Cash, Cash Equivalents and Short Term Investments (Tables)

6 Months Ended

Jun. 30, 2012

Cash, Cash Equivalents and Short-Term Investments [Abstract]

Short term investments classified by security type

	June 30, 2012
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses (1)		Estimated Fair Value
Corporate debt securities (2)	$	19,557	$	4	$	—	$	19,561
Municipal securities		3,470		—		—		3,470
Government-sponsored enterprise securities		22,873		27		1		22,899
Bank certificates of deposit		1,500		—		1		1,499
	$	47,400	$	31	$	2	$	47,429

	December 31, 2011
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses (1)		Estimated Fair Value
U.S. Treasury securities	$	3,258	$	9	$	—	$	3,267
Corporate debt securities (2)		22,454		6		2		22,458
Municipal securities		4,315		—		—		4,315
Government-sponsored enterprise securities		19,033		32		15		19,050
Bank certificates of deposit		2,650		—		—		2,650
	$	51,710	$	47	$	17	$	51,740

Effective maturities of short-term investments

	June 30, 2012				December 31, 2011
	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Due in one year or less	$	33,538	$	33,550	$	44,034	$	44,079
Due after one year through two years		13,862		13,879		7,676		7,661
	$	47,400	$	47,429	$	51,710	$	51,740

Fair value assets measurements on a recurring basis

	Fair value measurements at June 30, 2012 using:
	Level 1		Level 2		Level 3		Total estimated fair value
Cash	$	24,877	$	—	$	—	$	24,877
Corporate debt securities		—		21,060		—		21,060
Municipal securities		—		6,570		—		6,570
Government-sponsored enterprise securities		—		22,899		—		22,899
Money market funds		—		29,717		—		29,717
Bank certificates of deposit		—		1,499		—		1,499
	$	24,877	$	81,745	$	—	$	106,622

	Fair value measurements at December 31, 2011 using:
	Level 1		Level 2		Level 3		Total estimated fair value
Cash	$	25,359	$	—	$	—	$	25,359
U.S. Treasury securities		3,267		—		—		3,267
Corporate debt securities		—		27,208		—		27,208
Municipal securities		—		4,315		—		4,315
Government-sponsored enterprise securities		—		19,050		—		19,050
Money market funds		—		22,753		—		22,753
Bank certificates of deposit		—		3,050		—		3,050
	$	28,626	$	76,376	$	—	$	105,002