Fair Value (Tables)

9 Months Ended

Sep. 30, 2018

Fair Value Disclosures [Abstract]

Summary of Fair Value Hierarchy of Assets Carried at Fair Value and Measured on a Recurring Basis

The fair value hierarchy of assets carried at fair value and measured on a recurring basis was as follows (in thousands):

	Fair value measurement using:
	Quoted prices in active markets for identical assets (Level 1)		Other observable inputs (Level 2)		Significant unobservable inputs (Level 3)		Total
September 30, 2018
Short-term investments—U.S. Treasury securities	$	347,619	$	—	$	—	$	347,619
Long-term investments—U.S. Treasury securities	22,635		—		—		22,635
Other non-current assets—equity securities	169,358		—		—		169,358
Total	$	539,612	$	—	$	—	$	539,612
December 31, 2017
Short-term investments—U.S. Treasury securities	$	252,226	$	—	$	—	$	252,226
Other non-current assets—equity securities	188,358		—		—		188,358
Total	$	440,584	$	—	$	—	$	440,584