TAXES - Income Tax Provision Differences from Federal Statutory Rate (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income tax provision differences from that computed at federal statutory rate
Federal tax at the statutory rate (as a percent)	35.00%	35.00%	35.00%
State tax (net of federal benefits) (as a percent)	1.00%	0.70%	0.72%
Permanent items (as a percent)	0.34%	0.27%	0.34%
Tax credits (as a percent)	(0.82%)	(0.96%)	(0.73%)
Excess tax benefits on stock-based compensation (as a percent)	(3.42%)
Tax Cuts and Jobs Act of 2017 (as a percent)	3.57%
Other (as a percent)		0.23%	(0.14%)
Income tax provision (as a percent)	35.67%	35.24%	35.19%