Document and Entity Information Document - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2020	Feb. 19, 2021	Jun. 30, 2020
Document Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	001-33156
Entity Registrant Name	First Solar, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	20-4623678
Entity Address, Address Line One	350 West Washington Street, Suite 600
Entity Address, City or Town	Tempe
Entity Address, State or Province	AZ
Entity Address, Postal Zip Code	85281
City Area Code	602
Local Phone Number	414-9300
Title of 12(b) Security	Common stock, $0.001 par value
Trading Symbol	FSLR
Security Exchange Name	NASDAQ
Entity Well Known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 4.1
Entity Common Stock, Shares Outstanding		105,986,398
Documents Incorporated by Reference	The information required by Part III of this Form 10-K, to the extent not set forth herein, is incorporated by reference from the registrant’s definitive proxy statement relating to the Annual Meeting of Shareholders to be held in 2021, which will be filed with the Securities and Exchange Commission within 120 days after the end of the fiscal year to which this Form 10-K relates.
Entity Central Index Key	0001274494
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false