Basis of Presentation	3 Months Ended
Mar. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	1. Basis of Presentation The accompanying unaudited condensed consolidated financial statements of Arena Pharmaceuticals, Inc. should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2016, as filed with the Securities and Exchange Commission, or SEC, from which we derived our condensed consolidated balance sheet as of December 31, 2016. The accompanying condensed consolidated financial statements have been prepared in accordance with US generally accepted accounting principles, or GAAP, for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, since they are interim statements, the accompanying condensed consolidated financial statements do not include all of the information and notes required by GAAP for complete financial statements. The accompanying condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, that are, in the opinion of our management, necessary to a fair statement of the results for the interim periods presented. Interim results are not necessarily indicative of results for a full year. The accompanying consolidated financial statements include the balances and activity of our wholly owned subsidiaries and Beacon Discovery, Inc., or Beacon, a variable interest entity in which we have the controlling financial interest (see Note 13). The equity attributable to the noncontrolling interest in Beacon is presented as a separate component from the equity attributable to stockholders of Arena in the equity section of the condensed consolidated balance sheets. The results of operations and comprehensive loss attributable to the noncontrolling interest in Beacon are presented as separate components from the results of operations and comprehensive loss attributable to the stockholders of Arena in the condensed consolidated statements of operations and comprehensive loss. Liquidity. As of March 31, 2017, we had cash and cash equivalents of approximately $79.5 million. Subsequent to March 31, 2017, we raised approximately $75.5 million of net proceeds from sales of our common stock (see Note 7). We believe our cash and cash equivalents will be sufficient to fund our operations for at least the next 12 months. It will require substantial cash to achieve our objectives of discovering, developing and commercializing drugs, and this process typically takes many years and potentially several hundreds of millions of dollars for an individual drug. We may not have adequate available cash, or assets that could be readily turned into cash, to meet these objectives in the long term. We will need to obtain significant funds under our existing collaborations, under new collaboration, licensing or other commercial agreements for one or more of our drug candidates and programs or patent portfolios, or from other potential sources of liquidity, which may include the sale of equity, issuance of debt or other transactions. Recent Accounting Pronouncements Revenue Recognition. In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, Revenue from Contracts with Customers. ASU No. 2014-09 supersedes most current revenue recognition guidance and establishes a comprehensive revenue recognition model with a broad principle that would require an entity to recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve this principle, an entity identifies the contract with a customer, identifies the separate performance obligations in the contract, determines the transaction price, allocates the transaction price to the separate performance obligations and recognizes revenue when each separate performance obligation is satisfied. FASB has subsequently issued additional ASUs to clarify certain elements of the new revenue recognition guidance. The new guidance allows for two methods of adoption: (a) “full retrospective” adoption, meaning the standard is applied to all periods presented, or (b) “modified retrospective” adoption, meaning the cumulative effect of applying the new guidance is recognized as an adjustment to the opening retained earnings balance for the year of implementation. We plan to adopt the new revenue standard effective January 1, 2018, on a modified retrospective method with the cumulative effect of the change reflected in retained earnings as of January 1, 2018. We have continued to monitor FASB activity to assess certain interpretative issues and the associated implementation of the new standard. We are in the process of reviewing our revenue arrangements, which we expect to include product sales, manufacturing support payments, royalty payments, other collaboration payments and toll manufacturing, and are not yet able to estimate the anticipated impact to our consolidated financial statements from the implementation of the new standard as we continue to interpret the principles of the new standard. Other. In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities. ASU No. 2016-01 supersedes and amends the guidance to classify equity securities with readily determinable fair values into different categories (that is, trading or available-for-sale) and require equity securities to be measured at fair value with changes in the fair value recognized through net income. The amendments allow equity investments that do not have readily determinable fair values to be remeasured at fair value either upon the occurrence of an observable price change or upon identification of an impairment. The amendments also require enhanced disclosures about those investments. ASU No. 2016-01 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2017, and calls for prospective application, with early application permitted. We do not expect the adoption of ASU No. 2016-01 to have a material impact on our consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02, Leases. ASU No. 2016-02 amends the accounting guidance for leases. The amendments contain principles that will require lessees to recognize most leases on the balance sheet by recording a right-of-use asset and a lease liability, unless the lease is a short-term lease that has an accounting lease term of 12 months or less. The amendments also contain other changes to the current lease guidance that may result in changes to how entities determine which contractual arrangements qualify as a lease, the accounting for executory costs (such as property taxes and insurance), as well as which lease origination costs will be capitalizable. The new standard also requires expanded quantitative and qualitative disclosures. ASU No. 2016-02 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2018, with early adoption permitted. ASU No. 2016-02 requires the use of the modified retrospective transition method, whereby the new guidance will be applied at the beginning of the earliest period presented in the financial statements of the period of adoption. We are currently evaluating the impact of ASU No. 2016-02 on our consolidated financial statements. In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting. ASU No. 2016-09 modified certain aspects of the accounting for share-based payment transactions, including income taxes, classification of awards, and classification on the statement of cash flows. Previously, excess tax benefits or deficiencies from our equity awards were recorded as additional paid-in capital on the consolidated balance sheet. This guidance also requires excess tax benefits and deficiencies to be presented as an operating activity on the statement of cash flows and allows an entity to make an accounting policy election to either estimate expected forfeitures or to account for them as they occur. Upon adoption, any excess tax benefits or deficiencies from our equity awards are recorded on the consolidated statement of operations and comprehensive loss in the reporting periods in which stock options are exercised. We adopted ASU No. 2016-09 in the first quarter of 2017, which resulted in a $43,000 cumulative-effect adjustment to increase our accumulated deficit and additional paid-in capital as of January 1, 2017, due to our election to account for forfeitures as they occur. The adoption of this ASU did not have a material impact on our consolidated financial statements. Use of Estimates. The preparation of financial statements in accordance with GAAP requires our management to make estimates and assumptions that affect the reported amounts (including assets, liabilities, revenues and expenses) and related disclosures. The amounts reported could differ under different estimates and assumptions.

