Condensed Consolidated Statements of Comprehensive Earnings - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net earnings	$ 28,860	$ 28,011	$ 79,927	$ 85,100
Other comprehensive earnings (loss)
Unrealized gain (loss) on marketable securities, net of tax	1,337	8	10,419	(3,364)
Other comprehensive earnings (loss)	1,337	8	10,419	(3,364)
Comprehensive earnings	$ 30,197	$ 28,019	$ 90,346	$ 81,736