Employee and Agent Benefits - Supplemental Information, Qualified and Nonqualified Plans (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amount recognized in statement of financial position
Amount recognized in other liabilities	$ (841.4)	$ (865.4)
Total assets (liabilities) recognized in statement of financial position	(841.4)	(865.4)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	682.8	706.9
Prior service (benefit) cost	(23.7)	(2.9)
Pre-tax accumulated other comprehensive (income) loss	659.1	704.0
Components of Net Periodic Benefit Cost (Income)
Service cost	67.1	65.0	$ 63.2
Interest cost	124.4	134.9	120.3
Expected return on plan assets	(144.2)	(155.0)	(160.6)
Amortization of prior service (benefit) cost	(2.3)	(2.2)	(1.9)
Recognized net actuarial (gain) loss	68.0	77.0	102.4
Net periodic benefit cost (income)	113.0	119.7	$ 123.4
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	43.9	12.7
Prior service (benefit) cost	(23.1)
Amortization of net gain (loss)	(68.0)	(77.0)
Amortization of prior service benefit (cost)	2.3	2.2
Total recognized in pre-tax accumulated other comprehensive (income) loss	(44.9)	(62.1)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	68.1	57.6
Qualified benefit plans
Amount recognized in statement of financial position
Amount recognized in other liabilities	(340.0)	(410.0)
Total assets (liabilities) recognized in statement of financial position	(340.0)	(410.0)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	516.4	587.5
Prior service (benefit) cost	(12.5)	(1.6)
Pre-tax accumulated other comprehensive (income) loss	503.9	585.9
Components of Net Periodic Benefit Cost (Income)
Service cost	60.3	58.1
Interest cost	106.0	115.9
Expected return on plan assets	(144.2)	(155.0)
Amortization of prior service (benefit) cost	(1.2)	(1.1)
Recognized net actuarial (gain) loss	59.2	69.5
Net periodic benefit cost (income)	80.1	87.4
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	(11.9)	(2.0)
Prior service (benefit) cost	(12.2)
Amortization of net gain (loss)	(59.2)	(69.5)
Amortization of prior service benefit (cost)	1.2	1.1
Total recognized in pre-tax accumulated other comprehensive (income) loss	(82.1)	(70.4)
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	(2.0)	17.0
Nonqualified benefit plans
Amount recognized in statement of financial position
Amount recognized in other liabilities	(501.4)	(455.4)
Total assets (liabilities) recognized in statement of financial position	(501.4)	(455.4)
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	166.4	119.4
Prior service (benefit) cost	(11.2)	(1.3)
Pre-tax accumulated other comprehensive (income) loss	155.2	118.1
Components of Net Periodic Benefit Cost (Income)
Service cost	6.8	6.9
Interest cost	18.4	19.0
Amortization of prior service (benefit) cost	(1.1)	(1.1)
Recognized net actuarial (gain) loss	8.8	7.5
Net periodic benefit cost (income)	32.9	32.3
Other changes recognized in accumulated other comprehensive (income) loss
Net actuarial (gain) loss	55.8	14.7
Prior service (benefit) cost	(10.9)
Amortization of net gain (loss)	(8.8)	(7.5)
Amortization of prior service benefit (cost)	1.1	1.1
Total recognized in pre-tax accumulated other comprehensive (income) loss	37.2	8.3
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss	$ 70.1	$ 40.6