Short-Duration Insurance Contracts - Summary of Reinsurance Reserve Balances (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	[1]	$ 564.0
Less: reinsurance recoverables		462.5
Claims and claim expense payments for claims occurring during:
Plus: reinsurance recoverables		424.2	$ 462.5
Gross reserves, end of year		581.7	564.0	[1]
Group benefits
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less: reinsurance recoverables		352.4
Claims and claim expense payments for claims occurring during:
Plus: reinsurance recoverables		306.2	352.4
Property & Casualty
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year		388.7	362.4		$ 372.2
Less: reinsurance recoverables		100.8	110.3		112.9
Net reserves, beginning of year		287.9	252.1		259.3
Incurred claims and claim expenses:
Claims occurring in the current year		557.0	512.3		455.1
Decrease in estimated reserves for claims occurring in prior years		0.0	22.0		(7.2)
Total claims and claim expenses incurred		557.0	534.3		447.9
Claims and claim expense payments for claims occurring during:
Current year		353.1	320.0		307.1
Prior years		179.0	178.5		148.0
Total claims and claim expense payments		532.1	498.5		455.1
Net reserves, end of year		312.8	287.9		252.1
Plus: reinsurance recoverables		104.0	100.8		110.3
Gross reserves, end of year		416.8	388.7		362.4
Group benefits
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year		121.6	125.4
Less: reinsurance recoverables		27.9	28.5
Net reserves, beginning of year		93.7	96.9
Incurred claims and claim expenses:
Claims occurring in the current year		73.6	77.7
Decrease in estimated reserves for claims occurring in prior years		(13.9)	(10.9)
Total claims and claim expenses incurred		59.7	66.8
Claims and claim expense payments for claims occurring during:
Current year		33.8	35.0
Prior years		30.7	35.0
Total claims and claim expense payments		64.5	70.0
Net reserves, end of year		88.9	93.7		96.9
Plus: reinsurance recoverables		27.7	27.9		28.5
Gross reserves, end of year		$ 116.6	$ 121.6		$ 125.4

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements