2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Use of Estimates in Preparation of Financial Statements. The preparation of the accompanying Financial Statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of our Financial Statements, and the reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates.

Cash and Cash Equivalents. We consider all highly liquid investments with original maturities of three months or less at the date of the purchase to be cash equivalents. As of March 31, 2015 and December 31, 2014, our cash equivalents consist primarily of institutional money market funds, commercial paper, and time deposits held at major banks.

As of March 31, 2015 and December 31, 2014, we had $5.0 million and $4.7 million, respectively, of restricted cash that serves to collateralize outstanding letters of credit. This restricted cash is included in cash and cash equivalents in our Condensed Consolidated Balance Sheets (“Balance Sheets” or “Balance Sheet”).

Short-term Investments and Other Financial Instruments. Our financial instruments as of March 31, 2015 and December 31, 2014 include cash and cash equivalents, short-term investments, accounts receivable, accounts payable, and debt. Because of their short maturities, the carrying amounts of cash equivalents, accounts receivable, and accounts payable approximate their fair value.

Our short-term investments and certain of our cash equivalents are considered “available-for-sale” and are reported at fair value in our Balance Sheets, with unrealized gains and losses, net of the related income tax effect, excluded from earnings and reported in a separate component of stockholders’ equity. Realized and unrealized gains and losses were not material in any period presented.

Primarily all short-term investments held by us as of March 31, 2015 and December 31, 2014 have contractual maturities of less than two years from the time of acquisition. Our short-term investments as of March 31, 2015 and December 31, 2014 consisted almost entirely of fixed income securities. Proceeds from the sale/maturity of short-term investments for the first quarters of 2015 and 2014 were $49.5 million and $50.9 million, respectively.

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, 2015						December 31, 2014
	Level 1			Level 2		Total	Level 1		Level 2		Total
Assets:
Cash equivalents:
Money market funds	$	26,081	$	—	$	26,081	$	9,785	$	—	$	9,785
Commercial paper		—		7,564		7,564		—		12,248		12,248
Short-term investments:
Corporate debt securities		—		50,100		50,100		—		88,494		88,494
Municipal bonds		—		7,289		7,289		—		9,945		9,945
U.S. government agency bonds		—		16,091		16,091		—		11,313		11,313
Asset-backed securities		—		7,137		7,137				10,336		10,336
Total	$	26,081	$	88,181	$	114,262	$	9,785	$	132,336	$	142,121

Valuation inputs used to measure the fair values of our money market funds were derived from quoted market prices. The fair values of all other financial instruments are based upon pricing provided by third-party pricing services. These prices were derived from observable market inputs.

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, 2015				December 31, 2014
	Carrying Value		Fair Value		Carrying Value		Fair Value
Credit agreement (carrying value including current maturities)	$	148,125	$	148,125	$	120,000	$	120,000
Convertible debt (par value)		150,000		203,880		150,000		178,920

The fair value for our credit agreement was estimated using a discounted cash flow methodology, while the fair value for our convertible debt was estimated based upon quoted market prices or recent sales activity, both of which are considered Level 2 inputs. See Note 4 for discussion regarding the amendment to our credit agreement.

Accounting Pronouncement Issued But Not Yet Effective. In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606). This ASU is a single comprehensive model which supersedes nearly all existing revenue recognition guidance under U.S. GAAP. Under the new guidance, revenue is recognized when promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received for those goods or services. The ASU also requires additional disclosure about the nature, amount, timing, and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. The updated accounting guidance is effective for annual and interim reporting periods in fiscal years beginning after December 15, 2016. Early adoption is not permitted. An entity may choose to adopt this ASU either retrospectively or through a cumulative effect adjustment as of the start of the first period for which it applies the standard. We are currently in the process of evaluating the impact that this new guidance will have on our consolidated financial statements and our method of adoption.