Acquisitions - Schedule of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Mar. 01, 2021	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Goodwill		$ 676,440	$ 609,457	$ 609,457
Thryv Australia
Business Acquisition [Line Items]
Total cash consideration	$ 216,164
Consideration transferred to acquire business	216,164
Cash and cash equivalents	40,794
Accounts receivable and other current assets	88,529
Other assets	10,426
Fixed assets and capitalized software	40,957
Accounts payable	(31,163)
Accrued liabilities	(40,713)
Contract liabilities	(27,075)
Other current liabilities	(6,733)
Deferred tax liabilities	(42,121)
Other liabilities	(15,505)
Total identifiable net assets	144,113
Goodwill	72,051
Total net assets acquired	216,164
Thryv Australia | Client relationships
Business Acquisition [Line Items]
Intangible assets	$ 101,839
Intangible assets, useful life	3 years 6 months
Thryv Australia | Trademarks
Business Acquisition [Line Items]
Intangible assets	$ 24,877
Intangible assets, useful life	3 years 6 months