The fair value hierarchy prioritizes the inputs used in valuation techniques into three levels, as follows:

Pooled separate accounts – Pooled separate accounts invest primarily in domestic and international stocks, commercial paper or single mutual funds. The net asset value is used as a practical expedient to determine fair value for these accounts. Each pooled separate account provides for redemptions by the Retirement Plan at reported net asset values per share, with little to no advance notice requirement, therefore these funds are classified within Level 2 of the valuation hierarchy.

Long-Lived Assets Held-for-Sale – The Company recorded its long-lived assets associated with its property and equipment held-for-sale at fair value at December 31, 2020 and 2019 of $48,548,000 and $70,400,000, respectively. The fair values of these assets are based on observable values for the assets through corroboration with market data and are designated as Level 3 inputs.

Warrants issued to Senior Noteholders – The Company’s warrants issued December 22, 2019, were valued using the Black-Scholes Valuation Model and adjusted for quarterly. On August 5, 2020, these warrants were exercised in full and prior to exercise, the Company adjusted their fair value using the following assumptions (fair value dollars in thousands):

The assumptions used and related fair value for these warrants as of December 31, 2019 were as follows (fair value dollars in thousands):

Warrants issued in Equity Offering – The Company issued pre-funded warrants and other warrants with exercise prices of $0.001 and $9.757, respectively. The Company valued these warrants upon issuance using the Binomial valuation methodology. On November 16, 2020, the pre-funded warrants were exercised, and as a result, were revalued immediately prior to their exercise. Further, the other warrants were amended on November 24, 2020, resulting in equity accounting, and accordingly were revalued immediately prior to their amendment. The assumptions used were as follows (fair value dollars in thousands):

The fair values of the warrants are based on unobservable inputs and are designated as Level 3 inputs. The changes in the Company’s fair value of its Level 3 inputs with respect to its warrants were as follows (in thousands):

Other Derivative Instruments – The Company’s other derivative instruments consist of commodity positions. The fair values of the commodity positions are based on quoted prices on the commodity exchanges and are designated as Level 1 inputs.

The following table summarizes recurring and nonrecurring fair value measurements by level at December 31, 2020 (in thousands):

The following table summarizes recurring and nonrecurring fair value measurements by level at December 31, 2019 (in thousands):