Liability for Unpaid Claims and Claim Adjustment Expenses - Summary of Incurred and Paid Claims Development as Supplementary Information - Homeowners (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
Homeowners
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	1,644,632
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	1,565,779
All outstanding liabilities before period one, net of reinsurance	1,050
Liabilities for claims and claim adjustment expenses, net of reinsurance	79,903
Homeowners | Accident Year 2010
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	196,574	$ 196,639	$ 197,067	$ 197,465	$ 197,675	$ 197,443	$ 198,029	$ 198,111	$ 200,318	$ 206,606
IBNR plus expected development	$ 2
Cumulative number of reported claims | claim	37,075
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 196,501	196,480	196,504	196,419	196,281	195,714	195,365	193,006	189,046	$ 149,755
Homeowners | Accident Year 2011
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	197,020	197,070	197,239	197,451	197,381	197,581	198,757	200,356	203,301
IBNR plus expected development	$ 6
Cumulative number of reported claims | claim	38,761
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 196,787	196,757	196,717	196,628	196,575	195,327	194,237	190,946	$ 160,625
Homeowners | Accident Year 2012
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	174,276	174,611	175,032	175,178	175,509	175,523	177,664	181,284
IBNR plus expected development	$ 9
Cumulative number of reported claims | claim	30,997
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 174,256	174,247	174,139	173,676	173,170	171,842	169,415	$ 143,797
Homeowners | Accident Year 2013
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	147,359	147,444	147,927	148,231	149,272	149,080	152,208
IBNR plus expected development	$ 72
Cumulative number of reported claims | claim	20,038
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 147,233	147,145	146,920	146,650	145,152	140,309	$ 115,605
Homeowners | Accident Year 2014
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	130,747	130,895	131,546	130,287	131,634	132,651
IBNR plus expected development	$ 116
Cumulative number of reported claims | claim	18,176
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 130,305	130,059	129,467	126,245	122,601	$ 96,300
Homeowners | Accident Year 2015
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	123,731	123,824	123,619	124,199	125,430
IBNR plus expected development	$ 271
Cumulative number of reported claims | claim	17,744
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 122,955	122,585	119,331	114,696	$ 86,617
Homeowners | Accident Year 2016
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	145,019	144,376	145,373	147,264
IBNR plus expected development	$ 217
Cumulative number of reported claims | claim	21,535
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 144,000	140,972	136,796	$ 105,415
Homeowners | Accident Year 2017
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	172,491	172,274	164,284
IBNR plus expected development	$ 906
Cumulative number of reported claims | claim	23,504
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 166,009	159,107	$ 116,075
Homeowners | Accident Year 2018
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	179,561	174,495
IBNR plus expected development	$ 4,623
Cumulative number of reported claims | claim	22,382
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 165,203	$ 121,631
Homeowners | Accident Year 2019
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	177,854
IBNR plus expected development	$ 22,957
Cumulative number of reported claims | claim	18,757
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 122,530