Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Reconciliation of component parts of earnings per share [Abstract]
Net income	$ 9,168	$ 8,909
Less: Net income allocated to participating securities	10	10
Net income allocated to common shareholders	9,158	8,899
Basic EPS [Abstract]
Distributed earnings allocated to common stock	6,173	6,139
Undistributed earnings allocated to common stock	2,985	2,760
Net income allocated to common shareholders	9,158	8,899
Weighted average common shares outstanding including participating securities basic (in shares)	94,068,000	93,546,000
Less: Participating securities (in shares)	106,000	106,000
Weighted average common shares (in shares)	93,962,000	93,440,000
Basic EPS (in dollars per share)	$ 0.097	$ 0.095
Diluted EPS [Abstract]
Net income allocated to common shareholders	$ 9,158	$ 8,899
Weighted average common shares for basic EPS (in shares)	93,962,000	93,440,000
Effect of Dilutive Securities [Abstract]
Stock Options	5,000	1,000
Weighted average common shares including potential dilutive shares (in shares)	93,967,000	93,441,000
Diluted EPS (in dollars per share)	$ 0.097	$ 0.095
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