Earnings Per Share and Capital Stock (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic
Net Income		$ 20,211,000	$ 26,530,000	$ 25,612,000	$ 14,072,000	$ 16,700,000	$ 28,006,000	$ 26,784,000	$ 12,852,000	$ 86,425,000	$ 84,342,000	$ 63,871,000
Less: Distributed earnings to common shareholders										47,206,000	42,702,000	38,937,000
Less: Distributed earnings to participating securities										158,000	180,000	204,000
Undistributed earnings										39,061,000	41,460,000	24,730,000
Undistributed earnings allocated to common shareholders										38,930,000	41,285,000	24,601,000
Undistributed earnings allocated to participating securities											175,000	129,000
Total income available to common shareholders, basic										$ 86,136,000	$ 83,987,000	$ 63,538,000
Weighted average Common Shares outstanding, basic										36,880,000	36,814,000	36,733,000
Basic earnings per Common Share:
Earnings Per Share, Basic		$ 0.55	$ 0.72	$ 0.69	$ 0.38	$ 0.45	$ 0.76	$ 0.72	$ 0.35	$ 2.34	$ 2.28	$ 1.73
Diluted
Total income available to common shareholders, basic										$ 86,136,000	$ 83,987,000	$ 63,538,000
Undistributed earnings for dilutive stock options										131,000	175,000	129,000
Total common shareholders earnings, diluted										$ 86,267,000	$ 84,162,000	$ 63,667,000
Weighted average Common Shares outstanding, basic										36,880,000	36,814,000	36,733,000
Stock-based compensation (in shares)	[1]									115,000	150,000	203,000
Weighted average common shares outstanding, diluted										36,995,000	36,964,000	36,936,000
Diluted earnings per Common Share:
Earnings Per Share, Diluted		$ 0.54	$ 0.72	$ 0.69	$ 0.38	$ 0.45	$ 0.76	$ 0.72	$ 0.35	$ 2.33	$ 2.28	$ 1.72
Share based compensation arrangement
Proceeds from Stock Options Exercised, Distributed to Subsidiaries										$ 0	$ 0	$ 0
Dividend Reinvestment Plan Common Stock Capital Shares Reserved for Future Issuance		1,055,948								1,055,948
Common Shares authorized for issuance but unissued under 401(k) Plan		387,300								387,300
Restricted Stock Units
Share based compensation arrangement
Restricted stock units outstanding (in shares)		127,298								127,298

[1] (1)         In applying the treasury stock method of reflecting the dilutive effect of outstanding stock-based compensation in the calculation of diluted EPS, 127,298 restricted stock units, including performance awards, at December 31, 2020 were deemed to be outstanding in accordance with accounting guidance on earnings per share.