Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	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		27,450,190
Prior Period Reclassification Adjustment	$ 0						$ 397	[1]	$ (397)	[1]
Net income	91,688						$ 91,688
Other comprehensive income	(9,605)								(9,605)
Cash dividends declared per common share (in dollars per share)							$ 1.14
Common dividend declared	(31,380)						$ (31,380)
Stock Issued During Period, Value, Acquisitions	0
Proceeds from exercise of stock options, net of cash paid		23,195
Proceeds from exercise of stock options, net of cash paid	184		$ 0			$ 184
Stock based compensation	3,161					3,161
Restricted stock awards issued, net of awards surrendered		43,174
Restricted stock awards issued, net of awards surrendered	(1,344)		1			(1,345)
Shares issued under direct stock purchase plan		24,284
Shares issued under direct stock purchase plan	1,792					1,792
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits				$ (194)	$ 194
Increase (Decrease) in Deferred Compensation	0
Period End Balance at Sep. 30, 2018	998,305		274	(4,784)	4,784	483,222	527,473		(12,664)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding		27,532,524
Net income	33,015						$ 33,015
Other comprehensive income	(2,550)								(2,550)
Cash dividends declared per common share (in dollars per share)							$ 0.38
Common dividend declared	(10,468)						$ (10,468)
Stock based compensation	769					769
Restricted stock awards issued, net of awards surrendered		(43)
Restricted stock awards issued, net of awards surrendered	(6)		0			(6)
Shares issued under direct stock purchase plan		5,923
Shares issued under direct stock purchase plan	480					480
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits				(131)	131
Increase (Decrease) in Deferred Compensation	0
Period End Balance at Sep. 30, 2018	998,305		274	(4,784)	4,784	483,222	527,473		(12,664)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	0						831	[2]	(831)	[2]
Common Stock, Shares, Outstanding		27,540,843
Common Stock, Shares, Outstanding		28,080,408
Net income	117,698						$ 117,698
Other comprehensive income	27,375								27,375
Cash dividends declared per common share (in dollars per share)							$ 1.32
Common dividend declared	(42,603)						$ (42,603)
Stock Issued During Period, Shares, Acquisitions		6,166,010
Stock Issued During Period, Value, Acquisitions	499,693		61			499,632
Proceeds from exercise of stock options, net of cash paid		11,000
Proceeds from exercise of stock options, net of cash paid	281					281
Stock based compensation	3,413					3,413
Restricted stock awards issued, net of awards surrendered		50,431
Restricted stock awards issued, net of awards surrendered	(1,435)		1			(1,436)
Shares issued under direct stock purchase plan		58,932
Shares issued under direct stock purchase plan	4,412		1			4,411
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits				5	(5)
Increase (Decrease) in Deferred Compensation	0
Period End Balance at Sep. 30, 2019	1,682,324		342	(4,713)	4,713	1,033,949	621,831		26,202
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding		34,321,061
Net income	51,845						$ 51,845
Other comprehensive income	5,246								5,246
Cash dividends declared per common share (in dollars per share)							$ 0.44
Common dividend declared	(15,125)						$ (15,125)
Stock based compensation	981					981
Restricted stock awards issued, net of awards surrendered		(43)
Restricted stock awards issued, net of awards surrendered	(4)		(1)			(3)
Shares issued under direct stock purchase plan		45,763
Shares issued under direct stock purchase plan	3,378		1			3,377
Increase (Decrease) in Deferred Compensation and Other Retirement Benefits				(65)	65
Increase (Decrease) in Deferred Compensation	0
Period End Balance at Sep. 30, 2019	$ 1,682,324		$ 342	$ (4,713)	$ 4,713	$ 1,033,949	$ 621,831		$ 26,202
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Shares, Outstanding		34,366,781

[1] Represents adjustment needed to reflect the cumulative impact on retained earnings for reclassification of the income tax effects attributable to accumulated other comprehensive income, as a result of the Tax Cuts and Jobs Act (the "Tax Act"). Pursuant to the Company's adoption of Accounting Standards Update 2018-02, the Company has elected to reclassify amounts stranded in other comprehensive income to retained earnings.

[2] Represents adjustment needed to reflect the cumulative impact on retained earnings for the classification and measurement of investments in equity securities. Pursuant to the Company's adoption of Accounting Standards Update 2016-01, the Company's investments in equity securities will no longer be classified as available for sale, therefore the Company was required to reclassify the net unrealized gain recognized on the change in fair value of these equity securities from other comprehensive income to retained earnings.