Summary of Significant Accounting Policies (Changes in AOCI Balances by Component) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Balance	$ 1,365,505	$ 1,247,535	$ 1,143,864
Other comprehensive (loss) income	(547)	(8,239)	10,195
Balance	1,420,227	1,365,505	1,247,535
Accumulated Other Comprehensive (Loss) Income [Member]
Balance	(41,618)	(33,379)	(43,574)
Other comprehensive (loss) income	(547)	(8,239)	10,195
Balance	(42,165)	(41,618)	(33,379)
Defined Benefit Pension Plan [Member]
Balance	(40,268)	(32,113)	(43,023)
Other comprehensive (loss) income	2,026	(8,155)	10,910
Balance	(38,242)	(40,268)	(32,113)
Foreign Currency Translation [Member]
Balance	(1,389)	(751)	(13)
Other comprehensive (loss) income	(3,214)	(638)	(738)
Balance	(4,603)	(1,389)	(751)
Unrealized Gain (Loss) In Fair Value Of Investments [Member]
Balance	(110)	(315)	240
Other comprehensive (loss) income	766	205	(555)
Balance	656	(110)	(315)
Unrealized Gain (Loss) In Fair Value Of Interest Rate Swap [Member]
Balance	149	(200)	(778)
Other comprehensive (loss) income	(125)	349	578
Balance	$ 24	$ 149	$ (200)