Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost	$ 20,086.4	$ 18,103.9
Reinsurance recoverables	1,122.7	1,098.0
Accrued interest receivable	570.1	552.5
Commercial mortgage loans
Financing Receivable, Credit Quality Indicator
2021	2,109.3
2020	2,114.9	2,019.8
2019	2,741.0	2,769.9
2018	2,581.1	2,817.2
2017	1,806.3	2,049.3
2016		1,509.3
Prior	4,980.0	4,276.1
Mortgage loans, Total amortized cost	16,332.6	15,441.6
Accrued interest receivable	59.3	60.0
Commercial mortgage loans | A- and above
Financing Receivable, Credit Quality Indicator
2021	1,852.4
2020	1,812.6	1,807.6
2019	2,447.0	2,486.8
2018	2,411.6	2,464.7
2017	1,623.0	1,780.8
2016		1,417.8
Prior	4,504.4	3,697.0
Mortgage loans, Total amortized cost	14,651.0	13,654.7
Commercial mortgage loans | BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
2021	233.1
2020	296.9	149.1
2019	294.0	194.1
2018	160.5	352.5
2017	183.3	262.7
2016		75.8
Prior	384.8	499.7
Mortgage loans, Total amortized cost	1,552.6	1,533.9
Commercial mortgage loans | BB+ thru BB-
Financing Receivable, Credit Quality Indicator
2021	23.8
2020	5.4	23.7
2019		69.0
2016		9.1
Prior	56.1	43.9
Mortgage loans, Total amortized cost	85.3	145.7
Commercial mortgage loans | B+ and below
Financing Receivable, Credit Quality Indicator
2020		39.4
2019		20.0
2018	9.0
2017		5.8
2016		6.6
Prior	34.7	35.5
Mortgage loans, Total amortized cost	43.7	107.3
Direct financing leases
Financing Receivable, Credit Quality Indicator
2021	78.2
2020	117.3	209.7
2019	26.0	29.6
2018	58.6	65.8
2017	33.1	37.6
2016		19.3
Prior	310.2	348.8
Mortgage loans, Total amortized cost	623.4	710.8
Accrued interest receivable	1.1	0.5
Direct financing leases | A- and above
Financing Receivable, Credit Quality Indicator
2020	44.1	43.9
2019	1.4	1.6
2018	40.3	42.6
2017	17.1	19.2
2016		15.3
Prior	255.0	202.9
Mortgage loans, Total amortized cost	357.9	325.5
Direct financing leases | BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
2021	25.1
2020	59.4	94.9
2019	22.7	5.5
2018	18.3	11.3
2017	16.0	18.4
2016		3.0
Prior	53.1	35.5
Mortgage loans, Total amortized cost	194.6	168.6
Direct financing leases | BB+ thru BB-
Financing Receivable, Credit Quality Indicator
2021	51.6
2020	13.8	13.3
2019	1.9
Prior	2.1	1.9
Mortgage loans, Total amortized cost	69.4	15.2
Direct financing leases | B+ and below
Financing Receivable, Credit Quality Indicator
2021	1.5
2020		57.6
2019		22.5
2018		11.9
2016		1.0
Prior		108.5
Mortgage loans, Total amortized cost	1.5	201.5
Residential mortgage loans
Financing Receivable, Credit Quality Indicator
2021	1,665.9
2020	587.0	699.1
2019	182.8	338.7
2018	102.1	167.8
2017	118.3	166.5
2016		168.0
Prior	474.3	411.4
Mortgage loans, Total amortized cost	3,130.4	1,951.5
Accrued interest receivable	16.5	2.0
Residential mortgage loans | Performing
Financing Receivable, Credit Quality Indicator
2021	1,665.2
2020	585.2	699.1
2019	181.5	336.7
2018	101.5	167.0
2017	117.5	165.1
2016		167.6
Prior	470.3	398.7
Mortgage loans, Total amortized cost	3,121.2	1,934.2
Residential mortgage loans | Non-performing
Financing Receivable, Credit Quality Indicator
2021	0.7
2020	1.8
2019	1.3	2.0
2018	0.6	0.8
2017	0.8	1.4
2016		0.4
Prior	4.0	12.7
Mortgage loans, Total amortized cost	$ 9.2	$ 17.3
Mortgage loans, Days delinquent to be considered non-performing	90 days