Consolidated Statements Of Shareholders' Equity (unaudited) - USD ($) $ in Thousands	Total	Common Stock	Paid-In Capital	Treasury Stock	Unallocated Common Stock Held By ESOP	Retained Earnings	Accumulated other comprehensive income (loss)
Balance at Sep. 30, 2018	$ 1,758,404	$ 3,323	$ 1,726,992	$ (754,272)	$ (48,751)	$ 807,890	$ 23,222
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	40,463					40,463
Other comprehensive income (loss), net of tax	(36,752)					0	(36,752)
ESOP shares allocated or committed to be released	3,477		1,310		2,167
Compensation costs for equity incentive plans	2,227		2,227			0
Purchase of treasury stock	(6,208)			(6,208)
Treasury stock allocated to equity incentive plan	(917)		(1,030)	113		0
Dividends paid to common shareholders	(24,709)					(24,709)
Balance at Mar. 31, 2019	1,735,985	3,323	1,729,499	(760,367)	(46,584)	823,644	(13,530)
Balance at Dec. 31, 2018	1,743,780	3,323	1,727,909	(757,464)	(47,667)	815,918	1,761
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,130					20,130
Other comprehensive income (loss), net of tax	(15,291)					0	(15,291)
ESOP shares allocated or committed to be released	1,812		729		1,083
Compensation costs for equity incentive plans	1,146		1,146			0
Purchase of treasury stock	(2,432)			(2,432)
Treasury stock allocated to equity incentive plan	(756)		(285)	(471)		0
Dividends paid to common shareholders	(12,404)					(12,404)
Balance at Mar. 31, 2019	1,735,985	3,323	1,729,499	(760,367)	(46,584)	823,644	(13,530)
Balance at Sep. 30, 2019	1,696,754	3,323	1,734,154	(764,589)	(44,417)	837,662	(69,379)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	42,899					42,899
Other comprehensive income (loss), net of tax	(61,484)					0	(61,484)
ESOP shares allocated or committed to be released	4,227		2,061		2,166
Compensation costs for equity incentive plans	2,349		2,349			0
Purchase of treasury stock	(378)			(378)
Treasury stock allocated to equity incentive plan	(1,797)		959	(2,756)		0
Dividends paid to common shareholders	(27,406)					(27,406)
Balance at Mar. 31, 2020	1,655,164	3,323	1,739,523	(767,723)	(42,251)	853,155	(130,863)
Balance at Dec. 31, 2019	1,728,949	3,323	1,736,322	(766,492)	(43,334)	849,929	(50,799)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	17,257					17,257
Other comprehensive income (loss), net of tax	(80,064)					0	(80,064)
ESOP shares allocated or committed to be released	2,133		1,050		1,083
Compensation costs for equity incentive plans	1,160		1,160			0
Purchase of treasury stock	(19)			(19)
Treasury stock allocated to equity incentive plan	(221)		991	(1,212)		0
Dividends paid to common shareholders	(14,031)					(14,031)
Balance at Mar. 31, 2020	$ 1,655,164	$ 3,323	$ 1,739,523	$ (767,723)	$ (42,251)	$ 853,155	$ (130,863)