| Cover | Aug. 04, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Aug. 4, 2020 |
| Entity Registrant Name | CASTLIGHT HEALTH, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-36330 |
| Entity Tax Identification Number | 26-1989091 |
| Entity Address, Address Line One | 150 Spear Street |
| Entity Address, Address Line Two | Suite 400 |
| Entity Address, City or Town | San Francisco |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94105 |
| City Area Code | 415 |
| Local Phone Number | 829-1400 |
| Title of 12(b) Security | Class B Common Stock, par value $0.0001 per share |
| Trading Symbol | CSLT |
| Security Exchange Name | NYSE |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001433714 |
