Employee and Agent Benefits - Supplemental Information, Qualified and Nonqualified Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension benefits
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
Accumulated benefit obligation	3,390.9	3,209.4
Defined Benefit Plan Nonqualified Pension Plan Market Value Of Assets Held In Trust	342.2	336.7
Components of Net Periodic Benefit Cost (Income)
Service cost	55.6	78.4	$ 83.2
Interest cost	160.1	112.8	103.3
Expected return on plan assets	(163.2)	(178.9)	(182.7)
Amortization of prior service (benefit) cost	(16.8)	(16.8)	(16.8)
Recognized net actuarial (gain) loss	41.0	55.7	70.2
Net periodic benefit cost (income)	76.7	51.2	57.2
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	24.4	(90.3)
Amortization of gain (loss)	(41.0)	(55.7)
Amortization of prior service benefit (cost)	16.8	16.8
Total recognized in pre-tax accumulated other comprehensive (income) loss	0.2	(129.2)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	76.9	(78.0)
Pension benefits | Qualified benefit plans
Amount recognized in statement of financial position
Amount recognized in other liabilities	(58.3)	(70.7)
Total assets (liabilities) recognized in statement of financial position	(58.3)	(70.7)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	361.4	390.7
Prior service (benefit) cost	(60.0)	(72.6)
Pre-tax accumulated other comprehensive (income) loss	301.4	318.1
Components of Net Periodic Benefit Cost (Income)
Service cost	51.6	72.5
Interest cost	139.7	98.7
Expected return on plan assets	(163.2)	(178.9)
Amortization of prior service (benefit) cost	(12.6)	(12.6)
Recognized net actuarial (gain) loss	38.8	39.7
Net periodic benefit cost (income)	54.3	19.4
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	9.5	15.0
Amortization of gain (loss)	(38.8)	(39.7)
Amortization of prior service benefit (cost)	12.6	12.6
Total recognized in pre-tax accumulated other comprehensive (income) loss	(16.7)	(12.1)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	37.6	7.3
Pension benefits | Nonqualified benefit plans
Amount recognized in statement of financial position
Amount recognized in other liabilities	(423.0)	(412.4)
Total assets (liabilities) recognized in statement of financial position	(423.0)	(412.4)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	64.0	51.3
Prior service (benefit) cost	(10.6)	(14.8)
Pre-tax accumulated other comprehensive (income) loss	53.4	36.5
Components of Net Periodic Benefit Cost (Income)
Service cost	4.0	5.9
Interest cost	20.4	14.1
Amortization of prior service (benefit) cost	(4.2)	(4.2)
Recognized net actuarial (gain) loss	2.2	16.0
Net periodic benefit cost (income)	22.4	31.8
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	14.9	(105.3)
Amortization of gain (loss)	(2.2)	(16.0)
Amortization of prior service benefit (cost)	4.2	4.2
Total recognized in pre-tax accumulated other comprehensive (income) loss	16.9	(117.1)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	39.3	(85.3)
Other postretirement benefits
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)
Components of Net Periodic Benefit Cost (Income)
Interest cost	3.4	2.3	2.2
Expected return on plan assets	(4.6)	(4.9)	(4.8)
Amortization of prior service (benefit) cost	(0.8)	(1.1)	(1.1)
Recognized net actuarial (gain) loss	(1.0)	(1.1)	(0.5)
Net periodic benefit cost (income)	(3.0)	(4.8)	$ (4.2)
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	(5.2)	5.4
Amortization of gain (loss)	1.0	1.1
Amortization of prior service benefit (cost)	0.8	1.1
Total recognized in pre-tax accumulated other comprehensive (income) loss	(3.4)	7.6
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	$ (6.4)	$ 2.8