Non-Qualified Deferred Compensation Plan (Details) - Parent Company [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]
Deferred Compensation Plan Assets	$ 32,609	$ 31,662
Deferred Compensation Liability, Current and Noncurrent	$ 32,354	$ 31,383

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_DeferredCompensationPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant