Short-term Investments

In July 2017, the Company sold all short-term investments. As of September 30, 2017, $13.9 million and $1.3 million of money market funds, classified as Level 1 securities, were included in Cash and cash equivalents and Non-current assets, respectively, on the Company's Consolidated Balance Sheet. Additionally, $3.6 million of restricted cash is included in Non-current assets on the Company's Consolidated Balance Sheet.

The following table summarizes, by major security type, the Company’s assets at December 31, 2016 that were measured at fair value, the category using the fair value hierarchy and where they are classified on the Consolidated Balance Sheets:



	As of December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value		Cash and cash equivalents		Short-term investments		Non-current assets (1)
	(in thousands)
Cash	$	1,267,523	$	—	$	—		$	1,267,523	$	1,264,126	$	—	$	3,397
Level 1 securities:
Money market funds	204,967		—		—			204,967		203,450		—		1,517
Level 2 securities:
Corporate debt securities	199,843		110		(731		)	199,222		—		199,222		—
Government securities	35,944		—		(128		)	35,816		—		35,816		—
Certificates of deposit	9,833		—		—			9,833		—		9,833		—
Agency securities	21,563		—		(228		)	21,335		—		21,335		—
Total	$	1,739,673	$	110	$	(1,087	)	$	1,738,696	$	1,467,576	$	266,206	$	4,914

(1) Primarily restricted cash that is related to workers compensation deposits and letter of credit agreements.

Fair value is a market-based measurement that is determined based on the assumptions that market participants would use in pricing an asset or liability. The hierarchy level assigned to each security in the Company’s available-for-sale portfolio and cash equivalents is based on its assessment of the transparency and reliability of the inputs used in the valuation of such instrument at the measurement date. The fair value of available-for-sale securities and cash equivalents included in the Level 1 category is based on quoted prices that are readily and regularly available in an active market. The fair value of available-for-sale securities included in the Level 2 category is based on observable inputs, such as quoted prices for similar assets at the measurement date; quoted prices in markets that are not active; or other inputs that are observable, either directly or indirectly. These values were obtained from an independent pricing service and were evaluated using pricing models that vary by asset class and may incorporate available trade, bid and other market information and price quotes from well-established independent pricing vendors and broker-dealers. The Company's procedures include controls to ensure that appropriate fair values are recorded, such as comparing prices obtained from multiple independent sources. See Note 5 to the consolidated financial statements for further information regarding the fair value of the Company’s senior notes.

There were no material gross realized gains or losses in the three and nine months ended September 30, 2017 and 2016, respectively.