Basis of Presentation (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Cash and cash equivalents	$ 255,139		$ 248,904		$ 255,139
Restricted cash, current			$ 304
Restricted cash	304				$ 304
Federal statutory tax rate			21.00%		35.00%
Guarantees or other funding obligations	0		$ 0		$ 0
Defined Benefit Plan Net Periodic Benefit Cost		$ 400		$ 1,200
Other Expense [Member] | Adjustments for New Accounting Pronouncement [Member]
Defined Benefit Plan Net Periodic Benefit Cost		$ 200		$ 700
Retained Earnings (Accumulated Deficit) [Member]
Tax cuts and Jobs Act, reclassification tax effect	4,468
Accumulated Other Comprehensive Loss [Member]
Tax cuts and Jobs Act, reclassification tax effect	$ (4,468)				$ (4,468)