Fair Value Measurements (Fair Value, Marketable Securities Measured on Recurring and Nonrecurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash and cash equivalents, fair value disclosure	$ 63,377
Available-For-Sale Securities, Fair Value Disclosure	648,994	$ 392,912
Total financial assets	680,685	404,559
Fair Value, Inputs, Level 1 [Member]
Total financial assets	31,691	11,647
Fair Value, Inputs, Level 2 [Member]
Total financial assets	648,994	392,912
Money Market Funds [Member]
Cash and cash equivalents, fair value disclosure	31,691	11,647
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents, fair value disclosure	31,691	11,647
Commercial Paper [Member]
Available-For-Sale Securities, Fair Value Disclosure	70,063	34,327
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Available-For-Sale Securities, Fair Value Disclosure	70,063	34,327
Corporate Debt Securities [Member]
Available-For-Sale Securities, Fair Value Disclosure	574,935	349,177
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-For-Sale Securities, Fair Value Disclosure	574,935	349,177
US Government Agencies Debt Securities [Member]
Available-For-Sale Securities, Fair Value Disclosure	3,996	9,408
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-For-Sale Securities, Fair Value Disclosure	$ 3,996	$ 9,408