Liability for Unpaid Claims and Claim Adjustment Expenses - Summary of Incurred and Paid Claims Development as Supplementary Information - Commercial Multi-Peril (Detail) $ in Thousands	Dec. 31, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	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 ($)	Dec. 31, 2010 USD ($)
Claims Development [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	$ 995,986
Commercial Multi-Peril
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	362,832
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	269,890
All outstanding liabilities before period one, net of reinsurance	1,573
Liabilities for claims and claim adjustment expenses, net of reinsurance	94,515
Commercial Multi-Peril | Accident Year 2010
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	35,999	$ 35,923	$ 36,367	$ 37,215	$ 37,839	$ 35,914	$ 37,520	$ 40,243	$ 37,736	$ 41,116
IBNR plus expected development	$ 9
Cumulative number of reported claims | claim	3,592
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 35,881	34,903	34,764	34,888	33,384	31,378	27,152	22,135	17,490	$ 12,511
Commercial Multi-Peril | Accident Year 2011
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	35,964	36,638	37,014	36,945	38,456	38,160	39,037	40,825	42,185
IBNR plus expected development	$ 35
Cumulative number of reported claims | claim	3,563
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 34,939	34,782	34,177	32,692	30,458	28,291	22,616	18,390	$ 13,092
Commercial Multi-Peril | Accident Year 2012
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	22,045	22,481	23,090	23,385	23,258	26,805	28,548	35,169
IBNR plus expected development	$ 67
Cumulative number of reported claims | claim	2,717
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 21,415	21,352	21,026	20,716	18,670	16,263	14,454	$ 11,525
Commercial Multi-Peril | Accident Year 2013
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	24,790	26,028	25,948	26,970	27,867	27,592	33,979
IBNR plus expected development	$ 315
Cumulative number of reported claims | claim	2,226
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 23,210	21,718	20,816	18,406	15,426	12,723	$ 9,374
Commercial Multi-Peril | Accident Year 2014
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	34,279	36,132	33,962	34,911	31,220	36,852
IBNR plus expected development	$ 580
Cumulative number of reported claims | claim	2,308
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 31,448	27,339	24,602	20,199	16,484	$ 12,001
Commercial Multi-Peril | Accident Year 2015
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	31,285	32,282	29,023	31,488	33,997
IBNR plus expected development	$ 751
Cumulative number of reported claims | claim	2,226
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 25,188	21,680	16,402	12,956	$ 9,820
Commercial Multi-Peril | Accident Year 2016
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	31,615	33,080	33,475	38,115
IBNR plus expected development	$ 2,302
Cumulative number of reported claims | claim	4,775
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 22,339	19,085	17,193	$ 11,327
Commercial Multi-Peril | Accident Year 2017
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	40,611	37,079	42,411
IBNR plus expected development	$ 6,851
Cumulative number of reported claims | claim	6,737
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 23,294	20,828	$ 12,458
Commercial Multi-Peril | Accident Year 2018
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	50,182	50,784
IBNR plus expected development	$ 12,923
Cumulative number of reported claims | claim	5,455
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 30,078	$ 18,027
Commercial Multi-Peril | Accident Year 2019
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	56,062
IBNR plus expected development	$ 25,359
Cumulative number of reported claims | claim	2,928
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 22,098