Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 3,700	$ 2,920
Accumulated depreciation	$ 433,935	$ 398,039
Common stock - par value	$ 0.01	$ 0.01
Common stock - shares authorized	250,000,000
Common stock - shares issued	109,108,218	108,530,911
Treasury stock - shares at cost	56,369,752	50,495,378