Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost	$ 18,103.9	$ 16,513.9	$ 15,364.3
Reinsurance recoverables	1,098.0
Accrued interest receivable	552.5
Commercial mortgage loans
Financing Receivable, Credit Quality Indicator
2020	2,019.8
2019	2,769.9
2018	2,817.2
2017	2,049.3
2016	1,509.3
Prior	4,276.1
Mortgage loans, Total amortized cost	15,441.6	15,017.2	13,996.3
Accrued interest receivable	60.0
Commercial mortgage loans | A- and above
Financing Receivable, Credit Quality Indicator
2020	1,807.6
2019	2,486.8
2018	2,464.7
2017	1,780.8
2016	1,417.8
Prior	3,697.0
Mortgage loans, Total amortized cost	13,654.7	13,961.9
Commercial mortgage loans | BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
2020	149.1
2019	194.1
2018	352.5
2017	262.7
2016	75.8
Prior	499.7
Mortgage loans, Total amortized cost	1,533.9	1,026.9
Commercial mortgage loans | BB+ thru BB-
Financing Receivable, Credit Quality Indicator
2020	23.7
2019	69.0
2016	9.1
Prior	43.9
Mortgage loans, Total amortized cost	145.7	23.3
Commercial mortgage loans | B+ and below
Financing Receivable, Credit Quality Indicator
2020	39.4
2019	20.0
2017	5.8
2016	6.6
Prior	35.5
Mortgage loans, Total amortized cost	107.3	5.1
Brick and mortar
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		14,856.7
Brick and mortar | A- and above
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		13,885.2
Brick and mortar | BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		943.1
Brick and mortar | BB+ thru BB-
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		23.3
Brick and mortar | B+ and below
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		5.1
Credit tenant loans
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		160.5
Credit tenant loans | A- and above
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		76.7
Credit tenant loans | BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		83.8
Direct financing leases
Financing Receivable, Credit Quality Indicator
2020	209.7
2019	29.6
2018	65.8
2017	37.6
2016	19.3
Prior	348.8
Mortgage loans, Total amortized cost	710.8
Accrued interest receivable	0.5
Direct financing leases | A- and above
Financing Receivable, Credit Quality Indicator
2020	43.9
2019	1.6
2018	42.6
2017	19.2
2016	15.3
Prior	202.9
Mortgage loans, Total amortized cost	325.5
Direct financing leases | BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
2020	94.9
2019	5.5
2018	11.3
2017	18.4
2016	3.0
Prior	35.5
Mortgage loans, Total amortized cost	168.6
Direct financing leases | BB+ thru BB-
Financing Receivable, Credit Quality Indicator
2020	13.3
Prior	1.9
Mortgage loans, Total amortized cost	15.2
Direct financing leases | B+ and below
Financing Receivable, Credit Quality Indicator
2020	57.6
2019	22.5
2018	11.9
2016	1.0
Prior	108.5
Mortgage loans, Total amortized cost	201.5
Residential mortgage loans
Financing Receivable, Credit Quality Indicator
2020	699.1
2019	338.7
2018	167.8
2017	166.5
2016	168.0
Prior	411.4
Mortgage loans, Total amortized cost	1,951.5	1,496.7	$ 1,368.0
Accrued interest receivable	2.0
Residential mortgage loans | Performing
Financing Receivable, Credit Quality Indicator
2020	699.1
2019	336.7
2018	167.0
2017	165.1
2016	167.6
Prior	398.7
Mortgage loans, Total amortized cost	1,934.2	1,482.2
Residential mortgage loans | Non-performing
Financing Receivable, Credit Quality Indicator
2019	2.0
2018	0.8
2017	1.4
2016	0.4
Prior	12.7
Mortgage loans, Total amortized cost	$ 17.3	14.5
Mortgage loans, Days delinquent to be considered non-performing	90 days
First liens
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		1,485.7
First liens | Performing
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		1,474.2
First liens | Non-performing
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		11.5
Home equity
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		11.0
Home equity | Performing
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		8.0
Home equity | Non-performing
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost		$ 3.0