Fair Value Measurements

We base the fair value of our Level 1 financial instruments that are in active markets using quoted market prices for identical instruments.

We obtain the fair value of our Level 2 financial instruments, which are not in active markets, from a primary professional pricing source using quoted market prices for identical or comparable instruments, rather than direct observations of quoted prices in active markets. Fair value obtained from this professional pricing source can also be based on pricing models whereby all significant observable inputs, including maturity dates, issue dates, settlement date, benchmark yields, reported trades, broker-dealer quotes, issue spreads, benchmark securities, bids, offers or other market related data, are observable or can be derived from, or corroborated by, observable market data for substantially the full term of the asset.

We validate the quoted market prices provided by our primary pricing service by comparing the fair values of our Level 2 marketable securities portfolio balance provided by our primary pricing service against the fair values of our Level 2 marketable securities portfolio balance provided by our investment managers.

The following table represents our fair value hierarchy for our financial assets (cash equivalents and marketable securities) measured at fair value on a recurring basis as of December 31, 2017 (in millions):



	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Total
Cash equivalents	$	306.6	$	38.0	$	—	$	344.6
Marketable securities, available for sale
U.S. government agencies	—		87.3		—		87.3
Commercial paper	—		14.7		—		14.7
Corporate debt	—		5.1		—		5.1
Total marketable securities, available for sale	$	—	$	107.1	$	—	$	107.1



	Fair Value Measurements Using
	Level 1		Level 2		Level 3		Total
Cash equivalents	$	—	$	32.3	$	—	$	32.3
Marketable securities, available for sale
U.S. government agencies	—		22.2		—		22.2
Corporate debt	—		3.8		—		3.8
Commercial paper	—		3.2		—		3.2
Total marketable securities, available for sale	$	—	$	29.2	$	—	$	29.2

There were no transfers between Level 1 and Level 2 securities during the years ended December 31, 2017 and December 31, 2016. There were no transfers into or out of Level 3 securities during the years ended December 31, 2017 and 2016.