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Taxes on Income, Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Current deferred tax assets [Abstract]    
Compensated absences $ 829,000 $ 854,000
Allowance for doubtful accounts 3,288,000 3,111,000
Insurance accruals 4,234,000 2,858,000
Other 3,372,000 3,739,000
Current deferred tax assets, Total 11,723,000 10,562,000
Deferred tax assets [Abstract]    
Postretirement health care benefits 2,501,000 2,795,000
Insurance accruals 3,410,000 3,325,000
Deferred compensation 10,534,000 10,157,000
Unrealized loss on defined benefit plans 25,245,000 34,501,000
Operating loss carryforwards 5,188,000 5,219,000
Other 20,702,000 20,687,000
Valuation allowances (4,716,000) (4,704,000)
Deferred tax assets 62,864,000 71,980,000
Deferred tax liabilities [Abstract]    
Property (604,737,000) (547,388,000)
Deferred state taxes (53,542,000) (49,503,000)
Pension benefits (8,471,000) (11,198,000)
Goodwill and other intangibles (44,185,000) (37,936,000)
Other (21,737,000) (21,724,000)
Deferred tax liabilities (732,672,000) (667,749,000)
Deferred tax assets (liabilities) (669,808,000) $ (595,769,000)
Operating loss carryforwards, net of valuation allowance $ 472,000  
Expiration dates, operating loss carryforwards Dec. 31, 2030  
Unrecognized tax benefits including interest and penalties $ 2,479,000  
Amount that would impact the effective tax rate, if recognized $ 1,678,000  
Minimum [Member] | State and Local Jurisdiction [Member]    
Income Tax Examination [Line Items]    
Years open to audit under the statute of limitations 2009  
Minimum [Member] | Internal Revenue Service (IRS) [Member]    
Income Tax Examination [Line Items]    
Years open to audit under the statute of limitations 2012  
Maximum [Member] | State and Local Jurisdiction [Member]    
Income Tax Examination [Line Items]    
Years open to audit under the statute of limitations 2014  
Maximum [Member] | Internal Revenue Service (IRS) [Member]    
Income Tax Examination [Line Items]    
Years open to audit under the statute of limitations 2014