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Concentration of Credit Risk - Schedule Of Gross Written Premium From Major Brokers (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investment Holdings [Line Items] |  |  |
Gross written premiums, percentage | 100.00% | 100.00% | 100.00%
Gross written premiums | $ 3,360.9 | $ 3,147.0 | $ 2,997.3
Aon Corporation |  |  |
Investment Holdings [Line Items] |  |  |
Gross written premiums, percentage | 16.40% | 18.40% | 18.70%
Marsh & McLennan Companies, Inc. |  |  |
Investment Holdings [Line Items] |  |  |
Gross written premiums, percentage | 16.00% | 14.70% | 15.40%
Willis Group Holdings, Ltd. |  |  |
Investment Holdings [Line Items] |  |  |
Gross written premiums, percentage | 13.10% | 13.70% | 14.50%
Others |  |  |
Investment Holdings [Line Items] |  |  |
Gross written premiums, percentage | 54.50% | 53.20% | 51.40%

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- Definition Amount of direct and assumed premiums written. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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