Summary of Accounting Policies and Accounting Changes (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year		$ 13.1	$ 13.6
Provision for Expected Credit Losses		41.6	48.0
Write-offs of Uncollectible Receivables from Policyholders		(40.8)	(48.5)
Balance at End of Year		13.9	13.1		$ 13.6
Receivable Balance at End of Year		960.2
Receivables from Policyholders (Allowance for Credit Losses: 2023 - $13.9; 2022 - $13.1)		959.5	1,286.6	[1]
Basic (in dollars per share)	$ (1.92)				$ (1.92)
Diluted (in dollars per share)	$ (1.92)				$ (1.92)
Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Receivables from Policyholders (Allowance for Credit Losses: 2023 - $13.9; 2022 - $13.1)
Corporate, Non-Segment
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year		0.8	0.8
Provision for Expected Credit Losses		1.8	2.5
Write-offs of Uncollectible Receivables from Policyholders		(1.6)	(2.5)
Balance at End of Year		1.0	0.8		$ 0.8
Receivables from Policyholders (Allowance for Credit Losses: 2023 - $13.9; 2022 - $13.1)		73.5	108.7
Specialty and Life Segments | Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year		12.3	12.8
Provision for Expected Credit Losses		39.8	45.5
Write-offs of Uncollectible Receivables from Policyholders		(39.2)	(46.0)
Balance at End of Year		12.9	12.3		12.8
Receivables from Policyholders (Allowance for Credit Losses: 2023 - $13.9; 2022 - $13.1)		886.7	1,177.9
Speciality | Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year		12.3	12.8
Provision for Expected Credit Losses		39.3	44.4
Write-offs of Uncollectible Receivables from Policyholders		(38.7)	(44.9)
Balance at End of Year		12.9	12.3		12.8
Receivables from Policyholders (Allowance for Credit Losses: 2023 - $13.9; 2022 - $13.1)		875.4	1,166.9
Life Segment | Operating Segments
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Balance at Beginning of Year		0.0	0.0
Provision for Expected Credit Losses		0.5	1.1
Write-offs of Uncollectible Receivables from Policyholders		(0.5)	(1.1)
Balance at End of Year		0.0	0.0		$ 0.0
Receivables from Policyholders (Allowance for Credit Losses: 2023 - $13.9; 2022 - $13.1)		$ 11.3	$ 11.0

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.