Employee and Retiree Benefit Plans (Components of Net Periodic Benefit Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,037	$ 2,610	$ 6,099	$ 8,533
Interest cost	7,261	5,913	20,267	20,386
Expected return on plan assets	(8,070)	(6,626)	(21,730)	(21,709)
Amortization of prior service cost (credit)	259	323	767	963
Recognized actuarial loss	3,610	3,617	10,673	10,864
Curtailments				822
Net periodic benefit expense	5,097	5,837	16,076	19,859
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	427	674	1,276	2,022
Interest cost	966	1,109	2,899	3,324
Amortization of prior service cost (credit)	(18)	(21)	(55)	(62)
Amortization of transitional asset		2		4
Recognized actuarial loss		38		113
Curtailments				(19)
Net periodic benefit expense	$ 1,375	$ 1,802	$ 4,120	$ 5,382