STOCKHOLDERS' EQUITY (Schedule of Stock by Class) (Details) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Stockholders' equity:
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	83,956,890	83,724,916
Undesignated preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Undesignated preferred stock, shares issued (in shares)	0	0
Common stock, par value (in USD per share)	$ 0.000001	$ 0.000001