Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Financing Receivable, Credit Quality Indicator
Mortgage loans, Total amortized cost	$ 17,335.3	$ 16,513.9	$ 15,625.1
Reinsurance recoverables	510.4
Accrued interest receivable	569.8
Commercial mortgage loans
Financing Receivable, Credit Quality Indicator
2020	548.7
2019	2,787.6
2018	2,883.3
2017	2,188.5
2016	1,651.2
Prior	5,083.3
Mortgage loans, Total amortized cost	15,142.6	15,017.2	14,274.2
Accrued interest receivable	56.0
Commercial mortgage loans | A- and above
Financing Receivable, Credit Quality Indicator
2020	536.3
2019	2,593.6
2018	2,556.5
2017	2,006.5
2016	1,575.2
Prior	4,680.4
Mortgage loans, Total amortized cost	13,948.5	13,961.9
Commercial mortgage loans | BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
2020	12.4
2019	165.2
2018	322.5
2017	182.0
2016	76.0
Prior	398.4
Mortgage loans, Total amortized cost	1,156.5	1,026.9
Commercial mortgage loans | BB+ thru BB-
Financing Receivable, Credit Quality Indicator
2019	28.8
2018	4.3
Mortgage loans, Total amortized cost	33.1	23.3
Commercial mortgage loans | B+ and below
Financing Receivable, Credit Quality Indicator
Prior	4.5
Mortgage loans, Total amortized cost	4.5	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	55.5
2019	30.4
2018	77.4
2017	39.8
2016	46.1
Prior	396.4
Mortgage loans, Total amortized cost	645.6
Direct financing leases | A- and above
Financing Receivable, Credit Quality Indicator
2020	9.6
2019	1.4
2018	35.6
2017	16.0
2016	29.4
Prior	233.4
Mortgage loans, Total amortized cost	325.4
Direct financing leases | BBB+ thru BBB-
Financing Receivable, Credit Quality Indicator
2020	40.3
2019	22.3
2018	38.4
2017	23.8
2016	16.7
Prior	157.6
Mortgage loans, Total amortized cost	299.1
Direct financing leases | BB+ thru BB-
Financing Receivable, Credit Quality Indicator
2020	5.6
2019	6.7
2018	3.4
Prior	4.1
Mortgage loans, Total amortized cost	19.8
Direct financing leases | B+ and below
Financing Receivable, Credit Quality Indicator
Prior	1.3
Mortgage loans, Total amortized cost	1.3
Residential mortgage loans
Financing Receivable, Credit Quality Indicator
2020	73.6
2019	510.1
2018	211.4
2017	197.2
2016	138.2
Prior	416.6
Mortgage loans, Total amortized cost	1,547.1	1,496.7	$ 1,350.9
Residential mortgage loans | Performing
Financing Receivable, Credit Quality Indicator
2020	73.6
2019	509.4
2018	210.5
2017	196.6
2016	138.2
Prior	409.1
Mortgage loans, Total amortized cost	1,537.4	1,482.2
Residential mortgage loans | Non-performing
Financing Receivable, Credit Quality Indicator
2019	0.7
2018	0.9
2017	0.6
Prior	7.5
Mortgage loans, Total amortized cost	$ 9.7	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