Parent Financial Statements (Details Textual) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Accumulated depreciation	$ 203,124	$ 174,683
Parent Financial Statements (Textual) [Abstract]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares outstanding	46,878,474	48,167,229
Treasury stock at cost, shares	11,201,634	7,759,234
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Accumulated depreciation	$ 172,132	$ 145,375