Employee and Agent Benefits - Plan Changes and Gains/Losses, ABO in Excess of Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Pension benefits
Defined Benefit Plan Disclosures
Prior service cost (benefit) related to current year amendments to the qualified plan and agent nonqualified plan	$ (6.4)	$ (23.1)
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	3,239.2	3,383.6
Accumulated benefit obligation for defined benefit plans with an accumulated benefit obligation in excess of plan assets	3,029.0	3,139.3
Fair value of plan assets for defined benefit plans with an accumulated benefit obligation in excess of plan assets	2,498.2	2,542.2
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	2.1	2.2
Fair value of plan assets for defined benefit plans with an accumulated benefit obligation in excess of plan assets	$ 0.2	$ 0.4