Document and Entity Information	3 Months Ended
	Mar. 31, 2015	May 08, 2015
Class of Stock [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2015
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	CSLT
Entity Registrant Name	CASTLIGHT HEALTH, INC.
Entity Central Index Key	0001433714
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Class A [Member]
Class of Stock [Line Items]
Entity Common Stock, Shares Outstanding		58,411,009
Class B [Member]
Class of Stock [Line Items]
Entity Common Stock, Shares Outstanding		35,362,942