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Common Shareholders’ Equity and Non-controlling Interests (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Common Shares Repurchased and Retired [Abstract] |  |  |
Value of common shares repurchased and retired during the period | $ 723.9 | $ 887.2 | $ 284.2
Value of each common share repurchased and retired during the period (in dollars per share) | $ 869 | $ 802 | $ 733
Common Shares Issued [Abstract] |  |  |
Stock issued during period, shares, new issues | 25,086 | 47,030 | 25,016
Shares issued to directors (in shares) | 1,959 | 1,735 | 1,376
Stock issued, in satisfaction of performance shares |  | 5,000 |
Dividends, Common Stock [Abstract] |  |  |
Cash dividends declared and paid | $ 4.6 | $ 5.4 | $ 6.0
Dividends declared and paid (in dollars per share) | $ 1.00 | $ 1.00 | $ 1.00
Non-controlling Equity | $ (131.7) | $ 133.3 |
Total Non-Controlling Excluding Reciprocals |  |  |
Dividends, Common Stock [Abstract] |  |  |
Non-controlling Equity | $ 31.5 | $ 35.2 |
Restricted Stock |  |  |
Common Shares Issued [Abstract] |  |  |
Stock issued during period, shares, new issues | 17,985 | 25,365 | 23,640
Common Stock |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |
Common shares repurchased and retired during the period | 832,725 | 1,106,145 | 387,495
General Board Authorization |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |
Additional authorized repurchase of common shares | 643,130 |  |
Common shares repurchased and retired during the period | 821,732 | 1,098,123 | 361,839
Value of common shares repurchased and retired during the period | $ 713.1 | $ 881.4 | $ 267.4
Value of each common share repurchased and retired during the period (in dollars per share) | $ 870 | $ 803 | $ 739
Chief Executive Officer |  |  |
Common Shares Issued [Abstract] |  |  |
Stock issued during period, shares, new issues | 5,142 | 14,930 |
Repurchased for Employee Benefit Plans |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |
Common shares repurchased and retired during the period | 10,993 | 8,022 | 10,802
OneBeacon Ltd. |  |  |
Dividends, Common Stock [Abstract] |  |  |
Non-controlling Percentage | 0.00% | 23.90% |
Non-controlling Equity | $ 0.0 | $ 244.6 |
HG Global |  |  |
Dividends, Common Stock [Abstract] |  |  |
Non-controlling Percentage | 3.10% | 3.10% |
Non-controlling Equity | $ 15.9 | $ 16.6 |
MediaAlpha |  |  |
Dividends, Common Stock [Abstract] |  |  |
Non-controlling Percentage | 35.70% | 40.00% |
Non-controlling Equity | $ 13.1 | $ 11.7 |
Dewar |  |  |
Dividends, Common Stock [Abstract] |  |  |
Non-controlling Percentage | 0.00% | 18.80% |
Non-controlling Equity | $ 0.0 | $ 3.9 |
Buzzmove |  |  |
Dividends, Common Stock [Abstract] |  |  |
Non-controlling Percentage | 22.90% | 29.10% |
Non-controlling Equity | $ 2.5 | $ 3.0 |
BAM |  |  |
Dividends, Common Stock [Abstract] |  |  |
Non-controlling Percentage | 100.00% | 100.00% |
Non-controlling Equity | $ (163.2) | $ (150.9) |
Star & Shield Insurance Exchange |  |  |
Dividends, Common Stock [Abstract] |  |  |
Non-controlling Percentage | 0.00% | 100.00% |
Non-controlling Equity | $ 0.0 | $ 4.4 |
Total mutuals and reciprocals |  |  |
Dividends, Common Stock [Abstract] |  |  |
Non-controlling Equity | $ (163.2) | $ (146.5) |

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- Definition Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which is directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent (that is, noncontrolling interest, previously referred to as minority interest). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(22)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(24)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.31) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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