Investments - Collateral Property Type (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%
Office
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 2,504.6	$ 2,798.3
Percent of mortgage loans (as a percent)	16.30%	16.90%
Retail
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,484.1	$ 1,580.0
Percent of mortgage loans (as a percent)	9.70%	9.60%
Industrial
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 4,267.8	$ 4,288.0
Percent of mortgage loans (as a percent)	27.80%	26.00%
Apartments
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 6,406.8	$ 6,976.9
Percent of mortgage loans (as a percent)	41.70%	42.20%
Hotel
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 30.2	$ 31.0
Percent of mortgage loans (as a percent)	0.20%	0.20%
Mixed use/other
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 660.2	$ 843.9
Percent of mortgage loans (as a percent)	4.30%	5.10%