Stockholders' Equity (Details Textual) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Feb. 27, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash dividend per share	$ 0.09	$ 0.075
Stockholders' Equity (Textual) [Abstract]
Weighted average grant date fair value per share	$ 31.52
Sale of Common Stock, shares	12,000,000
Sale of Common Stock, par value	$ 0.01		$ 0.01	$ 0.01
Sale of Common Stock, Price Per Share			$ 30.83
Proceeds from sale of Common Stock	$ 369,460
Underwriting discounts and estimated transaction expenses payable	500
Cash dividends on common stock	10,010	8,515
Cash dividend per common share, increase	$ 0.015
Repurchase of common stock, maximum value	1,200,000
Repurchase of common stock		751,005
Payments for repurchase of common stock		20,613
Remaining value of common stock authorized under repurchase program	434,557
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash dividend per share	$ 0.09
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash dividend per share	$ 0.075
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense from restricted stock units	$ 7,034	$ 2,978