Future Policy Benefits and Claims - Liability for Unpaid Claims (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Change in unpaid claims
Balance at beginning of period, unpaid claims	$ 1,395.0	$ 1,370.9
Balance at beginning of period, reinsurance recoverables for unpaid claims	68.6	68.7
Net balance at beginning of period, unpaid claims	1,326.4	1,302.2
Incurred:
Incurred: Current year	1,233.1	1,176.6
Incurred: Prior years	(92.1)	(11.2)
Total incurred	1,141.0	1,165.4
Payments:
Payments: Current year	856.6	825.8
Payments: Prior years	295.4	318.8
Total payments	1,152.0	1,144.6
Net balance at end of period, unpaid claims	1,315.4	1,323.0
Balance at end of period, reinsurance recoverables for unpaid claims	66.5	71.0
Balance at end of period, unpaid claims	$ 1,381.9	$ 1,394.0