Business Combination & Acquisitions - Recapitalization (Details) - USD ($) shares in Thousands, $ in Thousands		9 Months Ended
	Jan. 12, 2021	Sep. 30, 2022	Sep. 30, 2021
Recapitalization
Cash - South Mountain (net of redemptions and non-contingent expenses)	$ 240,670
Cash - PIPE investors	200,000
Cash electing shares of Legacy Billtrust shareholders	(90,061)
Fees to underwriters and other transaction costs	(19,936)
Net cash received from reverse recapitalization	330,673	$ 0	$ 349,638
Net assets acquired and other adjustments	255	$ 0	$ 255
Net contributions from reverse recapitalization	$ 330,928
Recapitalization, Number of Shares
Common stock, shares outstanding (in shares)	145,266
Stock repurchased during period (in shares)	(9,006)
Common stock of south mountain (in shares)	30,498
Shares issued from PIPE (in shares)	20,000
Recapitalization shares (in shares)	41,492
Legal Billtrust stockholders (in shares)	103,774
South Mountain
Recapitalization, Number of Shares
Stock repurchased during period (in shares)	(2)
Common Shareholders | South Mountain
Recapitalization, Number of Shares
Common stock, shares outstanding (in shares)	25,000
Founder Shareholders | South Mountain
Recapitalization, Number of Shares
Common stock, shares outstanding (in shares)	5,500