Income Taxes - Reconciliation of Effective Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Income Tax Disclosure [Abstract]
Expected federal tax on income	$ 11.2	$ 3.5		$ 44.1
Add (deduct) tax effects of:
Tax-exempt interest	(2.7)	(3.3)		(3.9)
Dividend received deduction	(1.3)	(3.2)		(2.2)
Employee share-based compensation	0.1	(0.5)		(1.3)
Contingent consideration	0.0	(0.3)		0.0
Compensation deduction limitation	0.8	0.7		1.5
Research and development reserve	(0.2)	(0.4)		(0.2)
Prior year adjustments	0.3	0.1		0.1
Other, net	0.1	0.1		1.6
Total income tax expense (benefit)	$ 8.3	$ (3.3)	[1]	$ 39.7	[1]

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