CAPITAL STOCK AND SHARE-BASED COMPENSATION (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Employee Stock Purchase Plan Activity

The following is a detail of the purchases that were made or pending Board of Director approval under the plan:

Employee Stock Purchase Plan Period	Shares Issued	Price
2016 and Prior	3,039,200	$	14.72
January 1, 2017 to March 31, 2017	13,244	43.43
April 1, 2017 to June 30, 2017	11,446	53.39
July 1, 2017 to September 30, 2017	12,276	47.57
October 1, 2017 to December 31, 2017	13,323	44.80
January 1, 2018 to March 31, 2018	10,913	51.29
April 1, 2018 to June 30, 2018	8,673	72.64
July 1, 2018 to September 30, 2018	6,052	106.22
October 1, 2018 to December 31, 2018	7,856	99.54
	3,122,983

Schedule of Share-based Payment Award Valuation Assumptions

The fair values of the awards were estimated using the following assumptions for each 2018, 2017 and 2016:

	For the Years Ended December 31,
	2018	2017	2016
Risk Free Rate	2.56% - 3.04%	1.99% - 2.16%	1.19% - 1.58%
Expected Volatility	42.00% - 45.32%	50.18% - 51.81%	53.44% - 54.89%
Expected Term	4.12 - 6.25 years	5.78 - 6.25 years	5.86 - 6.25 years
Weighted Average Fair Value	$42.48	$28.02	$25.99
Dividend Yield	—%	—%	—%

Schedule of Stock Options Activity

The following table presents our non-vested stock option activity for 2018:

	Number of Shares		Weighted Average Grant Date Fair Value
Non-vested stock options at January 1, 2018	527,798		$	23.00
Granted	163,666		42.48
Vested	(246,442	)	23.11
Forfeited	(74,552	)	25.78
Non-vested stock options at December 31, 2018	370,470		$	30.97

The following table presents our stock option activity for 2018:

	Number of Shares		Weighted Average Exercise Price		Weighted Average Contractual Life (Years)
Outstanding options at January 1, 2018	909,730		$	33.25	7.62
Granted	163,666		55.87
Exercised	(162,690	)	36.59
Canceled, forfeited or expired	(77,391	)	35.95
Outstanding options at December 31, 2018	833,315		$	36.79	6.76
Exercisable options at December 31, 2018	462,845		$	27.97	6.17

Schedule of Non-Vested Stock Activity

The following table presents our non-vested stock activity for 2018:

	Number of Shares		Weighted Average Grant Date Fair Value
Non-vested stock at January 1, 2018	46,998		$	41.48
Granted	14,904		80.54
Vested	(46,998	)	41.48
Canceled, forfeited or expired	—		—
Non-vested stock at December 31, 2018	14,904		$	80.54

Schedule of Non-Vested Stock Unit Activity

The following table presents our service-based non-vested stock units activity for 2018:

	Number of  Shares		Weighted Average Grant Date Fair Value
Non-vested stock units at January 1, 2018	234,842		$	47.58
Granted	107,051		95.14
Vested	(71,658	)	46.55
Canceled, forfeited or expired	(29,835	)	44.20
Non-vested stock units at December 31, 2018	240,400		$	69.49

Schedule of Nonvested Performance-based Units Activity

The following table presents our performance-based non-vested stock units activity for 2018:

	Number of  Shares		Weighted Average Grant Date Fair Value
Non-vested stock units at January 1, 2018	252,948		$	51.15
Granted	115,338		79.59
Vested	(87,482	)	49.91
Canceled, forfeited or expired	(54,127	)	52.60
Non-vested stock units at December 31, 2018	226,677		$	65.76