SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Available-for-sale Investments) (Details)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) Securities
Schedule of Available-for-sale Securities [Line Items]
Weighted Average Interest Rate on Investments	1.23%	0.84%
Amortized cost	$ 188,543,000	$ 83,866,000
Gross unrealized gains	38,000	8,000
Gross unrealized loss	258,000	105,000
Estimated fair market value	188,323,000	83,769,000
Corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	46,338,000	38,586,000
Gross unrealized gains	2,000	7,000
Gross unrealized loss	2,000	10,000
Estimated fair market value	46,338,000	38,583,000
Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	78,900,000	33,654,000
Gross unrealized gains	15,000	1,000
Gross unrealized loss	76,000	36,000
Estimated fair market value	78,839,000	33,619,000
Investments due between 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	63,305,000	11,626,000
Gross unrealized gains	21,000	0
Gross unrealized loss	180,000	59,000
Estimated fair market value	$ 63,146,000	$ 11,567,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	0	0
Commercial paper [Member] | Corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 36,996,000
Gross unrealized gains	0
Gross unrealized loss	0
Estimated fair market value	36,996,000
Commercial paper [Member] | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	19,186,000
Gross unrealized gains	0
Gross unrealized loss	0
Estimated fair market value	19,186,000
Corporate securities [Member] | Corporate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	9,342,000	$ 38,586,000
Gross unrealized gains	2,000	7,000
Gross unrealized loss	2,000	10,000
Estimated fair market value	9,342,000	38,583,000
Corporate securities [Member] | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	59,714,000	33,654,000
Gross unrealized gains	15,000	1,000
Gross unrealized loss	76,000	36,000
Estimated fair market value	59,653,000	33,619,000
Corporate securities [Member] | Investments due between 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	63,305,000	11,626,000
Gross unrealized gains	21,000	0
Gross unrealized loss	180,000	59,000
Estimated fair market value	$ 63,146,000	$ 11,567,000