Employee and Agent Benefits - Benefit Obligation and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension benefits
Change in benefit obligation
Benefit obligation at beginning of year	$ (3,209.4)	$ (4,183.7)
Service cost	(55.6)	(78.4)	$ (83.2)
Interest cost	(160.1)	(112.8)	(103.3)
Actuarial gain (loss)	(119.7)	1,023.5
Benefits paid	153.9	142.0
Benefit obligation at end of year	(3,390.9)	(3,209.4)	(4,183.7)
Change in plan assets
Fair value of plan assets at beginning of year	2,726.3	3,550.1
Actual return on plan assets	258.5	(754.3)
Employer contribution	78.7	72.5
Benefits paid	(153.9)	(142.0)
Fair value of plan assets at end of year	2,909.6	2,726.3	3,550.1
Amount recognized in statement of financial position
Amount recognized in other liabilities	(481.3)	(483.1)
Total assets (liabilities) recognized in statement of financial position	(481.3)	(483.1)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	425.4	442.0
Prior service (benefit) cost	(70.6)	(87.4)
Pre-tax accumulated other comprehensive (income) loss	354.8	354.6
Additional defined benefit plan disclosures
Accumulated benefit obligation	3,390.9	3,209.4
Market value of assets held in Rabbi trusts for benefit of nonqualified pension plan participants	342.2	336.7
Other postretirement benefits
Change in benefit obligation
Benefit obligation at beginning of year	(71.0)	(95.3)
Interest cost	(3.4)	(2.3)	(2.2)
Actuarial gain (loss)	(2.9)	19.5
Participant contributions	(6.1)	(6.4)
Benefits paid	15.0	13.5
Benefit obligation at end of year	(68.4)	(71.0)	(95.3)
Change in plan assets
Fair value of plan assets at beginning of year	94.4	119.8
Actual return on plan assets	12.7	(20.0)
Employer contribution	1.6	1.7
Participant contributions	6.1	6.4
Benefits paid	(15.0)	(13.5)
Fair value of plan assets at end of year	99.8	94.4	$ 119.8
Amount recognized in statement of financial position
Amount recognized in other assets	31.4	23.4
Total assets (liabilities) recognized in statement of financial position	31.4	23.4
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	(24.4)	(20.2)
Prior service (benefit) cost	(4.3)	(5.1)
Pre-tax accumulated other comprehensive (income) loss	$ (28.7)	$ (25.3)