Short-term Investments

The Company’s investment policy is consistent with the definition of available-for-sale securities. The Company does not buy and hold securities principally for the purpose of selling them in the near future. The Company’s policy is focused on the preservation of capital, liquidity and investment return. From time to time, the Company may sell certain securities but the objectives are generally not to generate profits on short-term differences in price. The following tables summarize, by major security type, the Company’s assets that are measured at fair value on a recurring basis and are categorized using the fair value hierarchy and where they are classified on the Consolidated Balance Sheets:



	As of March 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	$	1,067,245	$	—	$	—		$	1,067,245	$	1,064,663	$	—	$	2,582
Level 1 securities:
Money market funds	543,544		—		—			543,544		540,581		—		2,963
Level 2 securities:
Corporate debt securities	225,081		711		(70		)	225,722		—		225,722		—
Government securities	231,048		333		(90		)	231,291		—		231,291		—
Agency securities	10,209		5		—			10,214		—		10,214		—
Total	$	2,077,127	$	1,049	$	(160	)	$	2,078,016	$	1,605,244	$	467,227	$	5,545



	As of December 31, 2015
	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	$	1,708,220	$	—	$	—		$	1,708,220	$	1,706,592	$	—	$	1,628
Level 1 securities:
Money market funds	107,199		—		—			107,199		102,738		—		4,461
Level 2 securities:
Corporate debt securities	240,867		154		(409		)	240,612		—		240,612		—
Government securities	235,252		—		(1,046		)	234,206		—		234,206		—
Agency securities	26,576		—		(9		)	26,567		—		26,567		—
Total	$	2,318,114	$	154	$	(1,464	)	$	2,316,804	$	1,809,330	$	501,385	$	6,089

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

Fair value is a market-based measurement that should be 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.

The Company does not intend to sell the investments that are in an unrealized loss position and it is not likely that the Company will be required to sell any investments before recovery of their amortized cost basis. As such, the Company does not consider those investments with an unrealized loss to be other-than-temporarily impaired at March 31, 2016 or December 31, 2015. There were no material other-than-temporary impairments or credit losses related to available-for-sale securities in the three months ended March 31, 2016 and 2015. In addition, there were no material gross realized gains or losses in the three months ended March 31, 2016 and 2015.

The estimated fair value of short-term investments by contractual maturity as of March 31, 2016 is as follows:



	(in thousands)
Due within one year	$	105,034
Due after one year and through five years	362,193
Total short-term investments	$	467,227