Postretirement Benefits - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net gain	$ (487)	$ (854)
Amortization of net loss	27	30
Total recognized in other comprehensive income (loss)	(460)	(824)
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax [Abstract]
Total recognized in net periodic pension cost and other comprehensive income (loss)	$ 128	$ (198)

Defined Benefit Plan, Accumulated Other Comprehensive Income, Amortization of Net Gains (Losses), Before Tax

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- Definition

Amount recognized in net periodic benefit cost (credit) and other comprehensive (income) loss.

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Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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