Investments - Additional Information (Details)	12 Months Ended
Dec. 31, 2017 USD ($)
Available For Sale Securities [Abstract]
Other-than-temporarily impairment, unrealized losses on available-for-sale investments	$ 0

The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=82896770&loc=SL6284422-111562

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration