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 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, 2015
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash	$	2,354,639	$	—	$	—		$	2,354,639
Level 1 securities:
Money market funds	105,060		—		—			105,060
Level 2 securities:
Corporate debt securities	270,196		726		(49		)	270,873
Government securities	198,815		479		(5		)	199,289
Certificate of deposits and commercial paper	6,100		—		—			6,100
Agency securities	26,644		25		—			26,669
Total	$	2,961,454	$	1,230	$	(54	)	$	2,962,630



	As of December 31, 2014
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash	$	1,007,543	$	—	$	—		$	1,007,543
Level 1 securities:
Money market funds	111,759		—		—			111,759
Level 2 securities:
Corporate debt securities	295,500		432		(199		)	295,733
Government securities	168,749		120		(95		)	168,774
Asset and mortgage-backed securities	112		—		—			112
Certificate of deposits	3,600		—		—			3,600
Agency securities	26,665		5		(1		)	26,669
Total	$	1,613,928	$	557	$	(295	)	$	1,614,190



	As of
	March 31, 2015		December 31, 2014
	(in thousands)
Cash and cash equivalents	$	2,454,777	$	1,113,608
Short-term investments	502,931		494,888
Other non-current assets (1)	4,922		5,694
Total	$	2,962,630	$	1,614,190

(1) Primarily restricted cash that is related to workers compensation deposits.

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 debt instruments.

There were no investments in a material unrealized loss position as of March 31, 2015 or December 31, 2014. There were no material other-than-temporary impairments or credit losses related to available-for-sale securities in the three months ended March 31, 2015 and 2014. In addition, there were no material gross realized gains or losses in the three months ended March 31, 2015 and 2014.

The estimated fair value of short-term investments by contractual maturity as of March 31, 2015 is as follows:



	(in thousands)
Due within one year	$	168,017
Due after one year and through five years	334,914
Total short-term investments	$	502,931