Fair Value Measurements - Available for Sale Investments at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 40,058	$ 38,960
Gross Unrealized Gains	0	8
Gross Unrealized Losses	(46)	(27)
Fair Value	40,012	38,941
U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,993	14,992
Gross Unrealized Gains	0	3
Gross Unrealized Losses	(28)	(13)
Fair Value	14,965	14,982
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,479	7,178
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	4,479	7,178
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,736	10,326
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(15)	(13)
Fair Value	15,721	10,314
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,850	6,464
Gross Unrealized Gains	0	4
Gross Unrealized Losses	(3)	(1)
Fair Value	$ 4,847	$ 6,467

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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