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EMPLOYEE RETIREMENT PLANS (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]          
Percentage of pool participation (in hundredths) 30.00% 30.00% 30.00%    
Other benefit plans expenses (including 401(k)) allocated to the Company $ 2,900 $ 2,700 $ 2,500    
Pension Plans [Member] | EMC Insurance Group, Inc. [Member]          
ASSETS          
Prepaid pension and postretirement benefits       $ 16,327 $ 9,065
Liability [Abstract]          
Pension and postretirement benefits       (4,185) (4,097)
Net amount recognized       12,142 4,968
Amounts recognized in the Company's consolidated balance sheets under the caption "accumulated other comprehensive income", before deferred income taxes [Abstract]          
Net actuarial loss       (11,598) (18,927)
Prior service (cost) credit       0 (6)
Net amount recognized       (11,598) (18,933)
Amounts that will be amortized from accumulated other comprehensive income (loss) in next fiscal year [Abstract]          
Amortization of net actuarial loss       150  
Amortization of prior service cost (credit)       0  
Amounts recognized in the Company's consolidated statements of comprehensive income [Abstract]          
Net actuarial gain (loss) 7,329 1,174      
Prior service (cost) credit 6 9      
Net amount recognized 7,335 1,183      
Components of net periodic benefit cost [Abstract]          
Net periodic benefit cost (income) 2,800 2,900 1,800    
Pension Plans [Member] | EMC Insurance Companies [Member]          
Change in projected benefit obligation [Abstract]          
Benefit obligation at beginning of year 284,194 269,904      
Service cost 15,135 14,432 13,962    
Interest cost 11,190 10,161 9,311    
Actuarial loss 12,577 5,361      
Benefits paid (14,852) (15,664)      
Medicare subsidy reimbursements 0 0      
Plan amendments 0 0      
Projected benefit obligation at end of year 308,244 284,194 269,904    
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 300,915 283,231      
Actual return on plan assets 54,255 23,081      
Employer contributions 8,686 10,267      
Benefits paid (14,852) (15,664)      
Fair value of plan assets at end of year 349,004 300,915 283,231    
Funded status       $ 40,760 $ 16,721
Components of net periodic benefit cost [Abstract]          
Service cost 15,135 14,432 13,962    
Interest cost 11,190 10,161 9,311    
Expected return on plan assets (20,765) (19,361) (20,298)    
Amortization of net actuarial loss 3,643 4,311 2,710    
Amortization of prior service cost (credit) 20 31 31    
Net periodic benefit cost (income) $ 9,223 $ 9,574 $ 5,716    
Weighted-average assumptions used to measure the benefit obligations [Abstract]          
Discount rate (in hundredths)       3.60% 4.07%
Weighted-average assumptions used to measure the net periodic benefit costs [Abstract]          
Discount rate (in hundredths) 4.07% 3.90% 3.57%    
Expected long-term rate of return on plan assets (in hundredths) 7.00% 7.00% 7.00%    
Expected future benefit payments to be paid from the plans over the next ten years [Abstract]          
2018       $ 20,276  
2019       22,628  
2020       23,050  
2021       19,841  
2022       22,655  
2023 - 2027       119,538  
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure $ 300,915 $ 283,231 $ 283,231 $ 349,004 $ 300,915
Qualified Pension Plan [Member] | EMC Insurance Companies [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Defined benefit plans, vest years of service 3 years        
Defined benefit plan, age of attainment years 55 years        
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year $ 300,915        
Fair value of plan assets at end of year $ 349,004 $ 300,915      
Weighted-average assumptions used to measure the benefit obligations [Abstract]          
Rate of compensation increase (in hundredths)       5.09% 5.07%
Weighted-average assumptions used to measure the net periodic benefit costs [Abstract]          
Rate of compensation increase (in hundredths) 5.08% 5.07% 4.73%    
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure $ 300,915 $ 300,915   $ 349,004 $ 300,915
Defined Benefit Plan, Expected Future Employer Contributions [Abstract]          
Defined benefit plan estimated future parent total contributions next fiscal year       8,000  
Qualified Pension Plan [Member] | EMC Insurance Companies [Member] | Pooled Separate Accounts [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 300,915        
Fair value of plan assets at end of year 349,004 300,915      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure $ 300,915 $ 300,915   349,004 300,915
Non-Qualified Pension Plan [Member] | EMC Insurance Companies [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Defined benefit plans, vest years of service 3 years        
Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]          
Projected benefit obligation       13,950 13,656
Accumulated benefit obligation       12,272 12,182
Fair value of plan assets       $ 0 $ 0
Weighted-average assumptions used to measure the benefit obligations [Abstract]          
Rate of compensation increase (in hundredths)       4.45% 4.53%
Weighted-average assumptions used to measure the net periodic benefit costs [Abstract]          
Rate of compensation increase (in hundredths) 4.47% 4.56% 4.68%    
Postretirement Benefit Plans [Member] | EMC Insurance Group, Inc. [Member]          
ASSETS          
Prepaid pension and postretirement benefits       $ 4,356 $ 3,249
Liability [Abstract]          
Pension and postretirement benefits       0 0
Net amount recognized       4,356 3,249
