Fair Value Measurements (Fair Value, Marketable Securities Measured on Recurring and Nonrecurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Cash and cash equivalents, fair value disclosure	$ 41,408	$ 70,055
Available-for-sale securities, fair value disclosure	507,628	590,065
Total financial assets	532,401	609,441
Fair Value, Inputs, Level 1 [Member]
Total financial assets	24,773	19,376
Fair Value, Inputs, Level 2 [Member]
Total financial assets	507,628	590,065
Money Market Funds [Member]
Cash and cash equivalents, fair value disclosure	24,773	19,376
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents, fair value disclosure	24,773	19,376
Commercial Paper [Member]
Cash and cash equivalents, fair value disclosure		7,498
Available-for-sale securities, fair value disclosure	18,488	42,848
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Cash and cash equivalents, fair value disclosure		7,498
Available-for-sale securities, fair value disclosure	18,488	42,848
Corporate Debt Securities [Member]
Available-for-sale securities, fair value disclosure	485,098	539,719
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities, fair value disclosure	485,098	$ 539,719
US Treasury Securities [Member]
Available-for-sale securities, fair value disclosure	4,042
US Treasury Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-for-sale securities, fair value disclosure	$ 4,042