Employee and Agent Benefits - Benefit Obligation and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension benefits
Change in benefit obligation
Benefit obligation at beginning of year	$ (3,383.6)	$ (3,056.2)
Service cost	(73.0)	(67.1)	$ (65.0)
Interest cost	(119.5)	(124.4)	(134.9)
Actuarial gain (loss)	220.9	(268.0)
Benefits paid	109.6	109.0
Plan amendments	6.4	23.1
Benefit obligation at end of year	(3,239.2)	(3,383.6)	(3,056.2)
Change in plan assets
Fair value of plan assets at beginning of year	2,542.2	2,190.8
Actual return on plan assets	(122.2)	368.3
Employer contribution	187.8	92.1
Benefits paid	(109.6)	(109.0)
Fair value of plan assets at end of year	2,498.2	2,542.2	2,190.8
Amount recognized in statement of financial position
Amount recognized in other liabilities	(741.0)	(841.4)
Total assets (liabilities) recognized in statement of financial position	(741.0)	(841.4)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	673.3	682.8
Prior service (benefit) cost	(26.7)	(23.7)
Pre-tax accumulated other comprehensive (income) loss	646.6	659.1
Additional defined benefit plan disclosures
Accumulated benefit obligation	3,029.0	3,139.3
Market value of assets held in Rabbi trusts for benefit of nonqualified pension plan participants	348.8	352.0
Other postretirement benefits
Change in benefit obligation
Benefit obligation at beginning of year	(109.1)	(109.8)
Service cost	(0.1)	(0.1)	(2.1)
Interest cost	(3.5)	(3.9)	(6.2)
Actuarial gain (loss)	7.9	(1.7)
Participant contributions	(3.7)	(3.5)
Benefits paid	9.7	9.9
Other	(0.1)
Benefit obligation at end of year	(98.9)	(109.1)	(109.8)
Change in plan assets
Fair value of plan assets at beginning of year	696.8	628.6
Actual return on plan assets	(47.7)	73.9
Employer contribution	0.8	0.7
Participant contributions	3.7	3.5
Benefits paid	(9.7)	(9.9)
Fair value of plan assets at end of year	643.9	696.8	$ 628.6
Amount recognized in statement of financial position
Amount recognized in other assets	546.9	589.5
Amount recognized in other liabilities	(1.9)	(1.8)
Total assets (liabilities) recognized in statement of financial position	545.0	587.7
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	40.2	(34.7)
Prior service (benefit) cost	(10.2)	(24.1)
Pre-tax accumulated other comprehensive (income) loss	$ 30.0	$ (58.8)