ACCOUNTING POLICIES (Details) - USD ($) $ in Thousands	Mar. 31, 2018		Dec. 31, 2017
Cash equivalents
Cash equivalents	$ 155,800		$ 25,500
Assets:
Cash equivalents - Money market mutual funds	155,763		25,455
Trading securities held in a "rabbi trust"	78	[1]	74
Total assets	155,841		25,529
Liabilities:
Deferred compensation accrual "rabbi trust"	86	[2]	92
Total liabilities	86		92
Level 1
Assets:
Cash equivalents - Money market mutual funds	155,763		25,455
Trading securities held in a "rabbi trust"	78	[1]	74
Total assets	155,841		25,529
Liabilities:
Deferred compensation accrual "rabbi trust"	86	[2]	92
Total liabilities	86		92
Level 2
Assets:
Cash equivalents - Money market mutual funds	0		0
Trading securities held in a "rabbi trust"	0	[1]	0
Total assets	0		0
Liabilities:
Deferred compensation accrual "rabbi trust"	0	[2]	0
Total liabilities	0		0
Level 3
Assets:
Cash equivalents - Money market mutual funds	0		0
Trading securities held in a "rabbi trust"	0	[1]	0
Total assets	0		0
Liabilities:
Deferred compensation accrual "rabbi trust"	0	[2]	0
Total liabilities	$ 0		$ 0

[1] Trading securities held in a rabbi trust are included in Prepaids and other current assets and Other long-term assets, net in the consolidated balance sheets.

[2] Non-qualified deferred compensation in a rabbi trust is included in Accrued liabilities and Other long-term liabilities in the consolidated balance sheets.