Employee and Agent Benefits - Contributions and Estimated Future Benefit Payments (Details) $ in Millions	Dec. 31, 2019 USD ($)
Pension benefits
Estimated Future Benefit Payments
Estimated future benefit payments, Year 1	$ 164.5
Estimated future benefit payments, Year 2	150.1
Estimated future benefit payments, Year 3	161.4
Estimated future benefit payments, Year 4	170.1
Estimated future benefit payments, Year 5	177.9
Estimated future benefit payments, Years 6-10	1,014.6
Pension benefits | Maximum
Defined Benefit Plan Disclosures
Possible contributions to be made during the next fiscal year to the qualified and nonqualified pension plans combined	35.0
Other postretirement benefits
Estimated Future Benefit Payments
Estimated future benefit payments, Year 1	14.0
Estimated future benefit payments, Year 2	12.2
Estimated future benefit payments, Year 3	11.5
Estimated future benefit payments, Year 4	10.6
Estimated future benefit payments, Year 5	9.8
Estimated future benefit payments, Years 6-10	$ 37.9