Investments

As of September 30, 2018 and December 31, 2017, we held common stock of Immunomedics, Inc., or Immunomedics, and Unum Therapeutics, Inc., or Unum, each holding purchased in connection with strategic collaborations with the respective company. The collaboration agreement with Immunomedics was terminated in 2017. As of September 30, 2018 and December 31, 2017, the fair values of these equity securities were $169.4 million and $188.4 million, respectively.

Investment and other income (loss), net includes the realized and unrealized holding gains and losses on these equity securities. During the three and nine months ended September 30, 2018, we recognized a net loss of $23.8 million and a net gain of $62.9 million, respectively, from changes in the fair values of these equity securities. This included the impact of selling a portion of our Immunomedics common stock holdings for $91.9 million during the nine months ended September 30, 2018. Net unrealized gains recognized during the nine months ended September 30, 2018 on equity securities still held at that date were $34.6 million.

In 2017, investment and other income (loss), net also included activity related to an Immunomedics warrant derivative prior to the warrant's exercise by us in December 2017. During the three and nine months ended September 30, 2017, we recognized net gains from changes in the fair value of the warrant derivative of $78.7 million and $76.7 million, respectively, and related income tax benefits of $2.7 million and $5.4 million, respectively.

We also held debt securities, which consisted of the following (in thousands):



	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
September 30, 2018
U.S. Treasury securities	$	370,551	$	—	$	(297	)	$	370,254
Contractual maturities (at date of purchase)
Due in one year or less	$	284,377						$	284,230
Due in one to two years	86,174							86,024
Total	$	370,551						$	370,254
December 31, 2017
U.S. Treasury securities	$	252,511	$	—	$	(285	)	$	252,226
Contractual maturities (at date of purchase)
Due in one year or less	$	151,903						$	151,842
Due in one to two years	100,608							100,384
Total	$	252,511						$	252,226