Summary of Significant Accounting Policies - Property and Equipment (Details)	Dec. 31, 2025
Computer hardware
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Computer software \| Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Computer software \| Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Software licenses \| Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	3 years
Software licenses \| Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Internally developed software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years
Buildings and improvements \| Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Buildings and improvements \| Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	40 years

SELECTIVE INSURANCE GROUP INC 10-K