Employee and Retiree Benefit Plans (Components of Net Periodic Benefit Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 8,279	$ 8,136	$ 17,948
Interest cost	27,047	25,185	33,168
Expected return on plan assets	(28,941)	(28,154)	(34,016)
Amortization of prior service cost (credit)	1,026	1,204	1,560
Amortization of transitional (asset) liability			(1)
Recognized actuarial loss	16,691	16,165	15,147
Gross periodic benefit expense	24,102	22,536	33,806
Termination benefits expense			8,606
Curtailment expense		822	306
Net periodic benefit expense	24,102	23,358	42,718
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,601	1,864	3,180
Interest cost	3,444	3,800	4,883
Amortization of prior service cost (credit)	(74)	(75)	(82)
Recognized actuarial loss		5	992
Gross periodic benefit expense	4,971	5,594	8,973
Curtailment expense		(19)
Net periodic benefit expense	$ 4,971	$ 5,575	$ 8,973