Liability for Losses and Loss Expenses - Summary of Incurred and Paid Claims Development, Net of Reinsurance (Detail) $ in Thousands	Dec. 31, 2018 USD ($) Claims	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Personal Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 1,417,312
Personal Automobile [Member] | 2009 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	106,685	$ 106,713	$ 106,499	$ 106,549	$ 106,705	$ 107,190	$ 107,589	$ 106,841	$ 106,313	$ 105,707
Total IBNR Plus Expected Development on Reported Claims	$ 10
Cumulative Number of Reported Claims | Claims	65
Personal Automobile [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 118,314	118,289	118,385	118,587	118,916	118,876	118,562	117,552	117,967
Total IBNR Plus Expected Development on Reported Claims	$ 35
Cumulative Number of Reported Claims | Claims	70
Personal Automobile [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 133,142	133,145	133,218	133,617	133,229	132,987	131,678	127,929
Total IBNR Plus Expected Development on Reported Claims	$ 76
Cumulative Number of Reported Claims | Claims	74
Personal Automobile [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 136,141	136,463	136,552	136,493	135,592	133,201	130,415
Total IBNR Plus Expected Development on Reported Claims	$ 159
Cumulative Number of Reported Claims | Claims	69
Personal Automobile [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 132,934	132,604	132,643	131,594	130,737	124,965
Total IBNR Plus Expected Development on Reported Claims	$ 254
Cumulative Number of Reported Claims | Claims	66
Personal Automobile [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 126,779	126,200	124,210	124,806	124,426
Total IBNR Plus Expected Development on Reported Claims	$ 539
Cumulative Number of Reported Claims | Claims	71
Personal Automobile [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 142,493	139,181	139,333	137,596
Total IBNR Plus Expected Development on Reported Claims	$ 1,719
Cumulative Number of Reported Claims | Claims	70
Personal Automobile [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 157,516	153,937	150,216
Total IBNR Plus Expected Development on Reported Claims	$ 4,833
Cumulative Number of Reported Claims | Claims	73
Personal Automobile [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 176,728	166,690
Total IBNR Plus Expected Development on Reported Claims	$ 9,368
Cumulative Number of Reported Claims | Claims	79
Personal Automobile [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 186,580
Total IBNR Plus Expected Development on Reported Claims	$ 31,914
Cumulative Number of Reported Claims | Claims	78
Homeowners [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 633,165
Homeowners [Member] | 2009 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 50,435	50,435	50,433	50,419	50,405	50,137	49,998	50,333	50,621	51,054
Cumulative Number of Reported Claims | Claims	18
Homeowners [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 60,624	60,542	60,443	60,440	60,355	59,972	60,248	60,729	60,315
Cumulative Number of Reported Claims | Claims	25
Homeowners [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 70,642	70,479	70,351	70,297	70,381	70,436	70,461	71,256
Cumulative Number of Reported Claims | Claims	27
Homeowners [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 54,523	54,381	54,281	54,351	54,468	54,794	53,962
Total IBNR Plus Expected Development on Reported Claims	$ (2)
Cumulative Number of Reported Claims | Claims	19
Homeowners [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 50,971	50,988	50,874	51,122	51,121	50,887
Total IBNR Plus Expected Development on Reported Claims	$ 8
Cumulative Number of Reported Claims | Claims	13
Homeowners [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 57,288	57,332	57,680	58,378	56,916
Total IBNR Plus Expected Development on Reported Claims	$ 11
Cumulative Number of Reported Claims | Claims	18
Homeowners [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 63,071	63,053	63,925	63,359
Total IBNR Plus Expected Development on Reported Claims	$ 112
Cumulative Number of Reported Claims | Claims	14
Homeowners [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 63,735	64,064	62,443
Total IBNR Plus Expected Development on Reported Claims	$ 365
Cumulative Number of Reported Claims | Claims	13
Homeowners [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 79,911	79,283
Total IBNR Plus Expected Development on Reported Claims	$ 1,211
Cumulative Number of Reported Claims | Claims	18
Homeowners [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 81,965
Total IBNR Plus Expected Development on Reported Claims	$ 4,886
Cumulative Number of Reported Claims | Claims	19
Commercial Automobile [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 455,872
Commercial Automobile [Member] | 2009 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	19,475	19,449	19,461	19,430	19,569	19,346	19,015	18,998	18,549	18,735
Total IBNR Plus Expected Development on Reported Claims	$ 2
Cumulative Number of Reported Claims | Claims	6
Commercial Automobile [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 21,726	21,713	21,746	21,756	21,490	21,477	20,695	19,913	19,315
Total IBNR Plus Expected Development on Reported Claims	$ 9
Cumulative Number of Reported Claims | Claims	7
Commercial Automobile [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 29,751	29,487	29,107	29,112	28,893	28,570	27,157	26,642
Total IBNR Plus Expected Development on Reported Claims	$ 1
Cumulative Number of Reported Claims | Claims	9
Commercial Automobile [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 31,803	31,648	31,278	31,435	30,606	27,720	26,557
Total IBNR Plus Expected Development on Reported Claims	$ 2
Cumulative Number of Reported Claims | Claims	8
Commercial Automobile [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 36,435	36,404	35,240	34,751	33,749	32,902
