Employee and Agent Benefits - Benefit Obligation and Funded Status (Details) - USD ($) $ in Millions		12 Months Ended
	May 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension benefits
Change in benefit obligation
Benefit obligation at beginning of year		$ (3,692.1)	$ (3,239.2)
Service cost		(72.7)	(66.0)	$ (73.0)
Interest cost		(117.3)	(126.5)	(119.5)
Actuarial gain (loss)	$ (59.0)	(462.4)	(506.4)
Benefits paid		134.3	123.6
Plan amendments			122.4
Benefit obligation at end of year		(4,210.2)	(3,692.1)	(3,239.2)
Change in plan assets
Fair value of plan assets at beginning of year		2,926.0	2,498.2
Actual return on plan assets	$ 178.6	521.3	520.9
Employer contribution		60.5	30.5
Benefits paid		(134.3)	(123.6)
Fair value of plan assets at end of year		3,373.5	2,926.0	2,498.2
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
Additional defined benefit plan disclosures
Accumulated benefit obligation		4,136.5	3,599.5
Market value of assets held in Rabbi trusts for benefit of nonqualified pension plan participants		394.8	377.8
Other postretirement benefits
Change in benefit obligation
Benefit obligation at beginning of year		(101.4)	(98.9)
Service cost				(0.1)
Interest cost		(2.8)	(3.7)	(3.5)
Actuarial gain (loss)		(11.7)	(4.8)
Participant contributions		(6.0)	(4.4)
Benefits paid		12.8	12.2
Plan amendments		1.0	(1.8)
Other		0.1
Benefit obligation at end of year		(108.0)	(101.4)	(98.9)
Change in plan assets
Fair value of plan assets at beginning of year		732.8	643.9
Actual return on plan assets		53.1	95.5
Employer contribution		1.5	1.2
Participant contributions		6.0	4.4
Benefits paid		(12.8)	(12.2)
Fair value of plan assets at end of year		780.6	732.8	$ 643.9
Amount recognized in statement of financial position
Amount recognized in other assets		675.5	635.1
Amount recognized in other liabilities		(2.9)	(3.7)
Total assets (liabilities) recognized in statement of financial position		672.6	631.4
Amount recognized in accumulated other comprehensive (income) loss
Total net actuarial (gain) loss		(22.8)	(17.4)
Prior service (benefit) cost		(7.3)	(7.3)
Pre-tax accumulated other comprehensive (income) loss		$ (30.1)	$ (24.7)