Reserve for Known and Incurred but Not Reported Claims - Summary of Incurred and Paid Claims Development Net of Reinsurance (Detail) $ in Thousands	Dec. 31, 2019 USD ($) Claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	[1]	Dec. 31, 2014 USD ($)	[1]	Dec. 31, 2013 USD ($)	[1]	Dec. 31, 2012 USD ($)	[1]	Dec. 31, 2011 USD ($)	[1]	Dec. 31, 2010 USD ($)	[1]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	$ 2,105,624
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	2,037,424
All outstanding liabilities before 2010, net of reinsurance	0
Liabilities for claims and claims adjustment expenses, net of reinsurance	68,200
Accident Year 2010 [Member]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	140,345	$ 140,324	$ 140,308	$ 140,353	$ 140,641		$ 140,128		$ 139,639		$ 139,991		$ 139,966		$ 140,621
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of reported claims | Claim	606
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 140,325	140,321	140,304	140,302	140,886		140,136		139,486		138,978		136,770		$ 113,513
Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	149,486	149,487	149,488	149,552	149,763		149,486		149,768		149,076		148,395
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of reported claims | Claim	641
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 149,486	149,486	149,485	149,495	149,358		148,984		146,952		144,367		$ 123,116
Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	161,634	161,650	160,911	160,517	160,579		159,918		158,981		157,287
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of reported claims | Claim	692
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 161,617	161,304	160,268	159,740	159,181		157,364		153,753		$ 130,623
Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	184,606	184,777	184,668	184,826	185,244		184,419		182,858
Total of IBNR liabilities plus expected development on reported claims	$ 27
Cumulative number of reported claims | Claim	762
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 184,552	184,711	184,473	183,957	182,565		180,277		$ 151,377
Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	191,218	190,944	191,025	191,120	190,738		190,985
Total of IBNR liabilities plus expected development on reported claims	$ 129
Cumulative number of reported claims | Claim	789
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 190,772	190,398	189,525	188,117	185,686		$ 156,536
Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	226,882	226,555	225,977	225,754	221,617
Total of IBNR liabilities plus expected development on reported claims	$ 349
Cumulative number of reported claims | Claim	867
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 226,201	225,041	223,045	217,618	$ 181,445
Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	253,258	251,506	249,358	245,859
Total of IBNR liabilities plus expected development on reported claims	$ 975
Cumulative number of reported claims | Claim	972
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 250,867	248,211	243,111	$ 205,857
Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	278,005	275,480	267,392
Total of IBNR liabilities plus expected development on reported claims	$ 3,101
Cumulative number of reported claims | Claim	1,014
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 270,705	266,653	$ 220,218
Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	268,931	264,088
Total of IBNR liabilities plus expected development on reported claims	$ 5,312
Cumulative number of reported claims | Claim	1,063
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 255,557	$ 222,966
Accident Year 2019 [Member]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	251,259
Total of IBNR liabilities plus expected development on reported claims	$ 8,661
Cumulative number of reported claims | Claim	1,078
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 207,342

[1] Amounts unaudited