Employee and Agent Benefits - Supplemental Information, Qualified and Nonqualified Plans (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amount recognized in statement of financial position
Amount recognized in other liabilities	$ (741.0)	$ (841.4)
Total assets (liabilities) recognized in statement of financial position	(741.0)	(841.4)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	673.3	682.8
Prior service (benefit) cost	(26.7)	(23.7)
Pre-tax accumulated other comprehensive (income) loss	646.6	659.1
Components of Net Periodic Benefit Cost (Income)
Service cost	73.0	67.1	$ 65.0
Interest cost	119.5	124.4	134.9
Expected return on plan assets	(157.0)	(144.2)	(155.0)
Amortization of prior service (benefit) cost	(3.4)	(2.3)	(2.2)
Recognized net actuarial (gain) loss	67.8	68.0	77.0
Net periodic benefit cost (income)	99.9	113.0	$ 119.7
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	58.3	43.9
Prior service (benefit) cost	(6.4)	(23.1)
Amortization of net gain (loss)	(67.8)	(68.0)
Amortization of prior service benefit (cost)	3.4	2.3
Total recognized in pre-tax accumulated other comprehensive (income) loss	(12.5)	(44.9)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	87.4	68.1
Qualified benefit plans
Amount recognized in statement of financial position
Amount recognized in other liabilities	(266.3)	(340.0)
Total assets (liabilities) recognized in statement of financial position	(266.3)	(340.0)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	544.4	516.4
Prior service (benefit) cost	(11.0)	(12.5)
Pre-tax accumulated other comprehensive (income) loss	533.4	503.9
Components of Net Periodic Benefit Cost (Income)
Service cost	65.7	60.3
Interest cost	102.0	106.0
Expected return on plan assets	(157.0)	(144.2)
Amortization of prior service (benefit) cost	(1.5)	(1.2)
Recognized net actuarial (gain) loss	53.6	59.2
Net periodic benefit cost (income)	62.8	80.1
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	81.7	(11.9)
Prior service (benefit) cost		(12.2)
Amortization of net gain (loss)	(53.6)	(59.2)
Amortization of prior service benefit (cost)	1.5	1.2
Total recognized in pre-tax accumulated other comprehensive (income) loss	29.6	(82.1)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	92.4	(2.0)
Nonqualified benefit plans
Amount recognized in statement of financial position
Amount recognized in other liabilities	(474.7)	(501.4)
Total assets (liabilities) recognized in statement of financial position	(474.7)	(501.4)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	128.9	166.4
Prior service (benefit) cost	(15.7)	(11.2)
Pre-tax accumulated other comprehensive (income) loss	113.2	155.2
Components of Net Periodic Benefit Cost (Income)
Service cost	7.3	6.8
Interest cost	17.5	18.4
Amortization of prior service (benefit) cost	(1.9)	(1.1)
Recognized net actuarial (gain) loss	14.2	8.8
Net periodic benefit cost (income)	37.1	32.9
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	(23.4)	55.8
Prior service (benefit) cost	(6.4)	(10.9)
Amortization of net gain (loss)	(14.2)	(8.8)
Amortization of prior service benefit (cost)	1.9	1.1
Total recognized in pre-tax accumulated other comprehensive (income) loss	(42.1)	37.2
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	$ (5.0)	$ 70.1