Borrowed Funds (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Debt Disclosure [Abstract]
Interest rate swaps, notional amount	$ 640,000,000.0		$ 640,000,000.0		$ 640,000,000
FHLB advances, variable rate	640,000,000.0		$ 640,000,000.0		$ 640,000,000
Interest rate swaps, remaining term to maturity			4 years		4 years 6 months
Interest rate swaps, fair value	(55,600,000)		$ (55,600,000)		$ (33,100,000)
Interest rate swaps, amount reclassified from AOCI	983,000	$ (100,000)	1,700,000	$ 51,000
Interest rate swaps, amount of hedge ineffectiveness recognized			0
Interest rate swaps, future amount to be reclassified from AOCI			13,000,000.0
Interest rate swaps, collateral posted	56,300,000		56,300,000		33,300,000
Debt Instrument [Line Items]
FHLB Advances, Amount	2,115,869,000		2,115,869,000		$ 2,239,989,000
Prepaid FHLB Advances [Member]
Debt Instrument [Line Items]
FHLB Advances, Amount	$ 350,000,000.0		$ 350,000,000.0
FHLB Advances, Interest Rate	2.42%		2.42%
FHLB Advances, Term	1 year
FHLB Advances, Prepayment Penalties	$ 4,200,000
Replacement FHLB Advances [Member]
Debt Instrument [Line Items]
FHLB Advances, Amount	$ 350,000,000.0		$ 350,000,000.0
FHLB Advances, Interest Rate	1.43%		1.43%
FHLB Advances, Term	4 years 8 months 12 days