Basis of Presentation, Significant Accounting Policies and Recently-Issued Accounting Pronouncements - Schedule of Revisions for the Financial Statement - Condensed Consolidated Statements of Stockholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Beginning Balance	$ 398,613	$ 319,267
Net loss attributable to BlackLine, Inc.	(12,843)	(8,781)
Ending Balance	396,189	318,036
Accumulated Deficit
Beginning Balance	(163,598)	(132,896)
Net loss attributable to BlackLine, Inc.	(10,642)	(8,781)
Ending Balance	$ (174,240)	(141,677)
As Previously Reported
Beginning Balance		321,569
Net loss attributable to BlackLine, Inc.		(8,583)
Ending Balance		320,536
As Previously Reported | Accumulated Deficit
Beginning Balance		(130,594)
Net loss attributable to BlackLine, Inc.		(8,583)
Ending Balance		(139,177)
Adjustments
Beginning Balance		(2,302)
Net loss attributable to BlackLine, Inc.		(198)
Ending Balance		(2,500)
Adjustments | Accumulated Deficit
Beginning Balance		(2,302)
Net loss attributable to BlackLine, Inc.		(198)
Ending Balance		$ (2,500)