STOCK-BASED COMPENSATION - RSU and RSA Activity (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jul. 31, 2017
RSUs
Nonvested RSUs and RSAs, Number of Shares
Nonvested at the beginning of the period (in shares)		144,939
Granted (in shares)		138,418
Vested (in shares)		(70,698)
Forfeited (in shares)		(17,973)
Nonvested at the end of the period (in shares)		194,686	144,939
Nonvested RSUs and RSAs, Weighted Average Grant Date Fair Value
Nonvested at the beginning of the period (in dollars per share)		$ 267.85
Granted (in dollars per share)		286.42
Vested (in dollars per share)		239.15
Forfeited (in dollars per share)		285.95
Nonvested at the end of the period (in dollars per share)		$ 289.82	$ 267.85
Unrecognized compensation cost		$ 35.2
Weighted-average period for recognition of unrecognized compensation cost		1 year 8 months 12 days
Vested fair value		$ 22.4	$ 27.2	$ 21.8
Performance Shares
Nonvested RSUs and RSAs, Number of Shares
Nonvested at the beginning of the period (in shares)		14,647
Granted (in shares)		0	0
Vested (in shares)		(8,319)
Forfeited (in shares)		0
Nonvested at the end of the period (in shares)		6,328	14,647
Nonvested RSUs and RSAs, Weighted Average Grant Date Fair Value
Nonvested at the beginning of the period (in dollars per share)		$ 210.55
Granted (in dollars per share)		0
Vested (in dollars per share)		200.40
Forfeited (in dollars per share)		0
Nonvested at the end of the period (in dollars per share)		$ 223.90	$ 210.55
Unrecognized compensation cost		$ 1.1
Weighted-average period for recognition of unrecognized compensation cost		9 months 18 days
Vested fair value		$ 2.6	$ 18.8	7.9
Performance-Based Restricted Stock
Nonvested RSUs and RSAs, Number of Shares
Nonvested at the beginning of the period (in shares)		47,608
Granted (in shares)		0	0
Vested (in shares)		(23,804)
Forfeited (in shares)		0
Nonvested at the end of the period (in shares)		23,804	47,608
Nonvested RSUs and RSAs, Weighted Average Grant Date Fair Value
Nonvested at the beginning of the period (in dollars per share)		$ 340.25
Granted (in dollars per share)		0
Vested (in dollars per share)		340.25
Forfeited (in dollars per share)		0
Nonvested at the end of the period (in dollars per share)		$ 340.25	$ 340.25
Unrecognized compensation cost		$ 4.4
Weighted-average period for recognition of unrecognized compensation cost		1 year
Vested fair value		$ 6.2	$ 8.2	13.6
Grant date fair value					$ 21.9
Expiration period	6 months
Market-Based Restricted Stock
Nonvested RSUs and RSAs, Number of Shares
Nonvested at the beginning of the period (in shares)		26,674
Granted (in shares)		0
Vested (in shares)		0
Forfeited (in shares)		0
Nonvested at the end of the period (in shares)		26,674	26,674
Nonvested RSUs and RSAs, Weighted Average Grant Date Fair Value
Nonvested at the beginning of the period (in dollars per share)		$ 340.25
Granted (in dollars per share)		0
Vested (in dollars per share)		0
Forfeited (in dollars per share)		0
Nonvested at the end of the period (in dollars per share)		$ 340.25	$ 340.25
Unrecognized compensation cost		$ 0.7
Weighted-average period for recognition of unrecognized compensation cost		1 year 9 months 18 days
Grant date fair value				$ 1.9
Return performance, percent of target stock price				110.00%
Maximum number of shares for earned (in shares)				44,545
Performance awards earned (in shares)		29,601
Minimum | Performance Shares
Nonvested RSUs and RSAs, Weighted Average Grant Date Fair Value
Vesting period		6 months
Minimum | Market-Based Restricted Stock
Nonvested RSUs and RSAs, Weighted Average Grant Date Fair Value
Return performance, percent of target stock price				70.00%