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Concentration of Credit Risk (Tables) | 12 Months Ended
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Dec. 31, 2017
Risks and Uncertainties [Abstract] |
Schedule Of Gross Written Premium From Major Brokers | The following table shows the largest brokers that the Company transacted business within the three years ended December 31, 2017 and the proportion of gross written premiums from each of those brokers.
|  |  | Twelve Months Ended December 31,
|  |  | 2017 |  | 2016 |  | 2015
|  |  | (in percentages)
Aon Corporation |  | 16.4 | % |  | 18.4 | % |  | 18.7 | %
Marsh & McLennan Companies, Inc. |  | 16.0 |  |  | 14.7 |  |  | 15.4 |
Willis Group Holdings, Ltd. |  | 13.1 |  |  | 13.7 |  |  | 14.5 |
Other brokers/non-broker sources(1) |  | 54.5 |  |  | 53.2 |  |  | 51.4 |
Total |  | 100.0 | % |  | 100.0 | % |  | 100.0 | %
Gross written premiums ($ millions) |  | $ | 3,360.9 |  |  | $ | 3,147.0 |  |  | $ | 2,997.3 |
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(1) | No other individual broker accounted for more than 10% of total gross written premiums.

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