Investments

12 Months Ended

Dec. 31, 2020

Investments Debt And Equity Securities [Abstract]

5.	Investments

Available-for-sale investments consisted of the following as of December 31, 2020 and 2019 (in thousands):

	December 31, 2020
	Amortized Cost		Unrealized Gain		Unrealized Loss			Estimated Fair Value
Short-term marketable securities
U.S. government debt securities	$	552,539	$	723	$	(10	)	$	553,252
Corporate bonds		11,497		86		(2	)		11,581
Total short-term marketable securities	$	564,036	$	809	$	(12	)	$	564,833
Long-term marketable securities
U.S. government debt securities	$	118,429	$	98	$	(2	)	$	118,525
Total long-term marketable securities	$	118,429	$	98	$	(2	)	$	118,525

	December 31, 2019
	Amortized Cost		Unrealized Gain		Unrealized Loss			Estimated Fair Value
Short-term marketable securities
Commercial paper	$	121,866	$	—	$	—		$	121,866
U.S. government debt securities		285,963		394		(1	)		286,356
Corporate bonds		71,962		109		(3	)		72,068
Total short-term marketable securities	$	479,791	$	503	$	(4	)	$	480,290
Long-term marketable securities
U.S. government debt securities	$	90,750	$	146	$	(9	)	$	90,887
Corporate bonds		14,513		35		—			14,548
Total long-term marketable securities	$	105,263	$	181	$	(9	)	$	105,435

All the commercial paper, U.S. government debt securities and corporate bonds designated as short-term marketable securities have an effective maturity date that is equal to or less than one year from the respective balance sheet date. Those that are designated as long-term marketable securities have an effective maturity date that is more than one year from the respective balance sheet date.

Accrued interest receivables are excluded from the amortized cost and estimated fair value of our marketable securities. Accrued interest receivables of $2.5 million and $2.2 million were presented separately within the prepaid expenses and other current assets line item on our balance sheet as of December 31, 2020 and 2019, respectively. We have made an accounting policy election to not measure an allowance for credit losses for accrued interest receivables.

The following table presents the gross unrealized holding losses and fair value for investments in an unrealized loss position and the length of time that individual securities have been in a continuous loss position as of December 31, 2020 (in thousands):

	Less Than 12 Months					12 Months Or Greater
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
U.S. government debt securities	$	177,554	$	(12	)	$	—	$	—
Corporate bonds		1,512		(2	)		—		—
Total available-for-sale securities	$	179,066	$	(14	)	$	—	$	—

We periodically review our available-for-sale securities to assess for credit impairment. Some of the factors considered in assessing impairment include the extent to which the fair value is less than the amortized cost basis, adverse conditions related to the security, an industry or geographic area, changes to security ratings or sector credit ratings and other relevant market data.

As of December 31, 2020, we did not intend, nor were we more likely than not to be required, to sell our available-for-sale investments before the recovery of their amortized cost basis, which may be maturity. Based on our assessment, we concluded all impairment as of December 31, 2020 to be due to factors other than credit loss, such as changes in interest rates. A credit allowance was not recognized and the impairment of our available-for-sale securities was recorded in other comprehensive loss.