INCOME TAXES (Details)	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Deferred tax assets:
Tax discounting of claim reserves	$ 19,143,000	$ 18,327,000
Unearned premium offset	18,755,000	17,864,000
Deferred compensation	2,981,000	2,700,000
Stock option expense	2,728,000	2,702,000
Deferred tax asset lease liabilities	5,140,000
Other	275,000	616,000
Deferred tax assets before allowance	49,022,000	42,209,000
Total deferred tax assets	49,022,000	42,209,000
Deferred tax liabilities:
Net unrealized appreciation of securities	56,532,000	22,177,000
Deferred policy acquisition costs	17,859,000	17,836,000
Deferred tax liability lease assets	4,690,000
Deferred Tax Liabilities Discounting Of Unpaid Losses TCJA Offset	3,817,000	5,203,000
Book/tax depreciation	3,008,000	3,133,000
Intangible assets	1,634,000	1,711,000
Undistributed earnings of unconsolidated investees	17,673,000	15,811,000
Other	536,000	576,000
Total deferred tax liabilities	105,749,000	66,447,000
Net deferred tax liability	$ (56,727,000)	$ (24,238,000)
U.S. federal tax rate (as a percent)	21.00%	21.00%	35.00%
Increase (reduction) in taxes resulting from:
Provision for income taxes at the statutory federal tax rate	$ 48,874,000	$ 14,192,000	$ 29,606,000
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act, Amount		(2,268,000)	(32,821,000)
Excess tax benefit on share-based compensation	(3,958,000)	(4,533,000)	(5,798,000)
Dividends received deduction	(823,000)	(775,000)	(2,025,000)
ESOP dividends paid deduction	(1,122,000)	(1,184,000)	(2,905,000)
Tax-exempt interest income	(1,238,000)	(1,795,000)	(4,671,000)
Unconsolidated investee dividends	(1,802,000)		(1,351,000)
Nondeductible expenses	1,649,000	389,000	276,000
Other items, net	(488,000)	(624,000)	(750,000)
Income tax expense	$ 41,092,000	$ 3,402,000	$ (20,439,000)
Reconciliation of income tax expense rate to the U.S. federal tax rate
U.S. federal tax rate (as a percent)	21.00%	21.00%	35.00%
Enactment of Tax Cuts and Jobs Act (TCJA) (as a percent)		(0.034)	(0.388)
Excess tax benefit on share-based compensation (as a percent)	(1.70%)	(6.70%)	(6.90%)
Effective rate reduction due to dividend received (as a percent)	(0.40%)	(1.10%)	(2.40%)
Effective rate reduction due to dividend paid to ESOP (as a percent)	(0.50%)	(1.80%)	(3.40%)
Effective rate reduction due to tax exempt interest income (as a percent)	(0.50%)	(2.70%)	(5.50%)
Effective rate reduction due to unconsolidated investee dividends (as a percent)	(0.80%)		(1.60%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other (as a percent)	0.70%	0.60%	0.30%
Effective rate reduction due to other items, net (as a percent)	(0.10%)	(0.90%)	(0.90%)
Effective Income Tax Rate Reconciliation, Percent, Total	17.70%	5.00%	(24.20%)
Income tax expense	$ 41,092,000	$ 3,402,000	$ (20,439,000)
Effective tax rates (as a percent)	17.70%	5.00%	(24.20%)
Capital gains rate (as a percent)	21.00%
Tax Cuts and Jobs Act, Measurement Period Adjustment, Income Tax Expense (Benefit)		$ 2,300,000	$ (32,800,000)
Tax Cuts and Jobs Act, Measurement Period Adjustment, Increase (Decrease) in Effective Tax Rate			(0.388)
Dividend received	$ 13,200,000	$ 9,900,000	$ 0
Lower tax rate (as a percent)	7.40%	7.40%
Effective rate reduction due to dividends paid to ESOP (as a percent)	0.50%	1.80%	3.40%
Expected effective tax rate when deferred items reverse in future years (as a percent)	21.00%
Federal and state income taxes paid	$ 25,600,000	$ 16,400,000	$ 10,400,000