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Benefit Plans Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 4,447 $ 5,072 $ 3,938
Interest cost 6,830 6,342 6,098
Expected return on assets (7,288) (6,984) (6,414)
Amortization of prior service cost 376 376 376
Recognized actuarial loss 3,527 4,005 1,879
Net periodic pension cost 7,892 8,811 5,877
Other Postretirement Benefit Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 527 379 284
Interest cost 655 504 508
Expected return on assets (292) (283) (268)
Amortization of prior service cost 197 197 197
Recognized actuarial loss 316 188 59
Net periodic pension cost $ 1,403 $ 985 $ 780