Investment Securities - Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Investment income:
Total investment income	$ 84.8	$ 132.2	$ 81.0
Third-party investment expenses	(2.3)	(1.2)	(1.3)
Net investment income, pre-tax	82.5	131.0	79.7
Fixed maturity investments
Investment income:
Total investment income	28.8	29.0	32.4
Short-term investments
Investment income:
Total investment income	0.6	1.1	5.0
Common equity securities
Investment income:
Total investment income	0.1	6.6	13.5
MediaAlpha
Investment income:
Total investment income	0.0	59.9	8.0
Other long-term investments
Investment income:
Total investment income	56.3	35.6	22.1
Amount attributable to TPC Providers
Investment income:
Total investment income	$ (1.0)	$ 0.0	$ 0.0