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Marketable Securities (Tables)
 3 Months Ended
Mar. 31, 2013
Investments, Debt and Equity Securities [Abstract]  
Available-for-sale Securities
The following tables summarize the amortized cost basis of debt and equity securities included in marketable securities, the aggregate fair value of those securities, and gross unrealized gains and losses on those securities at March 31, 2013 and December 31, 2012.
 
 
Amortized
 
Unrealized
 
Fair
At March 31, 2013
 
Cost Basis
 
Gains
 
Losses
 
Value
Unrestricted
 
 
 
 
 
 
 
 
U.S. government and government agency obligations
 
$
305,402

 
$
568

 
$
(3
)
 
$
305,967

Corporate bonds
 
123,015

 
3

 
(148
)
 
122,870

Commercial paper
 
25,172

 


 


 
25,172

Municipal bonds
 
17,159

 
6

 
(4
)
 
17,161

     Equity securities
 
4,044

 


 
(1,259
)
 
2,785

 
 
474,792

 
577

 
(1,414
)
 
473,955

Restricted
 
 
 
 
 
 
 
 
U.S. government obligations
 
8,350

 
9

 
(1
)
 
8,358

 
 
$
483,142

 
$
586

 
$
(1,415
)
 
$
482,313

At December 31, 2012
 
 
 
 
 
 
 
 
Unrestricted
 
 
 
 
 
 
 
 
U.S. government and government agency obligations
 
$
327,502

 
$
661

 
$
(17
)
 
$
328,146

Municipal bonds
 
17,542

 


 
(32
)
 
17,510

Equity securities
 
4,044

 

 
(651
)
 
3,393

 
 
349,088

 
661

 
(700
)
 
349,049

Restricted
 
 
 
 
 
 
 
 
U.S. government obligations
 
5,902

 
9

 
(2
)
 
5,909

 
 
$
354,990

 
$
670

 
$
(702
)
 
$
354,958
Marketable Securities, Based on Contractual Maturity Dates
The fair values of debt security investments by contractual maturity as of March 31, 2013 and December 31, 2012 consist of the following:
 
 
March 31,
 
December 31,
 
 
2013
 
2012
Unrestricted
 
 
 
 
Maturities within one year
 
$
127,349

 
$
77,819

Maturities after one year through five years
 
343,821

 
267,837

 
 
471,170

 
345,656

Restricted
 
 
 
 
Maturities within one year
 
6,868

 
2,781

Maturities after one year through five years
 
1,490

 
3,128

 
 
8,358

 
5,909

 
 
$
479,528

 
$
351,565
Fair Value and Unrealized Losses of Marketable Securities
The following table shows the fair value of the Company’s marketable securities that have unrealized losses and that are deemed to be only temporarily impaired, aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position, at March 31, 2013 and December 31, 2012.
 
Less than 12 Months
 
12 Months or Greater
 
Total
At March 31, 2013
Fair Value
 
Unrealized Loss
 
Fair Value
 
Unrealized Loss
 
Fair Value
 
Unrealized Loss
Unrestricted
 
 
 
 
 
 
 
 
 
 
 
U.S. government and government agency obligations
$
25,004

 
$
(3
)
 
 
 
 
 
$
25,004

 
$
(3
)
Corporate bonds
114,555

 
(148
)
 
 
 
 
 
114,555

 
(148
)
Municipal bonds
6,490

 
(4
)
 
 
 
 
 
6,490

 
(4
)
Equity security
 
 
 
 
$
2,786

 
$
(1,259
)
 
2,786

 
(1,259
)
 
146,049

 
(155
)
 
2,786

 
(1,259
)
 
148,835

 
(1,414
)
Restricted
 
 
 
 
 
 
 
 
 
 
 
U.S. government obligations
904

 
(1
)
 
 
 
 
 
904

 
(1
)
 
$
146,953

 
$
(156
)
 
$
2,786

 
$
(1,259
)
 
$
149,739

 
$
(1,415
)
 
 
 
 
 
 
 
 
 
 
 
 
At December 31, 2012
 
 
 
 
 
 
 
 
 
 
 
Unrestricted
 
 
 
 
 
 
 
 
 
 
 
U.S. government and government agency obligations
$
44,738

 
$
(17
)
 
 
 
 
 
$
44,738

 
$
(17
)
Municipal bonds
17,510

 
(32
)
 
 
 
 
 
17,510

 
(32
)
Equity security


 


 
$
3,393

 
$
(651
)
 
3,393

 
(651
)
 
62,248

 
(49
)
 
3,393

 
(651
)
 
65,641

 
(700
)
Restricted
 
 
 
 
 
 
 
 
 
 
 
U.S. government obligations
1,194

 
(2
)
 


 


 
1,194

 
(2
)
 
$
63,442

 
$
(51
)
 
$
3,393

 
$
(651
)
 
$
66,835

 
$
(702
)