Reinsurance - Effects Of Reinsurance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Premiums and other considerations:
Premiums and other considerations, Direct	$ 1,575.3	$ 1,591.6	$ 4,757.5	$ 4,068.8
Premiums and other considerations, Ceded	(117.4)	(105.1)	(358.3)	(328.7)
Net premiums and other considerations	1,457.9	1,486.5	4,399.2	3,740.1
Benefits, claims and settlement expenses:
Benefits, claims and settlement expenses, Direct	2,143.0	2,174.8	6,504.9	5,826.4
Benefits, claims and settlement expenses, Ceded	(432.4)	(373.2)	(1,186.3)	(1,090.9)
Net benefits, claims and settlement expenses	1,710.6	1,801.6	5,318.6	4,735.5
Liability for future policy benefits remeasurement (gain) loss:
Liability for future policy benefits remeasurement (gain) loss, Direct	697.4	(68.0)	721.7	(199.6)
Liability for future policy benefits remeasurement (gain) loss, Ceded	(739.7)	0.4	(768.9)	(68.4)
Net liability for future policy benefits remeasurement (gain) loss	$ (42.3)	$ (67.6)	$ (47.2)	$ (268.0)