Employee and Agent Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Pension benefits
Components of Net Periodic Benefit Cost (Income)
Service cost	$ 20.8	$ 18.2	$ 62.4	$ 54.5
Interest cost	$ 25.8	$ 29.3	$ 77.4	$ 88.0
Interest cost, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses	Operating expenses
Expected return on plan assets	$ (45.7)	$ (39.1)	$ (137.0)	$ (117.5)
Expected return on plan assets, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses	Operating expenses
Amortization of prior service (benefit) cost	$ (4.2)	$ (4.3)	$ (12.6)	$ (12.7)
Amortization of prior service (benefit) cost, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses	Operating expenses
Recognized net actuarial (gain) loss	$ 17.5	$ 18.9	$ 52.6	$ 56.6
Recognized net actuarial (gain) loss, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses	Operating expenses
Net periodic benefit cost (income)	$ 14.2	$ 23.0	$ 42.8	$ 68.9
Contributions made by employer to fund qualified and nonqualified pension plans	20.7		92.1
Pension benefits | Maximum
Components of Net Periodic Benefit Cost (Income)
Amount of possible contributions to be made during the current fiscal year to the qualified and nonqualified pension plans combined	125.0		125.0
Other postretirement benefits
Components of Net Periodic Benefit Cost (Income)
Interest cost	$ 0.5	$ 0.7	$ 1.6	$ 2.1
Interest cost, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses	Operating expenses
Expected return on plan assets	$ (1.1)	$ (9.0)	$ (3.5)	$ (27.0)
Expected return on plan assets, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses	Operating expenses
Amortization of prior service (benefit) cost	$ (0.3)	$ (0.3)	$ (0.8)	$ (0.8)
Amortization of prior service (benefit) cost, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses	Operating expenses
Recognized net actuarial (gain) loss	$ (0.2)		$ (0.4)
Recognized net actuarial (gain) loss, location on consolidated statements of operations	Operating expenses	Operating expenses	Operating expenses	Operating expenses
Net periodic benefit cost (income)	$ (1.1)	$ (8.6)	$ (3.1)	$ (25.7)
Contributions made by employer to fund qualified and nonqualified pension plans			$ 1.1		$ 1.5