Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic and Diluted Earnings Per Share
Net Income	$ 29,508	$ 29,614	$ 29,335	$ 27,670	$ 28,054	$ 29,294	$ 28,677	$ 26,730	$ 116,127	$ 112,755	$ 95,644
Average outstanding shares - basic									75,542,455	74,641,957	73,191,713
Add: dilutive stock options and awards									53,126	45,358	68,565
Average outstanding shares - diluted									75,595,581	74,687,315	73,260,278
Basic earnings per share	$ 0.39	$ 0.39	$ 0.39	$ 0.37	$ 0.37	$ 0.40	$ 0.38	$ 0.36	$ 1.54	$ 1.51	$ 1.31
Diluted earnings per share	$ 0.39	$ 0.39	$ 0.39	$ 0.37	$ 0.37	$ 0.40	$ 0.38	$ 0.36	$ 1.54	$ 1.51	$ 1.31
Earnings Per Share
Stock options and restricted stock awards excluded from the diluted average outstanding share calculation									0	0	38,915