, and deferred over the noncancelable term of the cloud computing arrangements plus any option renewal periods that are reasonably certain to be exercised by the customer or for which the exercise is controlled by the service provider. The ASU is effective for public entities for fiscal years beginning after December 15, 2019 and early adoption is permitted. The Company elected to early adopt this standard in the third quarter of 2019 on a prospective basis, which did not result in a material effect on the Company's consolidated financial statements. Recently Issued Accounting Pronouncements—In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820), which modifies the disclosures on fair value measurements by removing the requirement to disclose the amount and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy and the policy for timing of such transfers. The ASU expands the disclosure requirements for Level 3 fair value measurements, primarily focused on changes in unrealized gains and losses included in other comprehensive income. The ASU is effective for public entities for fiscal years beginning after December 15, 2019, with early adoption permitted. The Company has not yet completed its assessment of the impact of the new standard on the Company’s consolidated financial statements. In June 2016, the FASB issued authoritative guidance under ASU 2016-13, Financial Instruments—Credit Losses (Topic 326), which 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. For available-for-sale debt securities, an entity is required to recognize credit losses through an allowance for credit losses rather than as an impairment. The ASU is effective for interim and annual reporting periods beginning after December 15, 2019, and early adoption is permitted. 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 Company completed its assessment of the impact of the new standard on the Company’s consolidated financial statements and does not expect a material impact.

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2019

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”). All amounts are presented in thousands, except share and per share amounts.

The financial information as of December 31, 2018 that is included in this Quarterly Report is derived from the audited consolidated financial statements and notes for the year ended December 31, 2018 included in Item 8 in the Annual Report on Form 10-K (the “2018 Annual Report”) of Redfin Corporation (the "Company" or "Redfin"). Such financial information should be read in conjunction with the notes and management’s discussion and analysis of the consolidated financial statements included in the 2018 Annual Report.

The unaudited condensed consolidated financial statements, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, necessary to present fairly the Company's financial position as of September 30, 2019, the statements of comprehensive income (loss) and stockholders' equity for the three and nine months ended September 30, 2019 and 2018, and the statement of cash flows for the nine months ended September 30, 2019 and 2018. The results for the three and nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any interim period or for any other future year.

Principles of Consolidation

Principles of Consolidation—The unaudited condensed consolidated interim financial statements include the accounts of Redfin and its wholly owned subsidiaries. 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. The Company evaluates its estimates on an ongoing basis. During the nine months ended September 30, 2019, the estimated useful life of capitalized software for internal use was updated from one to two years. This change in estimate was not material. In addition, with the adoption of Accounting Standards Codification Topic 842, Leases, the Company estimated its incremental borrowing rate for the determination of the present value of lease payments. Further description of the impact of this pronouncement is included in Note 6. The amounts ultimately realized from the affected assets or liabilities will depend on, among other factors, general business conditions and could differ materially in the near term from the carrying amounts reflected in the consolidated financial statements.

Cash and Cash Equivalents

Cash and Cash Equivalents—Cash equivalents consist primarily of money market instruments and U.S. treasury securities. The Company considers all highly liquid investments originally purchased by the Company with original maturities of three months or less at the date of purchase to be cash equivalents and classified as available-for-sale.

Investments

Investments—The Company has two types of investments: (i) available-for-sale investments that are available to support operational needs of the Company, which are reported on the balance sheet 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

The Company's short-term and long-term investments consist primarily of U.S. treasury 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. The Company evaluates its available-for-sale securities, both ones classified as cash equivalents and as investments, for other-than-temporary impairment on a quarterly basis. Unrealized losses are charged against net earnings when a decline in fair value is determined to be other than temporary. The Company reviews factors to determine whether a loss is other than temporary, such as the length and extent of the fair value decline, the financial condition and near-term prospects of the issuer, and whether the Company has the intent to sell or will more likely than not be required to sell before the securities' anticipated recovery, which may be at maturity. Realized gains and losses are accounted for using the specific identification method. Purchases and sales are recorded on a trade date basis. There were no other than temporary impairments during the periods presented.

Cost Method Investments

In December 2018, the Company purchased an equity interest in a privately held company for approximately $2,000, which is classified as long-term. The investment is an equity security without a readily determinable fair value that is 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. The Company performs a qualitative assessment considering impairment indicators to evaluate whether the investment is impaired as of the end of each reporting period.

