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Loss And Loss Adjustment Expense Reserves - Incurred and Paid Claims (Details) number_of_claims in Thousands, $ in Thousands | Dec. 31, 2020 USD ($) number_of_claims | Dec. 31, 2019 USD ($) | 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 ($) | Dec. 31, 2011 USD ($)
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Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Net outstanding liabilities | $ 1,820,077 |  |  |  |  |  |  |  |  |
Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 16,138,435 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 14,695,210 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2011, net of reinsurance | (531) |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 1,442,694 |  |  |  |  |  |  |  |  |
Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 2,647,227 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 2,415,495 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2011, net of reinsurance | 1,457 |  |  |  |  |  |  |  |  |
Net outstanding liabilities | 233,189 |  |  |  |  |  |  |  |  |
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 | 1,401,543 | $ 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 | $ 59 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 181 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,401,592 | 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 2011 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 170,402 | 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 | $ 167 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 23 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 169,958 | 169,899 | 169,757 | 168,914 | 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 | 1,440,537 | 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 | $ 116 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 181 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,440,482 | 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 2012 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 193,002 | 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 | $ 120 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 25 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 192,454 | 192,362 | 191,660 | 190,649 | 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 | 1,468,747 | 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 | $ 816 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 185 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,466,356 | 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 2013 [Member] | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 187,683 | 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 | $ 142 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 23 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 186,816 | 186,494 | 186,132 | 185,168 | 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 | 1,501,713 | 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 | $ 718 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 180 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,496,814 | 1,490,366 | 1,476,944 | 1,432,472 | 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 | 208,743 | 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 | $ 592 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 25 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 207,524 | 203,333 | 202,193 | 198,758 | 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 | 1,649,170 | 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 | $ 4,577 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 170 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,634,780 | 1,614,188 | 1,560,480 | 1,466,368 | 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 | 239,941 | 238,652 | 236,855 | 233,501 | 234,881 | 234,800 |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 2,950 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 24 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 235,971 | 234,683 | 230,480 | 224,178 | 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 | 1,732,410 | 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 | $ 8,206 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 155 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,699,069 | 1,656,192 | 1,554,217 | 1,395,199 | 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 | 260,496 | 259,708 | 259,497 | 259,489 | 250,691 |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 1,821 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 24 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 256,642 | 253,919 | 245,878 | 234,215 | $ 173,537 |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2017 | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,733,425 | 1,741,825 | 1,727,277 | 1,703,857 |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 22,626 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 149 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,648,328 | 1,561,850 | 1,399,202 | 1,076,079 |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2017 | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 289,869 | 288,322 | 295,163 | 309,491 |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 2,702 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 30 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 283,311 | 278,341 | 269,254 | $ 217,900 |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2018 | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,785,071 | 1,773,502 | 1,781,817 |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 60,829 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 146 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,588,049 | 1,417,637 | 1,082,127 |  |  |  |  |  |  |
Short-duration Insurance Contracts, Accident Year 2018 | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 310,695 | 308,361 | 311,798 |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 9,704 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 25 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 286,658 | 271,534 | $ 213,038 |  |  |  |  |  |  |
Short-Duration Insurance Contract, Accident Year 2019 | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,911,268 | 1,916,269 |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 165,108 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 147 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 1,494,342 | 1,134,859 |  |  |  |  |  |  |  |
Short-Duration Insurance Contract, Accident Year 2019 | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 366,139 | 359,643 |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 17,580 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 30 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 324,953 | $ 240,240 |  |  |  |  |  |  |  |
Short-Duration Insurance Contract, Accident Year 2020 | Property insurance product line, automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 1,514,551 |  |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 407,969 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 85 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 825,398 |  |  |  |  |  |  |  |  |
Short-Duration Insurance Contract, Accident Year 2020 | Property insurance product line, homeowners' [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | 420,257 |  |  |  |  |  |  |  |  |
Total of Incurred But Not Reported Liabilities Plus Expected Development on Reported Claims | $ 77,396 |  |  |  |  |  |  |  |  |
Cumulative Number of Reported Claims | number_of_claims | 26 |  |  |  |  |  |  |  |  |
Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance | $ 271,208 |  |  |  |  |  |  |  |  |

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