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Pension and Other Postretirement Benefits - Weighted average assumptions used to determine net periodic benefit cost (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Rate of compensation increase, year one 3.00%    
Rate of compensation increase, year two and thereafter 3.50%    
Rate of compensation increase, year two   3.00%  
Rate of compensation increase, year three and thereafter   4.00%  
Rate of compensation increase, year one through three     3.00%
Rate of compensation increase, year four and thereafter     4.00%
Defined Benefit Pension Plans [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Measurement date 12/31/2017 12/31/2016 12/31/2015
Fiscal year end 12/31/2018 12/31/2017 12/31/2016
Discount rate 3.69% 4.15% 4.44%
Rate of compensation increase, group 1 3.00% 3.00% 3.00%
Rate of compensation increase, group 2 4.00% 4.00% 4.00%
Expected return on plan assets 7.18% 6.82% 7.10%
Other Postretirement Benefit Plans [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]      
Measurement date 12/31/2017 12/31/2016 12/31/2015
Fiscal year end 12/31/2018 12/31/2017 12/31/2016
Discount rate 3.66% 4.05% 4.50%
Rate of compensation increase, group 1 3.00% 3.00% 3.00%
Rate of compensation increase, group 2 4.00% 4.00% 4.00%
Expected return on plan assets 0.00% 0.00% 0.00%