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Investments in Unconsolidated Affiliates (Financial Information) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
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Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investments in and Advances to Affiliates |  |  |  |  |  |  |  |  |  |
Total assets |  |  |  | $ 6,520.2 |  |  |  | $ 3,659.2 | $ 6,520.2 |
Total liabilities |  |  |  | 2,804.2 |  |  |  | 298.4 | 2,804.2 |
Revenues | $ 87.5 | $ 83.5 | $ 88.8 | (5.5) | $ 56.8 | $ 48.4 | $ 58.0 | 373.8 | 157.7 | $ 440.0
White Mountains |  |  |  |  |  |  |  |  |  |
Investments in and Advances to Affiliates |  |  |  |  |  |  |  |  |  |
Total assets |  |  |  | 79.4 |  |  |  | 75.4 | 79.4 |
Total liabilities |  |  |  | $ 18.8 |  |  |  | 24.2 | 18.8 |
Revenues |  |  |  |  |  |  |  | 60.0 | 32.9 | 12.9
Expenses |  |  |  |  |  |  |  | $ (66.8) | $ (76.4) | $ (69.9)

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- Definition Sum of the carrying amounts as of the balance sheet date of all liabilities that are recognized. Liabilities are probable future sacrifices of economic benefits arising from present obligations of an entity to transfer assets or provide services to other entities in the future. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19-26) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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