Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reconciliation of component parts of earnings per share [Abstract]
Net income	$ 10,715	$ 11,011
Less: Net income allocated to participating securities	0	12
Net income allocated to common shareholders	10,715	10,999
Basic EPS [Abstract]
Distributed earnings allocated to common stock	6,228	6,206
Undistributed earnings allocated to common stock	4,487	4,793
Net income allocated to common shareholders	10,715	10,999
Weighted average common shares outstanding including participating securities (in shares)	94,947,000	94,558,000
Less: Participating securities (in shares)	0	106,000
Weighted average common shares (in shares)	94,947,000	94,452,000
Basic EPS (in dollars per share)	$ 0.113	$ 0.116
Diluted EPS [Abstract]
Net income allocated to common shareholders	$ 10,715	$ 10,999
Weighted average common shares for basic EPS (in shares)	94,947,000	94,452,000
Effect of Dilutive Securities [Abstract]
Stock Options (in shares)	127,000	129,000
Weighted average common shares including potential dilutive shares (in shares)	95,074,000	94,581,000
Diluted EPS (in dollars per share)	$ 0.113	$ 0.116
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)	1,400,000	2,400,000