Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($)	Total	Common Stock Outstanding	Common Stock	Value of Shares Held in Rabbi Trust at Cost	Deferred Compensation and Other Retirement Benefit Obligations	Additional Paid in Capital	Retained Earnings	Accumulated Other Comprehensive Income/(Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding		28,080,408
Net income	$ 35,225,000						$ 35,225,000
Other comprehensive income	8,054,000							$ 8,054,000
Cash dividends declared per common share (in dollars per share)							$ 0.44
Common dividend declared	(12,379,000)						$ (12,379,000)
Proceeds from exercise of stock options, net of cash paid		6,000
Proceeds from exercise of stock options, net of cash paid	165,000		$ 0			$ 165,000
Stock based compensation	915,000					915,000
Restricted stock awards issued, net of awards surrendered		44,407
Restricted stock awards issued, net of awards surrendered	(1,419,000)		1,000			(1,420,000)
Shares issued under direct stock purchase plan		6,689
Shares issued under direct stock purchase plan	487,000					487,000
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits				$ 119,000	$ (119,000)
Increase (Decrease) in Deferred Compensation	0
Period End Balance at Mar. 31, 2019	1,104,538,000		280,000	(4,599,000)	4,599,000	527,795,000	569,582,000	6,881,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding		28,137,504
Common Stock, Shares, Outstanding		34,377,388
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(1,137,000)
Net income	26,751,000						$ 26,751,000
Other comprehensive income	31,559,000							31,559,000
Cash dividends declared per common share (in dollars per share)							$ 0.46
Common dividend declared	(15,402,000)						$ (15,402,000)
Stock based compensation	850,000					850,000
Restricted stock awards issued, net of awards surrendered		42,531
Restricted stock awards issued, net of awards surrendered	(1,132,000)		1,000			(1,133,000)
Shares issued under direct stock purchase plan		7,009
Shares issued under direct stock purchase plan	560,000					560,000
Stock Repurchased During Period, Shares		(1,166,923)
Stock Repurchased During Period, Value	(73,226,000)		(12,000)			(73,214,000)
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits				131,000	(131,000)
Increase (Decrease) in Deferred Compensation	0
Period End Balance at Mar. 31, 2020	1,679,656,000		$ 331,000	$ (4,604,000)	$ 4,604,000	$ 962,513,000	$ 667,084,000	$ 49,728,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method, net of tax.	817,000
Off-Balance Sheet, Credit Loss, Liability, Change in Method, Credit Loss Expense (Reversal)	(1,000,000.0)
Off-Balance Sheet, Credit Loss, Liability, Change in Method, Credit Loss Expense (Reversal), net of tax.	$ 736,000
Common Stock, Shares, Outstanding		33,260,005