Other Comprehensive Income (Loss) (Accumulated Other Comprehensive Income (Loss) by Component) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	$ 1,728,949	$ 1,743,780	$ 1,696,754	$ 1,758,404
Other comprehensive income (loss)	(80,064)	(15,291)	(61,484)	(36,752)
Balance	1,655,164	1,735,985	1,655,164	1,735,985
Unrealized gain (losses) on securities available for sale
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	758	(8,527)	(2,165)	(13,624)
Other comprehensive loss before reclassifications, net of tax	9,253	3,381	12,176	8,478
Amounts reclassified from accumulated other comprehensive income (loss), net of tax	0	0	0	0
Other comprehensive income (loss)	9,253	3,381	12,176	8,478
Balance	10,011	(5,146)	10,011	(5,146)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(29,710)	25,092	(44,915)	51,914
Other comprehensive loss before reclassifications, net of tax	(90,388)	(15,731)	(74,416)	(40,231)
Amounts reclassified from accumulated other comprehensive income (loss), net of tax	619	(3,205)	(148)	(5,527)
Other comprehensive income (loss)	(89,769)	(18,936)	(74,564)	(45,758)
Balance	(119,479)	6,156	(119,479)	6,156
Defined Benefit Plan
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(21,847)	(14,804)	(22,299)	(15,068)
Other comprehensive loss before reclassifications, net of tax	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss), net of tax	452	264	904	528
Other comprehensive income (loss)	452	264	904	528
Balance	(21,395)	(14,540)	(21,395)	(14,540)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(50,799)	1,761	(69,379)	23,222
Other comprehensive loss before reclassifications, net of tax	(81,135)	(12,350)	(62,240)	(31,753)
Amounts reclassified from accumulated other comprehensive income (loss), net of tax	1,071	(2,941)	756	(4,999)
Other comprehensive income (loss)	(80,064)	(15,291)	(61,484)	(36,752)
Balance	$ (130,863)	$ (13,530)	$ (130,863)	$ (13,530)