CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Property and equipment, Accumulated depreciation	$ 60,697	$ 59,404
Common stock, shares authorized (in shares)	180,000,000	180,000,000
Common stock, issued (in shares)	65,897,866	74,342,826
Common stock, outstanding (in shares)	65,897,866	74,342,826