Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Basic earnings per share [Abstract]
Net income available to common shareholders	$ 54,558	$ 57,901	$ 57,404
Weighted average common shares, basic (in shares)	33,379,000	33,662,000	34,275,000
Basic EPS (in dollars per share)	$ 1.63	$ 1.72	$ 1.67
Diluted earnings per share [Abstract]
Net income available to common shareholders	$ 54,558	$ 57,901	$ 57,404
Dilutive effect of stock based compensation (in shares)	340,000	262,000	234,000
Weighted average common shares, diluted (in shares)	33,719,000	33,924,000	34,509,000
Diluted EPS (in dollars per share)	$ 1.62	$ 1.71	$ 1.66
Stock options excluded from calculation of diluted EPS (in shares)	1,193,000	1,258,000	1,309,000