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Retirement Plans, Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rate (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Increase $ 0 $ 0  
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Decrease 0 0  
SERP [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Increase 0 0  
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Decrease 0 0  
Other Postretirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Increase 74 107  
Effect of one-percentage-point change in assumed health care cost trend rate on postretirement obligation, Decrease (64) (93)  
Effect of one-percentage-point change in assumed health care cost trend rate on aggregate service and interest cost, Increase 4 5 $ 7
Effect of one-percentage-point change in assumed health care cost trend rate on aggregate service and interest cost, Decrease $ (3) $ (4) $ (6)