CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Tax provision (benefit) on net unrealized change on investments	$ 204	$ 489	$ (861)
Accumulated Other Comprehensive Income (Loss)
Tax provision (benefit) on net unrealized change on investments	$ 204	$ (1,172)	$ (861)

Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e640-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (e,f)
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580

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