Borrowed Funds (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Debt Disclosure [Abstract]
Interest rate swaps, notional amount	$ 640,000		$ 640,000		$ 640,000
FHLB advances, variable rate	640,000		$ 640,000		$ 640,000
Interest rate swaps, remaining term to maturity			3 years 9 months 18 days		4 years 6 months
Interest rate swaps, fair value	(57,100)		$ (57,100)		$ (33,100)
Interest rate swaps, amount reclassified from AOCI	1,710	$ 55	3,402	$ 106
Interest rate swaps, future amount to be reclassified from AOCI			15,400
Interest rate swaps, collateral posted	58,000		58,000		33,300
Debt Instrument [Line Items]
FHLB Advances, Amount	1,989,089		1,989,089		$ 2,239,989
Prepaid FHLB Advances [Member]
Debt Instrument [Line Items]
FHLB Advances, Amount	$ 350,000		$ 350,000
FHLB Advances, Interest Rate	2.42%		2.42%
FHLB Advances, Term			1 year
FHLB Advances, Prepayment Penalties			$ 4,200
Replacement FHLB Advances [Member]
Debt Instrument [Line Items]
FHLB Advances, Amount	$ 350,000		$ 350,000
FHLB Advances, Interest Rate	1.43%		1.43%
FHLB Advances, Term			4 years 8 months 12 days