Basis of Presentation and Significant Accounting Policies - Schedule of Changes in the Statement of Equity due to Adoption of ASU 2018-12 (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	[1]	Dec. 31, 2021	[1]	Dec. 31, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity	$ 1,175.3	$ 1,098.3		$ 1,499.0
Total
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity	(314.0)	(399.4)		(50.0)		$ 355.1	[1]
Total | Liability for future policy benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						(496.4)
Total | Deferred policy acquisition costs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						71.1
Total | Deferred sales inducements
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						0.0
Total | Market risk benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						(1.3)
Total | Revision of Prior Period, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						(426.6)
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity	$ 1,502.2	$ 1,512.4		$ 1,547.0		1,434.6	[1]
Retained earnings | Liability for future policy benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						(0.2)
Retained earnings | Deferred policy acquisition costs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						0.0
Retained earnings | Deferred sales inducements
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						0.0
Retained earnings | Market risk benefits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						(5.3)
Retained earnings | Revision of Prior Period, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity						$ (5.5)

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