SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)			9 Months Ended	12 Months Ended
	Mar. 27, 2020 USD ($)	Sep. 18, 2017 segment	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) segment state plan	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2021 USD ($)	Dec. 15, 2020 USD ($)	Dec. 31, 2017 USD ($)
Item Effected [Line Items]
Number of operating segments | segment		2		1
Percentage of the gains and profits from the Lloyds plan available to United Fire & Indemnity Company				100.00%
Securities on deposit as required by law				$ 21,628,000	$ 20,816,000
Payment for income taxes				138,000	1,575,000	$ 29,071,000
Federal tax refund received				0	5,401,000	1,503,000
Interest payment				0	0	0
Depreciation expense				5,947,000	10,482,000	4,455,000
Goodwill impairment			$ 15,091,000	15,091,000	0	0
Income tax benefit, CARES Act	$ 18,600,000
Unrecognized tax benefit				0	0	0
Assets				$ 3,069,678,000	3,013,472,000
Number of equity compensation plans | plan				2
Investment income accrual period				6 months
Credit loss allowance for reinsurance receivables				$ 190,000	0		$ 38,000
Cumulative effect adjustment				825,149,000	910,472,000	888,375,000			$ 973,373,000
Retaining earnings
Item Effected [Line Items]
Cumulative effect adjustment				555,854,000	697,116,000	715,472,000			608,700,000
Cumulative effect of change in accounting principle
Item Effected [Line Items]
Cumulative effect adjustment					(30,000)	(514,000)			0
Cumulative effect of change in accounting principle | Retaining earnings
Item Effected [Line Items]
Cumulative effect adjustment					(30,000)	(514,000)			$ 191,244,000
Post-retirement Benefits
Item Effected [Line Items]
Benefit obligation				31,666,000	31,001,000	30,973,000
Variable Interest Entity, Not Primary Beneficiary
Item Effected [Line Items]
Assets				3,300,000
Surplus Notes
Item Effected [Line Items]
Principal amount								$ 50,000,000
Debt issuance costs				24,000
Interest expense incurred				133,000
Surplus Notes | Federated Mutual
Item Effected [Line Items]
Principal amount				35,000,000
Surplus Notes | Federated Life
Item Effected [Line Items]
Principal amount				$ 15,000,000
Minimum
Item Effected [Line Items]
Useful life				2 years
Maximum
Item Effected [Line Items]
Useful life				15 years
Property and casualty business
Item Effected [Line Items]
Number of states in which we are licensed as insurer | state				49
Amortization expense				$ 709,000	$ 709,000	$ 719,000