Amounts recognized in the Company's consolidated balance sheets under the caption "accumulated other comprehensive income", before deferred income taxes [Abstract]          
Net actuarial loss       (4,950) (6,147)
Prior service (cost) credit       16,407 19,441
Net amount recognized       11,457 13,294
Amounts that will be amortized from accumulated other comprehensive income (loss) in next fiscal year [Abstract]          
Amortization of net actuarial loss       260  
Amortization of prior service cost (credit)       (3,200)  
Amounts recognized in the Company's consolidated statements of comprehensive income [Abstract]          
Net actuarial gain (loss) $ 1,197 $ 376      
Prior service (cost) credit (3,034) (4,221)      
Net amount recognized (1,837) (3,845)      
Components of net periodic benefit cost [Abstract]          
Net periodic benefit cost (income) $ (2,900) (3,000) $ (3,000)    
Postretirement Benefit Plans [Member] | EMC Insurance Companies [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Defined benefit plan, age of attainment years 55 years        
Change in projected benefit obligation [Abstract]          
Benefit obligation at beginning of year $ 55,651 51,449      
Service cost 1,362 1,273 1,411    
Interest cost 2,281 2,215 2,148    
Actuarial loss 3,278 357      
Benefits paid (2,455) (2,377)      
Medicare subsidy reimbursements 0 553      
Plan amendments $ (1,113) 2,181      
Amortization period for prior service credit from plan amendment 10 years        
Projected benefit obligation at end of year $ 59,004 55,651 51,449    
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 67,809 66,320      
Actual return on plan assets 10,304 3,866      
Employer contributions 0 0      
Benefits paid (2,455) (2,377)      
Fair value of plan assets at end of year 75,658 67,809 66,320    
Funded status       $ 16,654 $ 12,158
Components of net periodic benefit cost [Abstract]          
Service cost 1,362 1,273 1,411    
Interest cost 2,281 2,215 2,148    
Expected return on plan assets (4,311) (4,224) (4,416)    
Amortization of net actuarial loss 1,371 1,494 1,745    
Amortization of prior service cost (credit) (11,154) (11,338) (11,466)    
Net periodic benefit cost (income) $ (10,451) $ (10,580) $ (10,578)    
Weighted-average assumptions used to measure the benefit obligations [Abstract]          
Discount rate (in hundredths)       3.63% 4.21%
Weighted-average assumptions used to measure the net periodic benefit costs [Abstract]          
Discount rate (in hundredths) 4.21% 4.42% 4.04%    
Expected long-term rate of return on plan assets (in hundredths) 6.50% 6.50% 6.50%    
Expected future benefit payments to be paid from the plans over the next ten years [Abstract]          
2018       $ 3,175  
2019       3,383  
2020       3,560  
2021       3,636  
2022       3,690  
2023 - 2027       18,679  
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure $ 67,809 $ 66,320 $ 66,320 75,658 $ 67,809
Postretirement Benefit Plans [Member] | EMC Insurance Companies [Member] | HRA Maximum Adjustment [Member]          
Change in projected benefit obligation [Abstract]          
Plan amendments 2,700        
Postretirement Benefit Plans [Member] | EMC Insurance Companies [Member] | Life Insuance Amount Adjustment [Member]          
Change in projected benefit obligation [Abstract]          
Plan amendments (3,800)        
VEBA Trust [Member] | EMC Insurance Companies [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 67,809        
Fair value of plan assets at end of year 75,658 67,809      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 67,809 67,809   75,658 67,809
Defined Benefit Plan, Expected Future Employer Contributions [Abstract]          
Defined benefit plan estimated future parent total contributions next fiscal year       $ 0  
VEBA Trust [Member] | EMC Insurance Companies [Member] | Money Market Funds [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 1,485        
Fair value of plan assets at end of year 1,254 1,485      
Fair values of the plan assets held [Abstract]          
Net asset value per share (in dollars per share)       $ 1.00  
Assets, Fair Value Disclosure 1,485 1,485   $ 1,254 1,485
VEBA Trust [Member] | EMC Insurance Companies [Member] | Emerging Markets ETF [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 3,743        
Fair value of plan assets at end of year 4,803 3,743      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 3,743 3,743   4,803 3,743
VEBA Trust [Member] | EMC Insurance Companies [Member] | Mutual Fund [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 47,916        
Fair value of plan assets at end of year 54,180 47,916      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 47,916 47,916   $ 54,180 47,916
VEBA Trust [Member] | EMC Insurance Companies [Member] | Life Insurance Contracts [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 14,159        
Fair value of plan assets at end of year 14,524 14,159      
Fair values of the plan assets held [Abstract]          
Guaranteed interest rate life insurance policy       4.50%  
Assets, Fair Value Disclosure 14,159 14,159   $ 14,524 14,159
Reconciliation of the plan assets measured at fair value using significant unobservable inputs (Level 3) [Roll Forward]          
Beginning balance 14,159 13,792      
Actual return on plan assets [Abstract]          
Increase in cash surrender value of life insurance contracts 365 367      
Balance at end of period 14,524 14,159 $ 13,792    
VEBA Trust [Member] | EMC Insurance Companies [Member] | Cash [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 506        
