Receivables from Policyholders - Allowance for Expected Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	$ 4.8	$ 12.0	$ 13.9	$ 13.1
Provision for Expected Credit Losses	12.9	11.3	27.2	32.8
Write-offs of Uncollectible Receivables from Policyholders	(14.1)	(9.8)	(37.5)	(32.4)
Balance, End of Period	3.6	13.5	3.6	13.5
Receivable Balance, End of Period	982.8	1,102.6	982.8	1,102.6
Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	0.0
Provision for Expected Credit Losses	0.1
Write-offs of Uncollectible Receivables from Policyholders	(0.1)
Balance, End of Period	0.0	0.0	0.0	0.0
Receivable Balance, End of Period
Corporate and Other
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	0.5	0.6	1.0	0.8
Provision for Expected Credit Losses	0.1	0.4	0.6	1.2
Write-offs of Uncollectible Receivables from Policyholders	(0.2)	(0.3)	(1.2)	(1.3)
Balance, End of Period	0.4	0.7	0.4	0.7
Receivable Balance, End of Period	17.3	88.8	17.3	88.8
Specialty and Life Segments | Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	4.3	11.4	12.9	12.3
Provision for Expected Credit Losses	12.8	10.9	26.6	31.6
Write-offs of Uncollectible Receivables from Policyholders	(13.9)	(9.5)	(36.3)	(31.1)
Balance, End of Period	3.2	12.8	3.2	12.8
Receivable Balance, End of Period	965.5	1,013.8	965.5	1,013.8
Speciality | Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year	4.3	11.4	12.9	12.3
Provision for Expected Credit Losses	12.7	10.9	26.4	31.3
Write-offs of Uncollectible Receivables from Policyholders	(13.8)	(9.5)	(36.1)	(30.8)
Balance, End of Period	3.2	12.8	3.2	12.8
Receivable Balance, End of Period	954.1	1,002.4	954.1	1,002.4
Life Segment | Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance, Beginning of Year		0.0	0.0	0.0
Provision for Expected Credit Losses		0.0	0.2	0.3
Write-offs of Uncollectible Receivables from Policyholders		0.0	(0.2)	(0.3)
Balance, End of Period	0.0	0.0	0.0	0.0
Receivable Balance, End of Period	$ 11.4	$ 11.4	$ 11.4	$ 11.4