CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CONSOLIDATED BALANCE SHEETS
Allowance for doubtful accounts	$ 14,065	$ 13,472	$ 13,109	$ 13,611	$ 15,521	$ 14,418	$ 15,241	$ 16,296	$ 15,027	$ 9,944
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.01	$ 0.01
Common stock, shares authorized	60,000,000	60,000,000	60,000,000	60,000,000	60,000,000	60,000,000	60,000,000	60,000,000
Common stock, shares issued	36,515,232	36,486,490	36,478,597	36,402,093	36,183,894	36,158,347	36,144,560	36,044,247
Common stock, shares outstanding	36,372,411	36,343,669	36,335,776	36,259,272	36,041,073	36,015,526	36,001,739	35,901,426
Treasury stock, shares	142,821	142,821	142,821	142,821	142,821	142,821	142,821	142,821