Fair Value Measurements

3 Months Ended

Mar. 31, 2013

Fair Value Disclosures [Abstract]

Fair Value Measurements

Fair Value Measurements

The Company’s assets that are measured at fair value on a recurring basis, at March 31, 2013 and December 31, 2012, consist of the following:

			Fair Value Measurements at Reporting Date Using
At March 31, 2013	Fair Value		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)
Unrestricted
Available-for-sale marketable securities:
U.S. government and government agency obligations	$	305,967			$	305,967
Corporate bonds	122,870				122,870
Commercial paper	25,172				25,172
Municipal bonds	17,161				17,161
Equity security	2,785		$	2,785
	473,955		2,785		471,170
Restricted
Available-for-sale marketable securities:
U.S. government obligations	8,358				8,358
	$	482,313	$	2,785	$	479,528
At December 31, 2012
Unrestricted
Available-for-sale marketable securities:
U.S. government and government agency obligations	$	328,146			$	328,146
Municipal bonds	17,510				17,510
Equity security	3,393		$	3,393
	349,049		3,393		345,656
Restricted
Available-for-sale marketable securities:
U.S. government obligations	5,909				5,909
	$	354,958	$	3,393	$	351,565

Marketable securities included in Level 2 were valued using a market approach utilizing prices and other relevant information, such as interest rates, yield curves, prepayment speeds, loss severities, credit risks and default rates, generated by market transactions involving identical or comparable assets. The Company considers market liquidity in determining the fair value for these securities. The Company did not record any charges for other-than-temporary impairment of its Level 2 marketable securities during the three months ended March 31, 2013 and 2012.

There were no purchases, sales, or maturities of Level 3 marketable securities and no unrealized gains or losses related to Level 3 marketable securities for the three months ended March 31, 2013 and 2012. There were no transfers of marketable securities between Levels 1, 2, or 3 classifications during the three months ended March 31, 2013 and 2012.

As of March 31, 2013 and December 31, 2012, the Company had $400.0 million in aggregate principal amount of 1.875% convertible senior notes that will mature on October 1, 2016 unless earlier converted or repurchased. The fair value of the outstanding convertible senior notes was estimated to be $861.1 million and $843.2 million as of March 31, 2013 and December 31, 2012, respectively, and was determined based on Level 2 inputs.