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Fair Values and Percentage of Company's Pension Plan Assets by Asset Category (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Sep. 30, 2016
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 $ 65,676 $ 68,276
Corporate and U.S. government bond fund    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets [1] 80.00% 79.00%
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] $ 52,735 $ 54,267
U.S. large-cap equity    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets [1] 14.00% 15.00%
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] $ 9,270 $ 10,207
International equity    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets [1] 5.00% 5.00%
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] $ 3,063 $ 3,118
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 $ 608 $ 684
[1] Represent investments in Vanguard funds that seek to replicate the asset category description.