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 table summarizes, 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:



	As of March 31, 2013
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash	$	310,813	$	—	$	—		$	310,813
Level 1 securities:
Money market funds	7,017		—		—			7,017
Level 2 securities:
Corporate debt securities	318,573		1,760		(291		)	320,042
Government and agency securities classified as cash equivalents	105,823		—		—			105,823
Government and agency securities classified as short-term investments	114,110		179		—			114,289
Asset and mortgage-backed securities	173,219		469		(198		)	173,490
Total (1)	$	1,029,555	$	2,408	$	(489	)	$	1,031,474



	As of December 31, 2012
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
Cash	$	284,661	$	—	$	—		$	284,661
Level 1 securities:
Money market funds	10,500		—		—			10,500
Level 2 securities:
Corporate debt securities	150,322		1,605		(32		)	151,895
Government and agency securities	166,643		285		—			166,928
Asset and mortgage-backed securities	138,340		750		(125		)	138,965
Total (2)	$	750,466	$	2,640	$	(157	)	$	752,949


(1)	Includes $418.1 million that is included in cash and cash equivalents, $607.8 million included in short-term investments and $5.6 million of restricted cash that is included in other non-current assets related to workers compensation deposits.


(2)	Includes $290.3 million that is included in cash and cash equivalents, $457.8 million included in short-term investments and $4.8 million of restricted cash that is included in other non-current assets related to workers compensation deposits.

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 6 to the consolidated financial statements for further information regarding the fair value of the Company’s debt instruments.

Because 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, the Company does not consider those investments with an unrealized loss to be other-than-temporarily impaired at March 31, 2013. There were no material other-than-temporary impairments or credit losses related to available-for-sale securities in the three months ended March 31, 2013 and 2012. In addition, there were no material gross realized gains or losses in the three months ended March 31, 2013 and 2012.

The estimated fair value of short-term investments by contractual maturity as of March 31, 2013 is as follows:



	(in thousands)
Due within one year	$	70,895
Due after one year and through 5 years	486,043
Due after 5 years and through 10 years	6,633
Due after 10 years	44,250
Total short-term investments	$	607,821