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, 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

	December 31, 2016
(In thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
U.S. government agencies	$	12,428	$	1	$	—	$	12,429
U.S. commercial papers		72,065		—		—		72,065
Money market funds		64,319		—		—		64,319

Total	$	148,812	$	1	$	—	$	148,813

As of December 31, 2017, 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 the amortized cost basis. The estimated fair values were as follows:

	Estimated Fair Value Measurements as of December 31, 2017 Using:
	Quoted Price in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
Types of Instruments (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

	Estimated Fair Value Measurements as of December 31, 2016 Using:
	Quoted Price in Active Markets for Identical Assets		Significant Other Observable Inputs		Significant Unobservable Inputs
Types of Instruments (In thousands)	Level 1		Level 2		Level 3		Total
Assets
U.S. government agencies	$	—	$	12,429	$	—	$	12,429
U.S. commercial papers		—		72,065		—		72,065
Money market funds		64,319		—		—		64,319

Total assets measured at estimated fair value	$	64,319	$	84,494	$	—	$	148,813


Liabilities
2023 Notes	$	—	$	202,125	$	—	$	202,125
2029 Notes		—		487,189		—		487,189

Total fair value of liabilities	$	—	$	689,314	$	—	$	689,314

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 convertible subordinated notes due 2023 (the "2023 Notes"), of our 2025 Notes and of our previously outstanding 2029 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.