Investments - Mortgage Loans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Commercial mortgage loans
Mortgage loan disclosures
Mortgage loans, purchased	$ 0.5	$ 98.2	$ 108.4	$ 144.4
Mortgage loans, sold		0.1		0.1
Mortgage loans, Total amortized cost	$ 16,447.7		$ 16,447.7		$ 16,730.3
Percent of mortgage loans (as a percent)	100.00%		100.00%		100.00%
Commercial mortgage loans | Office
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 3,676.3		$ 3,676.3		$ 4,322.0
Percent of mortgage loans (as a percent)	22.30%		22.30%		25.90%
Commercial mortgage loans | Retail
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,503.4		$ 1,503.4		$ 1,499.4
Percent of mortgage loans (as a percent)	9.10%		9.10%		9.00%
Commercial mortgage loans | Industrial
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 3,507.4		$ 3,507.4		$ 3,235.9
Percent of mortgage loans (as a percent)	21.30%		21.30%		19.30%
Commercial mortgage loans | Apartments
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 6,925.7		$ 6,925.7		$ 6,827.1
Percent of mortgage loans (as a percent)	42.20%		42.20%		40.80%
Commercial mortgage loans | Hotel
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 69.5		$ 69.5		$ 72.5
Percent of mortgage loans (as a percent)	0.40%		0.40%		0.40%
Commercial mortgage loans | Mixed use/other
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 765.4		$ 765.4		$ 773.4
Percent of mortgage loans (as a percent)	4.70%		4.70%		4.60%
Commercial mortgage loans | New England
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 377.8		$ 377.8		$ 512.1
Percent of mortgage loans (as a percent)	2.30%		2.30%		3.10%
Commercial mortgage loans | Middle Atlantic
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 4,446.5		$ 4,446.5		$ 4,505.6
Percent of mortgage loans (as a percent)	26.90%		26.90%		26.90%
Commercial mortgage loans | East North Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 586.7		$ 586.7		$ 652.5
Percent of mortgage loans (as a percent)	3.60%		3.60%		3.90%
Commercial mortgage loans | West North Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 330.7		$ 330.7		$ 370.9
Percent of mortgage loans (as a percent)	2.00%		2.00%		2.20%
Commercial mortgage loans | South Atlantic
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 2,640.8		$ 2,640.8		$ 2,558.3
Percent of mortgage loans (as a percent)	16.10%		16.10%		15.30%
Commercial mortgage loans | East South Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 432.2		$ 432.2		$ 339.8
Percent of mortgage loans (as a percent)	2.60%		2.60%		2.00%
Commercial mortgage loans | West South Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,259.5		$ 1,259.5		$ 1,204.9
Percent of mortgage loans (as a percent)	7.70%		7.70%		7.20%
Commercial mortgage loans | Mountain
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 858.5		$ 858.5		$ 938.7
Percent of mortgage loans (as a percent)	5.20%		5.20%		5.60%
Commercial mortgage loans | Pacific
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 4,997.4		$ 4,997.4		$ 5,115.3
Percent of mortgage loans (as a percent)	30.50%		30.50%		30.60%
Commercial mortgage loans | International
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 517.6		$ 517.6		$ 532.2
Percent of mortgage loans (as a percent)	3.10%		3.10%		3.20%
Residential mortgage loans
Mortgage loan disclosures
Mortgage loans, purchased	$ 74.6	476.1	$ 497.0	1,657.5
Mortgage loans, sold	4.1	$ 4.8	25.4	$ 490.8
Mortgage loans, Total amortized cost	$ 3,896.7		$ 3,896.7		$ 3,983.0