CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Patient accounts receivable, allowance for doubtful accounts	$ 20,866	$ 17,716
Property and equipment, accumulated depreciation	146,814	138,650
Intangible assets, accumulated amortization	$ 30,610	$ 27,864
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)	35,747,134	35,253,577
Common stock, outstanding (shares)	33,964,767	33,597,215
Treasury stock at cost (shares)	1,782,367	1,656,362