Document and Entity Information - shares	3 Months Ended
	Mar. 30, 2019	Apr. 26, 2019
Entity Information [Line Items]
Entity Registrant Name	FITBIT INC
Entity Central Index Key	0001447599
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Mar. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Amendment Flag	false
Common Class A [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		223,736,591
Common Class B [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		31,267,322