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Liability for Losses and Loss Expenses - Summary of Cumulative Claim Frequency and Total of Incurred but not Reported Claims plus Expected Development on Reported Claims Incurred (Detail) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
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Personal Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | $ 1,347,094 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2009, net of reinsurance | 719 |  |  |  |  |  |  |  |  |
Liabilities for claims and claims adjustment expenses, net of reinsurance | 122,094 |  |  |  |  |  |  |  |  |
Personal Automobile [Member] | 2010 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 118,119 | $ 118,093 | $ 118,054 | $ 117,978 | $ 117,812 | $ 116,748 | $ 113,243 | $ 107,662 | $ 96,749 | $ 75,889
Personal Automobile [Member] | 2011 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 132,835 | 132,777 | 132,714 | 132,479 | 131,319 | 127,545 | 121,621 | 110,249 | 87,191 |
Personal Automobile [Member] | 2012 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 136,137 | 135,132 | 134,581 | 133,428 | 130,862 | 124,652 | 111,941 | 87,517 |  |
Personal Automobile [Member] | 2013 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 131,642 | 131,326 | 130,026 | 125,946 | 120,118 | 109,051 | 84,241 |  |  |
Personal Automobile [Member] | 2014 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 124,926 | 123,815 | 120,491 | 114,893 | 104,736 | 85,377 |  |  |  |
Personal Automobile [Member] | 2015 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 139,121 | 135,027 | 128,395 | 116,303 | 93,611 |  |  |  |  |
Personal Automobile [Member] | 2016 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 151,159 | 143,321 | 129,507 | 102,433 |  |  |  |  |  |
Personal Automobile [Member] | 2017 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 159,879 | 142,372 | 111,964 |  |  |  |  |  |  |
Personal Automobile [Member] | 2018 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 150,175 | 115,585 |  |  |  |  |  |  |  |
Personal Automobile [Member] | 2019 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 103,101 |  |  |  |  |  |  |  |  |
Homeowners [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 635,976 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2009, net of reinsurance | 30 |  |  |  |  |  |  |  |  |
Liabilities for claims and claims adjustment expenses, net of reinsurance | 20,762 |  |  |  |  |  |  |  |  |
Homeowners [Member] | 2010 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 60,557 | 60,540 | 60,525 | 60,501 | 60,486 | 60,239 | 59,875 | 59,283 | 57,334 | 47,419
Homeowners [Member] | 2011 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 70,692 | 70,692 | 70,648 | 70,626 | 70,541 | 70,351 | 70,125 | 69,345 | 57,588 |
Homeowners [Member] | 2012 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 54,557 | 54,557 | 54,356 | 54,317 | 54,298 | 54,028 | 53,619 | 46,566 |  |
Homeowners [Member] | 2013 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 50,967 | 50,902 | 51,043 | 50,478 | 50,210 | 49,410 | 40,949 |  |  |
Homeowners [Member] | 2014 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 57,243 | 56,995 | 57,195 | 56,990 | 56,255 | 45,823 |  |  |  |
Homeowners [Member] | 2015 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 62,844 | 62,590 | 62,204 | 61,542 | 51,885 |  |  |  |  |
Homeowners [Member] | 2016 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 63,144 | 62,760 | 61,145 | 50,125 |  |  |  |  |  |
Homeowners [Member] | 2017 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 78,006 | 77,663 | 67,077 |  |  |  |  |  |  |
Homeowners [Member] | 2018 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 79,892 | 70,385 |  |  |  |  |  |  |  |
Homeowners [Member] | 2019 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 58,074 |  |  |  |  |  |  |  |  |
Commercial Automobile [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 410,219 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2009, net of reinsurance | 47 |  |  |  |  |  |  |  |  |
Liabilities for claims and claims adjustment expenses, net of reinsurance | 118,372 |  |  |  |  |  |  |  |  |
Commercial Automobile [Member] | 2010 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 21,559 | 21,558 | 21,549 | 21,482 | 21,014 | 20,804 | 19,030 | 16,426 | 14,180 | 10,778
Commercial Automobile [Member] | 2011 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 29,474 | 29,102 | 28,836 | 28,758 | 28,014 | 26,973 | 24,267 | 19,106 | 13,876 |
Commercial Automobile [Member] | 2012 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 31,228 | 31,104 | 30,402 | 29,873 | 27,417 | 23,718 | 20,240 | 13,642 |  |
