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Fair Values and Percentage of Company's Pension Plan Assets by Asset Category (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets 100.00% 100.00%
Quoted Prices in Active Markets for Identical Assets Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Fair values of the Partnership's pension plan assets $ 66,838 $ 61,924
Corporate and U.S. government bond fund    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets [1] 90.00% 90.00%
Corporate and U.S. government bond fund | Quoted Prices in Active Markets for Identical Assets Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Fair values of the Partnership's pension plan assets [1] $ 60,720 $ 55,908
U.S. large-cap equity    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets [1] 7.00% 7.00%
U.S. large-cap equity | Quoted Prices in Active Markets for Identical Assets Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Fair values of the Partnership's pension plan assets [1] $ 4,632 $ 4,566
International equity    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets [1] 2.00% 2.00%
International equity | Quoted Prices in Active Markets for Identical Assets Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Fair values of the Partnership's pension plan assets [1] $ 1,119 $ 1,129
Cash    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets 1.00% 1.00%
Cash | Quoted Prices in Active Markets for Identical Assets Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Fair values of the Partnership's pension plan assets $ 367 $ 321
[1] Represent investments in Vanguard funds that seek to replicate the asset category description.