Employee and Agent Benefits - Benefit Obligation and Funded Status (Details) - Other postretirement benefits $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Change in plan assets
Fair value of plan assets at beginning of year	$ 780.6	$ 732.8
Actual return on plan assets	(2.9)	53.1
Employer contribution	1.1	1.5
Participant contributions	4.6	6.0
Benefits paid	(10.2)	(12.8)
Assets re-designated for non-retiree benefits	(656.5)
Fair value of plan assets at end of year	116.7	780.6
Amount recognized in statement of financial position
Amount recognized in other assets	11.7	675.5
Amount recognized in other liabilities	(1.8)	(2.9)
Total assets (liabilities) recognized in statement of financial position	$ 9.9	$ 672.6