Equity Method Eligible Investments - Balance Sheet and Income Statement Tables (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Investments in unconsolidated affiliates
Total assets	$ 7,000.7	$ 4,831.4
Total liabilities	3,296.0	1,013.5
Total revenues	944.8	1,180.7	$ 893.4
Net income (loss)	(275.4)	708.7	414.5
Total Revenues
Investments in unconsolidated affiliates
Out of period adjustments			0.1
Total Operating Expenses
Investments in unconsolidated affiliates
Out of period adjustments			1.2
Total Net Income (Loss)
Investments in unconsolidated affiliates
Out of period adjustments			(1.1)
White Mountains
Investments in unconsolidated affiliates
Total assets	1,845.7	1,328.5
Total liabilities	373.4	228.7
Total revenues	987.4	526.5	344.6
Total expenses	418.7	325.9	88.3
Net income (loss)	568.7	201.7	255.1
MediaAlpha
Investments in unconsolidated affiliates
Total assets	289.8	210.3
Total liabilities	351.4	315.5
Total revenues	645.3	584.8	408.0
Total expenses	653.8	574.2	390.2
Net income (loss)	$ (8.5)	$ 10.6	$ 17.8