Cash, Cash Equivalents and Investment Debt Securities (Cash, Cash Equivalents and Investment Debt Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Cash and cash equivalents:
Cash and Cash Equivalents, Amortized Cost	$ 41,408	$ 70,055
Cash and cash equivalents, Fair Value	41,408	70,055
Investment Debt Securities Abstract
Total investments, Amortized Cost	508,986	581,771
Total cash and cash equivalents and investment debt securities, Amortized Cost	550,394	651,826
Total investments, Gross Unrealized Gains	223	878
Total cash and cash equivalents and investment debt securities, Gross Unrealized Gains	223	878
Total investments, Gross Unrealized Losses	(1,581)	(82)
Total cash and cash equivalents and investment debt securities, Gross Unrealized Losses	(1,581)	(82)
Total investments, Fair Value	507,628	590,065
Total Investments, Fair Value		582,567
Total cash and cash equivalents and investment debt securities, Fair Value	549,036	652,622
Commercial Paper [Member]
Cash and cash equivalents:
Cash and Cash Equivalents, Amortized Cost		7,498
Cash and cash equivalents, Fair Value		7,498
Investment Debt Securities Abstract
Total investments, Amortized Cost	18,478	42,806
Total investments, Gross Unrealized Gains	10	43
Total investments, Gross Unrealized Losses		(1)
Total investments, Fair Value	18,488	42,848
Corporate Debt Securities [Member]
Investment Debt Securities Abstract
Total investments, Amortized Cost	486,490	538,965
Total investments, Gross Unrealized Gains	189	835
Total investments, Gross Unrealized Losses	(1,581)	(81)
Total investments, Fair Value	485,098	539,719
US Treasury Securities [Member]
Investment Debt Securities Abstract
Total investments, Amortized Cost	4,018
Total investments, Gross Unrealized Gains	24
Total investments, Fair Value	4,042
Cash and Money Market Funds [Member]
Cash and cash equivalents:
Cash and Cash Equivalents, Amortized Cost	41,408	62,557
Cash and cash equivalents, Fair Value	$ 41,408	$ 62,557