Liability for Losses and Loss Expenses - Reconciliation of Net Incurred and Paid Claims Development Tables to Liability for Claims and Claim Adjustment Expenses (Detail) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Claims Development [Line Items]
Net outstanding liabilities	$ 470,940,000
Reinsurance recoverable	348,332,000
Unallocated loss adjustment expenses	50,402,000
Gross liability for unpaid losses and loss expenses	869,673,849	$ 814,665,224	$ 676,671,727	$ 606,664,590
Personal Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	122,094,000
Reinsurance recoverable	119,364,000
Homeowners [Member]
Claims Development [Line Items]
Net outstanding liabilities	20,762,000
Reinsurance recoverable	10,216,000
Commercial Automobile [Member]
Claims Development [Line Items]
Net outstanding liabilities	118,372,000
Reinsurance recoverable	70,973,000
Commercial MultiPeril [Member]
Claims Development [Line Items]
Net outstanding liabilities	95,153,000
Reinsurance recoverable	58,765,000
Workers Compensation [Member]
Claims Development [Line Items]
Net outstanding liabilities	102,035,000
Reinsurance recoverable	81,837,000
Other [Member]
Claims Development [Line Items]
Net outstanding liabilities	12,524,000
Reinsurance recoverable	$ 7,177,000