Fair Value of Financial Instruments (Fair Value Hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	$ 334,238
Recurring Basis | Fair Value
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	334,296	$ 324,963
Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	58	113
Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	334,238	324,850
Commercial paper
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	5,638
Commercial paper | Recurring Basis | Fair Value
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	5,638	3,237
Commercial paper | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	0	0
Commercial paper | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	5,638	3,237
U.S. government agencies
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	220,362
U.S. government agencies | Recurring Basis | Fair Value
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	220,362	222,400
U.S. government agencies | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	0	0
U.S. government agencies | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	220,362	222,400
Corporate bonds
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	65,596
Corporate bonds | Recurring Basis | Fair Value
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	65,596	65,854
Corporate bonds | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	0	0
Corporate bonds | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	65,596	65,854
Asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	42,642
Asset-backed securities | Recurring Basis | Fair Value
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	42,642	33,359
Asset-backed securities | Level 1 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	0	0
Asset-backed securities | Level 2 | Recurring Basis
Assets, Fair Value Disclosure [Abstract]
Debt Securities, Available-for-sale	42,642	33,359
Money Market Funds [Member] | Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 58	$ 113