Cash and cash equivalents include bank demand deposits, certificates of deposit, investments in debt securities with maturities of three months or less when purchased, and an interest in money market funds which invest primarily in US government obligations. Investments in marketable securities consist of equity securities and corporate or government debt securities that have a readily ascertainable market value and are readily marketable. We report these investments at fair value. We designate all cash equivalents and marketable securities as available-for-sale, with unrealized gains and losses included as a component of equity.

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value

At March 31, 2012
Money market funds	$	1,437	$	—	$	—		$	1,437
U.S. corporate debt securities		36,922		1		—			36,923
Debt securities issued by U.S. government and U.S. government agencies		72,461		6		(5	)		72,462
Marketable equity securities		2,687		1,001		—			3,688

Total	$	113,507	$	1,008	$	(5	)	$	114,510

At December 31, 2011
Money market funds	$	4,469	$	—	$	—		$	4,469
U.S. corporate debt securities		32,830		—		(1	)		32,829
Debt securities issued by U.S. government and U.S. government agencies		72,901		14		(2	)		72,913
Marketable equity securities		2,726		—		(907	)		1,819

Total	$	112,926	$	14	$	(910	)	$	112,030

The available-for-sale securities are classified on the balance sheet as follows (in thousands):

	Fair Value

	March 31, 2012		December 31, 2011
Amounts included in cash and cash equivalents	$	23,042	$	23,767
Marketable securities, current		87,780		86,444
Marketable securities, non-current		3,688		1,819

Total	$	114,510	$	112,030

At March 31, 2012, all of our debt securities were due in one year or less based on their contractual maturities.

In the three month periods ended March 31, 2012 and 2011, realized gains and losses calculated using the specific identification method were immaterial.

We evaluate investments with unrealized losses to determine if the losses are other than temporary. In making this determination, we consider the financial condition and near-term prospects of the issuers, the magnitude of the losses compared to the investments' amortized cost, changes in the credit quality of the issuer, the length of time the investments have been in an unrealized loss position, and whether it is more likely than not that we will hold the investment for a period of time sufficient for a recovery of our cost basis.

Equity investments in securities without readily determinable fair value, which consist of investments in privately held companies, are carried at cost. As of March 31, 2012 and December 31, 2011, we held one such non-marketable security with a carrying amount of $500,000. This investment is included in other assets in the condensed consolidated balance sheets. We periodically review this investment and evaluate whether an impairment has occurred. We monitor the liquidity and financing activities of the issuer of this security and evaluate, among other factors, the financial condition and business outlook of the issuer, including key operational and cash flow metrics and current market conditions, as well as our intent and ability to retain the investment. We believe this equity investment continues to be realizable.