Financial Instruments (Details) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017 USD ($) position	Sep. 30, 2017 USD ($) position	Dec. 31, 2016 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 135,017	$ 135,017	$ 130,860
Other than temporary impairment		$ 0
Number of investments, unrealized loss position \| position	15	15
Interest income	$ 300	$ 800
Amortized Cost
Less than one year	86,061	86,061
Greater than one year but less than five years	49,352	49,352
Total	135,413	135,413
Fair Value
Less than one year	85,999	85,999
Greater than one year but less than five years	49,018	49,018
Total	135,017	135,017
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	135,699	135,699	142,362
Unrealized Gain	23	23
Unrealized Loss	(419)	(419)	(725)
Estimated Fair Value	135,303	135,303	141,637
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	286	286	10,777
Estimated Fair Value	286	286	10,777
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	84,550	84,550	57,846
Unrealized Gain	8	8
Unrealized Loss	(279)	(279)	(404)
Estimated Fair Value	84,279	84,279	57,442
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	39,106	39,106	56,261
Unrealized Loss	(136)	(136)	(147)
Estimated Fair Value	38,970	38,970	56,114
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	11,757	11,757	17,478
Unrealized Gain	15	15
Unrealized Loss	(4)	(4)	(174)
Estimated Fair Value	11,768	11,768	17,304
Cash equivalents \| Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	286	286	10,777
Short-term investments \| Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 135,017	$ 135,017	$ 130,860

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

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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