Liability for Unpaid Claims and Claim Adjustment Expenses - Summary of Incurred and Paid Claims Development as Supplementary Information - Non-Auto Liability (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
Non-Auto Liability
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	739,007
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	491,273
All outstanding liabilities before period one, net of reinsurance	19,158
Liabilities for claims and claim adjustment expenses, net of reinsurance	266,892
Non-Auto Liability | Accident Year 2010
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	77,215	$ 77,985	$ 78,279	$ 80,920	$ 82,087	$ 82,145	$ 82,287	$ 83,724	$ 85,498	$ 91,191
IBNR plus expected development	$ 1,196
Cumulative number of reported claims | claim	7,873
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 72,941	72,261	71,509	70,369	69,091	65,557	57,354	46,746	31,819	$ 16,473
Non-Auto Liability | Accident Year 2011
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	67,307	67,979	68,248	71,423	73,647	74,372	75,390	76,038	86,409
IBNR plus expected development	$ 1,611
Cumulative number of reported claims | claim	5,708
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 63,706	63,121	62,414	60,706	56,791	52,003	41,814	31,943	$ 13,848
Non-Auto Liability | Accident Year 2012
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	63,336	64,118	63,630	68,017	75,226	78,644	80,470	83,146
IBNR plus expected development	$ 2,380
Cumulative number of reported claims | claim	4,858
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 60,900	59,528	57,863	55,194	49,585	38,826	27,574	$ 13,862
Non-Auto Liability | Accident Year 2013
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	63,284	64,564	65,096	67,841	70,772	75,815	74,183
IBNR plus expected development	$ 2,722
Cumulative number of reported claims | claim	4,542
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 54,323	51,672	47,987	42,504	32,474	22,743	$ 12,794
Non-Auto Liability | Accident Year 2014
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	67,267	67,865	67,339	72,624	75,550	83,084
IBNR plus expected development	$ 3,904
Cumulative number of reported claims | claim	6,047
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 55,307	51,853	45,206	36,220	26,587	$ 11,201
Non-Auto Liability | Accident Year 2015
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	64,189	67,548	76,724	78,968	83,897
IBNR plus expected development	$ 6,682
Cumulative number of reported claims | claim	5,542
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 51,303	46,285	37,059	23,488	$ 11,979
Non-Auto Liability | Accident Year 2016
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	73,195	73,764	83,179	86,935
IBNR plus expected development	$ 12,700
Cumulative number of reported claims | claim	4,385
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 45,820	35,502	24,633	$ 12,733
Non-Auto Liability | Accident Year 2017
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	81,240	88,902	102,616
IBNR plus expected development	$ 16,401
Cumulative number of reported claims | claim	8,021
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 48,654	37,139	$ 14,865
Non-Auto Liability | Accident Year 2018
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	85,910	88,986
IBNR plus expected development	$ 36,008
Cumulative number of reported claims | claim	13,327
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 26,115	$ 13,156
Non-Auto Liability | Accident Year 2019
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	96,064
IBNR plus expected development	$ 57,352
Cumulative number of reported claims | claim	10,158
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 12,204