TAXES - Tax Cuts and Jobs Act Bill and Stock-Based Compensation Tax Benefits (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reasonable estimate of effect of enactment of Tax Act
U.S. federal corporate tax rate (as a percent)			35.00%	35.00%	35.00%
Provisional amount for reasonable estimate of effect of Tax Act on existing deferred tax balance	$ 1,500		$ 1,461
Tax benefits for employee stock-based compensation
Increase to contributed capital from certain tax benefits for employee stock-based compensation				$ 737	$ 865
Income tax expense (tax benefit)			14,161	$ 13,358	$ 11,217
Accounting Standards Update 2016-09
Tax benefits for employee stock-based compensation
Income tax expense (tax benefit)			$ (1,359)
Expected
Reasonable estimate of effect of enactment of Tax Act
U.S. federal corporate tax rate (as a percent)		21.00%