For investments which are quantified by principal amount, the principle balance held at close of period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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Name:	us-gaap_InvestmentOwnedBalancePrincipalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Investments (Details) - USD ($) $ in Millions	9 Months Ended
Investments (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2016
Minority interest in preferred stock in privately held company
Schedule of Cost-method Investments [Line Items]
Purchase price of investment			$ 0.3
Impairment on investments	$ 0.0
Investment in a convertible notes of a privately held company
Schedule of Cost-method Investments [Line Items]
Investment principle amount		$ 0.2