Employee and Agent Benefits - ABO in Excess of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Pension benefits
Information for Defined Benefit Plans With an Accumulated Benefit Obligation in Excess of Plan Assets
Projected benefit obligation for defined benefit plans with an accumulated benefit obligation in excess of plan assets	$ 4,210.2	$ 3,692.1
Accumulated benefit obligation for defined benefit plans with an accumulated benefit obligation in excess of plan assets	4,136.5	3,599.5
Fair value of plan assets for defined benefit plans with an accumulated benefit obligation in excess of plan assets	3,373.5	2,926.0
Other postretirement benefits
Information for Defined Benefit Plans With an Accumulated Benefit Obligation in Excess of Plan Assets
Accumulated benefit obligation for defined benefit plans with an accumulated benefit obligation in excess of plan assets	$ 2.9	$ 3.8