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Fair Value of Financial Instruments and Concentration of Credit Risk
12 Months Ended
Dec. 31, 2015
Fair Value Disclosures [Abstract]  
Fair Value of Financial Instruments and Concentration of Credit Risk
3. Fair Value of Financial Instruments and Concentration of Credit Risk

All of our financial instruments are recognized in our consolidated balance sheets. Carrying values approximate fair value of most financial assets and liabilities. The fair value of restricted marketable securities, consisting primarily of money market funds, certificates of deposit, municipal bonds, corporate bonds, and U.S. treasury bills, is recorded at amortized cost, which approximates fair value for similar instruments. The interest rates on our restricted marketable security investments approximate current market rates for these types of investments. Therefore, the recorded value of the restricted marketable securities approximates fair value.

Financial instruments that potentially subject us to concentration of credit risk consist primarily of cash, marketable securities, restricted marketable securities and trade accounts receivable. We limit investment of cash and marketable securities to financial institutions with high credit ratings, corporate bonds and to governmental debt instruments. Credit risk on trade receivables is minimized as a result of the large and diverse nature of our customer base.

At December 31, 2015, we had significant concentrations of credit risk as follows:

 

   

Marketable securities—$273.2 million, at fair value, in money market funds.

 

   

Marketable securities—$10.5 million, at fair value, in certificates of deposit.

 

   

Marketable securities—$6.7 million, at fair value, in municipal bonds.

 

   

Marketable securities—$5.9 million, at fair value, in corporate bonds.

 

   

Marketable securities—$4.8 million, at fair value, in U.S. treasuries.