Employee and Retiree Benefit Plans (Components of Net Periodic Benefit Expense) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,032	$ 3,153
Interest cost	6,719	5,608
Expected return on plan assets	(7,185)	(5,385)
Amortization of prior service cost (credit)	254	319
Recognized actuarial loss	3,554	3,529
Curtailments		822
Net periodic benefit expense	5,374	8,046
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	425	673
Interest cost	966	1,108
Amortization of prior service cost (credit)	(18)	(21)
Recognized actuarial loss		39
Curtailments		(19)
Net periodic benefit expense	$ 1,373	$ 1,780