LOSS AND LOSS ADJUSTMENT RESERVES (Activity in Liability for Loss and LAE Reserves) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning-of-period	$ 540,367	$ 324,362
Less: reinsurance recoverable	(840,346)	(349,221)	$ (358,128)	$ (164,429)
Net reserves, beginning-of-period	182,239	159,933
Incurred loss, net of reinsurance, related to:
Current year	183,307	291,741
Prior year loss development (redundancy)	1,863	6,912
Ceded losses subject to offsetting experience account adjustments	(68)	(744)
Prior years	1,795	6,168
Amortization of acquisition fair value adjustment	(12)	(47)
Total incurred loss and LAE, net of reinsurance	185,090	297,862
Paid loss, net of reinsurance, related to:
Current year	60,476	143,106
Prior years	109,662	109,930
Total paid loss and LAE, net of reinsurance	170,138	253,036
Net reserves, end-of-period	197,191	204,759
Plus: reinsurance recoverable	840,346	349,221	$ 358,128	$ 164,429
Gross reserves, end-of-period	$ 1,037,537	$ 553,980