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:

	September 30, 2017
(In thousands)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. government agencies	$	9,959	$	—	$	—		$	9,959
U.S. corporate notes	9,879		—		(3		)	9,876
U.S. commercial paper	24,582		—		—			24,582
Money market funds	118,163		—		—			118,163

Total	$	162,583	$	—	$	(3	)	$	162,580

	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 paper	72,065		—		—		72,065
Money market funds	64,319		—		—		64,319

Total	$	148,812	$	1	$	—	$	148,813

As of September 30, 2017, all of the available-for-sale securities had contractual maturities within one year and the weighted average maturity of marketable securities was approximately four 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 September 30, 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 agencies	$	—	$	9,959	$	—	$	9,959
U.S. corporate notes	—		9,876		—		9,876
U.S. commercial paper	—		24,582		—		24,582
Money market funds	118,163		—		—		118,163

Total assets measured at estimated fair value	$	118,163	$	44,417	$	—	$	162,580

Liabilities
Term B Loan	$	—	$	250,000	$	—	$	250,000
2023 Notes	—		234,658		—		234,658
2025 Notes	—		202,125		—		202,125

Total fair value of liabilities	$	—	$	686,783	$	—	$	686,783

	Estimated Fair Value Measurements as of December 31, 2016 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 agencies	$	—	$	12,429	$	—	$	12,429
U.S. commercial paper	—		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 2023 Notes, of our 2025 Notes and of our previously outstanding non-recourse fixed rate term notes due 2029 (the “2029 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.