v3.26.3
Debt (Narrative) (Details) - USD ($)
$ in Thousands
Sep. 30, 2026
Apr. 01, 2026
Dec. 31, 2025
Debt Instrument [Line Items]      
Long-term Debt $ 3,904,334   $ 3,903,377
Maximum borrowing capacity 500,000    
Finance Lease, Liability, Current 6   154
Finance Lease, Liability, Noncurrent 107,048   107,079
Obligations      
Debt Instrument [Line Items]      
Finance Lease, Liability, Current 0   200
Finance Lease, Liability, Noncurrent 107,000   107,100
Tax Exempt Refund Revenue Bonds Par Value      
Debt Instrument [Line Items]      
Other Long-Term Debt, Current   $ 30,000  
Tax Exempt Refund Revenue Bonds Outstanding Value      
Debt Instrument [Line Items]      
Other Long-Term Debt, Current   $ 60,000  
Subsidiaries [Member]      
Debt Instrument [Line Items]      
Long-term Debt 3,037,137   3,037,074
Subsidiaries [Member] | FMB Twenty - four      
Debt Instrument [Line Items]      
Secured Debt $ 55,000   55,000
Debt Instrument, Interest Rate, Stated Percentage 1.40%    
Subsidiaries [Member] | FMB Twenty - five      
Debt Instrument [Line Items]      
Secured Debt $ 0   60,000
Debt Instrument, Interest Rate, Stated Percentage 0.95%    
Subsidiaries [Member] | FMB Twenty Seven      
Debt Instrument [Line Items]      
Secured Debt $ 650,000   650,000
Debt Instrument, Interest Rate, Stated Percentage 5.70%    
Subsidiaries [Member] | $60 million Series 2020B tax-exempt Indiana Finance Authority Environmental Facilities Refunding Revenue Bonds      
Debt Instrument [Line Items]      
Debt Instrument, Interest Rate, Effective Percentage 515.00%    
Subsidiaries [Member] | $30M Series 2020B tax-exempt Indiana Finance Authority Environmental Facilities Refunding Revenue Bonds to fixed rates of 4.90% [Abstract]      
Debt Instrument [Line Items]      
Debt Instrument, Interest Rate, Effective Percentage 490.00%    
Parent Company [Member] | Three Point Four Five Percent Senior Secured Notes [Member]      
Debt Instrument [Line Items]      
Long-term Debt $ 475,000   475,000
Debt Instrument, Interest Rate, Stated Percentage 4.25%    
Line of Credit [Member] | Committed Line of Credit [Member]      
Debt Instrument [Line Items]      
Line of credit, outstanding borrowings $ 235,000   $ 0