Goodwill and Other Intangible Assets - Changes in carrying amount of goodwill (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Goodwill [Roll Forward]
Goodwill, gross, beginning of period	$ 633,015	$ 633,015	$ 633,015	$ 612,702
Accumulated Impairment Losses, beginning of period	(305,590)	(305,590)	(305,590)	(305,590)
Goodwill, net of Accumulated Impairment Losses, beginning of period	327,425	327,425	327,425	307,112
Acquisitions	41,593	0	41,593	20,313
Goodwill, gross, end of period	674,608	633,015	674,608	633,015
Accumulated Impairment Losses, end of period	(305,590)	(305,590)	(305,590)	(305,590)
Goodwill, net of Accumulated Impairment Losses, end of period	$ 369,018	$ 327,425	$ 369,018	$ 327,425