DEBT (Senior Secured Revolving Credit Facility) (Details)				3 Months Ended	12 Months Ended
	Jul. 21, 2020 USD ($)	Dec. 10, 2019 USD ($)	Oct. 26, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Revolving credit facility				$ 75,000,000	$ 0	$ 75,000,000
Interest rate				3.01%		3.01%
Credit Agreement, potential additional borrowing capacity		$ 185,000,000.0
Credit Agreement, ratio of debt to EBITDA		2.50
Letters of credit outstanding				$ 200,000	200,000	$ 200,000
Ratio of debt to EBITDA step-down		4.0
Ratio of debt to EBITDA Increase		0.5
Write-off of previously-capitalized debt issuance costs					0	333,000	$ 0
Amortization of debt issuance costs					3,474,000	1,974,000	1,776,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 500,000,000.0	$ 350,000,000.0
Revolving credit facility, term		5 years	5 years
Percent of domestic subsidiaries equity		100.00%
Percent of voting equity		66.00%
Percent of non-voting equity		100.00%
Revolving credit facility, collateral, percent of assets		100.00%
Revolving credit facility, collateral, percent of equity		100.00%
Revolving credit facility, letter of credit fronting fee percentage		0.125%
Write-off of previously-capitalized debt issuance costs				300,000
Revolving credit facility, debt issuance costs	$ 1,100,000	$ 2,800,000
Interest expense					4,300,000	6,100,000
Unused borrowing capacity fee					1,700,000	600,000	800,000
Amortization of debt issuance costs					1,300,000	600,000	400,000
Revolving Credit Facility | Line of Credit
Line of Credit Facility [Line Items]
Current borrowing capacity	$ 200,000,000.0
Floor interest rate	0.75%
Revolving credit facility, unused capacity, commitment fee percentage	0.50%
Swingline Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity		10,000,000.0
Letter of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 10,000,000.0
Fed Funds Effective Rate Overnight Index Swap Rate | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility, basis spread on variable rate		0.50%
LIBOR | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility, basis spread on variable rate		1.00%
Interest expense					$ 1,300,000	4,900,000	800,000
LIBOR | Revolving Credit Facility | Line of Credit
Line of Credit Facility [Line Items]
Revolving credit facility, basis spread on variable rate	2.25%
ABR | Revolving Credit Facility | Line of Credit
Line of Credit Facility [Line Items]
Revolving credit facility, basis spread on variable rate	1.25%
Minimum | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility, unused capacity, commitment fee percentage		0.25%
Revolving credit facility, letter of credit participation fee percentage		1.25%
Minimum | LIBOR | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility, basis spread on variable rate		1.25%
Minimum | ABR | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility, basis spread on variable rate		0.25%
Maximum
Line of Credit Facility [Line Items]
Ratio of debt to EBITDA		4.5
Maximum | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility, unused capacity, commitment fee percentage		0.45%
Revolving credit facility, letter of credit participation fee percentage		2.00%
Maximum | LIBOR | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility, basis spread on variable rate		2.00%
Maximum | ABR | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility, basis spread on variable rate		1.00%
31 Day Borrowing, first | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility				50,000,000.0		50,000,000.0
31 Day Borrowing, second | Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility				$ 25,000,000.0		$ 25,000,000.0
Revolving Credit Facility
Line of Credit Facility [Line Items]
Interest expense							$ 2,000,000.0