Commercial Automobile [Member] | 2013 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 36,106 | 36,004 | 34,083 | 31,053 | 26,879 | 23,557 | 16,306 |  |  |
Commercial Automobile [Member] | 2014 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 48,418 | 47,290 | 44,374 | 39,436 | 31,089 | 22,707 |  |  |  |
Commercial Automobile [Member] | 2015 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 51,605 | 48,261 | 41,678 | 35,342 | 23,875 |  |  |  |  |
Commercial Automobile [Member] | 2016 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 57,237 | 48,837 | 38,237 | 27,033 |  |  |  |  |  |
Commercial Automobile [Member] | 2017 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 49,703 | 40,213 | 28,707 |  |  |  |  |  |  |
Commercial Automobile [Member] | 2018 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 47,941 | 33,862 |  |  |  |  |  |  |  |
Commercial Automobile [Member] | 2019 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 36,948 |  |  |  |  |  |  |  |  |
Commercial MultiPeril [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 390,852 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2009, net of reinsurance | 425 |  |  |  |  |  |  |  |  |
Liabilities for claims and claims adjustment expenses, net of reinsurance | 95,153 |  |  |  |  |  |  |  |  |
Commercial MultiPeril [Member] | 2010 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 29,923 | 29,721 | 29,632 | 28,906 | 28,681 | 27,768 | 26,832 | 24,749 | 22,017 | 17,007
Commercial MultiPeril [Member] | 2011 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 38,413 | 38,366 | 38,240 | 37,759 | 36,722 | 33,526 | 30,286 | 24,767 | 18,773 |
Commercial MultiPeril [Member] | 2012 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 36,329 | 35,909 | 35,331 | 33,327 | 29,370 | 26,634 | 23,384 | 16,666 |  |
Commercial MultiPeril [Member] | 2013 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 37,333 | 36,321 | 35,104 | 33,614 | 29,159 | 26,216 | 19,875 |  |  |
Commercial MultiPeril [Member] | 2014 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 51,615 | 50,017 | 47,021 | 40,936 | 35,520 | 27,920 |  |  |  |
Commercial MultiPeril [Member] | 2015 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 42,849 | 39,162 | 34,323 | 29,419 | 21,837 |  |  |  |  |
Commercial MultiPeril [Member] | 2016 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 41,193 | 34,612 | 29,402 | 19,660 |  |  |  |  |  |
Commercial MultiPeril [Member] | 2017 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 42,691 | 36,926 | 27,399 |  |  |  |  |  |  |
Commercial MultiPeril [Member] | 2018 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 42,296 | 30,597 |  |  |  |  |  |  |  |
Commercial MultiPeril [Member] | 2019 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 28,210 |  |  |  |  |  |  |  |  |
Workers Compensation [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 346,316 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2009, net of reinsurance | 3,261 |  |  |  |  |  |  |  |  |
Liabilities for claims and claims adjustment expenses, net of reinsurance | 102,035 |  |  |  |  |  |  |  |  |
Workers Compensation [Member] | 2010 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 26,301 | 26,295 | 26,050 | 25,804 | 25,337 | 24,539 | 23,137 | 21,176 | 15,937 | $ 8,066
Workers Compensation [Member] | 2011 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 34,109 | 33,963 | 33,577 | 33,067 | 32,394 | 31,905 | 27,517 | 21,450 | $ 9,157 |
Workers Compensation [Member] | 2012 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 34,622 | 34,460 | 34,177 | 33,196 | 31,244 | 28,812 | 22,963 | $ 11,097 |  |
Workers Compensation [Member] | 2013 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 40,361 | 39,617 | 38,877 | 36,351 | 32,783 | 26,043 | $ 13,052 |  |  |
Workers Compensation [Member] | 2014 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 43,866 | 42,465 | 40,393 | 36,284 | 28,513 | $ 13,932 |  |  |  |
Workers Compensation [Member] | 2015 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 37,936 | 36,929 | 34,192 | 27,531 | $ 13,071 |  |  |  |  |
Workers Compensation [Member] | 2016 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 40,570 | 37,178 | 30,344 | $ 14,709 |  |  |  |  |  |
Workers Compensation [Member] | 2017 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 39,684 | 31,990 | $ 15,581 |  |  |  |  |  |  |
Workers Compensation [Member] | 2018 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | 31,928 | $ 17,644 |  |  |  |  |  |  |  |
Workers Compensation [Member] | 2019 [Member] |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance | $ 16,939 |  |  |  |  |  |  |  |  |

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