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Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended	6 Months Ended
	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Proceeds from sales of available-for-sale securities	$ 0	$ 491,000
Available-for-sale securities purchased		290,574,000
Available-for-sale securities purchased			$ 123,324,000
Held-to-maturity securities purchased		0	170,836,000
Proceeds from sale of held-to-maturity securities		$ 0	$ 0
Term of contractual due dates of substantially all mortgage-backed securities (in years)		10 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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Amount of cash outflow to acquire investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of cash outflow through purchase of long-term held-to-maturity securities.

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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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