Summary of Significant Accounting Policies (Changes in AOCI Balances by Component) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Balance	$ 1,553,023
Other comprehensive income (loss)	2,695	$ (3,248)	$ (547)
Balance	1,713,275	1,553,023
Accumulated Other Comprehensive Loss [Member]
Balance	(45,413)	(42,165)	(41,618)
Other comprehensive income (loss)		(4,347)	(547)
Other comprehensive loss before reclassifications	1,577
Amounts reclassified from AOCI	1,118	1,099
Balance	(42,718)	(45,413)	(42,165)
Defined Benefit Pension Plan [Member]
Balance	(40,865)	(38,242)	(40,268)
Other comprehensive income (loss)		(3,722)	2,026
Other comprehensive loss before reclassifications	306
Amounts reclassified from AOCI	1,118	1,099
Balance	(39,441)	(40,865)	(38,242)
Foreign Currency Translation [Member]
Balance	(4,864)	(4,603)	(1,389)
Other comprehensive income (loss)		(261)	(3,214)
Other comprehensive loss before reclassifications	1,273
Balance	(3,591)	(4,864)	(4,603)
Unrealized (Loss) Gain In Fair Value Of Investments [Member]
Balance	316	656	(110)
Other comprehensive income (loss)		(340)	766
Other comprehensive loss before reclassifications	(2)
Balance	$ 314	316	656
Unrealized Gain (Loss) In Fair Value Of Interest Rate Swap [Member]
Balance		24	149
Other comprehensive income (loss)		$ (24)	(125)
Balance			$ 24