The Company and Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Description Of Business	Description of Business Qualys, Inc. (the “Company”) was incorporated in the state of Delaware on December 30, 1999. The Company is headquartered in Redwood City, California and has majority-owned subsidiaries throughout the world. The Company is a pioneer and leading provider of cloud-based security and compliance solutions that enable organizations to identify security risks to their IT infrastructures, help protect their IT systems and applications from ever-evolving cyber-attacks and achieve compliance with internal policies and external regulations. The Company’s cloud solutions address the growing security and compliance complexities and risks that are amplified by the dissolving boundaries between internal and external IT infrastructures and web environments, the rapid adoption of cloud computing and the proliferation of geographically dispersed IT assets. Organizations can use the Company’s integrated suite of solutions delivered on its Qualys Cloud Platform to cost-effectively obtain a unified view of their security and compliance posture across globally-distributed IT infrastructures.
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements and condensed footnotes have been prepared by the Company in accordance with accounting principles generally accepted in the United States ("U.S. GAAP") for interim financial information as well as the instructions to Form 10-Q and the rules and regulations of the U.S. Securities and Exchange Commission ("SEC"). Certain information and note disclosures normally included in the financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to such rules and regulations. The condensed consolidated balance sheet as of December 31, 2016, included herein, was derived from the audited financial statements as of that date but does not include all disclosures, including notes required by U.S. GAAP. In the opinion of management, the accompanying unaudited condensed consolidated financial statements reflect all adjustments, which include only normal recurring adjustments, necessary for the fair presentation of the financial position, results of operations and cash flows for the interim periods. The results of operations for the three and nine month periods ended September 30, 2017 are not necessarily indicative of the results of operations expected for the entire year ending December 31, 2017 or for any other future annual or interim period. These condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and related notes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016, filed with the SEC on February 24, 2017.
Reclassification	Reclassification The Company reclassified certain information technology expenses across the functions that benefit from their support. For the three months ended September 30, 2016, the Company reclassified $0.8 million out of general and administrative expenses. Of this amount the Company reclassified $0.2 million to cost of revenues, $0.3 million to research and development and $0.3 million to sales and marketing. For the nine months ended September 30, 2016, the Company reclassified $2.2 million out of general and administrative expenses. Of this amount the Company reclassified $0.5 million to cost of revenues, $1.0 million to research and development and $0.7 million to sales and marketing.
Recently adopted Accounting Pronouncements and Recent Accounting Pronouncements	Recently adopted Accounting Pronouncements In March 2016, the FASB issued ASU 2016-09, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting, (ASU 2016-09) intended to simplify and improve various aspects related to how employee-share based payment transactions are accounted for and presented in the financial statements. The ASU addresses income tax consequences, classification of awards as either equity or liabilities, classification on the statement of cash flows and forfeiture rate calculations. This guidance is effective for public business entities in fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. The Company adopted this guidance in its first quarter of 2017 and elected to apply this adoption prospectively. Prior periods have not been adjusted. The Company has made the accounting policy election to continue to estimate forfeitures expected to occur to determine the amount of stock-based compensation expense to record each period. See Note 8 for information regarding the impact on the Company's financial statements. Recent Accounting Pronouncements Revenue from Contracts with Customers In May 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2014-09, Revenue from Contracts with Customers (ASU 2014-09), as amended, which will supersede nearly all existing revenue recognition guidance. Under ASU 2014-09, an entity is required to recognize revenue upon transfer of promised goods or services to customers in an amount that reflects the expected consideration received in exchange for those goods or services. ASU 2014-09 defines a five-step process in order to achieve this core principle, which may require the use of judgment and estimates, and also requires expanded qualitative and quantitative disclosures relating to the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers, including significant judgments and estimates used. The FASB has recently issued several amendments to the new standard, including clarification on accounting for licenses of intellectual property and identifying performance obligations. The amendments include ASU 2016-08, Revenue from Contracts with Customers (Topic 606)-Principal versus Agent Considerations, which was issued in March 2016, and clarifies the implementation guidance for principal versus agent considerations in ASU 2014-09, and ASU 2016-10, Revenue from Contracts with Customers (Topic 606)-Identifying Performance Obligations and Licensing, which was issued in April 2016, and amends the guidance in ASU 2014-09 related to identifying performance obligations and accounting for licenses of intellectual property. The new standard permits adoption either by using (i) a full retrospective approach for all periods presented in the period of adoption or (ii) a modified retrospective approach with the cumulative effect of initially applying the new standard recognized at the date of initial application and providing certain additional disclosures. The new standard is effective for annual reporting periods beginning after December 15, 2017, with