Investments

During the year ended December 31, 2012, we purchased commercial paper, corporate notes and bonds and U.S. government agency securities, all with maturities of less than twelve months. We had no investments as of December 31, 2011. The following is a summary of our investments (in thousands):



	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 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 was 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 category (in thousands):



	December 31, 2012
	Fair Value		Gross Unrealized Losses
Commercial paper	$	36,753	$	(15	)
Corporate notes and bonds	137,558		(98		)
Total	$	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 consolidated statements of comprehensive income (loss). For the year ended December 31, 2012, gross realized gains and losses on sales of our available-for-sale securities were immaterial.