Investment Securities - Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Investment income:
Total investment income	$ 126.5	$ 84.8	$ 132.2
Third-party investment expenses	(2.1)	(2.3)	(1.2)
Net investment income, pre-tax	124.4	82.5	131.0
Fixed maturity investments
Investment income:
Total investment income	41.4	28.8	29.0
Short-term investments
Investment income:
Total investment income	16.4	0.6	1.1
Common equity securities
Investment income:
Total investment income	1.6	0.1	6.6
MediaAlpha
Investment income:
Total investment income	0.0	0.0	59.9
Other long-term investments
Investment income:
Total investment income	68.1	56.3	35.6
Amount attributable to TPC Providers
Investment income:
Amount attributable to TPC Providers	$ (1.0)	$ (1.0)	$ 0.0