Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amounts owed to employees participating in the Deferred Compensation Plan	$ 3,400	$ 3,900
Obligations
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	3,000	4,000
New level 3 liability	0	0
Total (gains) losses reported in earnings	(400)	(1,000)
Transfers into or out of Level 3	0	0
Balance at end of period	$ 2,600	$ 3,000