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Investments at Fair Value for Company Sponsored Pension Benefit Plans Within Fair Value Hierarchy (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Cash and cash equivalents    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets $ 2,010,000 $ 2,002,000
defined benefit plan net asset value 0 0
Common and collective trusts    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets 308,956,000 283,636,000
defined benefit plan net asset value 308,956,000 283,636,000
Investments    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets 310,966,000 285,638,000
defined benefit plan net asset value 308,956,000 283,636,000
Fair Value, Inputs, Level 1 | Cash and cash equivalents    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets 2,010,000 2,002,000
Fair Value, Inputs, Level 1 | Common and collective trusts    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets 0 0
Fair Value, Inputs, Level 1 | Investments    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets $ 2,010,000 $ 2,002,000