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CAPITAL CONSTRUCTION FUND - CCF CASH and CASH EQUIVALENTS, and INVESTMENT ACCOUNT (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
CCF Cash and Cash Equivalents:      
CCF cash and cash equivalents balance at beginning of period $ 307.2 $ 230.7  
Proceeds from U.S. Treasury debt securities at maturity 157.3 168.4  
Interest income on cash and cash equivalents, and CCF investments 9.9 13.4  
Repurchase of assigned accounts receivable 2.2 100.7  
Qualifying withdrawal payments for vessel construction expenditures (248.7) (136.5)  
Total CCF cash and cash equivalents balance at end of period 227.9 376.7  
CCF Investments:      
CCF investments balance at beginning of period 225.5 411.9  
Sale of U.S. Treasury debt securities at maturity (157.3) (168.4)  
Accretion of CCF investments 2.5 7.7  
Total CCF investments balance at end of period 70.7 251.2  
Total CCF cash and cash equivalents, and investments balance at end of period $ 298.6 $ 627.9 $ 532.7
Weighted average life of securities held within the fund 93 days