Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Accounts receivable, allowance for doubtful accounts	$ 3,599	$ 3,513
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, authorized	1,000,000	1,000,000
Preferred stock, shares issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, authorized	300,000,000	300,000,000
Common stock, issued	186,446,351	184,059,900
Common stock, outstanding	147,340,925	145,913,162
Treasury stock, shares	39,105,426	38,146,738