Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 3,593	$ 3,614
Accumulated depreciation	$ 522,101	$ 482,448
Common stock - par value	$ 0.01	$ 0.01
Common stock - shares authorized	250,000,000	250,000,000
Common stock - shares issued	109,519,369	109,441,911
Treasury stock - shares at cost	56,369,752	56,369,752