Employee Benefit Plans (Schedule of Benefits Plans) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefit Plan [Member]
Periodic costs for employee retirement benefit plans
Service Cost		$ 607	$ 600	$ 1,213	$ 1,200
Interest Cost		765	786	1,530	1,572
Expected Return on Assets		(1,218)	(1,122)	(2,435)	(2,245)
Amortization of Unrecognized Losses		415	391	829	783
Amortization of Unrecognized Prior Service Cost (Credit)
Net Periodic Benefit Cost	[1]	569	655	1,137	1,310
Other Benefits Plan [Member]
Periodic costs for employee retirement benefit plans
Service Cost		284	272	567	544
Interest Cost		474	491	949	982
Expected Return on Assets		(637)	(601)	(1,275)	(1,203)
Amortization of Unrecognized Losses		447	445	894	890
Amortization of Unrecognized Prior Service Cost (Credit)		(402)	(432)	(803)	(864)
Net Periodic Benefit Cost	[1]	$ 166	$ 175	$ 332	$ 349

[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.