Business Combination & Acquisitions - Narrative (Details)					3 Months Ended			15 Months Ended
	Feb. 14, 2022 USD ($)	Oct. 07, 2021 USD ($)	Jan. 12, 2021 USD ($) $ / shares shares	Jan. 11, 2021 shares	Mar. 31, 2022 USD ($) $ / shares shares	Mar. 31, 2021 USD ($)	Mar. 31, 2021 shares	Mar. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) $ / shares shares	Nov. 18, 2021 $ / shares
Reverse Recapitalization [Line Items]
Purchase of business, net of acquired cash | $					$ 59,456,000	$ 0
Total revenues | $					$ 45,631,000	41,936,000
Reverse recapitalization, aggregate consideration | $			$ 1,190,000,000
Cash electing shares of Legacy Billtrust shareholders | $			90,061,000
Reverse recapitalization, equity interests issued and issuable | $			$ 1,099,000,000
Reverse recapitalization, equity interests issued and issuable, number of shares (shares)			109,900,000
Reverse recapitalization, equity interests issuable, number of shares (shares)			15,200,000
Recapitalization conversion ratio			7.2282662
Stock repurchased during period (USD per share) | $ / shares			$ 10.00
Common stock, shares outstanding (shares)			145,266,000
Warrants outstanding (shares)			12,500,000
Number of shares exercisable by each warrant (shares)			1
Warrants exercise price (USD per share) | $ / shares			$ 11.50
Stock converted conversion ratio			1
Common stock, par value (USD per share) | $ / shares			$ 0.0001
Common and preferred stock, shares authorized (shares)			575,000,000
Preferred stock, shares authorized (shares)			10,000,000		10,000,000			10,000,000	10,000,000
Preferred stock, par value (USD per share) | $ / shares			$ 0.0001		$ 0.0001			$ 0.0001	$ 0.0001
Number of shares issued (shares)			20,000,000
Aggregate purchase price | $			$ 200,000,000
Stock repurchased during period (shares)			9,006,000
Warrants exercised and converted (shares)			100,000
Contingent consideration liability, shares (shares)			12,000,000
Issuance and vesting of Earnout Shares (shares)							10,900,000
Amount paid for tax withholding | $					$ 49,000	4,013,000		$ 4,013,000
Unamortized debt discount | $					1,200,000			1,200,000
Loss on extinguishment of debt | $					1,600,000
Acquisition related costs | $					700,000	$ 0
Fair value, recurring
Reverse Recapitalization [Line Items]
Contingent consideration | $									$ 370,000
Anachron Beheer BV
Reverse Recapitalization [Line Items]
Equity interest acquired	100.00%
Purchase of business, net of acquired cash | $	$ 59,878,000
Contingent consideration | $	$ 11,500,000				$ 1,000,000			1,000,000
Weighted average useful life					17 years 10 months 24 days
Total revenues | $					$ 1,400,000
Direct expenses | $					1,400,000
Anachron Beheer BV | Fair value, recurring
Reverse Recapitalization [Line Items]
Contingent consideration | $					$ 586,000			586,000
iController BV
Reverse Recapitalization [Line Items]
Equity interest acquired		100.00%
Purchase of business, net of acquired cash | $		$ 57,020,000
Contingent consideration | $		$ 5,085,000
Weighted average useful life					13 years 4 months 24 days
iController BV | Fair value, recurring
Reverse Recapitalization [Line Items]
Contingent consideration | $					$ 5,018,000			5,018,000	$ 5,085,000
Restricted stock units
Reverse Recapitalization [Line Items]
Number of RSU's granted in period (shares)					1,996,000
Share price (USD per share) | $ / shares			$ 16.80
Earnout Restricted Stock Units
Reverse Recapitalization [Line Items]
Number of RSU's granted in period (shares)					800,000
Amount paid for tax withholding | $					$ 4,000,000
Earnout Shares
Reverse Recapitalization [Line Items]
Amount paid for tax withholding | $								4,013,000
Earnout Shares | Restricted stock units
Reverse Recapitalization [Line Items]
Share price (USD per share) | $ / shares			$ 16.80
Risk-free interest rate			0.50%
Expected volatility			42.00%
First Earnout
Reverse Recapitalization [Line Items]
Contingent consideration liability, shares (shares)			6,000,000
Contingent consideration liability, stock price trigger (USD per share) | $ / shares			$ 12.50
Contingent consideration liability, threshold trading days			20 days
Contingent consideration liability, threshold trading day period			30 days
Second Earnout
Reverse Recapitalization [Line Items]
Contingent consideration liability, shares (shares)			6,000,000
Contingent consideration liability, stock price trigger (USD per share) | $ / shares			$ 15.00
Contingent consideration liability, threshold trading days			20 days
Contingent consideration liability, threshold trading day period			30 days
Sponsor Vesting Shares
Reverse Recapitalization [Line Items]
Amount paid for tax withholding | $								$ 0
Billtrust
Reverse Recapitalization [Line Items]
Common stock, par value (USD per share) | $ / shares			$ 0.001
South Mountain
Reverse Recapitalization [Line Items]
Warrants cancelled (shares)			7,000,000
Stock repurchased during period (shares)				2,000
South Mountain | Common Shareholders
Reverse Recapitalization [Line Items]
Common stock, shares outstanding (shares)				25,000,000
Common Stock | Share-based Payment Arrangement
Reverse Recapitalization [Line Items]
Number of shares of common stock withheld from employees for tax withholdings (shares)					200,000
Common Class I
Reverse Recapitalization [Line Items]
Common stock, shares outstanding (shares)			138,700,000		159,954,000			159,954,000	159,413,000
Common stock, par value (USD per share) | $ / shares					$ 0.0001			$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares authorized (shares)			538,000,000		538,000,000			538,000,000	538,000,000
Common stock, shares issued (shares)					159,954,000			159,954,000	159,413,000
Common Class I | Prior South Mountain Shareholders
Reverse Recapitalization [Line Items]
Common stock, shares outstanding (shares)			2,400,000
Common Class I | South Mountain | Common Shareholders
Reverse Recapitalization [Line Items]
Common stock, shares outstanding (shares)					2,400,000			2,400,000
Common stock, shares issued (shares)					2,400,000			2,400,000
Common Class I | South Mountain | Common Shareholders | First Earnout
Reverse Recapitalization [Line Items]
Common stock, shares outstanding (shares)					1,200,000			1,200,000
Common Class I | South Mountain | Common Shareholders | Second Earnout
Reverse Recapitalization [Line Items]
Common stock, shares outstanding (shares)					1,200,000			1,200,000
Common Class 2
Reverse Recapitalization [Line Items]
Common stock, shares outstanding (shares)			6,500,000		3,396,000			3,396,000
Common stock, par value (USD per share) | $ / shares			$ 0.0001		$ 0.0001			$ 0.0001	$ 0.0001
Common stock, shares authorized (shares)			27,000,000		27,000,000			27,000,000	27,000,000
Common stock, shares issued (shares)					3,396,000			3,396,000