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Long-Term Debt (Narrative) (Details) - USD ($)
 1 Months Ended 3 Months Ended
Dec. 31, 2019
Jun. 30, 2021
Dec. 28, 2024
Sep. 30, 2017
Dec. 31, 2010
Debt Instrument [Line Items]          
Property and equipment with undepreciated cost pledge as collateral for long term debt     $ 248,800,000    
Debt Instrument, Variable Interest Rate, Type [Extensible Enumeration]     us-gaap:SecuredOvernightFinancingRateSofrOvernightIndexSwapRateMember    
Excess net worth calculated under covenants in the Notes, the Bonds, and the Line     $ 511,200,000    
Line of Credit [Member]          
Debt Instrument [Line Items]          
Line of credit facility borrowing capacity     150,000,000.0    
Borrowings outstanding     $ 0    
Debt maturity date     Jun. 01, 2026    
Letter of Credit [Member]          
Debt Instrument [Line Items]          
Line of credit facility borrowing capacity     $ 10,000,000.0    
Borrowings outstanding     0    
Cash Flow Hedging [Member]          
Debt Instrument [Line Items]          
Other comprehensive income (loss), net of income taxes     2,300,000    
Unrealized gains (losses) on cash flow hedge     12,000,000.0    
Senior Notes, Maturing 2031 [Member]          
Debt Instrument [Line Items]          
Debt instrument principal amount   $ 350,000,000.0      
Debt instrument stated interest rate   4.00%      
Maturity period of senior notes   2031      
Secured Debt Maturing October 2027 [Member]          
Debt Instrument [Line Items]          
Secured borrowing       $ 60,000,000  
Secured Debt Maturing October 2027 [Member] | Interest Rate Swap [Member]          
Debt Instrument [Line Items]          
Derivative notional amount     $ 17,000,000.0    
Derivative, fixed interest rate     3.962%    
Monthly principal amortization     $ 500,000    
Derivative maturity date     Oct. 01, 2027    
Debt instrument variable interest rate     1.75%    
Secured Debt Maturing January 2030 [Member]          
Debt Instrument [Line Items]          
Debt maturity date Jan. 01, 2030        
Derivative notional amount $ 155,000,000        
Secured Debt Maturing January 2030 [Member] | Interest Rate Swap [Member]          
Debt Instrument [Line Items]          
Derivative notional amount $ 115,000,000.0        
Derivative, fixed interest rate 2.998%        
Monthly principal amortization $ 650,000        
Debt instrument variable interest rate 1.60%        
Recovery Zone Facility Bonds [Member]          
Debt Instrument [Line Items]          
Debt maturity date     Jan. 01, 2036    
Annual amount of redemption of bonds     $ 4,500,000    
Mandatory bonds redemption beginning period     Jan. 01, 2014    
Mandatory bonds redemption period end date     Dec. 17, 2029    
Total amount of bonds funded     $ 49,900,000   $ 99,700,000