Fair Value Measurements
 3 Months Ended
Apr. 30, 2019
Fair Value Disclosures [Abstract]  
Fair Value Measurements Fair Value Measurements
We carry certain assets and liabilities at fair value. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The three-tier hierarchy for inputs used in measuring fair value, which prioritizes the inputs based on the observability as of the measurement date, is as follows:
Level 1
Quoted prices in active markets for identical assets or liabilities;
Level 2
Observable inputs other than the quoted prices in active markets for identical assets and liabilities; and
Level 3
Unobservable inputs for which there is little or no market data, which require us to develop assumptions of what market participants would use in pricing the asset or liability.

The following table summarizes our financial assets that are measured at fair value on a recurring basis during the period:
 
April 30, 2019
(in thousands)
Amortized Cost
 
Gross Unrealized Gains
 
Gross Unrealized Losses
 
Estimated Fair Value
Level 1:
 
 
 
 
 
 
 
Cash equivalents
 
 
 
 
 
 
 
Money market funds
$
91,330

 
$
—

 
$
—

 
$
91,330

Level 2:
 
 
 
 
 
 
 
Cash equivalents
 
 
 
 
 
 
 
Commercial paper
4,998

 
—

 
—

 
4,998

Available-for-sale securities
 
 
 
 
 
 
 
Commercial paper
62,695

 
31

 
(8
)
 
62,718

Corporate notes and bonds
334,912

 
582

 
(34
)
 
335,460

U.S. Treasury securities
232,152

 
32

 
(15
)
 
232,169

U.S. government agency securities
69,977

 
12

 
(5
)
 
69,984

Level 2 total
704,734

 
657

 
(62
)
 
705,329

Total
$
796,064

 
$
657

 
$
(62
)
 
$
796,659

 
 
 
 
 
 
 
 
 
January 31, 2019
(in thousands)
Amortized Cost
 
Gross Unrealized Gains
 
Gross Unrealized Losses
 
Estimated Fair Value
Level 1:
 
 
 
 
 
 
 
Cash equivalents
 
 
 
 
 
 
 
Money market funds
$
350,063

 
$
—

 
$
—

 
$
350,063

Level 2:
 
 
 
 
 
 
 
Cash equivalents
 
 
 
 
 
 
 
Commercial paper
76,828

 
—

 
(11
)
 
76,817

Corporate notes and bonds
2,998

 
—

 
—

 
2,998

U.S. government agency securities
6,491

 
—

 
—

 
6,491

Available-for-sale securities
 
 
 
 
 
 
 
Commercial paper
86,655

 
4

 
(21
)
 
86,638

Corporate notes and bonds
287,496

 
389

 
(105
)
 
287,780

U.S. Treasury securities
4,982

 
—

 
(1
)
 
4,981

U.S. government agency securities
36,021

 
7

 
(4
)
 
36,024

Level 2 total
501,471

 
400

 
(142
)
 
501,729

Total
$
851,534

 
$
400

 
$
(142
)
 
$
851,792



Money market funds consist of cash equivalents with original maturities of three months or less at the date of purchase. We use quoted prices in active markets for identical assets or liabilities to determine the fair value of our Level 1 investments in money market funds. The fair value of our Level 2 investments is determined using pricing based on quoted market prices or alternative market observable inputs.

The fair value of our available-for-sale marketable securities as of April 30, 2019, by remaining contractual maturities, were as follows (in thousands):
Due in one year or less
$
515,648

Due in one to two years
184,683

 
$
700,331


As of April 30, 2019, we had a total of 155 available-for-sale securities, none of which were considered to be other-than-temporarily impaired.

Convertible Senior Notes

As of April 30, 2019, the estimated fair value of our 0.5% Convertible Senior Notes with aggregate principal amount of $575.0 million was $615.4 million. We estimated the fair value based on the quoted market prices in an inactive market on the last trading day of the reporting period (Level 2). These Convertible Senior Notes are recorded at face value less unamortized debt discount and transaction costs as "Convertible senior notes, net" on our consolidated balance sheets. Refer to Note 9 for further information.