1. Basis of Presentation

The accompanying unaudited condensed consolidated financial statements of Arena Pharmaceuticals, Inc. should be read in conjunction with the audited consolidated financial statements and notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2016, as filed with the Securities and Exchange Commission, or SEC, from which we derived our condensed consolidated balance sheet as of December 31, 2016. The accompanying condensed consolidated financial statements have been prepared in accordance with US generally accepted accounting principles, or GAAP, for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, since they are interim statements, the accompanying condensed consolidated financial statements do not include all of the information and notes required by GAAP for complete financial statements. The accompanying condensed consolidated financial statements reflect all adjustments, consisting of normal recurring adjustments, that are, in the opinion of our management, necessary to a fair statement of the results for the interim periods presented. Interim results are not necessarily indicative of results for a full year.

The accompanying consolidated financial statements include the balances and activity of our wholly owned subsidiaries and Beacon Discovery, Inc., or Beacon, a variable interest entity in which we have the controlling financial interest (see Note 13). The equity attributable to the noncontrolling interest in Beacon is presented as a separate component from the equity attributable to stockholders of Arena in the equity section of the condensed consolidated balance sheets. The results of operations and comprehensive loss attributable to the noncontrolling interest in Beacon are presented as separate components from the results of operations and comprehensive loss attributable to the stockholders of Arena in the condensed consolidated statements of operations and comprehensive loss.

Liquidity.

As of March 31, 2017, we had cash and cash equivalents of approximately $79.5 million. Subsequent to March 31, 2017, we raised approximately $75.5 million of net proceeds from sales of our common stock (see Note 7). We believe our cash and cash equivalents will be sufficient to fund our operations for at least the next 12 months.

It will require substantial cash to achieve our objectives of discovering, developing and commercializing drugs, and this process typically takes many years and potentially several hundreds of millions of dollars for an individual drug. We may not have adequate available cash, or assets that could be readily turned into cash, to meet these objectives in the long term. We will need to obtain significant funds under our existing collaborations, under new collaboration, licensing or other commercial agreements for one or more of our drug candidates and programs or patent portfolios, or from other potential sources of liquidity, which may include the sale of equity, issuance of debt or other transactions.

Recent Accounting Pronouncements

Revenue Recognition.

