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Dividends-Additional Information (Details) | 12 Months Ended
--- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2016 | Dec. 31, 2015
Dividends Payable [Line Items] |  |  |
Target percentage used to determine annual variable dividend | 33.33% | 33.33% | 33.33%
Tax at statutory federal rate | 35.00% | 35.00% | 35.00%
Gainshare factor used to determine annual variable dividend range, minimum | 0 |  |
Gainshare factor used to determine annual variable dividend range, maximum | 2 |  |
Gainshare factor used to determine annual variable dividend | 1.79 | 1.67 | 1.60
Minimum Payments of Ordinary Dividends, Common Stock | $ 0 |  |
Tax Year 2018 |  |  |
Dividends Payable [Line Items] |  |  |
Tax at statutory federal rate | 21.00% |  |

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- Definition Company performance factor determined by comparing the company's operating performance to certain predetermined profitability and growth objectives approved by the Board of Directors. This factor is used in the computation of dividends payable to shareholders, as well as the variable cash incentive program for employees. + References No definition available. + Details
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- Definition Company Performance Factor Range, Maximum + References No definition available. + Details
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- Definition Dividend computation target rate as percent of underwriting after tax income. This rate is determined by the company's Board of Directors each year and is used in the computation of dividends payable to shareholders as part of the company's annual variable dividend policy. + References No definition available. + Details
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- Definition Percentage of domestic federal statutory tax rate applicable to pretax income (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319 + Details
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