Cover Page - shares	9 Months Ended
	Oct. 03, 2020	Oct. 28, 2020
Entity Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Oct. 03, 2020
Document Transition Report	false
Entity File Number	001-37444
Entity Registrant Name	FITBIT, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	20-8920744
Entity Address, Address Line One	199 Fremont Street,
Entity Address, Address Line Two	14th Floor
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	513-1000
Title of 12(b) Security	Class A Common Stock, $0.0001 par value
Trading Symbol	FIT
Security Exchange Name	NYSE
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
Entity Shell Company	false
Entity Central Index Key	0001447599
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Amendment Flag	false
Common Class A
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		244,867,751
Common Class B
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		27,600,601