Employee and Agent Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Plan actuarial gain and loss amortization disclosures
Allowable corridor not used for amortization of actuarial gains and losses of qualified pension plan (as a percent)	10.00%
Pension benefits
Components of Net Periodic Benefit Cost (Income)
Service cost	$ 55.6	$ 78.4	$ 83.2
Interest cost	$ 160.1	$ 112.8	$ 103.3
Interest cost, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses
Expected return on plan assets	$ (163.2)	$ (178.9)	$ (182.7)
Expected return on plan assets, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses
Amortization of prior service (benefit) cost	$ (16.8)	$ (16.8)	$ (16.8)
Amortization of prior service (benefit) cost, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses
Recognized net actuarial (gain) loss	$ 41.0	$ 55.7	$ 70.2
Recognized net actuarial (gain) loss, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses
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)
Other postretirement benefits
Components of Net Periodic Benefit Cost (Income)
Interest cost	$ 3.4	$ 2.3	$ 2.2
Interest cost, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses
Expected return on plan assets	$ (4.6)	$ (4.9)	$ (4.8)
Expected return on plan assets, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses
Amortization of prior service (benefit) cost	$ (0.8)	$ (1.1)	$ (1.1)
Amortization of prior service (benefit) cost, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses
Recognized net actuarial (gain) loss	$ (1.0)	$ (1.1)	$ (0.5)
Recognized net actuarial (gain) loss, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses
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