Employee and Retiree Benefit Plans (Components of Net Periodic Benefit Expense) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,255	$ 2,032
Interest cost	6,737	6,719
Expected return on plan assets	(7,506)	(7,185)
Amortization of prior service cost (credit)	257	254
Recognized actuarial loss	5,215	3,554
Net periodic benefit expense	6,958	5,374
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	494	425
Interest cost	874	966
Amortization of prior service cost (credit)	(10)	(18)
Net periodic benefit expense	$ 1,358	$ 1,373