Selected Quarterly Financial Data - Unaudited (Details) - USD ($) $ / shares in Units, $ 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
Revenues	$ 320,451	$ 318,567	$ 303,379	$ 328,607	$ 315,523	$ 307,471	$ 312,282	$ 330,932	$ 1,271,004	$ 1,266,208	$ 1,400,040
Gross Profit	119,874	113,924	106,284	121,415	114,140	118,552	103,869	135,303	461,497	471,864	570,028
Net Income	$ 26,142	$ 24,986	$ 18,908	$ 16,834	$ 20,641	$ 26,563	$ 22,833	$ 30,998	$ 86,870	$ 101,035	$ 28,793
Basic	$ 1.24	$ 1.19	$ 0.91	$ 0.81	$ 1.00	$ 1.29	$ 1.11	$ 1.51	$ 4.16	$ 4.90	$ 1.41
Diluted	$ 1.21	$ 1.16	$ 0.88	$ 0.78	$ 0.96	$ 1.24	$ 1.06	$ 1.44	$ 4.02	$ 4.69	$ 1.33