Investments - Mortgage Loans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 17,389.7	$ 16,164.9	$ 17,389.7	$ 16,164.9	$ 16,513.9
Commercial mortgage loans
Mortgage loan disclosures
Mortgage loans, purchased	73.5	103.0	86.3	115.9
Mortgage loans, sold			1.1	0.5
Mortgage loans, Total amortized cost	$ 15,095.5	14,830.6	$ 15,095.5	14,830.6	$ 15,017.2
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	$ 4,689.9		$ 4,689.9		$ 4,887.1
Percent of mortgage loans (as a percent)	31.10%		31.10%		32.60%
Commercial mortgage loans | Retail
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,921.3		$ 1,921.3		$ 2,052.6
Percent of mortgage loans (as a percent)	12.70%		12.70%		13.70%
Commercial mortgage loans | Industrial
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 2,310.0		$ 2,310.0		$ 2,268.5
Percent of mortgage loans (as a percent)	15.30%		15.30%		15.10%
Commercial mortgage loans | Apartments
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 5,616.7		$ 5,616.7		$ 5,246.9
Percent of mortgage loans (as a percent)	37.20%		37.20%		34.90%
Commercial mortgage loans | Hotel
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 89.9		$ 89.9		$ 90.8
Percent of mortgage loans (as a percent)	0.60%		0.60%		0.60%
Commercial mortgage loans | Mixed use/other
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 467.7		$ 467.7		$ 471.3
Percent of mortgage loans (as a percent)	3.10%		3.10%		3.10%
Commercial mortgage loans | New England
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 668.4		$ 668.4		$ 613.9
Percent of mortgage loans (as a percent)	4.40%		4.40%		4.10%
Commercial mortgage loans | Middle Atlantic
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 4,137.7		$ 4,137.7		$ 4,139.7
Percent of mortgage loans (as a percent)	27.40%		27.40%		27.50%
Commercial mortgage loans | East North Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 611.8		$ 611.8		$ 624.5
Percent of mortgage loans (as a percent)	4.10%		4.10%		4.20%
Commercial mortgage loans | West North Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 236.7		$ 236.7		$ 237.2
Percent of mortgage loans (as a percent)	1.60%		1.60%		1.60%
Commercial mortgage loans | South Atlantic
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 2,286.0		$ 2,286.0		$ 2,318.4
Percent of mortgage loans (as a percent)	15.10%		15.10%		15.40%
Commercial mortgage loans | East South Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 340.7		$ 340.7		$ 438.5
Percent of mortgage loans (as a percent)	2.30%		2.30%		2.90%
Commercial mortgage loans | West South Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,404.1		$ 1,404.1		$ 1,450.0
Percent of mortgage loans (as a percent)	9.30%		9.30%		9.70%
Commercial mortgage loans | Mountain
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 972.3		$ 972.3		$ 931.8
Percent of mortgage loans (as a percent)	6.40%		6.40%		6.20%
Commercial mortgage loans | Pacific
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 4,121.2		$ 4,121.2		$ 3,963.7
Percent of mortgage loans (as a percent)	27.30%		27.30%		26.40%
Commercial mortgage loans | International
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 316.6		$ 316.6		$ 299.5
Percent of mortgage loans (as a percent)	2.10%		2.10%		2.00%
Residential mortgage loans
Mortgage loan disclosures
Mortgage loans, purchased	$ 267.7	66.2	$ 494.5	99.4
Mortgage loans, sold	48.5	21.2	73.4	32.1
Mortgage loans, Total amortized cost	1,647.1	$ 1,334.3	1,647.1	$ 1,334.3	$ 1,496.7
Direct financing leases
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 647.1		$ 647.1