Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 3,614	$ 4,023
Accumulated depreciation	$ 482,448	$ 440,586
Common stock - par value	$ 0.01	$ 0.01
Common stock - shares authorized	250,000,000	250,000,000
Common stock - shares issued	109,441,911	109,353,001
Treasury stock - shares at cost	56,369,752	56,369,752