STOCKHOLDERS' EQUITY - Stock Option Activity (Details) - Stock Options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Option Activity [Abstract]
Outstanding at beginning of year (in shares)	946	1,066	1,017
Granted (in shares)	148	222	341
Exercised (in shares)	(198)	(268)	(236)
Forfeited (in shares)	(6)	(52)	(56)
Cancelled (in shares)	(3)	(22)
Outstanding at end of year (in shares)	887	946	1,066
Exercisable at end of year (in shares)	490	493	604
Weighted Average Exercise Price [Abstract]
Outstanding at beginning of year (in dollars per share)	$ 55.44	$ 45.32	$ 37.29
Granted (in dollars per share)	74.57	85.22	60.92
Exercised (in dollars per share)	41.71	36.36	30.44
Forfeited (in dollars per share)	74.90	72.29	58.23
Cancelled (in dollars per share)	48.54	57.48
Outstanding at end of year (in dollars per share)	61.59	55.44	45.32
Exercisable at end of period (in dollars per share)	$ 50.50	$ 41.01	$ 34.77
Aggregate intrinsic value for outstanding stock options	$ 16,192	$ 24,714	$ 41,161
Weighted average remaining contractual term for outstanding stock options	6 years 3 months 18 days
Aggregate intrinsic value for exercisable stock options outstanding	$ 13,887
Weighted average remaining contractual term for exercisable stock options outstanding	5 years
Weighted-average fair value of options granted (in dollars per share)	$ 18.62	$ 23.20	$ 18.48
Total intrinsic value of stock options exercised	$ 10,456	$ 11,900	$ 8,609