Unpaid Claims and Claim Expense Reserves - Summary of Reinsurance Reserve Balances (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	$ 425.9
Less: reinsurance recoverables	119.6
Claims and claim expense payments for claims occurring during:
Plus: reinsurance recoverables	462.5	$ 119.6
Gross reserves, end of year	585.1	425.9
Group benefits
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	135.2
Less: reinsurance recoverables	37.8
Net reserves, beginning of year	97.4
Incurred claims and claim expenses:
Claims occurring in the current year	78.3
Decrease in estimated reserves for claims occurring in prior years	(11.1)
Total claims and claim expenses incurred	67.2
Claims and claim expense payments for claims occurring during:
Current year	35.1
Prior years	35.1
Total claims and claim expense payments	70.2
Net reserves, end of year	94.4	97.4
Plus: reinsurance recoverables	38.2	37.8
Gross reserves, end of year	132.6	135.2
Property & Casualty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	362.4	372.2	$ 387.0
Less: reinsurance recoverables	110.3	112.9	120.5
Net reserves, beginning of year	252.1	259.3	266.5
Incurred claims and claim expenses:
Claims occurring in the current year	512.3	455.1	441.2
Decrease in estimated reserves for claims occurring in prior years	22.0	(7.2)	(10.2)
Total claims and claim expenses incurred	534.3	447.9	431.0
Claims and claim expense payments for claims occurring during:
Current year	320.0	307.1	291.4
Prior years	178.5	148.0	146.8
Total claims and claim expense payments	498.5	455.1	438.2
Net reserves, end of year	287.9	252.1	259.3
Plus: reinsurance recoverables	100.8	110.3	112.9
Gross reserves, end of year	$ 388.7	$ 362.4	$ 372.2