| Document and Entity Information | Aug. 06, 2020 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001351288 |
| Document Type | 8-K |
| Document Period End Date | Aug. 6, 2020 |
| Entity Registrant Name | GW PHARMACEUTICALS PLC |
| Entity Incorporation, State or Country Code | X0 |
| Entity File Number | 001-35892 |
| Entity Tax Identification Number | 00-0000000 |
| Entity Address, Address Line One | Sovereign House |
| Entity Address, Address Line Two | Vision Park Chivers Way |
| Entity Address, City or Town | Histon Cambridge |
| Entity Address, Postal Zip Code | CB24 9BZ |
| Entity Address, Country | GB |
| Country Region | 44 |
| City Area Code | 1223 |
| Local Phone Number | 266 800 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | American Depositary Shares, each representing 12 Ordinary Shares, par value £0.001 per share |
| Trading Symbol | GWPH |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
