Employee and Retiree Benefit Plans (Components of Net Periodic Benefit Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,030	$ 2,770	$ 4,062	$ 5,923
Interest cost	6,287	8,865	13,006	14,473
Expected return on plan assets	(6,475)	(9,698)	(13,660)	(15,083)
Amortization of prior service cost (credit)	254	321	508	640
Recognized actuarial loss	3,509	3,718	7,063	7,247
Curtailments				822
Net periodic benefit expense	5,605	5,976	10,979	14,022
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	424	675	849	1,348
Interest cost	967	1,107	1,933	2,215
Amortization of transitional asset		2		2
Amortization of prior service cost (credit)	(19)	(20)	(37)	(41)
Recognized actuarial loss		36		75
Curtailments				(19)
Net periodic benefit expense	$ 1,372	$ 1,800	$ 2,745	$ 3,580