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Losses And Loss Adjustment Expense Reserves - Incurred and Paid Claims (Details) $ in Thousands | Dec. 31, 2017 USD ($) number_of_claims | Dec. 31, 2016 USD ($) | 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 ($) | Dec. 31, 2008 USD ($)
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Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Loss and allocated loss adjustment expense reserves, net of reinsurance | $ 1,359,898 |  |  |  |  |  |  |  |  |
Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 15,002,453 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 13,860,983 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2008, net of reinsurance | (169) |  |  |  |  |  |  |  |  |
Loss and allocated loss adjustment expense reserves, net of reinsurance | 1,141,300 |  |  |  |  |  |  |  |  |
Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,987,298 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,852,242 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2008, net of reinsurance | 2,141 |  |  |  |  |  |  |  |  |
Loss and allocated loss adjustment expense reserves, net of reinsurance | 137,195 |  |  |  |  |  |  |  |  |
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 | 1,466,687 | $ 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 | $ 55 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 199,223 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,466,704 | 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 2007 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 139,188 | 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 | $ 0 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 16,497 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 138,963 | 138,922 | 138,809 | 138,680 | 138,137 | 137,284 | 135,500 | 129,821 | 122,239 | $ 86,954
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 | 1,371,812 | 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 | $ 82 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 185,624 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,371,933 | 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 2008 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 134,359 | 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 | $ 26 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 17,411 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 134,229 | 134,301 | 134,378 | 134,381 | 132,180 | 130,928 | 126,591 | 119,306 | $ 86,034 |
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 | 1,388,077 | 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 | $ 158 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 184,135 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,387,835 | 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 2009 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 159,662 | 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 | $ 117 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 20,776 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 159,268 | 158,943 | 158,053 | 156,853 | 155,191 | 149,084 | 137,628 | $ 95,057 |  |
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 | 1,405,112 | 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 | $ 9,367 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 180,857 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,394,684 | 1,391,101 | 1,378,920 | 1,347,082 | 1,277,808 | 1,152,459 | 926,983 |  |  |
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 | 170,126 | 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 | $ 234 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 23,070 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 168,914 | 168,621 | 167,806 | 166,375 | 162,870 | 153,845 | $ 111,909 |  |  |
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 | 1,436,034 | 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 | $ 1,756 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 180,654 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,422,705 | 1,409,911 | 1,372,828 | 1,304,511 | 1,194,648 | 955,647 |  |  |  |
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 | 191,804 | 192,057 | 191,548 | 190,376 | 188,010 | 196,063 |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 428 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 24,621 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 190,649 | 190,373 | 188,121 | 182,756 | 175,029 | $ 128,618 |  |  |  |
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 | 1,464,071 | 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 | $ 4,011 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 184,561 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,447,004 | 1,413,999 | 1,340,724 | 1,217,906 | 974,445 |  |  |  |  |
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 | 187,165 | 186,795 | 188,026 | 188,915 | 191,903 |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 524 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 23,436 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 185,168 | 183,860 | 180,858 | 174,295 | $ 133,528 |  |  |  |  |
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 | 1,486,322 | 1,473,545 | 1,454,366 | 1,467,175 |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 12,959 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 180,410 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,432,472 | 1,358,472 | 1,231,413 | 967,481 |  |  |  |  |  |
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 | 202,513 | 203,218 | 202,621 | 199,298 |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 1,330 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 24,826 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 198,758 | 194,605 | 186,996 | $ 139,615 |  |  |  |  |  |
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 | 1,610,839 | 1,588,443 | 1,551,105 |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 40,444 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 169,997 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,466,368 | 1,336,223 | 1,040,253 |  |  |  |  |  |  |
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 | 233,501 | 234,881 | 234,800 |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 4,368 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 23,955 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 224,178 | 213,994 | $ 163,196 |  |  |  |  |  |  |
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 | 1,669,642 | 1,672,853 |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 102,672 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 153,776 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,395,199 | 1,094,006 |  |  |  |  |  |  |  |
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 | 259,489 | 250,691 |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 9,995 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 24,254 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 234,215 | $ 173,537 |  |  |  |  |  |  |  |
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 | 1,703,857 |  |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 347,296 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 140,219 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,076,079 |  |  |  |  |  |  |  |  |
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 | 309,491 |  |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 47,502 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 28,182 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 217,900 |  |  |  |  |  |  |  |  |

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