Summary of Significant Accounting Policies (Changes in AOCI Balances by Component) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance	$ 1,713,275
Balance	1,809,177	$ 1,713,275
Accumulated Other Comprehensive Loss [Member]
Balance	(42,718)	(45,413)	$ (42,165)
Other comprehensive loss before reclassifications	(4,254)	1,577	(4,347)
Amounts reclassified from AOCI	1,523	1,118	1,099
Balance	(45,449)	(42,718)	(45,413)
Defined Benefit Pension Plan [Member]
Balance	(39,441)	(40,865)	(38,242)
Other comprehensive loss before reclassifications	(695)	306	(3,722)
Amounts reclassified from AOCI	1,466	1,118	1,099
Balance	(38,670)	(39,441)	(40,865)
Foreign Currency Translation [Member]
Balance	(3,591)	(4,864)	(4,603)
Other comprehensive loss before reclassifications	(2,724)	1,273	(261)
Balance	(6,315)	(3,591)	(4,864)
Unrealized (Loss) Gain In Fair Value Of Investments [Member]
Balance	314	316	656
Other comprehensive loss before reclassifications	(835)	(2)	(340)
Amounts reclassified from AOCI	57
Balance	$ (464)	$ 314	316
Unrealized Gain (Loss) In Fair Value Of Interest Rate Swap [Member]
Balance			24
Other comprehensive loss before reclassifications			$ (24)

Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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