Goodwill and Other Intangible Assets - Schedule of Goodwill and Other Intangible Assets Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Goodwill [Roll Forward]
Beginning balance	$ 142.3
Amortization	(5.2)	$ (4.6)	$ (1.6)
Ending balance	176.5	142.3
Insurance licenses	7,389.3	7,000.7
Goodwill
Goodwill [Roll Forward]
Beginning balance	142.3	19.2
Acquisition of businesses	59.5	15.8
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	(22.9)	(9.3)
Measurement period adjustments	(2.4)	(0.2)
Assets held for sale	0.0	0.0
Amortization	0.0	0.0
Ending balance	176.5	142.3	19.2
Goodwill | Ark
Goodwill [Roll Forward]
Ark Transaction	0.0	116.8
Total other intangible assets
Goodwill [Roll Forward]
Beginning balance	198.2	26.4
Acquisition of businesses	0.0	0.0
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	22.9	9.3
Measurement period adjustments	0.0	0.0
Assets held for sale	0.0	(8.6)
Amortization	(5.2)	(4.6)
Ending balance	215.9	198.2	26.4
Total other intangible assets | Ark
Goodwill [Roll Forward]
Ark Transaction	0.0	175.7
Total Goodwill and Other Intangible Assets
Goodwill [Roll Forward]
Beginning balance	340.5	45.6
Acquisition of businesses	59.5	15.8
Attribution of acquisition date fair value estimates between goodwill and other intangible assets	0.0	0.0
Measurement period adjustments	(2.4)	(0.2)
Assets held for sale	0.0	(8.6)
Amortization	(5.2)	(4.6)
Ending balance	392.4	340.5	$ 45.6
Total Goodwill and Other Intangible Assets | Ark
Goodwill [Roll Forward]
Ark Transaction	$ 0.0	292.5
Insurance Licenses | Disposal Group, Held-for-sale, Not Discontinued Operations | Other Operations
Goodwill [Roll Forward]
Insurance licenses		$ 8.6