Fair Value Measurements. Cash Equivalents and Marketable Securities - Fair Value Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Initial fair value of contingent consideration at acquisition date	$ 1,065	$ 0	$ 0
Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	10,734	25,796
Debt securities, short-term	379,574	278,417
Debt securities, long-term	268,783	77,563
Total assets	659,091	381,776
Initial fair value of contingent consideration at acquisition date	1,365
Total liabilities	1,365
Fair value, measurements, recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	10,734	25,796
Debt securities, short-term	0	0
Debt securities, long-term	0	0
Total assets	10,734	25,796
Initial fair value of contingent consideration at acquisition date	0
Total liabilities	0
Fair value, measurements, recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Debt securities, short-term	379,574	278,417
Debt securities, long-term	268,783	77,563
Total assets	648,357	355,980
Initial fair value of contingent consideration at acquisition date	0
Total liabilities	0
Fair value, measurements, recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Debt securities, short-term	0	0
Debt securities, long-term	0	0
Total assets	0	0
Initial fair value of contingent consideration at acquisition date	1,365
Total liabilities	1,365
Corporate bonds | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	16,690	38,397
Debt securities, long-term		3,805
Corporate bonds | Fair value, measurements, recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	0	0
Debt securities, long-term		0
Corporate bonds | Fair value, measurements, recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	16,690	38,397
Debt securities, long-term		3,805
Corporate bonds | Fair value, measurements, recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	0	0
Debt securities, long-term		0
U.S. government debt securities | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	362,884	235,016
Debt securities, long-term	268,783	73,758
U.S. government debt securities | Fair value, measurements, recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	0	0
Debt securities, long-term	0	0
U.S. government debt securities | Fair value, measurements, recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	362,884	235,016
Debt securities, long-term	268,783	73,758
U.S. government debt securities | Fair value, measurements, recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	0	0
Debt securities, long-term	0	0
U.S. government agency bonds | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term		5,004
U.S. government agency bonds | Fair value, measurements, recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term		5,004
Money market funds | Fair value, measurements, recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	10,734	25,796
Money market funds | Fair value, measurements, recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	10,734	25,796
Money market funds | Fair value, measurements, recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Money market funds | Fair value, measurements, recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 0	$ 0