Pension, Profit Sharing and Other Benefit Plans - Fair value by asset category and hierarchy (Details) - Defined Benefit Pension Plans - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	$ 232,132	$ 221,311	$ 214,530
Net Asset Value
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	230,305	218,403
Cash equivalents
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	1,827	2,908
Cash equivalents | Level 1
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	1,827	2,908
Equity securities
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	99,768	100,302
Equity securities | Net Asset Value
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	99,768	100,302
Fixed Income Funds
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	113,310	105,651
Fixed Income Funds | Net Asset Value
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	113,310	105,651
Diversified growth fund
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	3,626	8,886
Diversified growth fund | Net Asset Value
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	3,626	8,886
Real Estate
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	13,601	3,564
Real Estate | Net Asset Value
Fair value of assets of defined benefit pension plans by asset category
Fair value of the assets	$ 13,601	$ 3,564