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Critical accounting estimates and judgments - Key assumptions to fair value less costs of disposals calculation (Details) - I-Systems
 9 Months Ended 12 Months Ended
Sep. 30, 2025
Dec. 31, 2024
Disclosure of intangible assets material to entity [line items]    
Discount rate 10.40% 9.40%
Terminal growth rate 4.60% 4.70%
Forecast period for calculation of fair value less costs of disposal 10 years  
Minimum    
Disclosure of intangible assets material to entity [line items]    
Homes connected 600,000 700,000
Maximum    
Disclosure of intangible assets material to entity [line items]    
Homes connected 1,800,000 3,100,000