			Gross Unrealized
					Losses
	Amortized				Less than			Greater than			Allowance for			Estimated
December 31, 2020	Cost		Gains		One Year			One Year			Credit Losses			Fair Value
Short-term investments:
Available-for-sale securities:
Corporate debt securities	$	176,937	$	1,105	$	(7	)	$	—		$	(977	)	$	177,058
U.S. government and agency debt securities		103,011		336		(2	)		—			—			103,345
International government agency debt securities		79,346		469		(6	)		—			—			79,809
		359,294		1,910		(15	)		—			(977	)		360,212
Held-to-maturity securities:
Fixed term deposit account		1,667		187		—			—			—			1,854
Total short-term investments		360,961		2,097		(15	)		—			(977	)		362,066
Long-term investments:
Available-for-sale securities:
Corporate debt securities		7,908		—		(10	)		—			—			7,898
International government agency debt securities		15,077		—		(15	)		—			—			15,062
		22,985		—		(25	)		—			—		$	22,960
Held-to-maturity securities:
Certificates of deposit		1,820		—		—			—			—			1,820
Total long-term investments		24,805		—		(25	)		—			—			24,780
Total investments	$	385,766	$	2,097	$	(40	)	$	—		$	(977	)	$	386,846

December 31, 2019
Short-term investments:
Available-for-sale securities:
Corporate debt securities	$	144,161	$	676	$	—		$	—		$	—		$	144,837
U.S. government and agency debt securities		112,948		434		(1	)		(1	)		—			113,380
International government agency debt securities		72,753		248		(10	)		—			—			72,991
Total short-term investments		329,862		1,358		(11	)		(1	)		—			331,208
Long-term investments:
Available-for-sale securities:
Corporate debt securities		51,070		—		(45	)		(7	)		—		$	51,018
International government agency debt securities		20,806		—		(18	)		—			—			20,788
U.S. government and agency debt securities		4,000		—		(4	)		—			—			3,996
		75,876		—		(67	)		(7	)		—			75,802
Held-to-maturity securities:
Certificates of deposit		1,820		—		—			—			—			1,820
Fixed term deposit account		1,667		102		—			—			—			1,769
		3,487		102		—			—			—			3,589
Total long-term investments		79,363		102		(67	)		(7	)		—			79,391
Total investments	$	409,225	$	1,460	$	(78	)	$	(8	)	$	—		$	410,599

At December 31, 2020, the Company reviewed its investment portfolio to assess whether the unrealized losses on its available-for-sale investments were other-than-temporary. Investments with unrealized losses consisted primarily of corporate debt securities and debt securities issued by foreign agencies and backed by foreign governments. In making the determination whether the decline in fair value of these securities was other-than-temporary, the Company evaluated whether it intended to sell the security and whether it was more likely than not that the Company would be required to sell the security before recovering its amortized cost basis.

If the Company intends to sell a security, or it is more likely than not that the Company would be required to sell a security prior to recovering its amortized cost basis, an other-than-temporary impairment is deemed to have occurred. The amount of an other-than-temporary impairment related to a credit loss, or investments that the Company intends to sell before recovery, is recognized in earnings. The amount of an other-than-temporary impairment on available-for-sale investments related to other factors is recorded as a component of accumulated other comprehensive income.

In September 2019, the Company purchased $1.9 million of convertible promissory notes from Synchronicity Pharma, Inc. (“Synchronicity”), a related party. The notes mature on the earlier of June 30, 2021 or the closing of a preferred equity financing or closing of a merger, business combination or sale of stock resulting in Synchronicity’s stockholders owning less than 50% of the surviving entity. The note is classified as a corporate debt instrument and is classified as available-for-sale. During the year ended December 31, 2020, the Company recorded an other-than-temporary credit loss of $1.0 million against the value of this investment. The loss was recorded within “Other income (expense), net” in the accompanying consolidated statements of operations and comprehensive loss.

In May 2014, the Company entered into an agreement whereby it is committed to provide up to €7.4 million to a partnership, Fountain Healthcare Partners II, L.P. of Ireland (“Fountain”), which was created to carry on the business of investing exclusively in companies and businesses engaged in the healthcare, pharmaceutical and life sciences sectors. As of December 31, 2020, the Company’s total contribution in Fountain was equal to €7.0 million, and its commitment represented approximately 7% of the partnership’s total funding. The Company is accounting for its investment in Fountain under the equity method.

During the year ended December 31, 2020, two of the companies within the Fountain portfolio were acquired by third parties. The Company’s proportional share of the proceeds from these transactions was $11.1 million, of which $10.4 million was received in September 2020 and $0.7 million is being held in escrow. The Company expects to collect the funds in escrow in 2021. The transactions were accounted for under the cumulative earnings approach whereby the return on investment of $8.3 million was recorded as a gain within “Other income (expense), net” in the accompanying consolidated statements of operations and comprehensive loss and the return of investment of $2.8 million was recorded as a reduction in the Company’s net investment in Fountain. The Company’s net investment in Fountain was $6.2 million and $5.9 million at December 31, 2020 and 2019, respectively, and was included within “Other assets” in the accompanying consolidated balance sheets.

During the years ended December 31, 2020, 2019 and 2018, the Company recorded an increase in its investment in Fountain of $0.3 million, a reduction of $0.4 million and an increase of $0.5 million, respectively, which represented the Company’s proportional share of Fountain’s net gains or losses for such periods.

Realized gains and losses on the sales and maturities of marketable securities, which were identified using the specific identification method, were as follows:

The Company’s available‑for‑sale and held‑to‑maturity securities at December 31, 2020 had contractual maturities in the following periods: