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Employee Benefit Plans (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Transition obligation period of recognition 20 years    
Change in plan assets [Roll Forward]      
Employer contributions $ 0 $ 5,600  
Assumptions used to determine benefit obligations [Abstract]      
Expected long-term return on plan assets 7.00% 7.00% 7.00%
Rate of compensation increase 3.00% 3.00% 3.00%
Assumptions used to determine net periodic pension cost [Abstract]      
Expected long-term return on plan assets 7.00% 7.00% 7.50%
Rate of compensation increase 3.00% 3.00%  
Other changes in plan assets and benefit obligation recognized in OCI (pre-tax) [Abstract]      
Amortization of unrecognized net (loss) $ 2,401 $ 3,154 $ (6,144)
Net actuarial loss and prior service costs that will be amortized from Accumulated other comprehensive income (loss) in next fiscal year $ 1,200    
Estimate future benefit payments [Abstract]      
Ultimate health care cost trend rate 3.90%    
Effect of one-percentage point change in assumed health care cost trend rates [Abstract]      
One-percentage point increase on total service and interest cost components $ 40    
One-percentage point (decrease) on total service and interest cost components (34)    
One-percentage point increase on post-retirement accumulated benefit obligation 855    
One-percentage point (decrease) on post-retirement accumulated benefit obligation $ (738)    
Actual plan asset allocations [Abstract]      
Actual plan asset allocations 100.00% 100.00%  
Minimum [Member]      
Assumptions used to determine benefit obligations [Abstract]      
Discount rate 4.20% 4.76% 4.69%
Assumptions used to determine net periodic pension cost [Abstract]      
Discount rate 4.76% 4.69% 4.19%
Rate of compensation increase     3.00%
Estimate future benefit payments [Abstract]      
Annual rates of increase in the per capita cost of covered medical and prescription drug benefits, 6.30%    
Maximum [Member]      
Assumptions used to determine benefit obligations [Abstract]      
Discount rate 4.21% 4.84% 4.71%
Assumptions used to determine net periodic pension cost [Abstract]      
Discount rate 4.84% 4.71% 4.30%
Rate of compensation increase     3.75%
Estimate future benefit payments [Abstract]      
Annual rates of increase in the per capita cost of covered medical and prescription drug benefits, 10.50%    
Pension Benefits [Member]      
Accumulated other comprehensive income (loss), before tax [Abstract]      
Net actuarial loss $ 23,585 $ 28,328  
Prior service cost (credit) 94 140  
Total amounts recognized in AOCI (pre-tax) 23,679 28,468  
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year 90,477 92,445  
Service cost 1,511 2,162 $ 2,677
Interest cost 4,168 4,223 3,977
Plan participants' contributions 0 0  
Actuarial loss (gain) 4,028 (1,635)  
Curtailments/ settlement 0 (715)  
Benefits paid (9,234) (6,003)  
Projected benefit obligation at end of year 90,950 90,477 92,445
Change in plan assets [Roll Forward]      
Fair value of plan assets at beginning of year 116,216 107,529  
Actual return on plan assets 15,032 8,259  
Employer contributions 2,212 6,431  
Plan participants' contributions 0 0  
Benefits paid (9,234) (6,003)  
Fair value of plan assets at end of year 124,226 116,216 107,529
Funded (unfunded) status at year end 33,276 25,739  
Accumulated benefit obligation 91,000 90,500  
Amounts recognized in Balance Sheet [Abstract]      
Funded (unfunded) status at year end 33,276 25,739  
Components of net periodic benefit cost [Abstract]      
Service cost 1,511 2,162 2,677
Interest cost 4,168 4,223 3,977
Expected return on plan assets (7,929) (7,430) (8,589)
Amortization of gain due to curtailment 0 (768) (154)
Amortization of prior service cost (credit) 46 32 21
Amortization of unrecognized net loss 1,668 2,235 2,174
Total (benefit) cost (536) 454 106
Other changes in plan assets and benefit obligation recognized in OCI (pre-tax) [Abstract]      
Net (gain) loss (3,075) (2,464) 6,523
Prior service cost 0 96 0
Amortization of gain due to settlement 0 (43) (46)
Amortization of prior service (cost) credit (46) (32) (21)
Amortization of unrecognized net (loss) (1,668) (2,235) (2,174)
Total recognized in other comprehensive income (4,789) (4,678) 4,282
