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Loss And Loss Adjustment Expense Reserves - Incurred and Paid Claims (Details) number_of_claims in Thousands, $ in Thousands | Dec. 31, 2019 USD ($) number_of_claims | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2011 USD ($) | Dec. 31, 2010 USD ($)
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Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Net outstanding liabilities | $ 1,745,170 |  |  |  |  |  |  |  |  |
Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 16,007,752 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 14,567,335 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2010, net of reinsurance | (557) |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 1,439,861 |  |  |  |  |  |  |  |  |
Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 2,372,452 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 2,190,296 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2010, net of reinsurance | (576) |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 182,731 |  |  |  |  |  |  |  |  |
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 | 1,388,086 | $ 1,388,055 | $ 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 | $ 43 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 184 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,388,322 | 1,388,140 | 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 2007 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 159,652 | 159,720 | 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 | $ 1 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 21 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 159,491 | 159,435 | 159,268 | 158,943 | 158,053 | 156,853 | 155,191 | 149,084 | 137,628 | $ 95,057
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 | 1,401,151 | 1,401,178 | 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 | $ 73 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 181 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,400,958 | 1,400,441 | 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 2008 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 170,174 | 170,334 | 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 | $ 75 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 23 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 169,899 | 169,757 | 168,914 | 168,621 | 167,806 | 166,375 | 162,870 | 153,845 | $ 111,909 |
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 | 1,439,660 | 1,438,250 | 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 | $ 493 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 181 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,438,686 | 1,434,956 | 1,422,705 | 1,409,911 | 1,372,828 | 1,304,511 | 1,194,648 | 955,647 |  |
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 | 192,790 | 192,905 | 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 | $ 6 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 25 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 192,362 | 191,660 | 190,649 | 190,373 | 188,121 | 182,756 | 175,029 | $ 128,618 |  |
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 | 1,468,237 | 1,468,294 | 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 | $ 1,149 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 185 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,464,277 | 1,460,352 | 1,447,004 | 1,413,999 | 1,340,724 | 1,217,906 | 974,445 |  |  |
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 | 187,147 | 188,014 | 187,165 | 186,795 | 188,026 | 188,915 | 191,903 |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 42 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 23 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 186,494 | 186,132 | 185,168 | 183,860 | 180,858 | 174,295 | $ 133,528 |  |  |
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 | 1,501,075 | 1,498,504 | 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 | $ 3,469 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 180 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,490,366 | 1,476,944 | 1,432,472 | 1,358,472 | 1,231,413 | 967,481 |  |  |  |
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 | 208,003 | 204,986 | 202,513 | 203,218 | 202,621 | 199,298 |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 3,259 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 25 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 203,333 | 202,193 | 198,758 | 194,605 | 186,996 | $ 139,615 |  |  |  |
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 | 1,645,950 | 1,634,435 | 1,610,839 | 1,588,443 | 1,551,105 |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 9,132 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 170 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,614,188 | 1,560,480 | 1,466,368 | 1,336,223 | 1,040,253 |  |  |  |  |
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 | 238,652 | 236,855 | 233,501 | 234,881 | 234,800 |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 1,978 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 24 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 234,683 | 230,480 | 224,178 | 213,994 | $ 163,196 |  |  |  |  |
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 | 1,731,997 | 1,713,696 | 1,669,642 | 1,672,853 |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 22,097 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 154 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,656,192 | 1,554,217 | 1,395,199 | 1,094,006 |  |  |  |  |  |
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 | 259,708 | 259,497 | 259,489 | 250,691 |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 2,092 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 24 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 253,919 | 245,878 | 234,215 | $ 173,537 |  |  |  |  |  |
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 | 1,741,825 | 1,727,277 | 1,703,857 |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 58,648 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 149 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,561,850 | 1,399,202 | 1,076,079 |  |  |  |  |  |  |
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 | 288,322 | 295,163 | 309,491 |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 4,310 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 30 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 278,341 | 269,254 | $ 217,900 |  |  |  |  |  |  |
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 | 1,773,502 | 1,781,817 |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 139,022 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 146 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,417,637 | 1,082,127 |  |  |  |  |  |  |  |
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 | 308,361 | 311,798 |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 15,686 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 25 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 271,534 | $ 213,038 |  |  |  |  |  |  |  |
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 | 1,916,269 |  |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 438,858 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 140 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,134,859 |  |  |  |  |  |  |  |  |
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 | 359,643 |  |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 64,136 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 28 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 240,240 |  |  |  |  |  |  |  |  |

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