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EMPLOYEE RETIREMENT PLANS (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Pension plans [Member] | Emc Insurance Companies [Member]        
Components of net periodic benefit cost [Abstract]        
Service cost $ 4,213 $ 3,783 $ 12,639 $ 11,351
Interest cost 2,681 2,798 8,044 8,393
Expected return on plan assets (6,013) (5,192) (18,039) (15,574)
Amortization of net actuarial loss 135 911 403 2,732
Amortization of prior service cost (credit) 0 5 0 15
Net periodic benefit cost (income) 1,016 2,305 3,047 6,917
Pension plans [Member] | EMC Insurance Group, Inc. [Member]        
Components of net periodic benefit cost [Abstract]        
Net periodic benefit cost (income) 305 692 914 2,075
Postretirement benefit plans [Member] | Emc Insurance Companies [Member]        
Components of net periodic benefit cost [Abstract]        
Service cost 369 341 1,105 1,022
Interest cost 521 570 1,563 1,710
Expected return on plan assets (1,204) (1,077) (3,611) (3,233)
Amortization of net actuarial loss 234 342 701 1,028
Amortization of prior service cost (credit) (2,783) (2,788) (8,347) (8,365)
Net periodic benefit cost (income) (2,863) (2,612) (8,589) (7,838)
Postretirement benefit plans [Member] | EMC Insurance Group, Inc. [Member]        
Components of net periodic benefit cost [Abstract]        
Net periodic benefit cost (income) (806) $ (736) (2,418) $ (2,208)
Entity's share of estimated contributions to employee benefit plan $ 0   $ 0