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Investment Securities - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities purchased	$ 272,780,000	$ 76,367,000	$ 137,591,000
Payments to acquire held-to-maturity securities	170,836,000	466,058,000	0
Proceeds from sales of available-for-sale investment securities	0	$ 362,829,000	$ 50,741,000
Proceeds from sale of held-to-maturity securities	$ 0
Term of contractual due dates (in years)	25 years

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The cash outflow to acquire debt and equity securities not classified as either held-to-maturity securities or trading securities which would be classified as available-for-sale securities and reported at fair value, with unrealized gains and losses excluded from earnings and reported in a separate component of shareholders' equity.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
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Amount of cash outflow through purchase of long-term held-to-maturity securities.

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-Name Accounting Standards Codification
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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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-Name Accounting Standards Codification
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The cash inflow associated with the sale of securities that had been designated as held-to-maturity. Excludes proceeds from maturities, prepayments and calls by the issuer.

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Term of Contractual Due Dates of Substantially All Mortgage-Backed Securities

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