Impact of Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements [Abstract]
Operating expenses	$ 6,708	$ 6,420	$ 13,212	$ 12,548
Other pension costs	321	305	642	610
ASU No. 2017-07 [Member]
New Accounting Pronouncements [Abstract]
Operating expenses	(321)	(305)	(642)	(610)
Other pension costs	$ 321	$ 305	$ 642	$ 610

Amount of expense (reversal of expense) for net periodic benefit cost components, excluding service cost component, of defined benefit plan. Amount includes, but is not limited to, interest cost, expected (return) loss on plan asset, amortization of prior service cost (credit), amortization of (gain) loss, amortization of transition (asset) obligation, settlement (gain) loss, curtailment (gain) loss and certain termination benefits.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 45
-Paragraph 3A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109237824&loc=SL108413299-114919

Name:	us-gaap_NetPeriodicDefinedBenefitsExpenseReversalOfExpenseExcludingServiceCostComponent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration