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	58,720					58,720
Other comprehensive income (loss), net of tax	(69,199)					0	(69,199)
ESOP shares allocated or committed to be released	5,326		2,076		3,250
Compensation costs for equity incentive plans	3,343		3,343			0
Purchase of treasury stock	(7,915)			(7,915)
Treasury stock allocated to equity incentive plan	(1,095)		(1,082)	(13)		0
Dividends paid to common shareholders	(37,102)					(37,102)
Balance at Jun. 30, 2019	1,710,482	3,323	1,731,329	(762,200)	(45,501)	829,508	(45,977)
Balance at Mar. 31, 2019	1,735,985	3,323	1,729,499	(760,367)	(46,584)	823,644	(13,530)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	18,257					18,257
Other comprehensive income (loss), net of tax	(32,447)					0	(32,447)
ESOP shares allocated or committed to be released	1,849		766		1,083
Compensation costs for equity incentive plans	1,116		1,116			0
Purchase of treasury stock	(1,707)			(1,707)
Treasury stock allocated to equity incentive plan	(178)		(52)	(126)		0
Dividends paid to common shareholders	(12,393)					(12,393)
Balance at Jun. 30, 2019	1,710,482	3,323	1,731,329	(762,200)	(45,501)	829,508	(45,977)
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	69,739					69,739
Other comprehensive income (loss), net of tax	(76,614)					0	(76,614)
ESOP shares allocated or committed to be released	5,788		2,538		3,250
Compensation costs for equity incentive plans	3,527		3,527			0
Purchase of treasury stock	(378)			(378)
Treasury stock allocated to equity incentive plan	(1,904)		784	(2,688)		0
Dividends paid to common shareholders	(41,436)					(41,436)
Balance at Jun. 30, 2020	1,655,476	3,323	1,741,003	(767,655)	(41,167)	865,965	(145,993)
Balance at Mar. 31, 2020	1,655,164	3,323	1,739,523	(767,723)	(42,251)	853,155	(130,863)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	26,840					26,840
Other comprehensive income (loss), net of tax	(15,130)					0	(15,130)
ESOP shares allocated or committed to be released	1,561		477		1,084
Compensation costs for equity incentive plans	1,178		1,178			0
Purchase of treasury stock	0
Treasury stock allocated to equity incentive plan	(107)		(175)	68		0
Dividends paid to common shareholders	(14,030)					(14,030)
Balance at Jun. 30, 2020	$ 1,655,476	$ 3,323	$ 1,741,003	$ (767,655)	$ (41,167)	$ 865,965	$ (145,993)