Fair Value Measurements. Cash Equivalents and Marketable Securities - Measured on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 35,592	$ 10,734
Debt securities, short-term	367,853	379,574
Debt securities, long-term	238,206	268,783
Total assets	641,651	659,091
Initial fair value of contingent consideration at acquisition date	1,175	1,365
Financial and nonfinancial liabilities, fair value disclosure	1,175	1,365
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	35,592	10,734
Debt securities, short-term	0	0
Debt securities, long-term	0	0
Total assets	35,592	10,734
Initial fair value of contingent consideration at acquisition date	0
Financial and nonfinancial liabilities, fair value disclosure	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Debt securities, short-term	367,853	379,574
Debt securities, long-term	238,206	268,783
Total assets	606,059	648,357
Initial fair value of contingent consideration at acquisition date	0	0
Financial and nonfinancial liabilities, fair value disclosure	0	0
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	1,175	1,365
Financial and nonfinancial liabilities, fair value disclosure	1,175	1,365
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	4,035	16,690
Corporate bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	0	0
Corporate bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	4,035	16,690
Corporate bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	0	0
U.S. government debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	363,818	362,884
Debt securities, long-term	238,206	268,783
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
U.S. government debt securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, short-term	363,818	362,884
Debt securities, long-term	238,206	268,783
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
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	35,592	10,734
Money market funds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	35,592	10,734
Money market funds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Money market funds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 0	$ 0