Consolidated Statement of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss), net of taxes	Retained Earnings	Treasury Stock
Beginning balance at Dec. 31, 2015				$ 543
Comprehensive income:
Net income for the period	$ 37,386
Unrealized gain on available-for-sale marketable securities, net of taxes	1,133
Total comprehensive income	38,519
Ending balance at Jun. 30, 2016				1,676
Balance (in shares) at Dec. 31, 2016		74,204
Beginning balance at Dec. 31, 2016	338,842	$ 742	$ 217,664	(319)	$ 130,940	$ (10,185)
Comprehensive income:
Net income for the period	44,568				44,568
Unrealized gain on available-for-sale marketable securities, net of taxes	1,081			1,081
Total comprehensive income	45,649
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		348
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	7,257	$ 3	7,254
Share-based compensation expense — stock options, restricted stock and restricted stock units	2,624		2,624
Treasury shares issued for Deferred Compensation Plan funding and redemptions	501		502			(1)
Shares issued pursuant to Employee Stock Plan	2,091		1,752			339
Cash dividends	(27,374)				(27,374)
Shares issued pursuant to Dividend Reinvestment Plans	48		41			7
Shares issued pursuant to acquisition (in shares)		59
Shares issued pursuant to acquisition	2,501	$ 1	2,500
Contingent shares issuable pursuant to acquisition	3,938		3,938
Balance (in shares) at Jun. 30, 2017		74,611
Ending balance at Jun. 30, 2017	$ 376,077	$ 746	$ 236,275	$ 762	$ 148,134	$ (9,840)