Reportable Segments - Operations of Segments (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]
Revenue	$ 354,048	$ 365,424	$ 343,020	$ 335,208	$ 336,230	$ 345,558	$ 338,861	$ 326,738	$ 1,397,700	$ 1,347,387	$ 1,303,937
Gross profit	$ 100,461	$ 103,878	$ 97,361	$ 94,524	$ 98,025	$ 102,811	$ 101,026	$ 93,176	396,224	395,038	386,487
Operating and other expenses									321,012	323,642	318,607
Income from continuing operations, before income taxes									75,212	71,396	67,880
Tech
Segment Reporting Information [Line Items]
Revenue									1,049,628	1,057,859	990,089
Gross profit									289,720	292,980	277,388
FA
Segment Reporting Information [Line Items]
Revenue									348,072	289,528	313,848
Gross profit									$ 106,504	$ 102,058	$ 109,099