Earnings Per Share (Details 2) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Earnings Per Share
Net income available to common shareholders	$ 20,363	$ 17,600	$ 13,478	$ 15,298	$ 16,325	$ 15,693	$ 16,361	$ 15,181	$ 15,175	$ 16,720	$ 15,003	$ 17,253	$ 66,739	$ 63,560	$ 64,151
Weighted average common shares outstanding													90,626	93,913	99,802
Dilutive potential shares due to effect of stock options													845	288	376
Total weighted average common shares and dilutive potential shares													91,471	94,201	100,178
Basic earnings per share (in dollars per share)	$ 0.22	$ 0.19	$ 0.15	$ 0.17	$ 0.18	$ 0.17	$ 0.17	$ 0.16	$ 0.16	$ 0.17	$ 0.15	$ 0.16	$ 0.74	$ 0.68	$ 0.64
Diluted earnings per share (in dollars per share)	$ 0.22	$ 0.19	$ 0.15	$ 0.17	$ 0.18	$ 0.17	$ 0.17	$ 0.16	$ 0.16	$ 0.17	$ 0.15	$ 0.16	$ 0.73	$ 0.68	$ 0.64