Employee and Agent Benefits - Supplemental Information, Qualified and Nonqualified Plans (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amount recognized in statement of financial position
Amount recognized in other liabilities	$ (836.7)	$ (766.1)
Total assets (liabilities) recognized in statement of financial position	(836.7)	(766.1)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	760.0	737.5
Prior service (benefit) cost	(121.0)	(137.8)
Pre-tax accumulated other comprehensive (income) loss	639.0	599.7
Components of Net Periodic Benefit Cost (Income)
Service cost	72.7	66.0	$ 73.0
Interest cost	117.3	126.5	119.5
Expected return on plan assets	(156.8)	(148.8)	(157.0)
Amortization of prior service (benefit) cost	(16.8)	(11.3)	(3.4)
Recognized net actuarial (gain) loss	75.4	70.1	67.8
Net periodic benefit cost (income)	91.8	102.5	$ 99.9
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	97.9	134.3
Prior service (benefit) cost		(122.4)
Amortization of net gain (loss)	(75.4)	(70.1)
Amortization of prior service benefit (cost)	16.8	11.3
Total recognized in pre-tax accumulated other comprehensive (income) loss	39.3	(46.9)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	131.1	55.6
Qualified benefit plans
Amount recognized in statement of financial position
Amount recognized in other liabilities	(294.9)	(258.7)
Total assets (liabilities) recognized in statement of financial position	(294.9)	(258.7)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	563.5	567.6
Prior service (benefit) cost	(97.8)	(110.3)
Pre-tax accumulated other comprehensive (income) loss	465.7	457.3
Components of Net Periodic Benefit Cost (Income)
Service cost	66.1	59.7
Interest cost	101.2	108.0
Expected return on plan assets	(156.8)	(148.8)
Amortization of prior service (benefit) cost	(12.6)	(7.8)
Recognized net actuarial (gain) loss	59.9	57.6
Net periodic benefit cost (income)	57.8	68.7
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	55.7	80.8
Prior service (benefit) cost		(107.2)
Amortization of net gain (loss)	(59.8)	(57.6)
Amortization of prior service benefit (cost)	12.6	7.8
Total recognized in pre-tax accumulated other comprehensive (income) loss	8.5	(76.2)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	66.3	(7.5)
Nonqualified benefit plans
Amount recognized in statement of financial position
Amount recognized in other liabilities	(541.8)	(507.4)
Total assets (liabilities) recognized in statement of financial position	(541.8)	(507.4)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	196.5	169.9
Prior service (benefit) cost	(23.2)	(27.5)
Pre-tax accumulated other comprehensive (income) loss	173.3	142.4
Components of Net Periodic Benefit Cost (Income)
Service cost	6.6	6.3
Interest cost	16.1	18.5
Amortization of prior service (benefit) cost	(4.2)	(3.5)
Recognized net actuarial (gain) loss	15.5	12.5
Net periodic benefit cost (income)	34.0	33.8
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	42.2	53.5
Prior service (benefit) cost		(15.2)
Amortization of net gain (loss)	(15.6)	(12.5)
Amortization of prior service benefit (cost)	4.2	3.5
Total recognized in pre-tax accumulated other comprehensive (income) loss	30.8	29.3
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	$ 64.8	$ 63.1