Employee and Agent Benefits - Supplemental Information, Qualified and Nonqualified Plans (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Amount recognized in statement of financial position
Amount recognized in other liabilities	$ (766.1)	$ (741.0)
Total assets (liabilities) recognized in statement of financial position	(766.1)	(741.0)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial loss	737.5	673.3
Prior service benefit	(137.8)	(26.7)
Pre-tax accumulated other comprehensive (income) loss	599.7	646.6
Components of Net Periodic Benefit Cost (Income)
Service cost	66.0	73.0	$ 67.1
Interest cost	126.5	119.5	124.4
Expected return on plan assets	(148.8)	(157.0)	(144.2)
Amortization of prior service (benefit) cost	(11.3)	(3.4)	(2.3)
Recognized net actuarial (gain) loss	70.1	67.8	68.0
Net periodic benefit cost (income)	102.5	99.9	$ 113.0
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	134.3	58.3
Prior service (benefit) cost	(122.4)	(6.4)
Amortization of net gain (loss)	(70.1)	(67.8)
Amortization of prior service benefit (cost)	11.3	3.4
Total recognized in pre-tax accumulated other comprehensive (income) loss	(46.9)	(12.5)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	55.6	87.4
Qualified benefit plans
Amount recognized in statement of financial position
Amount recognized in other liabilities	(258.7)	(266.3)
Total assets (liabilities) recognized in statement of financial position	(258.7)	(266.3)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial loss	567.6	544.4
Prior service benefit	(110.3)	(11.0)
Pre-tax accumulated other comprehensive (income) loss	457.3	533.4
Components of Net Periodic Benefit Cost (Income)
Service cost	59.7	65.7
Interest cost	108.0	102.0
Expected return on plan assets	(148.8)	(157.0)
Amortization of prior service (benefit) cost	(7.8)	(1.5)
Recognized net actuarial (gain) loss	57.6	53.6
Net periodic benefit cost (income)	68.7	62.8
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	80.8	81.7
Prior service (benefit) cost	(107.2)
Amortization of net gain (loss)	(57.6)	(53.6)
Amortization of prior service benefit (cost)	7.8	1.5
Total recognized in pre-tax accumulated other comprehensive (income) loss	(76.2)	29.6
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	(7.5)	92.4
Nonqualified benefit plans
Amount recognized in statement of financial position
Amount recognized in other liabilities	(507.4)	(474.7)
Total assets (liabilities) recognized in statement of financial position	(507.4)	(474.7)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial loss	169.9	128.9
Prior service benefit	(27.5)	(15.7)
Pre-tax accumulated other comprehensive (income) loss	142.4	113.2
Components of Net Periodic Benefit Cost (Income)
Service cost	6.3	7.3
Interest cost	18.5	17.5
Amortization of prior service (benefit) cost	(3.5)	(1.9)
Recognized net actuarial (gain) loss	12.5	14.2
Net periodic benefit cost (income)	33.8	37.1
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	53.5	(23.4)
Prior service (benefit) cost	(15.2)	(6.4)
Amortization of net gain (loss)	(12.5)	(14.2)
Amortization of prior service benefit (cost)	3.5	1.9
Total recognized in pre-tax accumulated other comprehensive (income) loss	29.3	(42.1)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	$ 63.1	$ (5.0)