Liability for Unpaid Claims and Claim Adjustment Expenses - Summary of Incurred and Paid Claims Development as Supplementary Information - 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
Auto Liability
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	2,582,245
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	2,116,479
All outstanding liabilities before period one, net of reinsurance	715
Liabilities for claims and claim adjustment expenses, net of reinsurance	466,481
Auto Liability | Accident Year 2010
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	261,042	$ 261,207	$ 261,092	$ 261,930	$ 263,040	$ 264,104	$ 265,949	$ 266,223	$ 270,935	$ 288,166
IBNR plus expected development	$ 24
Cumulative number of reported claims | claim	47,101
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 260,693	260,404	259,966	259,173	257,021	250,647	237,540	208,531	164,298	$ 92,589
Auto Liability | Accident Year 2011
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	240,650	240,997	241,711	242,619	244,436	244,519	248,865	250,659	263,411
IBNR plus expected development	$ 109
Cumulative number of reported claims | claim	47,074
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 240,085	239,409	238,510	236,187	230,585	217,640	197,326	161,387	$ 93,245
Auto Liability | Accident Year 2012
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	228,486	228,864	228,559	229,426	228,013	231,312	242,255	251,593
IBNR plus expected development	$ 178
Cumulative number of reported claims | claim	44,686
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 222,865	222,117	219,170	214,467	204,980	183,448	150,323	$ 82,531
Auto Liability | Accident Year 2013
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	227,234	226,608	228,285	231,301	233,068	236,432	242,364
IBNR plus expected development	$ 448
Cumulative number of reported claims | claim	38,841
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 223,739	219,303	215,280	204,480	181,535	143,709	$ 79,358
Auto Liability | Accident Year 2014
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	214,557	214,870	215,419	217,819	223,386	232,146
IBNR plus expected development	$ 1,049
Cumulative number of reported claims | claim	35,986
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 209,401	204,057	187,375	166,947	134,376	$ 72,838
Auto Liability | Accident Year 2015
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	244,798	245,775	239,421	240,697	237,578
IBNR plus expected development	$ 2,614
Cumulative number of reported claims | claim	36,066
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 231,530	211,908	186,281	149,366	$ 78,861
Auto Liability | Accident Year 2016
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	259,128	261,400	256,080	259,177
IBNR plus expected development	$ 6,330
Cumulative number of reported claims | claim	37,050
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 225,869	198,326	153,911	$ 86,492
Auto Liability | Accident Year 2017
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	275,850	280,012	269,803
IBNR plus expected development	$ 17,886
Cumulative number of reported claims | claim	38,417
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 218,435	175,175	$ 88,357
Auto Liability | Accident Year 2018
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	299,512	314,467
IBNR plus expected development	$ 44,542
Cumulative number of reported claims | claim	37,354
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 185,317	$ 95,777
Auto Liability | Accident Year 2019
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	330,988
IBNR plus expected development	$ 95,556
Cumulative number of reported claims | claim	33,442
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 98,545