Financial Instruments (Details) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2016 USD ($) position	Sep. 30, 2016 USD ($) position	Dec. 31, 2015 USD ($)
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 157,717	$ 157,717	$ 201,586
Realized gain		$ 200
Number of investments, unrealized loss position \| position	9	9
Interest income	$ 400	$ 1,200
Amortized Cost
Less than one year	44,540	44,540
Greater than one year but less than five years	112,843	112,843
Total	157,383	157,383
Fair Value
Less than one year	44,539	44,539
Greater than one year but less than five years	113,178	113,178
Total	157,717	157,717
Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	167,194	167,194	208,968
Unrealized Gain	375	375	3
Unrealized Loss	(41)	(41)	(1,419)
Estimated Fair Value	167,528	167,528	207,552
Money market deposits
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	9,811	9,811	5,966
Estimated Fair Value	9,811	9,811	5,966
U.S. Treasury securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	75,455	75,455	104,537
Unrealized Gain	131	131	1
Unrealized Loss	(21)	(21)	(725)
Estimated Fair Value	75,565	75,565	103,813
U.S. agency securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	64,299	64,299	79,491
Unrealized Gain	183	183
Unrealized Loss	(13)	(13)	(638)
Estimated Fair Value	64,469	64,469	78,853
Municipal securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Cost	17,629	17,629	18,974
Unrealized Gain	61	61	2
Unrealized Loss	(7)	(7)	(56)
Estimated Fair Value	17,683	17,683	18,920
Cash equivalents \| Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	9,811	9,811	5,966
Short-term investments \| Available-for-sale securities
Available-for-sale Securities, Fair Value to Amortized Cost Basis [Abstract]
Estimated Fair Value	$ 157,717	$ 157,717	$ 201,586

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.