Fair value

We have certain assets that are measured at fair value on a recurring basis according to a fair value hierarchy that prioritizes the inputs, assumptions and valuation techniques used to measure fair value. The three levels of the fair value hierarchy are:



Level 1:		Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities.
Level 2:		Quoted prices in markets that are not active or financial instruments for which all significant inputs are observable, either directly or indirectly.
Level 3:		Prices or valuations that require inputs that are both significant to the fair value measurement and unobservable.

The determination of a financial instrument’s level within the fair value hierarchy is based on an assessment of the lowest level of any input that is significant to the fair value measurement. We consider observable data to be market data which is readily available, regularly distributed or updated, reliable and verifiable, not proprietary, and provided by independent sources that are actively involved in the relevant market.

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