Employee and Agent Benefits - Benefit Obligation and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension benefits
Change in benefit obligation
Benefit obligation at beginning of year	$ (3,056.2)	$ (3,052.2)
Service cost	(67.1)	(65.0)	$ (63.2)
Interest cost	(124.4)	(134.9)	(120.3)
Actuarial gain (loss)	(268.0)	(39.3)
Benefits paid	109.0	235.2
Plan amendments	23.1
Benefit obligation at end of year	(3,383.6)	(3,056.2)	(3,052.2)
Change in plan assets
Fair value of plan assets at beginning of year	2,190.8	2,166.3
Actual return on plan assets	368.3	181.6
Employer contribution	92.1	78.1
Benefits paid	(109.0)	(235.2)
Fair value of plan assets at end of year	2,542.2	2,190.8	2,166.3
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
Additional defined benefit plan disclosures
Accumulated benefit obligation	3,139.3	2,821.1
Market value of assets held in Rabbi trusts for benefit of nonqualified pension plan participants	352.0	341.0
Other postretirement benefits
Change in benefit obligation
Benefit obligation at beginning of year	(109.8)	(165.7)
Service cost	(0.1)	(2.1)	(2.0)
Interest cost	(3.9)	(6.2)	(6.6)
Actuarial gain (loss)	(1.7)	5.8
Participant contribution	(3.5)	(3.9)
Benefits paid	9.9	10.7
Plan amendments		51.6
Benefit obligation at end of year	(109.1)	(109.8)	(165.7)
Change in plan assets
Fair value of plan assets at beginning of year	628.6	627.0
Actual return on plan assets	73.9	8.0
Employer contribution	0.7	0.4
Participant contributions	3.5	3.9
Benefits paid	(9.9)	(10.7)
Fair value of plan assets at end of year	696.8	628.6	$ 627.0
Amount recognized in statement of financial position
Amount recognized in other assets	589.5	520.8
Amount recognized in other liabilities	(1.8)	(2.0)
Total assets (liabilities) recognized in statement of financial position	587.7	518.8
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss	(34.7)	10.0
Prior service (benefit) cost	(24.1)	(58.7)
Pre-tax accumulated other comprehensive (income) loss	$ (58.8)	$ (48.7)