Deposits and Borrowed Funds (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Noninterest-bearing deposits	$ 451,400	$ 357,300
Time Deposits, $250,000 or more	643,000	610,000
FHLB borrowings, Amount	1,789,313	2,239,989
FHLB Advances, Fixed Rate	1,150,000	1,400,000
FHLB Advances, Variable Rate	640,000	640,000
FHLB line of credit, outstanding balance	$ 0	100,000
Expiration date for FHLB line of credit	Nov. 12, 2021
Interest rate swaps, notional amount	$ 640,000	$ 640,000
Interest rate swaps, remaining term to maturity	3 years 6 months	4 years 6 months
Interest rate swaps, fair value	$ (53,100)	$ (33,100)
Interest rate swaps, amount reclassified from AOCI	6,274	438	$ 515
Interest rate swaps, future amount to be reclassified from AOCI	15,600
Interest rate swaps, collateral posted	$ 54,600	33,300
FHLB borrowings threshold percentage of regulatory assets	40.00%
FHLB borrowings threshold percentage of regulatory assets, temporary increase	50.00%
FHLB percentage of regulatory assets	19.00%
Repurchase Agreements	$ 0	$ 100,000
Weighted average rate of repurchase agreements		2.53%
Repurchase Agreements, Description of Potential Risks	See Note 4 for information regarding the amount of securities pledged as collateral in conjunction with repurchase agreements. Securities are delivered to the party with whom each transaction is executed and the party agrees to resell the same securities to the Bank at the maturity of the agreement. The Bank retains the right to substitute similar or like securities throughout the terms of the agreements. The repurchase agreements and collateral are subject to valuation at current market levels and the Bank may ask for the return of excess collateral or be required to post additional collateral due to changes in the market values of these items. The Bank may also be required to post additional collateral as a result of principal payments received on the securities pledged.
Replacement FHLB Advances [Member]
FHLB borrowings, Amount	$ 350,000
FHLB Advances, Interest Rate	1.43%
FHLB Advances, Term	4 years 8 months 12 days
Prepaid FHLB Advances [Member]
FHLB borrowings, Amount	$ 350,000
FHLB Advances, Interest Rate	2.42%
FHLB Advances, Term	1 year
FHLB Advances, Prepayment Penalties	$ 4,200
Leverage Strategy [Member]
FHLB line of credit available for leverage strategy, maximum	2,100,000
FHLB Advances [Member]
FHLB borrowings, Amount	$ 1,789,313	$ 2,039,989
Subordinated Debentures Subject to Mandatory Redemption [Member]
Junior subordinated debentures			$ 10,100