Advertising and Advertising Production Costs

Advertising and Advertising Production Costs—The Company expenses advertising costs as they are incurred and production costs as of the first date the advertisement takes place. Advertising costs totaled $5,279 and $5,936 for the three months ended September 30, 2019 and 2018, respectively, and $59,357 and $29,167 for the nine months ended September 30, 2019 and 2018, respectively, and are included in marketing expenses. Advertising production costs totaled $17 and $7 for the three months ended September 30, 2019 and 2018, respectively, and $166 and $20 for the nine months ended September 30, 2019 and 2018, respectively, and are included in marketing expenses.

Recently Adopted and Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements—In January 2019, the Company adopted ASU 2016-02, Leases (Topic 842), using the optional alternative transition method under ASU 2018-11, Leases (Topic 842) Targeted Improvements. The optional alternative transition method applies the new leases standard at the adoption date and recognizes a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company evaluated its portfolio of leases and determined a cumulative-effect adjustment to the opening balance of retained earnings was not needed, as the portfolio of leases contains only operating leases.

The Company elected the package of practical expedients permitted under the transition guidance within the standard, allowing the Company to carry forward the historical lease classification, carry forward the conclusions on whether current or expired contracts contain leases, and carry forward the accounting for initial direct costs for existing leases. Additionally, the Company elected the practical expedient for use of hindsight to determine the lease term for existing leases whereby it evaluated the performance of existing leases in relation to our leasing strategy and determined that most renewal options would not be reasonably certain to be exercised. This resulted in the shortening of lease terms for the existing leases.

Adoption of the standard resulted in the recording of right of use assets and corresponding lease liabilities of $33,953 and $49,395, respectively, as of January 1, 2019, the difference of which is due to lease incentives. Further description of the impact of this pronouncement is included in Note 6.

In January 2019, the Company adopted the guidance in the U.S. Securities and Exchange Commission (the "SEC") final rule under SEC Release No. 33-10532, Disclosure Update and Simplification. In August 2018, the SEC issued the final rule amending certain disclosure requirements that were redundant, duplicative, overlapping, outdated, or superseded. In addition, the amendments expanded the disclosure requirements on the analysis of stockholders' equity for interim financial statements. Under the amendments, an analysis of changes in each caption of stockholders' equity presented in the balance sheet must be provided in a note or separate statement. The analysis should present a reconciliation of the

beginning balance to the ending balance of each period for which a statement of comprehensive income is required to be filed.

In August 2018, the Financial Accounting Standards Board ("FASB") issued authoritative guidance under ASU 2018-15, Intangibles—Goodwill and Other—Internal-Use Software: Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract. The ASU requires implementation costs incurred by customers in cloud computing arrangements (i.e., hosting arrangements) to be capitalized under the same premises of authoritative guidance for internal-use software, and deferred over the noncancelable term of the cloud computing arrangements plus any option renewal periods that are reasonably certain to be exercised by the customer or for which the exercise is controlled by the service provider. The ASU is effective for public entities for fiscal years beginning after December 15, 2019 and early adoption is permitted. The Company elected to early adopt this standard in the third quarter of 2019 on a prospective basis, which did not result in a material effect on the Company's consolidated financial statements.

Recently Issued Accounting Pronouncements—In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820), which modifies the disclosures on fair value measurements by removing the requirement to disclose the amount and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy and the policy for timing of such transfers. The ASU expands the disclosure requirements for Level 3 fair value measurements, primarily focused on changes in unrealized gains and losses included in other comprehensive income. The ASU is effective for public entities for fiscal years beginning after December 15, 2019, with early adoption permitted. The Company has not yet completed its assessment of the impact of the new standard on the Company’s consolidated financial statements.

In June 2016, the FASB issued authoritative guidance under ASU 2016-13, Financial Instruments—Credit Losses (Topic 326), which 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. For available-for-sale debt securities, an entity is required to recognize credit losses through an allowance for credit losses rather than as an impairment. The ASU is effective for interim and annual reporting periods beginning after December 15, 2019, and early adoption is permitted. 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 Company completed its assessment of the impact of the new standard on the Company’s consolidated financial statements and does not expect a material impact.