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Pension and OPEB - Periodic Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]    
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) $ (749) $ (148)
Amortization of actuarial gain 6,303 5,511
Defined Benefit Plan, Net Periodic Benefit Cost 5,554 5,363
Pension Plan [Member]    
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]    
Service cost 846 482
Interest cost 6,428 6,841
Defined Benefit Plan, Expected Return on Plan Assets 10,596 11,869
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) (148) (147)
Amortization of actuarial gain 2,351 2,116
Defined Benefit Plan, Net Periodic Benefit Cost (1,119) (2,577)
Other Postretirement Benefit Plan [Member]    
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]    
Service cost 0 0
Interest cost 5,986 6,060
Defined Benefit Plan, Expected Return on Plan Assets 0 0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) (601) 0
Amortization of actuarial gain 5,778 4,792
Defined Benefit Plan, Net Periodic Benefit Cost $ 11,163 $ 10,852