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Investments in Unconsolidated Affiliates (Balance Sheet and Income Statement Tables) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments in unconsolidated affiliates
Assets	$ 3,362.6				$ 3,659.2				$ 3,362.6	$ 3,659.2
Liabilities	644.4				298.4				644.4	298.4
Revenues	6.0	$ 198.7	$ 122.3	$ 42.1	114.0	$ 87.5	$ 83.5	$ 88.8	369.1	373.8	$ 157.7
White Mountains
Investments in unconsolidated affiliates
Assets	218.8				75.4				218.8	75.4
Liabilities	$ 46.7				$ 24.2				46.7	24.2
Revenues									134.1	60.0	32.9
Expenses									$ (110.1)	$ (66.8)	$ (76.4)

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