Fair Value Measurements. Cash Equivalents and Marketable Securities - Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
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
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	8,066	$ 25,796
Debt securities, short-term	375,768	278,417
Debt securities, long-term	302,624	77,563
Total assets	686,458	381,776
Initial fair value of contingent consideration at acquisition date	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	8,066	25,796
Debt securities, short-term	0	0
Debt securities, long-term	0	0
Total assets	8,066	25,796
Initial fair value of contingent consideration at acquisition date	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure	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	375,768	278,417
Debt securities, long-term	302,624	77,563
Total assets	678,392	355,980
Initial fair value of contingent consideration at acquisition date	1,065
Financial and Nonfinancial Liabilities, Fair Value Disclosure	1,065
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	0
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0
Fair Value, Measurements, Recurring | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	23,029	38,397
Debt securities, long-term		3,805
Fair Value, Measurements, Recurring | Corporate bonds | 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
Fair Value, Measurements, Recurring | Corporate bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	23,029	38,397
Debt securities, long-term		3,805
Fair Value, Measurements, Recurring | Corporate bonds | 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
Fair Value, Measurements, Recurring | U.S. government debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	352,739	235,016
Debt securities, long-term	302,624	73,758
Fair Value, Measurements, Recurring | U.S. government debt securities | 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
Fair Value, Measurements, Recurring | U.S. government debt securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	352,739	235,016
Debt securities, long-term	302,624	73,758
Fair Value, Measurements, Recurring | U.S. government debt securities | 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
Fair Value, Measurements, Recurring | U.S. government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term		5,004
Fair Value, Measurements, Recurring | U.S. government agency bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term		0
Fair Value, Measurements, Recurring | U.S. government agency bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term		5,004
Fair Value, Measurements, Recurring | U.S. government agency bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term		0
Fair Value, Measurements, Recurring | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	8,066	25,796
Fair Value, Measurements, Recurring | Money market funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	8,066	25,796
Fair Value, Measurements, Recurring | Money market funds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Fair Value, Measurements, Recurring | Money market funds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 0	$ 0