Earnings Per Share (Details) (USD $) In Thousands, except 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, 2013	Dec. 31, 2012	Dec. 31, 2011
Basic and Diluted Earnings Per Share
Net Income	$ 26,546	$ 25,628	$ 22,702	$ 20,768	$ 20,758	$ 19,444	$ 18,981	$ 16,333	$ 95,644	$ 75,516	$ 17,471
Average outstanding shares - basic									73,191,713	71,928,570	71,915,073
Add: dilutive stock options and awards									68,565	86	0
Average outstanding shares - diluted									73,260,278	71,928,656	71,915,073
Basic earnings per share	$ 0.36	$ 0.35	$ 0.31	$ 0.29	$ 0.29	$ 0.27	$ 0.26	$ 0.23	$ 1.31	$ 1.05	$ 0.24
Diluted earnings per share	$ 0.36	$ 0.35	$ 0.31	$ 0.29	$ 0.29	$ 0.27	$ 0.26	$ 0.23	$ 1.31	$ 1.05	$ 0.24
Earnings Per Share
Options excluded from the diluted average outstanding share calculation									38,915	879,525	1,567,561