Investments - Financing Receivable, Non-Accrual and Aging (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	$ 12.7	$ 11.8
Financing receivables, Non-accrual assets without a valuation allowance	0.7
Financing receivables, Nonaccrual, Interest Income	0.0
Past due	122.9	68.2
Current	17,266.8	16,445.7
Financing receivables, Total amortized cost	17,389.7	16,513.9	$ 16,164.9
Recorded investment 90 days or more past due and accruing	8.9	2.7
Reinsurance recoverables	0.0
30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	59.0	47.4
60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	48.5	9.3
90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	15.4	11.5
Commercial mortgage loans
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	4.6	5.0
Past due	57.3
Current	15,038.2
Financing receivables, Total amortized cost	15,095.5	15,017.2	14,830.6
Recorded investment 90 days or more past due and accruing	7.7
Commercial mortgage loans | 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	14.9
Commercial mortgage loans | 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	30.1
Commercial mortgage loans | 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	12.3
Brick and mortar
Financing receivables, non-accrual and aging disclosures
Current		14,856.7
Financing receivables, Total amortized cost		14,856.7
Credit tenant loans
Financing receivables, non-accrual and aging disclosures
Current		160.5
Financing receivables, Total amortized cost		160.5
Direct financing leases
Financing receivables, non-accrual and aging disclosures
Past due	3.5
Current	643.6
Financing receivables, Total amortized cost	647.1
Direct financing leases | 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	3.5
Residential mortgage loans
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	8.1	6.8
Financing receivables, Non-accrual assets without a valuation allowance	0.7
Past due	62.1
Current	1,585.0
Financing receivables, Total amortized cost	1,647.1	1,496.7	$ 1,334.3
Recorded investment 90 days or more past due and accruing	1.2
Residential mortgage loans | 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	40.6
Residential mortgage loans | 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	18.4
Residential mortgage loans | 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	$ 3.1
First liens
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status		8.8
Past due		67.1
Current		1,418.6
Financing receivables, Total amortized cost		1,485.7
Recorded investment 90 days or more past due and accruing		2.7
First liens | 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due		46.6
First liens | 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due		9.3
First liens | 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due		11.2
Home equity
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status		3.0
Past due		1.1
Current		9.9
Financing receivables, Total amortized cost		11.0
Home equity | 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due		0.8
Home equity | 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due		$ 0.3