early adoption permitted for annual reporting periods beginning after December 15, 2016. The Company will adopt the new standard effective January 1, 2018. We have reached preliminary conclusions on key accounting assessments related to the standard. The Company currently believes the new standard will have the most significant impact relating to the potential deferral of sales commissions and contract costs. We are finalizing our assessment and quantifying the impacts related to accounting for costs incurred to obtain a contract. The Company currently plans to adopt using the modified retrospective approach. In January 2016, the FASB issued ASU 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which will impact certain aspects of recognition, measurement, presentation and disclosure of financial instruments. The guidance will impact the accounting for equity investments, financial liabilities under the fair value option, and the presentation and disclosure requirements for financial instruments. This guidance is effective for public business entities in fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017. The Company is currently evaluating the impact this ASU will have on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), which requires, among other things, lease assets and lease liabilities arising from leases, including operating leases, to be recognized on the balance sheet. In addition, this ASU requires disclosing key information about leasing arrangements. This guidance supersedes existing lease guidance and is effective for public business entities in fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Pursuant to the leasing criteria, most of our leased space and equipment leases will be capitalized assets on the balance sheet with an offsetting financing obligation. In the statement of operations, what was formerly rent expense will be bifurcated into depreciation and interest expense. The Company is currently evaluating the impact of the adoption of this update on its consolidated financial statements. In August 2016, the FASB issued ASU 2016-15, Classification of Certain Cash Receipts and Cash Payments (a consensus of the Emerging Issues Task Force), to provide guidance on the presentation of certain cash receipts and cash payments in the statement of cash flows in order to reduce diversity in existing practice. The amendments in this ASU are effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. The Company is currently evaluating the impact of the adoption of this update on its consolidated financial statements. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash, which requires the statement of cash flows to explain the change during the period in the total of cash, cash equivalents and restricted cash or restricted cash equivalents. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. This guidance is effective for public business entities in fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017. Early adoption is permitted. The Company is currently evaluating the impact this ASU will have on its consolidated financial statements. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business, which revises the definition of a business and provides new guidance in evaluating when a set of transferred assets and activities is a business. This guidance will be effective for public business entities in fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017 on a prospective basis. Early adoption is permitted. The Company does not expect the standard to have a material impact on our consolidated financial statements. In January 2017, the FASB issued ASU 2017-04, Simplifying the Test for Goodwill Impairment (Topic 350). This standard eliminates Step 2 from the goodwill impairment test, instead requiring an entity to recognize a goodwill impairment charge for the amount by which the goodwill carrying amount exceeds the reporting unit’s fair value. This guidance is effective for interim and annual goodwill impairment tests in fiscal years beginning after December 15, 2019 with early adoption permitted. This guidance must be applied on a prospective basis. The Company does not expect the adoption of this guidance to have a material impact on its consolidated financial statements.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. For certain of the Company’s financial instruments, including certain cash equivalents, accounts receivable, accounts payable, and other current liabilities, the carrying amounts approximate their fair values due to the relatively short maturity of these balances. The Company measures and reports certain cash equivalents, investments and derivative foreign currency forward contracts at fair value in accordance with the provisions of the authoritative accounting guidance that addresses fair value measurements. This guidance establishes a hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available. The hierarchy is broken down into three levels based on the reliability of inputs as follows: Level 1—Valuations based on quoted prices in active markets for identical assets or liabilities. Level 2—Valuations based on other than quoted prices in active markets for identical assets and liabilities, quoted prices for identical or similar assets or liabilities in inactive markets, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3—Valuations based on inputs that are generally unobservable and typically reflect management’s estimates of assumptions that market participants would use in pricing the asset or liability. The Company's financial instruments consist of assets and liabilities measured using Level 1 and 2 inputs. Level 1 assets include a highly liquid money market fund, which is valued using unadjusted quoted prices that are available in an active market for an identical asset. Level 2 assets include fixed-income U.S. government agency securities, commercial paper, corporate bonds, asset-backed securities and derivative financial instruments consisting of foreign currency forward contracts. The securities, bonds and commercial paper are valued using prices from independent pricing services based on quoted prices in active markets for similar instruments or on industry models using data inputs such as interest rates and prices that can be directly observed or corroborated in active markets. The foreign currency forward contracts are valued using observable inputs, such as quotations on forward foreign exchange points and foreign interest rates.