Investments (Tables)

9 Months Ended

Sep. 30, 2018

Investments, Debt and Equity Securities [Abstract]

Summary of Debt and Equity Securities

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