v3.25.4
Income taxes - Additional Information (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Jul. 18, 2025
Dec. 10, 2024
Sep. 30, 2025
Dec. 31, 2024
Dec. 31, 2017
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2018
Income Taxes Disclosure [Line Items]                  
Nondeductible Portion of Interest Carryforwards       $ 23.1   $ 30.6 $ 23.1    
Deferred income tax asset valuation allowance       14.1   $ 45.7 14.1    
Business interest income, adjusted taxable income           30.00%      
Deferred income taxes expense (benefit)       16.5   $ (9.5) 43.4 $ (50.7)  
Deferred income taxes       57.7   55.8 57.7    
Final installment payment           $ 18.6      
Pre transition gain   $ 77.1              
Gain on final regulation, amortization period           10 years      
Current tax expense           $ 1.6      
Deferred foreign income tax expense (benefit)           14.3 2.1 (35.9)  
Non-cash income tax benefit           8.6      
Germany                  
Income Taxes Disclosure [Line Items]                  
Effective income tax rate, before change in enacted rate 15.00%                
Effective income tax rate, reduction in enacted tax rate for each year, percentage 1.00%                
Effective Income Tax Rate, After Change In Enacted Rate 10.00%                
Deferred foreign income tax expense (benefit)     $ 19.3            
Germany | Corporate Tax Purposes                  
Income Taxes Disclosure [Line Items]                  
Net operating loss carryforwards           57.2      
Net operating loss carryforwards           510.8      
Germany | Trade Tax Purposes                  
Income Taxes Disclosure [Line Items]                  
Net operating loss carryforwards           5.0      
Net operating loss carryforwards           46.3      
Belgium                  
Income Taxes Disclosure [Line Items]                  
Deferred income tax asset valuation allowance       $ 8.2     8.2    
Belgium | Corporate Tax Purposes                  
Income Taxes Disclosure [Line Items]                  
Net operating loss carryforwards           27.2      
Net operating loss carryforwards           109.0      
U.S.                  
Income Taxes Disclosure [Line Items]                  
Net operating loss carryforwards           12.2      
Net operating loss carryforwards           58.1      
Canada | Corporate Tax Purposes                  
Income Taxes Disclosure [Line Items]                  
Net operating loss carryforwards           4.6      
Net operating loss carryforwards           30.9      
Canada | Provincial Tax Purposes                  
Income Taxes Disclosure [Line Items]                  
Net operating loss carryforwards           3.8      
Net operating loss carryforwards           33.5      
German, U.S., and Canadian                  
Income Taxes Disclosure [Line Items]                  
Deferred income tax asset valuation allowance           0.0      
Kronos | Direct Investment in Subsidiary                  
Income Taxes Disclosure [Line Items]                  
Deferred income taxes expense (benefit)           (7.5) $ 4.6 $ (6.4)  
Deferred income taxes           49.7      
Kronos | Direct Investment in Subsidiary | Maximum                  
Income Taxes Disclosure [Line Items]                  
Deferred income taxes           153.6      
Chemicals                  
Income Taxes Disclosure [Line Items]                  
Deferred income taxes expense (benefit)           $ 8.5      
Chemicals | Transition Tax | European Subsidiaries                  
Income Taxes Disclosure [Line Items]                  
Current income tax expense provisional pursuant to transition tax         $ 74.1        
Income tax liability payable period                 8 years