Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2019

Fair Value Measurements [Abstract]

Fair Value, Marketable Securities Measured on Recurring and Nonrecurring Basis

Financial assets carried at fair value are classified in the tables below in one of the three categories described above:

			Fair Value Measurements Using
	Total		Level 1		Level 2		Level 3
	(in thousands)
September 30, 2019
Assets
Cash and cash equivalents:
Money market funds (included in cash and cash equivalents)	$	31,691	$	31,691	$	—	$	—
Available-for-sale debt securities:
Commercial paper		70,063		—		70,063		—
Corporate debt securities		574,935		—		574,935		—
U.S. government and agency securities		3,996		—		3,996		—
Total financial assets	$	680,685	$	31,691	$	648,994	$	—
December 31, 2018
Assets
Money market funds (included in cash and cash equivalents)	$	11,647	$	11,647	$	—	$	—
Available-for-sale debt securities:
Commercial paper		34,327		—		34,327		—
Corporate debt securities		349,177		—		349,177		—
U.S. government and agency securities		9,408		—		9,408		—
Total financial assets	$	404,559	$	11,647	$	392,912	$	—

Schedule of Available for Sale Securities Debt Maturities

The estimated fair value of the available-for-sale debt securities (commercial paper, corporate debt securities and U.S. government and agency securities), by contractual maturity, are as follows:

	Fair Value as of
	September 30, 2019		December 31, 2018
	(in thousands)
Due in one year or less	$	486,971	$	319,717
Due after one year through two years		162,023		73,195
Total investments in debt securities	$	648,994	$	392,912