Acquisitions - Pro-forma Information (Details) - NIC - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Business Acquisition
Revenues	$ 459,873	$ 420,336	$ 1,322,055	$ 1,152,675
Net income	$ 44,170	$ 50,257	$ 103,330	$ 130,555
Basic earnings per share (usd per share)	$ 1.08	$ 1.25	$ 2.53	$ 3.26
Diluted earnings per share (usd per share)	$ 1.04	$ 1.21	$ 2.45	$ 3.15