Consolidated Statement of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss), net of taxes	Treasury Stock
Beginning balance at Dec. 31, 2016					$ (319)
Comprehensive income:
Net income for the period	$ 68,040
Unrealized loss on available-for-sale marketable securities, net of taxes	1,163
Total comprehensive income	69,203
Ending balance at Sep. 30, 2017					844
Balance (in shares) at Dec. 31, 2017		74,960
Beginning balance at Dec. 31, 2017	399,952	$ 750	$ 244,363	$ 163,860	837	$ (9,858)
Comprehensive income:
Net income for the period	51,972			51,972
Unrealized loss on available-for-sale marketable securities, net of taxes	(1,549)				(1,549)
Total comprehensive income	50,423
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		292
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	6,554	$ 3	6,551
Share-based compensation expense — stock options, restricted stock	4,174		4,174
Treasury shares issued for Deferred Compensation Plan funding and redemptions	475		597			(122)
Shares issued pursuant to Employee Stock Plan	2,819		2,473			346
Dividends paid and accrued	(42,868)			(42,868)
Shares issued pursuant to Dividend Reinvestment Plan	68		57			11
Contingent shares settled pursuant to acquisition	(2,291)		(2,291)
Other (in shares)		4
Other	274		205	69
Balance (in shares) at Sep. 30, 2018		75,256
Ending balance at Sep. 30, 2018	$ 419,580	$ 753	$ 256,129	$ 173,033	$ (712)	$ (9,623)