CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
CONSOLIDATED BALANCE SHEETS
Property and equipment, Accumulated depreciation	$ 61,631	$ 60,697
Common stock, par value	$ 0	$ 0
Common stock, shares authorized (in shares)	180,000,000	180,000,000
Common stock, issued (in shares)	65,907,822	65,897,866
Common stock, outstanding (in shares)	61,842,662	65,897,866
Treasury stock (in shares)	4,065,160