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	$ 57,097
Unrealized gain on available-for-sale marketable securities, net of taxes	765
Total comprehensive income	57,862
Ending balance at Sep. 30, 2016				1,308
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	68,040				68,040
Unrealized gain on available-for-sale marketable securities, net of taxes	1,163			1,163
Total comprehensive income	69,203
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)		528
Exercise of stock options and other stock-based compensation, net of shares tendered for payment	10,338	$ 5	10,333
Share-based compensation expense — stock options, restricted stock and restricted stock units	3,773		3,773
Treasury shares issued for Deferred Compensation Plan funding and redemptions	501		487			14
Shares issued pursuant to Employee Stock Plan	2,091		1,752			339
Cash dividends	(41,302)				(41,302)
Shares issued pursuant to Dividend Reinvestment Plans	71		61			10
Shares issued pursuant to acquisition (in shares)		59
Shares issued pursuant to acquisition	2,501	$ 1	2,500
Contingent shares issuable pursuant to acquisition	4,438		4,438
Balance (in shares) at Sep. 30, 2017		74,791
Ending balance at Sep. 30, 2017	$ 390,456	$ 748	$ 241,008	$ 844	$ 157,678	$ (9,822)