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Investments (Tables)
 3 Months Ended
Apr. 30, 2018
Investments, Debt and Equity Securities [Abstract]  
Summary of Marketable Securities
At April 30, 2018, marketable securities consisted of the following (in thousands):
 
Amortized Cost
 
Unrealized Gains
 
Unrealized Losses
 
Aggregate Fair Value
U.S. agency obligations
$
553,880

 
$
—

 
$
(1,002
)
 
$
552,878

U.S. treasury securities
1,391,222

 
7

 
(1,428
)
 
1,389,801

Corporate bonds
420,352

 
19

 
(1,527
)
 
418,844

Commercial paper
760,993

 
—

 
(9
)
 
760,984

 
$
3,126,447

 
$
26

 
$
(3,966
)
 
$
3,122,507

Included in cash and cash equivalents
$
792,889

 
$
4

 
$
(9
)
 
$
792,884

Included in marketable securities
$
2,333,558

 
$
22

 
$
(3,957
)
 
$
2,329,623

At January 31, 2018, marketable securities consisted of the following (in thousands):
 
Amortized Cost
 
Unrealized Gains
 
Unrealized Losses
 
Aggregate Fair Value
U.S. agency obligations
$
683,551

 
$
—

 
$
(1,127
)
 
$
682,424

U.S. treasury securities
797,977

 
—

 
(1,142
)
 
796,835

Corporate bonds
470,259

 
16

 
(1,154
)
 
469,121

Commercial paper
602,727

 
—

 
—

 
602,727

 
$
2,554,514

 
$
16

 
$
(3,423
)
 
$
2,551,107

Included in cash and cash equivalents
$
417,613

 
$
—

 
$
(1
)
 
$
417,612

Included in marketable securities
$
2,136,901

 
$
16

 
$
(3,422
)
 
$
2,133,495
Equity Investments
Equity investments consisted of the following (in thousands):
 
April 30, 2018
 
January 31, 2018
Marketable equity investments
$
88,571

 
$
551,804

Non-marketable equity investments
29,405

 
28,005

 
$
117,976

 
$
579,809