Marketable Securities and Assets Measured at Fair Value - Maturities of marketable securities available for sale (Details) - Certificates of Deposit [Member]	Sep. 30, 2019
Minimum [Member]
Debt Securities, Available-for-sale [Line Items]
Maturities of marketable securities, by date	Oct. 14, 2019
Maximum [Member]
Debt Securities, Available-for-sale [Line Items]
Maturities of marketable securities, by date	Jul. 28, 2020

Maturity date of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in CCYY-MM-DD format.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

Name:	us-gaap_DebtSecuritiesAvailableForSaleMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	instant