Summary of Significant Accounting Policies (Changes in AOCI Balances by Component) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Balance	$ 1,809,177
Balance	1,440,142	$ 1,809,177
Accumulated Other Comprehensive Loss [Member]
Balance	(45,449)	(42,718)	$ (45,413)
Other comprehensive loss before reclassifications	2,026	(4,254)	1,577
Amounts reclassified from AOCI	1,323	1,523	1,118
Balance	(42,100)	(45,449)	(42,718)
Defined Benefit Pension Plan [Member]
Balance	(38,670)	(39,441)	(40,865)
Other comprehensive loss before reclassifications	(539)	(695)	306
Amounts reclassified from AOCI	1,383	1,466	1,118
Balance	(37,826)	(38,670)	(39,441)
Foreign Currency Translation [Member]
Balance	(6,315)	(3,591)	(4,864)
Other comprehensive loss before reclassifications	944	(2,724)	1,273
Balance	(5,371)	(6,315)	(3,591)
Unrealized Gain (Loss) In Fair Value Of Investments [Member]
Balance	(464)	314	316
Other comprehensive loss before reclassifications	1,621	(835)	(2)
Amounts reclassified from AOCI	(60)	57
Balance	$ 1,097	$ (464)	$ 314

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

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