Investments In Securities (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Investments, Debt and Equity Securities [Abstract]
Number of securities with unrealized losses \| security	51	32
Impairment amount for cost-method investments \| $	$ 0	$ 0

Loss recognized as a result of the initial and subsequent write-down of a long-lived asset's carrying amount to fair value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84176398&loc=d3e2921-110230

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