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Losses And Loss Adjustment Expense Reserves - Incurred and Paid Claims (Details) number_of_claims in Thousands, $ in Thousands | Dec. 31, 2016 USD ($) number_of_claims | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2011 USD ($) | Dec. 31, 2010 USD ($) | Dec. 31, 2009 USD ($) | Dec. 31, 2008 USD ($) | Dec. 31, 2007 USD ($)
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Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Loss and allocated loss adjustment expense reserves, net of reinsurance | $ 1,196,434 |  |  |  |  |  |  |  |  |
Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 14,879,081 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 13,862,037 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2007, net of reinsurance | (212) |  |  |  |  |  |  |  |  |
Loss and allocated loss adjustment expense reserves, net of reinsurance | 1,016,832 |  |  |  |  |  |  |  |  |
Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,780,518 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,667,129 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2007, net of reinsurance | 2,661 |  |  |  |  |  |  |  |  |
Loss and allocated loss adjustment expense reserves, net of reinsurance | 116,050 |  |  |  |  |  |  |  |  |
Short-duration insurance contracts, accident year 2007 [Member] | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,636,065 | $ 1,636,379 | $ 1,635,529 | $ 1,634,127 | $ 1,633,169 | $ 1,633,606 | $ 1,624,875 | $ 1,614,648 | $ 1,610,874 | $ 1,591,217
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 52 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 230 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,636,204 | 1,635,631 | 1,634,888 | 1,633,178 | 1,628,413 | 1,615,964 | 1,581,001 | 1,496,543 | 1,365,854 | 1,040,933
Short-duration insurance contracts, accident year 2007 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 110,204 | 110,142 | 110,161 | 110,092 | 110,052 | 109,836 | 111,378 | 112,042 | 116,950 | 119,505
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 42 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 14 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 110,075 | 110,064 | 109,949 | 109,932 | 109,874 | 109,305 | 108,404 | 104,340 | 100,774 | $ 71,165
Short-duration insurance contracts, accident year 2008 [Member] | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,466,137 | 1,466,108 | 1,465,623 | 1,463,659 | 1,461,084 | 1,455,858 | 1,442,691 | 1,440,301 | 1,505,732 |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 82 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 199 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,465,832 | 1,464,763 | 1,463,384 | 1,459,216 | 1,450,172 | 1,418,274 | 1,345,354 | 1,226,787 | 992,844 |
Short-duration insurance contracts, accident year 2008 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 139,216 | 139,156 | 139,149 | 138,803 | 138,190 | 139,142 | 138,605 | 139,549 | 146,486 |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 5 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 16 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 138,922 | 138,809 | 138,680 | 138,137 | 137,284 | 135,500 | 129,821 | 122,239 | $ 86,954 |
Short-duration insurance contracts, accident year 2009 [Member] | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,372,259 | 1,372,694 | 1,371,779 | 1,365,551 | 1,361,652 | 1,361,116 | 1,349,025 | 1,372,833 |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 166 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 186 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,371,956 | 1,370,564 | 1,363,526 | 1,355,210 | 1,326,439 | 1,260,424 | 1,137,807 | 913,340 |  |
Short-duration insurance contracts, accident year 2009 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 134,478 | 134,597 | 134,718 | 133,121 | 133,087 | 131,680 | 135,000 | 135,889 |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ (3) |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 17 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 134,301 | 134,378 | 134,381 | 132,180 | 130,928 | 126,591 | 119,306 | $ 86,034 |  |
Short-duration insurance contracts, accident year 2010 [Member] | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,386,105 | 1,381,056 | 1,379,336 | 1,374,638 | 1,364,307 | 1,357,750 | 1,367,547 |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 390 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 184 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,384,333 | 1,375,799 | 1,365,464 | 1,335,871 | 1,268,142 | 1,143,984 | 908,954 |  |  |
Short-duration insurance contracts, accident year 2010 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 159,608 | 160,015 | 160,206 | 160,472 | 160,983 | 157,566 | 165,727 |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ (17) |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 21 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 158,943 | 158,053 | 156,853 | 155,191 | 149,084 | 137,628 | $ 95,057 |  |  |
Short-duration insurance contracts, accident year 2011 [Member] | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,398,518 | 1,393,878 | 1,388,363 | 1,380,557 | 1,367,000 | 1,343,919 |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 4,069 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 181 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,391,101 | 1,378,920 | 1,347,082 | 1,277,808 | 1,152,459 | 926,983 |  |  |  |
Short-duration insurance contracts, accident year 2011 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 169,621 | 169,390 | 169,600 | 170,052 | 170,623 | 167,414 |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 325 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 23 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 168,621 | 167,806 | 166,375 | 162,870 | 153,845 | $ 111,909 |  |  |  |
Short-duration insurance contracts, accident year 2012 [Member] | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,426,735 | 1,414,878 | 1,409,104 | 1,408,222 | 1,424,754 |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 4,531 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 181 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,409,911 | 1,372,828 | 1,304,511 | 1,194,648 | 955,647 |  |  |  |  |
Short-duration insurance contracts, accident year 2012 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 192,057 | 191,548 | 190,376 | 188,010 | 196,063 |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 585 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 25 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 190,373 | 188,121 | 182,756 | 175,029 | $ 128,618 |  |  |  |  |
Short-duration insurance contracts, accident year 2013 [Member] | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,458,421 | 1,447,881 | 1,431,058 | 1,448,567 |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 10,873 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 185 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,413,999 | 1,340,724 | 1,217,906 | 974,445 |  |  |  |  |  |
Short-duration insurance contracts, accident year 2013 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 186,795 | 188,026 | 188,915 | 191,903 |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 775 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 23 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 183,860 | 180,858 | 174,295 | $ 133,528 |  |  |  |  |  |
Short-duration insurance contracts, accident year 2014 [Member] | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,473,545 | 1,454,366 | 1,467,175 |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 32,532 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 180 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,358,472 | 1,231,413 | 967,481 |  |  |  |  |  |  |
Short-duration insurance contracts, accident year 2014 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 203,218 | 202,621 | 199,298 |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 2,993 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 25 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 194,605 | 186,996 | $ 139,615 |  |  |  |  |  |  |
Short-duration insurance contracts, accident year 2015 [Member] | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,588,443 | 1,551,105 |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 90,010 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 169 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,336,223 | 1,040,253 |  |  |  |  |  |  |  |
Short-duration insurance contracts, accident year 2015 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 234,881 | 234,800 |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 9,431 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 24 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 213,994 | $ 163,196 |  |  |  |  |  |  |  |
Short-duration insurance contracts, accident year 2016 [Member] | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,672,853 |  |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 303,353 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 145 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,094,006 |  |  |  |  |  |  |  |  |
Short-duration insurance contracts, accident year 2016 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 250,440 |  |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 36,538 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 23 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 173,435 |  |  |  |  |  |  |  |  |

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