STOCK-BASED COMPENSATION - Schedules - Stock Options (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Aggregate Intrinsic Value
Share price (in dollars per share)	$ 273.79
Stock Options
Number of Options
Outstanding at the beginning of the period (in shares)	777,871
Granted (in shares)	203,582
Exercised (in shares)	(47,630)
Forfeited (in shares)	(5,396)
Expired (in shares)	(3,717)
Outstanding at the end of the period (in shares)	924,710	777,871
Options exercisable at the end of the period (in shares)	663,239
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 69.87
Granted (in dollars per share)	290.27
Exercised (in dollars per share)	161.25
Forfeited (in dollars per share)	285.56
Expired (in dollars per share)	221.99
Outstanding at the end of the period (in dollars per share)	111.82	$ 69.87
Options exercisable at the end of the period (in dollars per share)	$ 44.49
Weighted Average Remaining Contractual Term
Outstanding at the end of the period	4 years 5 months 19 days
Options exercisable at the end of the period	2 years 7 months 17 days
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 155,411,000
Options exercisable at the end of the period	$ 153,243,000
Weighted average assumptions
Dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum	52.00%	51.00%	50.00%
Expected volatility, maximum	60.00%	55.00%	53.00%
Risk-free interest rate, minimum	0.33%	1.46%	2.33%
Risk-free interest rate, maximum	0.96%	2.55%	3.06%
Performance Options
Number of Options
Outstanding at the beginning of the period (in shares)	463,440
Granted (in shares)	236,769
Exercised (in shares)	0
Forfeited (in shares)	0
Expired (in shares)	0
Outstanding at the end of the period (in shares)	700,209	463,440
Options exercisable at the end of the period (in shares)	0
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 204.31
Granted (in dollars per share)	298.05
Exercised (in dollars per share)	0
Forfeited (in dollars per share)	0
Expired (in dollars per share)	0
Outstanding at the end of the period (in dollars per share)	236.01	$ 204.31
Options exercisable at the end of the period (in dollars per share)	$ 0
Weighted Average Remaining Contractual Term
Outstanding at the end of the period	7 years 9 months
Options exercisable at the end of the period	0 years
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 36,238,000
Options exercisable at the end of the period	$ 0
Weighted average assumptions
Expected term	7 years	7 years
Dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum	51.00%	51.00%	50.00%
Expected volatility, maximum	51.00%	51.00%	50.00%
Risk-free interest rate, minimum	1.03%	2.54%	2.38%
Risk-free interest rate, maximum	1.03%	2.54%	2.81%
Dividends	$ 0	$ 0	$ 0
Minimum \| Stock Options
Weighted average assumptions
Expected term	5 years	5 years	5 years
Minimum \| Performance Options
Weighted average assumptions
Expected term			7 years
Maximum \| Stock Options
Weighted average assumptions
Expected term	6 years 3 months	6 years 3 months	6 years 8 months 15 days
Maximum \| Performance Options
Weighted average assumptions
Expected term			7 years 1 month 24 days

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The estimated amount of dividends to be paid to holders of the underlying shares (expected dividends) over the option's term. Dividends are taken into account because payment of dividends to shareholders reduces the fair value of the underlying shares, and option holders generally do not receive dividends.

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The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

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Number of options outstanding, including both vested and non-vested options.

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