Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($)		Total	Cumulative Effect, Period of Adoption, Adjustment	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	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income/(Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding				28,080,408
Net income		$ 117,698,000							$ 117,698,000
Other comprehensive income		27,375,000									$ 27,375,000
Cash dividends declared per common share (in dollars per share)									$ 1.32
Common dividend declared		(42,603,000)							$ (42,603,000)
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		3,413,000						3,413,000
Restricted stock awards issued, net of awards surrendered				50,431
Restricted stock awards issued, net of awards surrendered		(1,435,000)			1,000			(1,436,000)
Shares issued under direct stock purchase plan				58,932
Shares issued under direct stock purchase plan		4,412,000			1,000			4,411,000
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits						$ 5,000	$ (5,000)
Increase (Decrease) in Deferred Compensation		0
Period End Balance at Sep. 30, 2019		1,682,324,000			342,000	(4,713,000)	4,713,000	1,033,949,000	621,831,000		26,202,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding				34,321,061
Net income		51,845,000							$ 51,845,000
Other comprehensive income		5,246,000									5,246,000
Cash dividends declared per common share (in dollars per share)									$ 0.44
Common dividend declared		(15,125,000)							$ (15,125,000)
Stock Issued During Period, Shares, Acquisitions				0
Stock Issued During Period, Value, Acquisitions		0			0			0
Proceeds from exercise of stock options, net of cash paid				0
Proceeds from exercise of stock options, net of cash paid		0			0			0
Stock based compensation		981,000						981,000
Restricted stock awards issued, net of awards surrendered				(43)
Restricted stock awards issued, net of awards surrendered		(4,000)			(1,000)			(3,000)
Shares issued under direct stock purchase plan				45,763
Shares issued under direct stock purchase plan		3,378,000			1,000			3,377,000
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits						(65,000)	65,000
Increase (Decrease) in Deferred Compensation		0
Period End Balance at Sep. 30, 2019		$ 1,682,324,000			342,000	(4,713,000)	4,713,000	1,033,949,000	621,831,000		26,202,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding				34,366,781
Common Stock, Shares, Outstanding		34,377,388		34,377,388
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method		$ (1,137,000)
Net income		86,526,000							$ 86,526,000
Other comprehensive income		30,463,000									30,463,000
Cash dividends declared per common share (in dollars per share)									$ 1.38
Common dividend declared		(45,715,000)							$ (45,715,000)
Stock Issued During Period, Value, Acquisitions		0
Proceeds from exercise of stock options, net of cash paid				5,873
Stock Repurchased During Period, Value, as a result of Stock Options Exercised		114,000						114,000
Stock based compensation		3,297,000						3,297,000
Restricted stock awards issued, net of awards surrendered				49,249
Restricted stock awards issued, net of awards surrendered		(1,186,000)			1,000			(1,187,000)
Shares issued under direct stock purchase plan				23,037
Shares issued under direct stock purchase plan		1,620,000						1,620,000
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						23,000	(23,000)
Increase (Decrease) in Deferred Compensation		0
Period End Balance at Sep. 30, 2020		1,689,724,000			328,000	(4,712,000)	4,712,000	944,218,000	696,546,000		48,632,000
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				32,942,110
Net income		34,873,000							$ 34,873,000
Other comprehensive income		(3,213,000)									(3,213,000)
Cash dividends declared per common share (in dollars per share)									$ 0.46
Common dividend declared		(15,161,000)							$ (15,161,000)
Proceeds from exercise of stock options, net of cash paid				5,000
Proceeds from exercise of stock options, net of cash paid					0
Stock Repurchased During Period, Value, as a result of Stock Options Exercised		140,000						140,000
Stock based compensation		868,000						868,000
Restricted stock awards issued, net of awards surrendered				(43)
Restricted stock awards issued, net of awards surrendered		(3,000)			0			(3,000)
Shares issued under direct stock purchase plan				8,480
Shares issued under direct stock purchase plan		528,000						528,000
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits						(63,000)	63,000
Increase (Decrease) in Deferred Compensation		0
Period End Balance at Sep. 30, 2020		1,689,724,000			$ 328,000	$ (4,712,000)	$ 4,712,000	$ 944,218,000	$ 696,546,000		$ 48,632,000
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		32,955,547		32,955,547
Cumulative Effect of New Accounting Principle in Period of Adoption (Deprecated 2020-01-31)	[1]		$ 1,553,000							$ 1,553,000

[1] Represents adjustment needed to reflect the cumulative impact on retained earnings pursuant to the Company's adoption of Accounting Standards Update 2016-13. The adjustment presented includes $1.1 million ($817,000, net of tax) attributable to the change in accounting methodology for estimating the allowance for credit losses and $1.0 million ($736,000, net of tax) related to the reserve for unfunded commitments resulting from the Company's adoption of the standard. Amount shown in the table above is presented net of tax.