Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Reconciliation of component parts of earnings per share [Abstract]
Net income	$ 10,252	$ 9,753	$ 29,183	$ 27,728
Less: Net income allocated to participating securities	11	11	33	31
Net income allocated to common shareholders	10,241	9,742	29,150	27,697
Basic EPS [Abstract]
Distributed earnings allocated to common stock	6,191	6,156	18,546	18,443
Undistributed earnings allocated to common stock	4,050	3,586	10,604	9,254
Net income allocated to common shareholders	10,241	9,742	29,150	27,697
Weighted average common shares outstanding including participating securities (in shares)	94,334,000	93,798,000	94,202,000	93,674,000
Less: Participating securities (in shares)	106,000	106,000	106,000	106,000
Weighted average common shares (in shares)	94,228,000	93,692,000	94,096,000	93,568,000
Basic EPS (in dollars per share)	$ 0.109	$ 0.104	$ 0.310	$ 0.296
Diluted EPS [Abstract]
Net income allocated to common shareholders	$ 10,241	$ 9,742	$ 29,150	$ 27,697
Weighted average common shares for basic EPS (in shares)	94,228,000	93,692,000	94,096,000	93,568,000
Effect of Dilutive Securities [Abstract]
Stock Options	47,000	7,000	19,000	4,000
Weighted average common shares including potential dilutive shares (in shares)	94,275,000	93,699,000	94,115,000	93,572,000
Diluted EPS (in dollars per share)	$ 0.109	$ 0.104	$ 0.310	$ 0.296
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)	2,100,000	2,600,000	2,500,000	3,000,000