Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2016

The following table represents the fair value hierarchy based upon three levels of inputs, of which Levels 1 and 2 are considered observable and Level 3 is unobservable, for our financial assets and liabilities measured at fair value (in thousands):

	March 31, 2016						December 31, 2015
	Level 1			Level 2		Total	Level 1		Level 2		Total
Assets:
Cash equivalents:
Money market funds	$	79,156	$	—	$	79,156	$	35,730	$	—	$	35,730
Commercial paper		—		—		—		—		63,890		63,890
Short-term investments:
Corporate debt securities		—		56,623		56,623		—		31,253		31,253
Corporate equity securities		3,024		—		3,024		—		—		—
Municipal bonds		—		2,763		2,763		—		2,763		2,763
U.S. government agency bonds		—		23,245		23,245		—		16,201		16,201
Asset-backed securities		—		9,841		9,841		—		11,443		11,443
Total	$	82,180	$	92,472	$	174,652	$	35,730	$	125,550	$	161,280

Carrying Value and Estimated Fair Value of Debt

We have chosen not to measure our debt at fair value, with changes recognized in earnings each reporting period. The following table indicates the carrying value and estimated fair value of our debt as of the indicated periods (in thousands):

	March 31, 2016				December 31, 2015
	Carrying Value		Fair Value		Carrying Value		Fair Value
Credit agreement (carrying value including current maturities)	$	140,625	$	140,625	$	142,500	$	142,500
2010 Convertible debt (par value)		110,000		218,878		150,000		237,900
2016 Convertible debt (par value)		230,000		246,100		—		—