Employee and Retiree Benefit Plans (Effects of Fair Value Measurements Using Significant Unobservable Inputs on Changes in Level 3 Plan Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total beginning balance	$ 519,357
Total ending balance	563,825	$ 519,357
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total beginning balance	34,114
Total ending balance	37,950	34,114
Defined Benefit Plan, Hedged Funds And Other Alternative Strategies [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total beginning balance	34,114	33,929
Relating to assets held at the reporting date	3,836	185
Relating to assets sold during the period
Purchases, sales and settlements
Total ending balance	$ 37,950	$ 34,114