Receivables from Policyholders - Allowance for Expected Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	$ 13.9	$ 13.1
Provision for Expected Credit Losses	6.5	11.0
Write-offs of Uncollectible Receivables from Policyholders	(14.0)	(12.8)
Balance, End of Period	6.4	11.3
Receivable Balance, End of Period	953.1	1,344.0
Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Receivable Balance, End of Period
Corporate and Other
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	1.0	0.8
Provision for Expected Credit Losses	0.2	0.4
Write-offs of Uncollectible Receivables from Policyholders	(0.2)	(0.5)
Balance, End of Period	1.0	0.7
Receivable Balance, End of Period	49.8	101.6
Specialty and Life Segments | Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	12.9	12.3
Provision for Expected Credit Losses	6.3	10.6
Write-offs of Uncollectible Receivables from Policyholders	(13.8)	(12.3)
Balance, End of Period	5.4	10.6
Receivable Balance, End of Period	903.3	1,242.4
Speciality | Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	12.9	12.3
Provision for Expected Credit Losses	6.2	10.3
Write-offs of Uncollectible Receivables from Policyholders	(13.7)	(12.0)
Balance, End of Period	5.4	10.6
Receivable Balance, End of Period	892.8	1,232.7
Life Segment | Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	0.0	0.0
Provision for Expected Credit Losses	0.1	0.3
Write-offs of Uncollectible Receivables from Policyholders	(0.1)	(0.3)
Balance, End of Period	0.0	0.0
Receivable Balance, End of Period	$ 10.5	$ 9.7