v3.26.1
Commitments and Contingencies - Senior Notes (Details) - Senior Notes - USD ($)
$ in Millions
6 Months Ended
Jun. 02, 2026
Jun. 30, 2026
Jun. 09, 2026
Debt Instrument [Line Items]      
Gain on repurchase of senior notes $ 16.0    
Senior Notes Due 2036      
Debt Instrument [Line Items]      
Aggregate principal amount     $ 600.0
Stated interest rate of Senior Notes (as a percent)     5.25%
Stated redemption price (as a percent)   100.00%  
Senior Notes Due 2056      
Debt Instrument [Line Items]      
Aggregate principal amount     $ 1,400.0
Stated interest rate of Senior Notes (as a percent)     5.95%
Stated redemption price (as a percent)   100.00%  
Senior Notes Due 2026 | Arch Finance      
Debt Instrument [Line Items]      
Stated interest rate of Senior Notes (as a percent)   4.011%  
Senior Notes Due 2043 | Arch-U.S.      
Debt Instrument [Line Items]      
Stated interest rate of Senior Notes (as a percent) 5.144%    
Aggregate principal amount repurchased $ 218.7    
Senior Notes Due 2046 | Arch Finance      
Debt Instrument [Line Items]      
Stated interest rate of Senior Notes (as a percent) 5.031%    
Aggregate principal amount repurchased $ 199.1