The available-for-sale marketable securities are primarily comprised of investments in commercial paper, corporate bonds and government securities and consist of the following, measured at fair value on a recurring basis (in thousands):

Accrued interest receivable on available-for-sale securities is included in amounts receivable and was $9,000 and $34,000 as of June 30, 2021 and December 31, 2020, respectively.

There were no transfers of assets between Level 1 and Level 2 of the fair value measurement hierarchy during the six months ended June 30, 2021.

Under the terms of the Company’s investment policy, purchases of marketable securities are limited to investment grade governmental and corporate obligations and bank certificates of deposit with a primary objective of principal preservation. Maturities of individual securities are less than one year and the amortized cost of all securities approximated fair value as of June 30, 2021 and December 31, 2020.