Employee and Retiree Benefit Plans (Components of Net Periodic Benefit Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,252	$ 2,037	$ 6,761	$ 6,099
Interest cost	6,716	7,261	20,160	20,267
Expected return on plan assets	(7,476)	(8,070)	(22,435)	(21,730)
Amortization of prior service cost (credit)	254	259	767	767
Recognized actuarial loss	5,197	3,610	15,593	10,673
Net periodic benefit expense	6,943	5,097	20,846	16,076
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	492	427	1,479	1,276
Interest cost	874	966	2,622	2,899
Amortization of prior service cost (credit)	(10)	(18)	(29)	(55)
Net periodic benefit expense	$ 1,356	$ 1,375	$ 4,072	$ 4,120