Long-term Debt (Schedule Of Line Of Credit Facilities) (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Apr. 04, 2017
Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility used			$ 400,000,000
Letter Of Credit
Debt Instrument [Line Items]
Revolving credit facility used			$ 15,000,000
Less: Outstanding letters of credit	$ (2,830,000)	$ (2,830,000)
Line of Credit | Revolving Credit Facility
Debt Instrument [Line Items]
Revolving credit facility used	249,830,000	249,830,000
Outstanding revolving credit facility	247,000,000	247,000,000
Unamortized loan origination fees	(2,186,000)	(2,085,000)
Long-term debt	$ 244,814,000	$ 244,915,000