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

Our available-for-sale securities include:

	Fair Value
	Hierarchy	Estimated Fair Value
(In thousands)	Level	September 30, 2016		December 31, 2015
U.S. government securities	Level 1	$	42,125	$	47,043
U.S. government agency securities	Level 2	73,957		31,465
Corporate notes	Level 2	58,971		19,089
Commercial paper	Level 2	27,626		4,990

Marketable securities		202,679		102,587
Money market funds	Level 1	67,841		69,126

Total		$	270,520	$	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. Net unrealized gains and losses were immaterial at September 30, 2016 and December 31, 2015.

At September 30, 2016, all of the marketable securities had contractual maturities within two years and the weighted average maturity of the marketable securities was approximately nine months. There were no transfers between Level 1 and Level 2 during the periods presented and there have been no changes to our valuation techniques during the three and nine months ended September 30, 2016.

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 September 30, 2016 were temporary in nature. All marketable securities with unrealized losses at September 30, 2016 have been in a loss position for less than twelve months.

At September 30, 2016, our accumulated other comprehensive income (loss) on our condensed consolidated balance sheets consisted of net unrealized gains on available-for-sale investments. During the three and nine months ended September 30, 2016, we did not sell any of our marketable securities.

Restricted cash pertained to certain lease agreements and letters of credit where we have pledged cash and cash equivalents as collateral.