Equity Method Eligible Investments - Balance Sheet and Income Statement Tables (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Investments in unconsolidated affiliates
Total assets	$ 7,389.3	$ 7,000.7
Total liabilities	3,454.3	3,296.0
Total revenues	1,157.9	614.4	$ 895.6
Net income (loss)	792.8	(275.4)	708.7
White Mountains
Investments in unconsolidated affiliates
Total assets	2,252.5	1,845.7
Total liabilities	526.9	373.4
Total revenues	735.3	987.4	526.5
Total expenses	515.9	418.7	325.9
Net income (loss)	219.4	568.7	200.6
MediaAlpha
Investments in unconsolidated affiliates
Total assets	170.1	289.8
Total liabilities	256.2	351.4
Total revenues	459.1	645.3	584.8
Total expenses	531.5	653.8	574.2
Net income (loss)	$ (72.4)	$ (8.5)	$ 10.6