Computation of Basic and Diluted Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Net Income (Loss) Available to Common Stockholders, Basic
Net income	$ 16,155	$ 11,921	$ 41,379	$ 31,281
Preferred stock dividends	863	862	2,569	2,559
Net income available to common stockholders	$ 15,292	$ 11,059	$ 38,810	$ 28,722
Weighted Average Number of Shares Outstanding, Basic and Diluted
Weighted average common shares outstanding for basic earnings per share computation	42,989,564	42,774,259	42,943,588	42,737,986
Dilutive effect of stock-based compensation	130,513	67,404	102,968	111,368
Weighted average common shares outstanding for diluted earnings per common share computation	43,120,077	42,841,663	43,046,556	42,849,354
Earnings Per Share, Basic and Diluted
Basic earnings per common share	$ 0.36	$ 0.26	$ 0.90	$ 0.67
Diluted earnings per common share	$ 0.35	$ 0.26	$ 0.90	$ 0.67
Stock Options
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			2,814,277	2,888,192
Unvested Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			48,196	33,068