Employee Benefit Plans (Schedule of Benefits Plans) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Pension Benefit Plan [Member]
Periodic costs for employee retirement benefit plans
Service Cost		$ 607	$ 600
Interest Cost		765	786
Expected Return on Assets		(1,218)	(1,122)
Amortization of Unrecognized Losses		415	391
Amortization of Unrecognized Prior Service Cost (Credit)
Net Periodic Benefit Cost	[1]	569	655
Other Benefits Plan [Member]
Periodic costs for employee retirement benefit plans
Service Cost		284	272
Interest Cost		474	491
Expected Return on Assets		(637)	(601)
Amortization of Unrecognized Losses		447	445
Amortization of Unrecognized Prior Service Cost (Credit)		(402)	(432)
Net Periodic Benefit Cost	[1]	$ 166	$ 175

[1] Service cost is included in Operations and Maintenance expense on Consolidated Statements of Income; all other amounts are included in Other Income/Expense, net.