Fair Value of Financial Instruments

6 Months Ended

Jun. 30, 2013

4.	Fair Value of Financial Instruments

The carrying values of our financial instruments including cash equivalents, accounts receivable and accounts payable approximate their fair value due to their short-term nature. As a basis for determining the fair value of certain of our assets and liabilities, we established a three-tier fair value hierarchy which prioritizes the inputs used in measuring fair value as follows: (Level I) observable inputs such as quoted prices in active markets; (Level II) inputs other than the quoted prices in active markets that are observable either directly or indirectly; and (Level III) unobservable inputs in which there is little or no market data which requires us to develop our own assumptions. This hierarchy requires us to use observable market data, when available, and to minimize the use of unobservable inputs when determining fair value. Our financial assets that are measured at fair value on a recurring basis consist of cash equivalents. Our liabilities that were measured at fair value on a recurring basis consisted of our common stock warrant liability.

All of our cash equivalents and current restricted cash, which are comprised primarily of money market funds, are classified within Level I of the fair value hierarchy because they are valued using quoted market prices or market prices for similar securities. Our common stock warrant liability (see Note 6) is classified within Level III of the fair value hierarchy.

As of June 30, 2013 and December 31, 2012, the fair value hierarchy for our financial assets and financial liabilities that are carried at fair value was as follows (in thousands):


	June 30, 2013								December 31, 2012
	Fair Value		Level I		Level II		Level III		Fair Value		Level I		Level II		Level III

Money market funds	$	581,209	$	581,209	$	—	$	—	$	60,272	$	60,272	$	—	$	—
Common stock warrant liability		—		—		—		—		10,692		—		—		10,692

The changes in the fair value of our common stock warrant liability (see Note 6) were as follows (in thousands):


	Three Months Ended June 30,					Six Months Ended June 30,
	2013		2012			2013			2012
Fair value, beginning of period	$	—	$	8,683		$	10,692		$	8,838
Change in fair value		—		(154	)		(10,692	)		(309	)

Fair value, end of period	$	—	$	8,529		$	—		$	8,529

The estimated fair value of our convertible senior notes based on a market approach was approximately $578.7 million (par value of $660.0 million) as of June 30, 2013, and represents a Level II valuation. The estimated fair value of our DOE Loan based on a market approach was approximately $366.9 million (par value of $452.3 million) as of December 31, 2012, and represents a Level II valuation. When determining the estimated fair value of our long-term debt, we used a commonly accepted valuation methodology and market-based risk measurements that are indirectly observable, such as credit risk.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

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