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Segment Information (Net Premiums Earned By Geographic Region) (Details) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting Information [Line Items] |  |  |  |
Net Premiums Earned by Geographic Region |  |
| North America |  |  | Europe (1) |  |  | Asia Pacific / Far East |  |  | Latin America |
2017 | 70 | % |  | 11 | % |  | 12 | % |  | 7 | %
2016 | 70 | % |  | 12 | % |  | 11 | % |  | 7 | %
2015 | 60 | % |  | 15 | % |  | 15 | % |  | 10 | %
North America [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Percentage of net premiums earned by geographic region |  | 70.00% | 70.00% | 60.00%
Europe [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Percentage of net premiums earned by geographic region | [1] | 11.00% | 12.00% | 15.00%
Asia Pacific and Far East [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Percentage of net premiums earned by geographic region |  | 12.00% | 11.00% | 15.00%
Latin America [Member] |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |
Percentage of net premiums earned by geographic region |  | 7.00% | 7.00% | 10.00%
[1] | (1) Europe includes Eurasia and Africa region.

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- Definition Tabular disclosure of the names of foreign countries from which revenue is material and the amount of revenue from external customers attributed to those countries. An entity may also provide subtotals of geographic information about groups of countries. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 280 -SubTopic 10 -Section 50 -Paragraph 41 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e9038-108599 Reference 2: 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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