DEBT (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)		Jul. 19, 2019 USD ($)	Jan. 22, 2019 USD ($)
Debt instruments [Line Items]
Capital lease obligations		$ 0	[1]	$ 1,864
Long-term debt, principal amount		883,500		911,864
Less current portion of long-term debt		0		(791)
Long-term debt		883,500		911,073
Long-term debt due within one year		0		791
Total debt		$ 883,500		911,864
Total debt less unrestricted cash to EBITDA ratio		2.75
Revolving credit facility, current capacity		$ 1,150,000		950,000					$ 1,150,000
Line of credit facility increase in current capacity									$ 200,000
Revolving credit facility, maximum capacity		$ 1,425,000
Leverage ratio		3.5
Ratio of EBIT to interest expense		3.0
Outstanding letters of credit	[2]	$ (5,408)
Net available for borrowing as of December 31, 2019		$ 261,092
Interest rate swap [Member]
Debt instruments [Line Items]
Interest rate swap notional amount								$ 200,000
Minimum [Member]
Debt instruments [Line Items]
Revolving credit facility, commitment fee		0.175%
Maximum [Member]
Debt instruments [Line Items]
Revolving credit facility, commitment fee		0.35%
Revolving credit facility [Member]
Debt instruments [Line Items]
Principal amount		$ 883,500		$ 910,000
Interest rate at period end		3.03%		3.79%
Daily average amount outstanding		$ 925,715		$ 731,110		$ 436,588
Weighted-average interest rate		3.54%		3.24%		2.55%
Capital lease obligations [Member]
Debt instruments [Line Items]
Less current portion of long-term debt		$ 0	[1]	$ (791)
Term loan facility [Member]
Debt instruments [Line Items]
Daily average amount outstanding		$ 0		$ 63,638	[3]	$ 315,862	[3]
Weighted-average interest rate		0.00%		2.97%	[3]	2.57%	[3]

[1] Upon adoption of ASU No. 2016-02, Leasing, and related amendments on January 1, 2019 (Note 2), we reclassified our capital lease obligations, now known as finance lease obligations, to accrued liabilities and other non-current liabilities on the consolidated balance sheet.

[2] We use standby letters of credit primarily to collateralize certain obligations related to our self-insured workers' compensation claims, as well as claims for environmental matters, as required by certain states. These letters of credit reduce the amount available for borrowing under our revolving credit facility.

[3] During 2018 and 2017, we had borrowings outstanding under a variable rate term loan facility. These amounts were repaid in March 2018.