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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, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Investment Holdings [Line Items] |  |  |
Gross written premiums, percentage | 100.00% | 100.00% | 100.00%
Gross written premiums | $ 3,442.4 | $ 3,446.9 | $ 3,360.9
Aon Corporation (1) |  |  |
Investment Holdings [Line Items] |  |  |
Gross written premiums, percentage | 13.40% | 15.80% | 16.40%
Marsh & McLennan Companies, Inc. |  |  |
Investment Holdings [Line Items] |  |  |
Gross written premiums, percentage | 13.60% | 15.80% | 16.00%
Willis Group Holdings, Ltd. |  |  |
Investment Holdings [Line Items] |  |  |
Gross written premiums, percentage | 10.30% | 12.40% | 13.10%
Others |  |  |
Investment Holdings [Line Items] |  |  |
Gross written premiums, percentage | 62.70% | 56.00% | 54.50%

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- Definition Amount of direct and assumed premiums written. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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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