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Fair Value Measurements
9 Months Ended
Sep. 30, 2021
Fair Value Disclosures [Abstract]  
Fair Value, Measurement Inputs, Disclosure Fair Value Measurements
The Company utilizes the market approach to measure the fair value of its financial assets. The market approach uses prices and other relevant information generated by market transactions involving identical or comparable assets. The assets classified as Level 2 primarily utilize quoted market prices or alternative pricing sources including transactions involving identical or comparable assets and models utilizing market observable inputs for valuation of these securities. No changes in valuation techniques or inputs occurred during the nine months ended September 30, 2021.

Financial instruments measured at fair value on a recurring basis:
September 30, 2021December 31, 2020
(in thousands)TotalLevel 1Level 2TotalLevel 1Level 2
Cash equivalents   
Money market funds$100,965 $100,965 $— $5,340 $5,340 $— 
Commercial Paper25,199 — 25,199 48,908 — 48,908 
Municipal debt securities16,674 — 16,674 34,338 — 34,338 
Federal agency debt securities— — — 51,400 — 51,400 
Total cash equivalents142,838 100,965 41,873 139,986 5,340 134,646 
Short-term     
Commercial paper387,044 — 387,044 229,821 — 229,821 
Corporate debt securities267,821 — 267,821 166,768 — 166,768 
Municipal debt securities219,479 — 219,479 87,290 — 87,290 
Federal agency debt securities3,000 — 3,000 48,598 — 48,598 
Total short-term877,344 — 877,344 532,477 — 532,477 
Total financial instruments$1,020,182 $100,965 $919,217 $672,463 $5,340 $667,123 

None of the Company's debt is publicly held and as a result, the Company has determined the estimated fair value of these notes to be Level 3. Certain inputs used to determine fair value are unobservable and, therefore, could be sensitive to changes in inputs. The Company utilizes the discounted cash flow method to estimate the fair value of Level 3 debt.

Carrying value and estimated fair value of long-term debt, including current maturities and without reduction for related costs, are as follows:
September 30, 2021December 31, 2020
(in thousands)Carrying ValueEstimated Fair ValueCarrying ValueEstimated Fair ValueHierarchy Level
Non-publicly held debt$1,329,907 $1,157,257 $1,555,637 $1,191,008 3

Due to their short-term nature, the carrying amounts of cash, cash equivalents, restricted cash, accounts receivable and accounts payable approximate fair value.