CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Property and equipment, accumulated depreciation	$ 101,128	$ 146,814
Intangible assets, accumulated amortization	$ 31,864	$ 30,610
Preferred stock, par value (usd per share)	$ 0.001	$ 0.001
Preferred stock, authorized (shares)	5,000,000	5,000,000
Preferred stock, issued (shares)	0	0
Preferred stock, outstanding (shares)	0	0
Common stock, par value (usd per share)	$ 0.001	$ 0.001
Common stock, authorized (shares)	60,000,000	60,000,000
Common stock, issued (shares)	36,044,177	35,747,134
Common stock, outstanding (shares)	31,801,357	33,964,767
Treasury stock at cost (shares)	4,242,820	1,782,367