Fair value of plan assets at end of year 897 506      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 506 506   897 506
VEBA Trust [Member] | EMC Insurance Companies [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 53,650        
Fair value of plan assets at end of year 61,134 53,650      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 53,650 53,650   61,134 53,650
VEBA Trust [Member] | EMC Insurance Companies [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Money Market Funds [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 1,485        
Fair value of plan assets at end of year 1,254 1,485      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 1,485 1,485   1,254 1,485
VEBA Trust [Member] | EMC Insurance Companies [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Emerging Markets ETF [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 3,743        
Fair value of plan assets at end of year 4,803 3,743      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 3,743 3,743   4,803 3,743
VEBA Trust [Member] | EMC Insurance Companies [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Mutual Fund [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 47,916        
Fair value of plan assets at end of year 54,180 47,916      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 47,916 47,916   54,180 47,916
VEBA Trust [Member] | EMC Insurance Companies [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Life Insurance Contracts [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 0        
Fair value of plan assets at end of year 0 0      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 0 0   0 0
VEBA Trust [Member] | EMC Insurance Companies [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Cash [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 506        
Fair value of plan assets at end of year 897 506      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 506 506   897 506
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant other observable inputs (Level 2) [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 0        
Fair value of plan assets at end of year 0 0      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 0 0   0 0
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant other observable inputs (Level 2) [Member] | Money Market Funds [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 0        
Fair value of plan assets at end of year 0 0      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 0 0   0 0
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant other observable inputs (Level 2) [Member] | Emerging Markets ETF [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 0        
Fair value of plan assets at end of year 0 0      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 0 0   0 0
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant other observable inputs (Level 2) [Member] | Mutual Fund [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 0        
Fair value of plan assets at end of year 0 0      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 0 0   0 0
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant other observable inputs (Level 2) [Member] | Life Insurance Contracts [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 0        
Fair value of plan assets at end of year 0 0      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 0 0   0 0
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant other observable inputs (Level 2) [Member] | Cash [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 0        
Fair value of plan assets at end of year 0 0      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 0 0   0 0
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant unobservable inputs (Level 3) [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 14,159        
Fair value of plan assets at end of year 14,524 14,159      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 14,159 14,159   14,524 14,159
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant unobservable inputs (Level 3) [Member] | Money Market Funds [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 0        
Fair value of plan assets at end of year 0 0      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 0 0   0 0
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant unobservable inputs (Level 3) [Member] | Emerging Markets ETF [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 0        
Fair value of plan assets at end of year 0 0      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 0 0   0 0
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant unobservable inputs (Level 3) [Member] | Mutual Fund [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 0        
Fair value of plan assets at end of year 0 0      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 0 0   0 0
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant unobservable inputs (Level 3) [Member] | Life Insurance Contracts [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 14,159        
Fair value of plan assets at end of year 14,524 14,159      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure 14,159 14,159   14,524 14,159
VEBA Trust [Member] | EMC Insurance Companies [Member] | Significant unobservable inputs (Level 3) [Member] | Cash [Member]          
Change in plan assets [Abstract]          
Fair value of plan assets at beginning of year 0        
Fair value of plan assets at end of year 0 0      
Fair values of the plan assets held [Abstract]          
Assets, Fair Value Disclosure $ 0 $ 0   $ 0 $ 0