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Fair Values and Percentage of Company's Pension Plan Assets by Asset Category (Detail) - USD ($)
$ in Thousands
Sep. 30, 2024
Sep. 30, 2023
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets 100.00% 100.00%
Significant Other Observable Inputs Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Fair values of the Partnership's pension plan assets $ 41,330  
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 $ 5,660 $ 44,206
Corporate and U.S. government bond fund    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets [1] 88.00% 90.00%
Corporate and U.S. government bond fund | Significant Other Observable Inputs Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Fair values of the Partnership's pension plan assets [1] $ 41,330  
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]   $ 39,852
U.S. large-cap equity    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets [1] 12.00% 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] $ 5,660 $ 3,244
International equity    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets [1] 0.00% 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] $ 0 $ 808
Cash    
Defined Benefit Plan Disclosure [Line Items]    
Percentage of the Partnership's pension plan assets 0.00% 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 $ 0 $ 302
[1] Represent investments in Principal funds at September 30, 2024 and Vanguard funds at September 30, 2023 that seek to replicate the asset category description.