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	$ 65,853,000						$ 65,853,000
Other comprehensive income	22,129,000							$ 22,129,000
Cash dividends declared per common share (in dollars per share)							$ 0.88
Common dividend declared	(27,478,000)						$ (27,478,000)
Stock Issued During Period, Shares, Acquisitions		6,166,010
Stock Issued During Period, Value, Acquisitions	499,693,000		$ 61,000			$ 499,632,000
Proceeds from exercise of stock options, net of cash paid		11,000
Proceeds from exercise of stock options, net of cash paid	281,000		0			281,000
Stock based compensation	2,432,000					2,432,000
Restricted stock awards issued, net of awards surrendered		50,474
Restricted stock awards issued, net of awards surrendered	(1,431,000)		2,000			(1,433,000)
Shares issued under direct stock purchase plan		13,169
Shares issued under direct stock purchase plan	1,034,000					1,034,000
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits				$ 70,000	$ (70,000)
Increase (Decrease) in Deferred Compensation	0
Period End Balance at Jun. 30, 2019	1,636,003,000		342,000	(4,648,000)	4,648,000	1,029,594,000	585,111,000	20,956,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding		28,137,504
Net income	30,628,000						$ 30,628,000
Other comprehensive income	14,075,000							14,075,000
Cash dividends declared per common share (in dollars per share)							$ 0.44
Common dividend declared	(15,099,000)						$ (15,099,000)
Stock Issued During Period, Shares, Acquisitions		6,166,010
Stock Issued During Period, Value, Acquisitions	499,693,000		61,000			499,632,000
Proceeds from exercise of stock options, net of cash paid		5,000
Proceeds from exercise of stock options, net of cash paid	116,000		0			116,000
Stock based compensation	1,517,000					1,517,000
Restricted stock awards issued, net of awards surrendered		6,067
Restricted stock awards issued, net of awards surrendered	(12,000)		1,000			(13,000)
Shares issued under direct stock purchase plan		6,480
Shares issued under direct stock purchase plan	547,000					547,000
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits				(49,000)	49,000
Increase (Decrease) in Deferred Compensation	0
Period End Balance at Jun. 30, 2019	1,636,003,000		342,000	(4,648,000)	4,648,000	1,029,594,000	585,111,000	20,956,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding		34,321,061
Common Stock, Shares, Outstanding		34,377,388
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(1,137,000)
Net income	51,653,000						$ 51,653,000
Other comprehensive income	33,676,000							33,676,000
Cash dividends declared per common share (in dollars per share)							$ 0.92
Common dividend declared	(30,554,000)						$ (30,554,000)
Stock Issued During Period, Value, Acquisitions	0
Proceeds from exercise of stock options, net of cash paid		873
Stock Repurchased During Period, Value, as a result of Stock Options Exercised	(26,000)					(26,000)
Stock based compensation	2,429,000					2,429,000
Restricted stock awards issued, net of awards surrendered		49,292
Restricted stock awards issued, net of awards surrendered	(1,183,000)		1,000			(1,184,000)
Shares issued under direct stock purchase plan		14,557
Shares issued under direct stock purchase plan	1,092,000					1,092,000
Stock Repurchased During Period, Shares		(1,500,000)
Stock Repurchased During Period, Value	(95,091,000)		(15,000)			(95,076,000)
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits				86,000	(86,000)
Increase (Decrease) in Deferred Compensation	0
Period End Balance at Jun. 30, 2020	1,671,692,000		328,000	(4,649,000)	4,649,000	942,685,000	676,834,000	51,845,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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
Net income	24,902,000						$ 24,902,000
Other comprehensive income	2,117,000							2,117,000
Cash dividends declared per common share (in dollars per share)							$ 0.46
Common dividend declared	(15,152,000)						$ (15,152,000)
Proceeds from exercise of stock options, net of cash paid		873
Proceeds from exercise of stock options, net of cash paid			0
Stock Repurchased During Period, Value, as a result of Stock Options Exercised	(26,000)					(26,000)
Stock based compensation	1,579,000					1,579,000
Restricted stock awards issued, net of awards surrendered		6,761
Restricted stock awards issued, net of awards surrendered	(51,000)		0			(51,000)
Shares issued under direct stock purchase plan		7,548
Shares issued under direct stock purchase plan	532,000					532,000
Stock Repurchased During Period, Shares		(333,077)
Stock Repurchased During Period, Value	(21,865,000)		(3,000)			(21,862,000)
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits				(45,000)	45,000
Increase (Decrease) in Deferred Compensation	0
Period End Balance at Jun. 30, 2020	1,671,692,000		$ 328,000	$ (4,649,000)	$ 4,649,000	$ 942,685,000	$ 676,834,000	$ 51,845,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method, net of tax.	$ 817,000
Common Stock, Shares, Outstanding		32,942,110