Investments

The following is a summary of our short-term investments (in thousands):



	March 31, 2013
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-sale securities:
Commercial paper	$	80,565	$	2	$	(6	)	$	80,561
Corporate notes and bonds	173,395		4		(93		)	173,306
Total available-for-sale securities	$	253,960	$	6	$	(99	)	$	253,867



	December 31, 2012
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-sale securities:
Commercial paper	$	72,850	$	—	$	(15	)	$	72,835
Corporate notes and bonds	158,038		8		(98		)	157,948
U.S. government agency securities	1,001		—		—			1,001
Total available-for-sale securities	$	231,889	$	8	$	(113	)	$	231,784

As of March 31, 2013 and December 31, 2012, we had certain available-for-sale securities in a gross unrealized loss position, all of which had been in such position for less than twelve months. There were no impairments considered other-than-temporary as it is more likely than not we will hold the securities until maturity or a recovery of the cost basis. The following table shows the fair values and the gross unrealized losses of these available-for-sale securities aggregated by investment types (in thousands):



	March 31, 2013					December 31, 2012
	Fair Value		Gross Unrealized Losses			Fair Value		Gross Unrealized Losses
Commercial paper	$	14,185	$	(6	)	$	36,753	$	(15	)
Corporate notes and bonds	139,242		(93		)	137,558		(98		)
Total	$	153,427	$	(99	)	$	174,311	$	(113	)

Realized gains and losses are determined based on the specific identification method and are reported in interest and other income (expense), net in our condensed consolidated statements of comprehensive loss.