Total recognized in net periodic benefit cost and OCI pre-tax (5,325) (4,224) 4,388
Estimate future benefit payments [Abstract]      
2018 7,256    
2019 7,087    
2020 6,997    
2021 6,728    
2022 6,666    
2023 - 2027 37,688    
Pension Benefits [Member] | Other Assets [Member]      
Amounts recognized in Balance Sheet [Abstract]      
Assets recognized 52,775 45,344  
Pension Benefits [Member] | Other Liabilities [Member]      
Amounts recognized in Balance Sheet [Abstract]      
Liabilities recognized $ (19,499) $ (19,605)  
Pension Benefits [Member] | Cash and Cash Equivalents [Member]      
Actual plan asset allocations [Abstract]      
Actual plan asset allocations 3.00% 2.00%  
Pension Benefits [Member] | Cash and Cash Equivalents [Member] | Minimum [Member]      
Target asset allocations [Abstract]      
Target allocation percentage of assets 0.00%    
Pension Benefits [Member] | Cash and Cash Equivalents [Member] | Maximum [Member]      
Target asset allocations [Abstract]      
Target allocation percentage of assets 20.00%    
Pension Benefits [Member] | Fixed Income Securities [Member]      
Actual plan asset allocations [Abstract]      
Actual plan asset allocations 45.00% 46.00%  
Pension Benefits [Member] | Fixed Income Securities [Member] | Minimum [Member]      
Target asset allocations [Abstract]      
Target allocation percentage of assets 25.00%    
Pension Benefits [Member] | Fixed Income Securities [Member] | Maximum [Member]      
Target asset allocations [Abstract]      
Target allocation percentage of assets 55.00%    
Pension Benefits [Member] | Equities [Member]      
Actual plan asset allocations [Abstract]      
Actual plan asset allocations 52.00% 52.00%  
Pension Benefits [Member] | Equities [Member] | Minimum [Member]      
Target asset allocations [Abstract]      
Target allocation percentage of assets 40.00%    
Pension Benefits [Member] | Equities [Member] | Maximum [Member]      
Target asset allocations [Abstract]      
Target allocation percentage of assets 65.00%    
Other Benefits [Member]      
Accumulated other comprehensive income (loss), before tax [Abstract]      
Net actuarial loss $ 1,731 $ 1,430  
Prior service cost (credit) 196 (38)  
Total amounts recognized in AOCI (pre-tax) 1,927 1,392  
Change in benefit obligation [Roll Forward]      
Benefit obligation at beginning of year 7,478 8,322  
Service cost 12 14 17
Interest cost 357 353 374
Plan participants' contributions 218 234  
Actuarial loss (gain) 388 (786)  
Curtailments/ settlement 286 0  
Benefits paid (689) (659)  
Projected benefit obligation at end of year 8,050 7,478 8,322
Change in plan assets [Roll Forward]      
Fair value of plan assets at beginning of year 0 0  
Actual return on plan assets 0 0  
Employer contributions 471 425  
Plan participants' contributions 218 234  
Benefits paid (689) (659)  
Fair value of plan assets at end of year 0 0 0
Funded (unfunded) status at year end (8,050) (7,478)  
Accumulated benefit obligation 8,050 7,500  
Amounts recognized in Balance Sheet [Abstract]      
Funded (unfunded) status at year end (8,050) (7,478)  
Components of net periodic benefit cost [Abstract]      
Service cost 12 14 17
Interest cost 357 353 374
Expected return on plan assets 0 0 0
Amortization of gain due to curtailment 0 0 0
Amortization of prior service cost (credit) 51 (57) (219)
Amortization of unrecognized net loss 87 117 263
Total (benefit) cost 507 427 435
Other changes in plan assets and benefit obligation recognized in OCI (pre-tax) [Abstract]      
Net (gain) loss 388 (786) (333)
Prior service cost 286 0 0
Amortization of gain due to settlement 0 0 0
Amortization of prior service (cost) credit (51) 57 219
Amortization of unrecognized net (loss) (87) (117) (263)
Total recognized in other comprehensive income 536 (846) (377)
Total recognized in net periodic benefit cost and OCI pre-tax 1,043 (419) $ 58
Estimate future benefit payments [Abstract]      
2018 606    
2019 598    
2020 583    
2021 559    
2022 566    
2023 - 2027 2,818    
Other Benefits [Member] | Other Assets [Member]      
Amounts recognized in Balance Sheet [Abstract]      
Assets recognized 0 0  
Other Benefits [Member] | Other Liabilities [Member]      
Amounts recognized in Balance Sheet [Abstract]      
Liabilities recognized $ (8,050) $ (7,478)