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Equity-Method Eligible Investments (Balance Sheet and Income Statement Tables) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investments in unconsolidated affiliates
Assets	$ 3,983.2				$ 3,362.6				$ 3,983.2	$ 3,362.6
Liabilities	838.5				644.4				838.5	644.4
Revenues	175.3	$ 155.5	$ 129.0	$ 433.6	6.0	$ 198.7	$ 122.3	$ 42.1	893.4	369.1	$ 373.8
White Mountains
Investments in unconsolidated affiliates
Assets	1,959.2				1,373.3				1,959.2	1,373.3
Liabilities	653.2				509.2				653.2	509.2
Revenues									344.6	226.0	143.3
Expenses									88.3	141.4	80.5
Net Income									255.1	83.6	65.3
MediaAlpha
Investments in unconsolidated affiliates
Assets	106.9				88.5				106.9	88.5
Liabilities	$ 144.6				$ 46.9				144.6	46.9
Revenues									407.9	297.1	163.2
Expenses									389.0	288.2	163.5
Net Income									$ 18.9	$ 8.9	$ (0.3)