Investments - Mortgage Loans By Geographic Region (Details) - Commercial mortgage loans - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 15,353.7	$ 16,518.1
Percent of mortgage loans (as a percent)	100.00%	100.00%
New England
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 388.5	$ 440.8
Percent of mortgage loans (as a percent)	2.50%	2.70%
Middle Atlantic
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 4,593.6	$ 4,402.6
Percent of mortgage loans (as a percent)	29.90%	26.60%
East North Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 594.3	$ 463.2
Percent of mortgage loans (as a percent)	3.90%	2.80%
West North Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 274.0	$ 284.4
Percent of mortgage loans (as a percent)	1.80%	1.70%
South Atlantic
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 2,732.2	$ 3,068.4
Percent of mortgage loans (as a percent)	17.80%	18.60%
East South Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 291.2	$ 366.4
Percent of mortgage loans (as a percent)	1.90%	2.20%
West South Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,396.4	$ 1,422.2
Percent of mortgage loans (as a percent)	9.10%	8.60%
Mountain
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,290.4	$ 1,217.9
Percent of mortgage loans (as a percent)	8.40%	7.40%
Pacific
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 3,793.1	$ 4,327.1
Percent of mortgage loans (as a percent)	24.70%	26.20%
International
Mortgage loan disclosures
Mortgage loans, Total amortized cost		$ 525.1
Percent of mortgage loans (as a percent)		3.20%