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:

	March 31, 2018
(In thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. government securities	$	9,978	$	—	$	(3	)	$	9,975
U.S. corporate notes	11,884		—		(19		)	11,865
U.S. commercial paper	12,476		—		—			12,476
Money market funds	17,118		—		—			17,118

Total	$	51,456	$	—	$	(22	)	$	51,434

	December 31, 2017
(In thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated 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 papers	29,945		—		—			29,945
Money market funds	61,971		—		—			61,971

Total	$	122,727	$	—	$	(18	)	$	122,709

As of March 31, 2018, all of the available-for-sale securities had contractual maturities within one year and the weighted average maturity of marketable securities was approximately two months.

Fair Value Measurements

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

	Estimated Fair Value Measurements as of March 31, 2018 Using:
Types of Instruments	Quoted Price in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
(In thousands)	Level 1		Level 2		Level 3		Total
Assets
U.S. government securities	$	—	$	9,975	$	—	$	9,975
U.S. corporate notes	—		11,865		—		11,865
U.S. commercial paper	—		12,476		—		12,476
Money market funds	17,118		—		—		17,118

Total assets measured at estimated fair value	$	17,118	$	34,316	$	—	$	51,434

Liabilities
Term B Loan	$	—	$	123,750	$	—	$	123,750
2023 Notes	—		259,058		—		259,058
2025 Notes	—		223,106		—		223,106

Total fair value of liabilities	$	—	$	605,914	$	—	$	605,914

	Estimated Fair Value Measurements as of December 31, 2017 Using:
Types of Instruments	Quoted Price in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable 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 papers	—		29,945		—		29,945
Money market funds	61,971		—		—		61,971

Total assets measured at estimated fair value	$	61,971	$	60,738	$	—	$	122,709

Liabilities
Term B Loan	$	—	$	243,750	$	—	$	243,750
2023 Notes	—		241,259		—		241,259
2025 Notes	—		205,975		—		205,975

Total fair value of liabilities	$	—	$	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 senior secured term loan (the “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.