Net Loss per Share - Calculation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Class of Stock [Line Items]
Net loss	$ (2,032)	$ (447)	$ (7,417)	$ (60,986)
Weighted-average shares used to compute basic net loss per share (in usd per share)	161,103	152,146	158,961	150,372
Weighted-average shares used to compute diluted net loss per share (in usd per share)	161,103	152,146	158,961	150,372
Basic net loss per share (in usd per share)	$ (0.01)	$ 0	$ (0.05)	$ (0.41)
Diluted net loss per share (in usd per share)	$ (0.01)	$ 0	$ (0.05)	$ (0.41)
Class A
Class of Stock [Line Items]
Net loss	$ (358)	$ (103)	$ (1,395)	$ (14,207)
Weighted-average shares used to compute basic net loss per share (in usd per share)	28,397	35,028	29,896	35,030
Weighted-average shares used to compute diluted net loss per share (in usd per share)	28,397	35,028	29,896	35,030
Basic net loss per share (in usd per share)	$ (0.01)	$ 0	$ (0.05)	$ (0.41)
Diluted net loss per share (in usd per share)	$ (0.01)	$ 0	$ (0.05)	$ (0.41)
Class B
Class of Stock [Line Items]
Net loss	$ (1,674)	$ (344)	$ (6,022)	$ (46,779)
Weighted-average shares used to compute basic net loss per share (in usd per share)	132,706	117,118	129,065	115,342
Weighted-average shares used to compute diluted net loss per share (in usd per share)	132,706	117,118	129,065	115,342
Basic net loss per share (in usd per share)	$ (0.01)	$ 0	$ (0.05)	$ (0.41)
Diluted net loss per share (in usd per share)	$ (0.01)	$ 0	$ (0.05)	$ (0.41)

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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CASTLIGHT HEALTH, INC. 10-Q