Derivatives and Hedging Activities - Summary of Fair Value, Notional Amount and Balance Sheet Classification (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Other assets
Derivatives, Fair Value [Line Items]
Notional	$ 656,074	$ 625,607
Derivative assets, fair value	48,201	21,989
Other liabilities
Derivatives, Fair Value [Line Items]
Notional	2,849,390	1,221,252
Derivative liabilities, fair value	90,129	32,546
Client swap program hedges \| Other assets
Derivatives, Fair Value [Line Items]
Notional	656,074	425,607
Derivative assets, fair value	48,201	20,381
Client swap program hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional	656,074	425,607
Derivative liabilities, fair value	48,201	20,381
Commercial loan fair value hedges \| Other assets
Derivatives, Fair Value [Line Items]
Notional	0	0
Derivative assets, fair value	0	0
Commercial loan fair value hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional	93,316	95,645
Derivative liabilities, fair value	8,492	4,288
Mortgage loan fair value hedges \| Other assets
Derivatives, Fair Value [Line Items]
Notional	0	200,000
Derivative assets, fair value	0	1,608
Mortgage loan fair value hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional	500,000	0
Derivative liabilities, fair value	16,061	0
Borrowings cash flow hedges \| Other assets
Derivatives, Fair Value [Line Items]
Notional	0	0
Derivative assets, fair value	0	0
Borrowings cash flow hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional	1,600,000	700,000
Derivative liabilities, fair value	$ 17,375	$ 7,877

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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