Allowance for Loan and Lease Losses - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans at fair value under the fair value option	$ 735,363		$ 735,363			$ 812,991
Loan held for sale	12,840		12,840			7,351
Financing receivable, after allowance for credit loss	10,165,841		10,165,841			9,635,989
Allowance for loan and lease losses	148,158	$ 76,546	148,158	$ 76,546	$ 135,950	70,774	$ 68,003	$ 64,540
Net impairment ((reversal) of provision	40	$ (123)	188	$ (241)
Unfunded Loan Commitment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reserve for unfunded loan commitments	3,300		3,300			500
Receivables acquired with deteriorated credit quality
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan and lease losses	700		700			600
Loans Guaranteed by US Government Authorities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, after allowance for credit loss	865,800		865,800			154,200
Loans Guaranteed by US Government Authorities | Guaranteed Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, after allowance for credit loss	$ 858,700		$ 858,700			$ 145,900