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Changes in Benefit Obligation, Plan Assets and Funded Status for Company-Sponsored Benefit Plans (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]      
Defined Benefit Plan, Settlements, Benefit Obligation   $ 3,500  
Pension Benefit Plans      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year $ 304,388 318,444  
Service cost 2,069 1,562 $ 1,244
Interest cost 13,149 14,072 13,931
Actuarial losses (gains) 19,130 6,225  
Benefits paid (20,922) (25,286)  
Benefit obligation at end of year 317,814 304,388 318,444
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at beginning of year 285,638 294,076  
Actual return on plan assets 45,796 27,056  
Employer contribution 454 421  
Fair value of plan assets at end of year 310,966 285,638 294,076
Defined Benefit Plan, Settlements, Plan Assets 0 (10,629)  
Funded status at end of year (6,848) (18,750)  
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]      
Noncurrent assets 8,144 1,740  
Current liabilities (441) (397)  
Noncurrent liabilities (14,551) (20,093)  
Net amount recognized (6,848) (18,750)  
Defined Benefit Plan, Settlements, Benefit Obligation 0 3,482 0
Other Postretirement Employee Benefit Plans      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation at beginning of year 69,163 71,672  
Service cost 163 249 363
Interest cost 2,745 3,075 3,881
Actuarial losses (gains) (1,254) 816  
Benefits paid (5,689) (6,649)  
Benefit obligation at end of year 65,128 69,163 71,672
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of plan assets at beginning of year 20 20  
Actual return on plan assets 0 0  
Employer contribution 5,689 6,649  
Fair value of plan assets at end of year 20 20 20
Defined Benefit Plan, Settlements, Plan Assets 0 0  
Funded status at end of year (65,108) (69,143)  
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]      
Noncurrent assets 0 0  
Current liabilities (7,190) (7,424)  
Noncurrent liabilities (57,918) (61,719)  
Net amount recognized (65,108) (69,143)  
Defined Benefit Plan, Settlements, Benefit Obligation $ 0 $ 0 $ 0