The Company’s investment policy defines allowable investments and establishes guidelines relating to credit quality, diversification, and maturities of its investments to preserve principal and maintain liquidity. In accordance with the Company’s investment policy, it has invested funds in available-for-sale securities as of March 31, 2021 and December 31, 2020.

The cost, gross unrealized holding gains, gross unrealized holding losses and fair value of available-for-sale investments by types and classes of security at March 31, 2021 and December 31, 2020 consisted of the following (in thousands):

At March 31, 2021	Amortized Cost		Unrealized Gains		Unrealized Losses			Estimated Fair Value
U.S. treasury securities	$	173,916	$	17	$	—		$	173,933
Total	$	173,916	$	17	$	—		$	173,933
At December 31, 2020
U.S. treasury securities	$	215,185	$	—	$	(31	)	$	215,154
Total	$	215,185	$	—	$	(31	)	$	215,154

The Company reviews its investments to identify and evaluate investments that have an indication of possible other-than-temporary impairment. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition and near-term prospects of the investee, and the Company’s intent and ability to hold the investment for a period of time sufficient to allow for any anticipated recovery in market value. These securities have not been in a continuous loss position for more than 12 months. Further, the Company does not generally intend to sell these investments and it is not more likely than not that the Company will be required to sell these investments before recovery of their amortized cost basis which may be at maturity.

None of the Company’s available-for-sale investments had maturities of greater than 12 months at March 31, 2021 and December 31, 2020. The Company has determined that there were no material declines in fair value of its investments due to credit-related factors as of March 31, 2021 and December 31, 2020.