Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 05, 2020
Document Information [Line Items]
Document Type	10-Q
Document Period End Date	Mar. 31, 2020
Entity Registrant Name	CASTLIGHT HEALTH, INC.
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001433714
Current Fiscal Year End Date	--12-31
Class A common stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		35,032,053
Class B
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		114,484,826