INCOME TAXES - Provision for Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	[1]	Sep. 30, 2016	[2]	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income taxes
Federal											$ 22,533	$ 20,433	$ 23,876
State											2,550	2,478	3,358
Total											25,083	22,911	27,234
Deferred income taxes
Federal											1,576	(857)	(1,754)
State											64	(29)	(164)
Total											1,640	(886)	(1,918)
Total income tax provision	$ 6,583	$ 6,066	$ 6,950	$ 7,124	$ 3,130		$ 4,153		$ 7,280	$ 7,462	$ 26,723	$ 22,025	$ 25,316

[1] The Company’s lower effective tax rate in the fourth quarter of 2016 (19%) was primarily attributable to the Company amending its 2014 and 2013 federal income tax returns to recognize favorable benefits related to the domestic production activities deduction, which increased basic and diluted earnings per share by approximately $0.03 during the quarter. (See Note 9)

[2] The Company’s lower effective tax rate in the third quarter of 2016 (20%) was primarily attributable to favorable benefits related to the domestic production activities deduction and federal research and development credit, which increased basic and diluted earnings per share by approximately $0.05 during the quarter. (See Note 9)