Employee and Agent Benefits - Plan Changes and Gains/Losses, ABO in Excess of Plan Assets (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Oct. 31, 2016	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Pension benefits
Defined Benefit Plan Disclosures
Prior service cost (benefit) related to current year amendments to the qualified plan and agent nonqualified plan				$ (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				3,383.6	$ 3,056.2
Accumulated benefit obligation for defined benefit plans with an accumulated benefit obligation in excess of plan assets				3,139.3	2,821.1
Fair value of plan assets for defined benefit plans with an accumulated benefit obligation in excess of plan assets				2,542.2	2,190.8
Other postretirement benefits
Defined Benefit Plan Disclosures
Reduction of accumulated postretirement benefit obligation due to elimination of subsidies for pre-65 employees and agents	$ 51.6
Reduction of accumulated postretirement benefit obligation due to change in post-65 medical plan			$ 15.5
Adjustment to accumulated post-retirement benefit obligation related to dental plan benefits		$ 5.8
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				2.2	2.6
Fair value of plan assets for defined benefit plans with an accumulated benefit obligation in excess of plan assets				$ 0.4	$ 0.6