4. Available-for-Sale Securities and Fair Value Measurements

Available-for-Sale Securities

The following table summarizes the classification of the available-for-sale securities in our consolidated balance sheets:

	December 31,
(In thousands)	2016		2015
Cash equivalents	$	323,602	$	69,126
Short-term marketable securities		156,387		59,727
Long-term marketable securities		91,565		42,860

Total	$	571,554	$	171,713

The estimated fair value of marketable securities is based on quoted market prices for these or similar investments that were based on prices obtained from a commercial pricing service. 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.

Available-for-sale securities are summarized below:

		December 31, 2016
(In thousands)		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. government securities	Level 1	$	69,963	$	39	$	(47	)	$	69,955
U.S. government agency securities	Level 2		60,783		9		(45	)		60,747
Corporate notes	Level 2		98,522		4		(213	)		98,313
Commercial paper	Level 2		18,937		—		—			18,937

Marketable securities			248,205		52		(305	)		247,952
Money market funds	Level 1		323,602		—		—			323,602

Total		$	571,807	$	52	$	(305	)	$	571,554

		December 31, 2015
(In thousands)		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. government securities	Level 1	$	47,068	$	4	$	(29	)	$	47,043
U.S. government agency securities	Level 2		31,502		—		(37	)		31,465
Corporate notes	Level 2		19,098		2		(11	)		19,089
Commercial paper	Level 2		4,990		—		—			4,990

Marketable securities			102,658		6		(77	)		102,587
Money market funds	Level 1		69,126		—		—			69,126

Total		$	171,784	$	6	$	(77	)	$	171,713

At December 31, 2016, all of the available-for-sale securities had contractual maturities within two years and the weighted average maturity of marketable securities was approximately 10 months. There were no transfers between Level 1 and Level 2 during the periods presented.

We do not intend to sell the investments that are in an unrealized loss position, and it is unlikely that we will be required to sell the investments before recovery of their amortized cost basis, which may be maturity. We have determined that the gross unrealized losses on our marketable securities at December 31, 2016 were temporary in nature. All marketable securities with unrealized losses at December 31, 2016 have been in a loss position for less than twelve months or the loss is not material.

There were no sales in 2016 and 2015. During 2014, we sold available-for-sale securities totaling $0.9 million, and the related realized gains and losses were not material.