Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2014

Fair Value Disclosures [Abstract]

Summary of Amortized Cost and Estimated Fair Values

The principal amounts of short-term investments are summarized in the tables below:

	Less Than 12 Months to Maturity
	Amortized Cost		Unrealized Gains		Unrealized Gains (Losses)			Fair Value
Balance at March 31, 2014:
U.S. Treasury securities	$	422,858	$	—	$	29		$	422,887
Total	$	422,858	$	—	$	29		$	422,887
Balance at December 31, 2013:
U.S. Treasury securities	$	346,618	$	—	$	(22	)	$	346,596
Total	$	346,618	$	—	$	(22	)	$	346,596

Financial Assets and Liabilities Measured at Fair Value on Recurring Basis

The following table presents information about the Company’s financial assets and liabilities measured at fair value on a recurring basis as of March 31, 2014:

	March 31,  2014		Quoted Price in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets
Cash equivalents
Money market fund	$	111,778	$	111,778	$	—	$	—
US treasury securities	$	29,009	$	—	$	29,009	$	—
Marketable securities
US treasury securities	$	422,887	$	—	$	422,887	$	—
Total	$	563,674	$	111,778	$	451,896	$	—

The following table presents information about the Company’s financial assets and liabilities measured at fair value on a recurring basis as of December 31, 2013:

	December 31,  2013		Quoted Price in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets
Cash equivalents
Money market fund	$	27,899	$	27,899	$	—	$	—
US treasury securities	$	21,021	$	—	$	21,021	$	—
Marketable securities
US treasury securities	$	346,596	$	—	$	346,596	$	—
Total	$	395,516	$	27,899	$	367,617	$	—