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Marketable Securities
 9 Months Ended
Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]  
Marketable securities
Marketable Securities

The amortized cost, gross unrealized holding gains, gross unrealized holding losses and estimated fair value of available-for-sale investments by type of security at September 30, 2016 and December 31, 2015 were as follows:
 
 
September 30, 2016
 
 
Amortized Cost
 
Gross Unrealized Holding Gains
 
Gross Unrealized Holding Losses
 
Estimated Fair Value
Commercial paper
 
$
238,092

 
$
—

 
$
—

 
$
238,092

Corporate bonds
 
188,009

 
32

 
(237
)
 
187,804

Municipal bonds
 
155,421

 
1

 
(116
)
 
155,306

Other government-related obligations:
 
 
 
 
 
 
 
 
U.S.
 
30,503

 
—

 
(20
)
 
30,483

Foreign
 
133,530

 
4

 
(105
)
 
133,429

Bank certificates of deposit
 
14,751

 
—

 
—

 
14,751

Total available-for-sale debt securities
 
$
760,306

 
$
37

 
$
(478
)
 
$
759,865

Equity securities
 
—

 
1,775

 
—

 
1,775

Total available-for-sale securities
 
$
760,306

 
$
1,812

 
$
(478
)
 
$
761,640



 
 
December 31, 2015
 
 
Amortized Cost Basis
 
Gross Unrealized Holding Gains
 
Gross Unrealized Holding Losses
 
Aggregate Fair Value
Commercial paper
 
$
254,396

 
$
—

 
$
—

 
$
254,396

Corporate bonds
 
133,062

 
23

 
(336
)
 
132,749

Municipal bonds
 
87,173

 
1

 
(63
)
 
87,111

Other government-related obligations:
 
 
 
 
 
 
 
 
U.S.
 
25,244

 
—

 
(94
)
 
25,150

Foreign
 
163,403

 
—

 
(504
)
 
162,899

Bank certificates of deposit
 
27,000

 
—

 
—

 
27,000

Total available-for-sale securities
 
$
690,278

 
$
24

 
$
(997
)
 
$
689,305



The aggregate fair value of available-for-sale securities in an unrealized loss position as of September 30, 2016 and December 31, 2015 were $348,525 and $293,947, respectively. Investments that have been in a continuous unrealized loss position for more than 12 months were not material. As of September 30, 2016, we believe that the cost basis of our available-for-sale investments is recoverable.
The fair values of available-for-sale securities by classification in the condensed consolidated balance sheet were as follows:
 
September 30, 2016
 
December 31, 2015
Cash and cash equivalents
$
224,179

 
$
323,218

Marketable securities
537,461

 
366,087

 
$
761,640

 
$
689,305



The fair values of available-for-sale debt securities at September 30, 2016, by contractual maturity, are summarized as follows:
 
September 30, 2016
Due in one year or less
$
508,813

Due after one year through three years
251,052

 
$
759,865



As of September 30, 2016 and December 31, 2015, the fair value of our trading securities was $13,421 and $8,817, respectively.
We utilize the specific identification method in computing realized gains and losses. Realized gains and losses on our available-for-sale and trading securities were not material for the three and nine months ended September 30, 2016 and 2015.