Statutory Financial Information - Additional Details (Detail) £ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2024 GBP (£)	Dec. 31, 2023 USD ($)	Dec. 31, 2025 GBP (£)	Dec. 31, 2024 GBP (£)
Fairwind Insurance Company
Statutory Accounting Practices
Statutory Accounting Practices, Statutory to NAIC, Permitted, Difference, Amount	$ 185.0	$ 334.0
Unum Limited
Statutory Accounting Practices
Statutory Net Income | £			£ 106.7
Statutory Combined Capital and Surplus | £						£ 749.8
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments | £					£ 125.0
Traditional U.S. Life Insurance Subsidiaries
Statutory Accounting Practices
Statutory Net Income	634.0	1,322.4		$ 1,329.9
Statutory Combined Capital and Surplus	3,770.6	3,909.7
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments	631.0
Cash and Securities Segregated under Federal and Other Regulations	$ 123.4	$ 119.3