Liability for Unpaid Losses and Loss Adjustment Expenses - Summary of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance (Detail) $ in Thousands	Dec. 31, 2020 USD ($) Claim	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Claims Development [Line Items]
Incurred Loss and Defense & Cost Containment Expenses, Net of Reinsurance	$ 2,185,444
Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Loss and Defense & Cost Containment Expenses, Net of Reinsurance	362,155	$ 351,487	$ 324,577	$ 286,252	$ 269,814
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development (Redundancy) on Reported Claims	$ 628
Cumulative Number of Reported Claims | Claim	40,568
Accident Year 2017 [Member]
Claims Development [Line Items]
Incurred Loss and Defense & Cost Containment Expenses, Net of Reinsurance	$ 404,673	375,123	334,734	$ 303,944
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development (Redundancy) on Reported Claims	$ 5,128
Cumulative Number of Reported Claims | Claim	132,576
Accident Year 2018 [Member]
Claims Development [Line Items]
Incurred Loss and Defense & Cost Containment Expenses, Net of Reinsurance	$ 348,792	335,946	$ 334,368
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development (Redundancy) on Reported Claims	$ (1,444)
Cumulative Number of Reported Claims | Claim	54,185
Accident Year 2019 [Member]
Claims Development [Line Items]
Incurred Loss and Defense & Cost Containment Expenses, Net of Reinsurance	$ 452,029	$ 446,419
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development (Redundancy) on Reported Claims	$ (2,589)
Cumulative Number of Reported Claims | Claim	47,437
Accident Year 2020 [Member]
Claims Development [Line Items]
Incurred Loss and Defense & Cost Containment Expenses, Net of Reinsurance	$ 617,795
Total of Incurred-but-Not-Reported Liabilities Plus Expected Development (Redundancy) on Reported Claims	$ 102,141
Cumulative Number of Reported Claims | Claim	76,800