Investments - Mortgage Loans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 20,086.4		$ 20,086.4		$ 18,103.9
Commercial mortgage loans
Mortgage loan disclosures
Mortgage loans, purchased	1.5	$ 65.0	106.3	$ 151.3
Mortgage loans, sold		3.3	66.3	4.4
Mortgage loans, Total amortized cost	$ 16,332.6		$ 16,332.6		$ 15,441.6
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	$ 4,839.0		$ 4,839.0		$ 4,491.7
Percent of mortgage loans (as a percent)	29.70%		29.70%		29.00%
Commercial mortgage loans | Retail
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,689.7		$ 1,689.7		$ 1,815.3
Percent of mortgage loans (as a percent)	10.30%		10.30%		11.80%
Commercial mortgage loans | Industrial
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 2,970.6		$ 2,970.6		$ 2,488.7
Percent of mortgage loans (as a percent)	18.20%		18.20%		16.10%
Commercial mortgage loans | Apartments
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 6,059.0		$ 6,059.0		$ 5,958.0
Percent of mortgage loans (as a percent)	37.10%		37.10%		38.60%
Commercial mortgage loans | Hotel
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 86.6		$ 86.6		$ 89.4
Percent of mortgage loans (as a percent)	0.50%		0.50%		0.60%
Commercial mortgage loans | Mixed use/other
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 687.7		$ 687.7		$ 598.5
Percent of mortgage loans (as a percent)	4.20%		4.20%		3.90%
Commercial mortgage loans | New England
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 588.4		$ 588.4		$ 593.9
Percent of mortgage loans (as a percent)	3.60%		3.60%		3.80%
Commercial mortgage loans | Middle Atlantic
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 4,508.0		$ 4,508.0		$ 4,438.2
Percent of mortgage loans (as a percent)	27.50%		27.50%		28.80%
Commercial mortgage loans | East North Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 596.4		$ 596.4		$ 572.6
Percent of mortgage loans (as a percent)	3.70%		3.70%		3.70%
Commercial mortgage loans | West North Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 290.4		$ 290.4		$ 267.5
Percent of mortgage loans (as a percent)	1.80%		1.80%		1.70%
Commercial mortgage loans | South Atlantic
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 2,455.1		$ 2,455.1		$ 2,368.9
Percent of mortgage loans (as a percent)	15.00%		15.00%		15.30%
Commercial mortgage loans | East South Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 374.9		$ 374.9		$ 316.6
Percent of mortgage loans (as a percent)	2.30%		2.30%		2.10%
Commercial mortgage loans | West South Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,249.6		$ 1,249.6		$ 1,315.9
Percent of mortgage loans (as a percent)	7.70%		7.70%		8.50%
Commercial mortgage loans | Mountain
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,006.9		$ 1,006.9		$ 936.2
Percent of mortgage loans (as a percent)	6.20%		6.20%		6.10%
Commercial mortgage loans | Pacific
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 4,825.0		$ 4,825.0		$ 4,183.0
Percent of mortgage loans (as a percent)	29.50%		29.50%		27.10%
Commercial mortgage loans | International
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 437.9		$ 437.9		$ 448.8
Percent of mortgage loans (as a percent)	2.70%		2.70%		2.90%
Residential mortgage loans
Mortgage loan disclosures
Mortgage loans, purchased	$ 1,142.0	242.0	$ 1,955.0	736.5
Mortgage loans, sold	29.5	$ 24.6	73.1	$ 98.0
Mortgage loans, Total amortized cost	$ 3,130.4		$ 3,130.4		$ 1,951.5