Segment Information - Reconciliation of Total Segment Operating Income to Consolidated Operating Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Revenues	$ 469,240	$ 356,976	$ 330,984	$ 332,393	$ 405,588	$ 344,141	$ 322,432	$ 299,893	$ 1,489,593	$ 1,372,054	$ 1,207,295
Gross profit	$ 409,213	$ 301,154	$ 274,182	$ 273,262	$ 341,260	$ 282,425	$ 262,166	$ 248,880	1,257,811	1,134,731	1,006,221
Direct costs									923,200	857,651	761,941
Income from operations									334,611	277,080	244,280
Reportable segments
Segment Reporting Information [Line Items]
Revenues									1,489,592	1,372,046	1,207,289
Gross profit									1,257,858	1,134,723	1,006,216
Direct costs									869,575	809,918	722,736
Income from operations									388,283	324,805	283,480
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues									(229)	(300)	(60)
Gross profit									(229)	(300)	(60)
Direct costs									(229)	(300)	(60)
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Revenues									1	8	6
Gross profit									(47)	8	5
Direct costs									53,625	47,733	39,205
Income from operations									(53,672)	(47,725)	(39,200)
Cloud Services Total \| Reportable segments
Segment Reporting Information [Line Items]
Revenues									678,461	661,835	597,975
Gross profit									524,200	517,565	475,821
Direct costs									274,997	270,025	239,629
Income from operations									249,203	247,540	236,192
Fax and Martech \| Reportable segments
Segment Reporting Information [Line Items]
Revenues									386,276	378,444	360,479
Gross profit									320,714	318,677	311,534
Direct costs									116,923	119,574	125,963
Income from operations									203,791	199,103	185,571
Voice, Backup, Security, and CPP \| Reportable segments
Segment Reporting Information [Line Items]
Revenues									292,185	283,391	237,496
Gross profit									203,486	198,888	164,287
Direct costs									158,074	150,451	113,666
Income from operations									45,412	48,437	50,621
Digital Media \| Reportable segments
Segment Reporting Information [Line Items]
Revenues									811,360	710,511	609,374
Gross profit									733,887	617,458	530,455
Direct costs									594,807	540,193	483,167
Income from operations									$ 139,080	$ 77,265	$ 47,288