Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	$ 57,457	$ 75,476
Recurring
Liabilities that are measured at fair value on a recurring basis
Repurchase agreements	0	5,000
Assets, fair value	57,457	75,476
Recurring \| Level 1
Liabilities that are measured at fair value on a recurring basis
Repurchase agreements	0	0
Assets, fair value	5,381	5,838
Recurring \| Level 2
Liabilities that are measured at fair value on a recurring basis
Repurchase agreements	0	5,000
Assets, fair value	52,076	69,638
Recurring \| Level 3
Liabilities that are measured at fair value on a recurring basis
Repurchase agreements	0	0
Assets, fair value	0	0
Money market funds \| Recurring
Liabilities that are measured at fair value on a recurring basis
Money market funds	5,381	5,838
Money market funds \| Recurring \| Level 1
Liabilities that are measured at fair value on a recurring basis
Money market funds	5,381	5,838
Money market funds \| Recurring \| Level 2
Liabilities that are measured at fair value on a recurring basis
Money market funds	0	0
Money market funds \| Recurring \| Level 3
Liabilities that are measured at fair value on a recurring basis
Money market funds	0	0
Commercial paper \| Recurring
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	11,892	30,710
Commercial paper \| Recurring \| Level 1
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	0	0
Commercial paper \| Recurring \| Level 2
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	11,892	30,710
Commercial paper \| Recurring \| Level 3
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	0	0
Corporate Note Securities \| Recurring
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	18,380	13,144
Corporate Note Securities \| Recurring \| Level 1
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	0	0
Corporate Note Securities \| Recurring \| Level 2
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	18,380	13,144
Corporate Note Securities \| Recurring \| Level 3
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	0	0
US Government Debt Securities \| Recurring
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	11,295	10,166
US Government Debt Securities \| Recurring \| Level 1
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	0	0
US Government Debt Securities \| Recurring \| Level 2
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	11,295	10,166
US Government Debt Securities \| Recurring \| Level 3
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	0	0
Asset-backed Securities
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	10,509	10,618
Asset-backed Securities \| Recurring
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	10,509	10,618
Asset-backed Securities \| Recurring \| Level 1
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	0	0
Asset-backed Securities \| Recurring \| Level 2
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	10,509	10,618
Asset-backed Securities \| Recurring \| Level 3
Liabilities that are measured at fair value on a recurring basis
Debt securities, fair value	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of asset associated with funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price.

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