Deferred Policy Acquisition Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Business Acquisition [Line Items]
Deferred beginning of year	$ 382.5	$ 350.4	$ 328.0
Acquisition costs deferred; additions	715.3	623.5	524.6
Amortization charged to income	(680.1)	(591.4)	(502.2)
Change for the year	35.2	32.0	22.4
Deferred end of year	417.8	382.5	350.4
Commissions, net of reinsurance [Member]
Business Acquisition [Line Items]
Acquisition costs deferred; additions	495.0	423.3	340.9
Premium taxes [Member]
Business Acquisition [Line Items]
Acquisition costs deferred; additions	161.0	147.1	135.2
Salaries and other marketing expenses [Member]
Business Acquisition [Line Items]
Acquisition costs deferred; additions	$ 59.3	$ 53.0	$ 48.3