Equity-Method Eligible Investments - Balance Sheet and Income Statement Tables (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments in unconsolidated affiliates
Total assets	$ 4,831.4				$ 3,983.2				$ 4,831.4	$ 3,983.2
Total liabilities	1,013.5				838.5				1,013.5	838.5
Total revenues	520.8	$ 459.5	$ 259.6	$ (59.2)	175.3	$ 155.5	$ 129.0	$ 433.6	1,180.7	893.4	$ 369.1
MediaAlpha
Investments in unconsolidated affiliates
Total assets	212.7				105.4				212.7	105.4
Total liabilities	315.8				144.9				315.8	144.9
Total revenues									584.8	408.0	296.9
Total expenses									575.4	390.2	278.8
Net income (loss)									9.4	17.8	18.1
White Mountains
Investments in unconsolidated affiliates
Total assets	1,328.5				1,959.2				1,328.5	1,959.2
Total liabilities	$ 228.7				$ 653.2				228.7	653.2
Total revenues									526.5	344.6	226.0
Total expenses									325.9	88.3	141.4
Net income (loss)									$ 201.7	$ 255.1	$ 83.6