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Employee Postretirement Benefits - Allocation of Plan Assets (Details) - USD ($)
$ in Thousands
Dec. 31, 2021
Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets $ 387,212 $ 360,678
Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 374,415 351,458
Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
NAV    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 12,797 9,220
Equity Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 149,479 180,169
Equity Funds | Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 149,479 180,169
Equity Funds | Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Equity Funds | Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Fixed Income Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 184,987 159,556
Fixed Income Funds | Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 184,987 159,556
Fixed Income Funds | Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Fixed Income Funds | Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Hybrid Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 11,776 11,729
Hybrid Funds | Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 11,776 11,729
Hybrid Funds | Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Hybrid Funds | Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
U.S. Treasury Securities    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 28,173  
U.S. Treasury Securities | Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 28,173  
U.S. Treasury Securities | Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0  
U.S. Treasury Securities | Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0  
SEI Energy Debt Collective Fund    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 12,797 9,220
SEI Energy Debt Collective Fund | Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
SEI Energy Debt Collective Fund | Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
SEI Energy Debt Collective Fund | Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
SEI Energy Debt Collective Fund | NAV    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets $ 12,797 9,220
Cash Equivalents    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets   4
Cash Equivalents | Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets   4
Cash Equivalents | Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets   0
Cash Equivalents | Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets   $ 0