Accounting Standards and Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2020
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The accompanying unaudited condensed consolidated financial statements include Castlight and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) and applicable rules and regulations of the Securities and Exchange Commission (“SEC”) regarding interim financial reporting. In the opinion of management, the information herein reflects all adjustments, consisting only of normal recurring adjustments except as otherwise noted, considered necessary for a fair statement of results of operations, financial position, stockholders’ equity and cash flows. The results for the interim periods presented are not necessarily indicative of the results expected for any future period. The following information should be read in conjunction with the audited financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019. Other than as described below, there have been no changes to the Company's significant accounting policies described in the Company's Annual Report that have had a material impact on the Company's consolidated financial statements and related notes.
Use of Estimates	Use of Estimates The preparation of condensed consolidated financial statements in conformity with GAAP requires the Company to make certain estimates, judgments and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements, as well as the reported amounts of revenue and expenses during the reporting period. These estimates include, but are not limited to the determination of: •Variable consideration included in the transaction price of the Company’s contracts with customers; •The standalone selling price of the performance obligations in the Company’s contracts with customers; •Assumptions used in the valuation of certain equity awards; and •Assumptions used in the calculation of goodwill impairment, including the forecast of future cash flows and discount rate. Actual results could differ from those estimates, and such differences could be material to the Company’s consolidated financial position and results of operations.
Marketable Securities	Marketable Securities The Company's marketable securities consist of U.S. agency obligations and U.S. treasury securities, with maturities at the time of purchase of greater than three months. Marketable securities with remaining maturities in excess of one year are classified as non-current. The Company classifies its marketable securities as available-for-sale at the time of purchase based on its intent and are recorded at their estimated fair value. Unrealized gains for available-for-sale securities are recorded in other comprehensive income/loss. Unrealized losses for available-for-sale securities are recorded in other comprehensive income/loss, unless the losses relate to deterioration in credit risk or if it is likely securities will be sold before the recovery of their cost basis. In these cases, the unrealized losses are reported in other income, net in the consolidated statement of operations. Realized gains and losses are determined based on the specific identification method and are reported in other income, net in the consolidated statements of operations.
Recently Adopted Accounting Pronouncements and Recently Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements Effective January 1, 2020, the Company adopted Accounting Standards Update (“ASU”) 2016-13, Financial Instruments-Credit Losses, and subsequent amendments (“ASC 326”). The standard changes how entities will measure credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. The adoption of this standard did not have a material impact on the Company’s financial statements. The Company will continue to actively monitor the impact of the current coronavirus (“COVID-19”) pandemic on expected credit losses. Effective January 1, 2020, the Company adopted ASU 2018-15, Intangibles – Goodwill and Other – Internal-Use Software (“ASU 2018-15”), which aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. The adoption of this standard did not have a material impact on the Company’s financial position or results of operations. Recently Issued Accounting Pronouncements The Company considers the applicability and impact of all ASUs issued by the FASB. The Company determined that the ASUs issued by the FASB during the six months ended June 30, 2020 are either not applicable or are expected to have minimal impact on the Company's condensed consolidated financial results.