Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Reconciliation of component parts of earnings per share [Abstract]
Net income	$ 9,763	$ 9,066	$ 18,931	$ 17,975
Less: Net income allocated to participating securities	11	10	21	20
Net income allocated to common shareholders	9,752	9,056	18,910	17,955
Basic EPS [Abstract]
Distributed earnings allocated to common stock	6,182	6,139	12,356	12,287
Undistributed earnings allocated to common stock	3,570	2,917	6,554	5,668
Net income allocated to common shareholders	9,752	9,056	18,910	17,955
Weighted average common shares outstanding including participating securities basic (in shares)	94,204,000	93,667,000	94,134,000	93,609,000
Less: Participating securities (in shares)	106,000	106,000	106,000	106,000
Weighted average common shares (in shares)	94,098,000	93,561,000	94,028,000	93,503,000
Basic EPS (in dollars per share)	$ 0.104	$ 0.097	$ 0.201	$ 0.192
Diluted EPS [Abstract]
Net income allocated to common shareholders	$ 9,752	$ 9,056	$ 18,910	$ 17,955
Weighted average common shares for basic EPS (in shares)	94,098,000	93,561,000	94,028,000	93,503,000
Effect of Dilutive Securities [Abstract]
Stock Options	7,000	1,000	6,000	1,000
Weighted average common shares including potential dilutive shares (in shares)	94,105,000	93,562,000	94,034,000	93,504,000
Diluted EPS (in dollars per share)	$ 0.104	$ 0.097	$ 0.201	$ 0.192
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average number of antidilutive stock options excluded from diluted earnings per share (in shares)	2,700,000	3,100,000	2,700,000	3,100,000