INDEBTEDNESS (Details) - USD ($)		12 Months Ended
	May 20, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Long-term debt
Total long-term debt		$ 192,000,000	$ 220,000,000
Less: Current maturities of long-term debt		10,000,000	10,000,000
Less: Deferred financing fees, net		952,000	1,617,000
Long-term debt		$ 181,048,000	$ 208,383,000
Interest rate		2.40%	2.00%
Amortization expense related to the deferred financing fees		$ 700,000	$ 700,000	$ 700,000
Eurocurrency
Long-term debt
Basis spread on variable rate (as a percent)	1.00%
Credit agreement
Long-term debt
Capacity	$ 500,000,000.0
Credit agreement | Term Loan
Long-term debt
Borrowing capacity available increase	200,000,000
Quarterly payment	2,500,000.0
Credit agreement | Revolving Credit Facility
Long-term debt
Capacity		300,000,000
Borrowed		27,000,000	45,000,000
Letter of Credit		5,367,000	5,766,000
Unused capacity		267,633,000	249,234,000
Borrowing capacity available increase	$ 300,000,000
Balance of revolving credit facility
Long-term debt
Total long-term debt		27,000,000	45,000,000
Balance of revolving credit facility | Federal funds rate
Long-term debt
Basis spread on variable rate (as a percent)	0.50%
Term Loan
Long-term debt
Total long-term debt		$ 165,000,000	$ 175,000,000