4. AVAILABLE-FOR-SALE SECURITIES AND FAIR VALUE MEASUREMENTS

Available-for-Sale Securities

The estimated fair value of available-for-sale securities is based on quoted market prices for these or similar investments that were based on prices obtained from a commercial pricing service. Available-for-sale securities are summarized below:


	December 31, 2018
			Gross		Gross
			Unrealized		Unrealized		Estimated
(In thousands)	Amortized Cost		Gains		Losses		Fair Value
U.S. government securities	$	29,736	$	—	$	(3)	$	29,733
U.S. government agencies		4,971		—		—		4,971
U.S. corporate notes		2,875		—		—		2,875
U.S. commercial paper		22,037		—		—		22,037
Money market funds		49,358		—		—		49,358
Total	$	108,977	$	—	$	(3)	$	108,974


	December 31, 2017
			Gross		Gross
			Unrealized		Unrealized		Estimated
(In thousands)	Amortized Cost		Gains		Losses		Fair Value
U.S. government securities	$	9,943	$	—	$	(1)	$	9,942
U.S. government agencies		9,987		—		(2)		9,985
U.S. corporate notes		10,881		—		(15)		10,866
U.S. commercial paper		29,945		—		—		29,945
Money market funds		61,971		—		—		61,971
Total	$	122,727	$	—	$	(18)	$	122,709

As of December 31, 2018, all of the available‑for‑sale debt securities had contractual maturities within one year, and the average duration of debt securities was approximately three months.

Fair Value Measurements

Our available‑for‑sale securities are measured at fair value on a recurring basis and our debt is carried at amortized cost basis. The estimated fair values were as follows:


	Estimated Fair Value Measurements as of December 31, 2018 Using:
	Quoted Price in
	Active Markets		Significant Other		Significant
	for Identical		Observable		Unobservable
Types of Instruments	Assets		Inputs		Inputs
(In thousands)	Level 1		Level 2		Level 3		Total
Assets
U.S. government securities	$	—	$	29,733	$	—	$	29,733
U.S. government agencies		—		4,971		—		4,971
U.S. corporate notes		—		2,875		—		2,875
U.S. commercial paper		—		22,037		—		22,037
Money market funds		49,358		—		—		49,358
Total assets measured at estimated fair value	$	49,358	$	59,616	$	—	$	108,974
Debt
Term B Loan	$	—	$	13,750	$	—	$	13,750
2023 Notes		—		258,918		—		258,918
2025 Notes		—		230,692		—		230,692
Total fair value of debt	$	—	$	503,360	$	—	$	503,360


	Estimated Fair Value Measurements as of December 31, 2017 Using:
	Quoted Price in
	Active Markets		Significant Other		Significant
	for Identical		Observable		Unobservable
Types of Instruments	Assets		Inputs		Inputs
(In thousands)	Level 1		Level 2		Level 3		Total
Assets
U.S. government securities	$	—	$	9,942	$	—	$	9,942
U.S. government agencies		—		9,985		—		9,985
U.S. corporate notes		—		10,866		—		10,866
U.S. commercial paper		—		29,945		—		29,945
Money market funds		61,971		—		—		61,971
Total assets measured at estimated fair value	$	61,971	$	60,738	$	—	$	122,709
Debt
Term B Loan	$	—	$	243,750	$	—	$	243,750
2023 Notes		—		241,259		—		241,259
2025 Notes		—		205,975		—		205,975
Total fair value of debt	$	—	$	690,984	$	—	$	690,984

The fair value of our marketable securities classified within Level 2 is based upon observable inputs that may include benchmark yields, reported trades, broker/dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids, offers and reference data including market research publications.

The fair value of our 2023 Notes and of our 2025 Notes is based on recent trading prices of the instruments. The carrying amount of our initial Term B Loan before deducting debt issuance costs approximates fair value as the loan carries a variable interest rate that is tied to the LIBOR rate plus an applicable spread.