Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Basic and Diluted Earnings Per Share
Net Income	$ 28,054	$ 29,294	$ 28,677	$ 26,730	$ 26,546	$ 25,628	$ 22,702	$ 20,768	$ 112,755	$ 95,644	$ 75,516
Average outstanding shares - basic									74,641,957	73,191,713	71,928,570
Add: dilutive stock options and awards									45,358	68,565	86
Average outstanding shares - diluted									74,687,315	73,260,278	71,928,656
Basic earnings per share	$ 0.37	$ 0.40	$ 0.38	$ 0.36	$ 0.36	$ 0.35	$ 0.31	$ 0.29	$ 1.51	$ 1.31	$ 1.05
Diluted earnings per share	$ 0.37	$ 0.40	$ 0.38	$ 0.36	$ 0.36	$ 0.35	$ 0.31	$ 0.29	$ 1.51	$ 1.31	$ 1.05
Earnings Per Share
Options excluded from the diluted average outstanding share calculation									0	38,915	879,525