Total IBNR Plus Expected Development on Reported Claims	$ 76
Cumulative Number of Reported Claims | Claims	9
Commercial Automobile [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 49,284	48,213	47,326	44,544	42,760
Total IBNR Plus Expected Development on Reported Claims	$ 380
Cumulative Number of Reported Claims | Claims	11
Commercial Automobile [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 54,259	51,412	48,323	46,526
Total IBNR Plus Expected Development on Reported Claims	$ 1,440
Cumulative Number of Reported Claims | Claims	12
Commercial Automobile [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 65,905	57,353	54,302
Total IBNR Plus Expected Development on Reported Claims	$ 3,985
Cumulative Number of Reported Claims | Claims	13
Commercial Automobile [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 67,927	61,484
Total IBNR Plus Expected Development on Reported Claims	$ 10,847
Cumulative Number of Reported Claims | Claims	14
Commercial Automobile [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 79,307
Total IBNR Plus Expected Development on Reported Claims	$ 24,366
Cumulative Number of Reported Claims | Claims	15
Commercial Multi-Peril [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 438,538
Commercial Multi-Peril [Member] | 2009 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 27,590	27,508	27,484	27,625	27,959	27,739	27,357	27,357	26,454	26,712
Cumulative Number of Reported Claims | Claims	6
Commercial Multi-Peril [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 29,925	29,779	29,463	29,453	29,373	29,169	29,390	29,656	28,745
Cumulative Number of Reported Claims | Claims	6
Commercial Multi-Peril [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 38,160	38,105	38,270	37,385	37,576	35,942	35,411	33,054
Cumulative Number of Reported Claims | Claims	7
Commercial Multi-Peril [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 36,525	36,214	35,755	34,117	32,449	30,716	29,789
Total IBNR Plus Expected Development on Reported Claims	$ 4
Cumulative Number of Reported Claims | Claims	6
Commercial Multi-Peril [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 37,365	37,981	37,205	37,292	35,679	35,683
Total IBNR Plus Expected Development on Reported Claims	$ 2
Cumulative Number of Reported Claims | Claims	6
Commercial Multi-Peril [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 53,294	52,336	51,843	50,135	48,204
Total IBNR Plus Expected Development on Reported Claims	$ 204
Cumulative Number of Reported Claims | Claims	7
Commercial Multi-Peril [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 45,104	44,728	43,874	42,070
Total IBNR Plus Expected Development on Reported Claims	$ 1,540
Cumulative Number of Reported Claims | Claims	6
Commercial Multi-Peril [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 48,267	46,988	43,005
Total IBNR Plus Expected Development on Reported Claims	$ 3,652
Cumulative Number of Reported Claims | Claims	6
Commercial Multi-Peril [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 56,043	56,185
Total IBNR Plus Expected Development on Reported Claims	$ 8,127
Cumulative Number of Reported Claims | Claims	6
Commercial Multi-Peril [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 66,265
Total IBNR Plus Expected Development on Reported Claims	$ 17,175
Cumulative Number of Reported Claims | Claims	6
Workers' Compensation [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 421,246
Workers' Compensation [Member] | 2009 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	22,053	22,079	22,114	22,172	22,278	22,848	21,826	21,894	22,497	$ 21,571
Total IBNR Plus Expected Development on Reported Claims	$ 32
Cumulative Number of Reported Claims | Claims	4
Workers' Compensation [Member] | 2010 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 26,801	26,984	27,037	27,121	26,944	26,637	27,010	27,859	$ 27,304
Total IBNR Plus Expected Development on Reported Claims	$ 63
Cumulative Number of Reported Claims | Claims	5
Workers' Compensation [Member] | 2011 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 34,926	35,194	35,039	35,670	36,369	36,614	35,757	$ 32,490
Total IBNR Plus Expected Development on Reported Claims	$ 82
Cumulative Number of Reported Claims | Claims	6
Workers' Compensation [Member] | 2012 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 35,954	36,468	37,163	37,926	38,827	39,516	$ 39,142
Total IBNR Plus Expected Development on Reported Claims	$ 132
Cumulative Number of Reported Claims | Claims	6
Workers' Compensation [Member] | 2013 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 42,555	42,327	42,828	44,289	47,027	$ 46,325
Total IBNR Plus Expected Development on Reported Claims	$ 307
Cumulative Number of Reported Claims | Claims	6
Workers' Compensation [Member] | 2014 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 47,540	48,537	49,288	51,553	$ 51,508
Total IBNR Plus Expected Development on Reported Claims	$ 431
Cumulative Number of Reported Claims | Claims	6
Workers' Compensation [Member] | 2015 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 44,986	45,991	49,615	$ 53,332
Total IBNR Plus Expected Development on Reported Claims	$ 1,613
Cumulative Number of Reported Claims | Claims	6
Workers' Compensation [Member] | 2016 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 47,883	49,802	$ 58,814
Total IBNR Plus Expected Development on Reported Claims	$ 2,869
Cumulative Number of Reported Claims | Claims	6
Workers' Compensation [Member] | 2017 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 56,351	$ 60,450
Total IBNR Plus Expected Development on Reported Claims	$ 7,290
Cumulative Number of Reported Claims | Claims	6
Workers' Compensation [Member] | Short-duration Insurance Contracts, Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 62,197
Total IBNR Plus Expected Development on Reported Claims	$ 19,729
Cumulative Number of Reported Claims | Claims	6