In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, Revenue from Contracts with Customers. ASU No. 2014-09 supersedes most current revenue recognition guidance and establishes a comprehensive revenue recognition model with a broad principle that would require an entity to recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve this principle, an entity identifies the contract with a customer, identifies the separate performance obligations in the contract, determines the transaction price, allocates the transaction price to the separate performance obligations and recognizes revenue when each separate performance obligation is satisfied. FASB has subsequently issued additional ASUs to clarify certain elements of the new revenue recognition guidance.

The new guidance allows for two methods of adoption: (a) “full retrospective” adoption, meaning the standard is applied to all periods presented, or (b) “modified retrospective” adoption, meaning the cumulative effect of applying the new guidance is recognized as an adjustment to the opening retained earnings balance for the year of implementation. We plan to adopt the new revenue standard effective January 1, 2018, on a modified retrospective method with the cumulative effect of the change reflected in retained earnings as of January 1, 2018.

We have continued to monitor FASB activity to assess certain interpretative issues and the associated implementation of the new standard. We are in the process of reviewing our revenue arrangements, which we expect to include product sales, manufacturing support payments, royalty payments, other collaboration payments and toll manufacturing, and are not yet able to estimate the anticipated impact to our consolidated financial statements from the implementation of the new standard as we continue to interpret the principles of the new standard.

Other.

In January 2016, the FASB issued ASU No. 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities. ASU No. 2016-01 supersedes and amends the guidance to classify equity securities with readily determinable fair values into different categories (that is, trading or available-for-sale) and require equity securities to be measured at fair value with changes in the fair value recognized through net income. The amendments allow equity investments that do not have readily determinable fair values to be remeasured at fair value either upon the occurrence of an observable price change or upon identification of an impairment. The amendments also require enhanced disclosures about those investments. ASU No. 2016-01 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2017, and calls for prospective application, with early application permitted. We do not expect the adoption of ASU No. 2016-01 to have a material impact on our consolidated financial statements.

In February 2016, the FASB issued ASU No. 2016-02, Leases. ASU No. 2016-02 amends the accounting guidance for leases. The amendments contain principles that will require lessees to recognize most leases on the balance sheet by recording a right-of-use asset and a lease liability, unless the lease is a short-term lease that has an accounting lease term of 12 months or less. The amendments also contain other changes to the current lease guidance that may result in changes to how entities determine which contractual arrangements qualify as a lease, the accounting for executory costs (such as property taxes and insurance), as well as which lease origination costs will be capitalizable. The new standard also requires expanded quantitative and qualitative disclosures. ASU No. 2016-02 is effective for annual reporting periods, and interim periods within those periods, beginning after December 15, 2018, with early adoption permitted. ASU No. 2016-02 requires the use of the modified retrospective transition method, whereby the new guidance will be applied at the beginning of the earliest period presented in the financial statements of the period of adoption. We are currently evaluating the impact of ASU No. 2016-02 on our consolidated financial statements.

In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting. ASU No. 2016-09 modified certain aspects of the accounting for share-based payment transactions, including income taxes, classification of awards, and classification on the statement of cash flows. Previously, excess tax benefits or deficiencies from our equity awards were recorded as additional paid-in capital on the consolidated balance sheet. This guidance also requires excess tax benefits and deficiencies to be presented as an operating activity on the statement of cash flows and allows an entity to make an accounting policy election to either estimate expected forfeitures or to account for them as they occur. Upon adoption, any excess tax benefits or deficiencies from our equity awards are recorded on the consolidated statement of operations and comprehensive loss in the reporting periods in which stock options are exercised. We adopted ASU No. 2016-09 in the first quarter of 2017, which resulted in a $43,000 cumulative-effect adjustment to increase our accumulated deficit and additional paid-in capital as of January 1, 2017, due to our election to account for forfeitures as they occur. The adoption of this ASU did not have a material impact on our consolidated financial statements.

Use of Estimates.

The preparation of financial statements in accordance with GAAP requires our management to make estimates and assumptions that affect the reported amounts (including assets, liabilities, revenues and expenses) and related disclosures. The amounts reported could differ under different estimates and assumptions.