Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands |
Apr. 30, 2019 |
Jan. 31, 2019 |
| Cash equivalents |
|
|
| Amortized Cost |
$ 236,476
|
$ 517,811
|
| Available-for-sale securities |
|
|
| Gross Unrealized Gains |
657
|
400
|
| Estimated Fair Value |
700,331
|
|
| Total |
|
|
| Amortized Cost |
796,064
|
851,534
|
| Gross Unrealized Losses |
(62)
|
(142)
|
| Estimated Fair Value |
796,659
|
851,792
|
| Level 2 |
|
|
| Available-for-sale securities |
|
|
| Amortized Cost |
704,734
|
501,471
|
| Gross Unrealized Gains |
657
|
400
|
| Gross Unrealized Losses |
(62)
|
(142)
|
| Estimated Fair Value |
705,329
|
501,729
|
| Commercial paper | Level 2 |
|
|
| Available-for-sale securities |
|
|
| Amortized Cost |
62,695
|
86,655
|
| Gross Unrealized Gains |
31
|
4
|
| Gross Unrealized Losses |
(8)
|
(21)
|
| Estimated Fair Value |
62,718
|
86,638
|
| Corporate notes and bonds | Level 2 |
|
|
| Available-for-sale securities |
|
|
| Amortized Cost |
334,912
|
287,496
|
| Gross Unrealized Gains |
582
|
389
|
| Gross Unrealized Losses |
(34)
|
(105)
|
| Estimated Fair Value |
335,460
|
287,780
|
| U.S. Treasury securities | Level 2 |
|
|
| Available-for-sale securities |
|
|
| Amortized Cost |
232,152
|
4,982
|
| Gross Unrealized Gains |
32
|
0
|
| Gross Unrealized Losses |
(15)
|
(1)
|
| Estimated Fair Value |
232,169
|
4,981
|
| U.S. government agency securities | Level 2 |
|
|
| Available-for-sale securities |
|
|
| Amortized Cost |
69,977
|
36,021
|
| Gross Unrealized Gains |
12
|
7
|
| Gross Unrealized Losses |
(5)
|
(4)
|
| Estimated Fair Value |
69,984
|
36,024
|
| Money market funds | Level 1 |
|
|
| Cash equivalents |
|
|
| Amortized Cost |
91,330
|
350,063
|
| Estimated Fair Value |
91,330
|
350,063
|
| Commercial paper | Level 2 |
|
|
| Cash equivalents |
|
|
| Amortized Cost |
4,998
|
76,828
|
| Gross Unrealized Losses |
0
|
(11)
|
| Estimated Fair Value |
$ 4,998
|
76,817
|
| Corporate notes and bonds | Level 2 |
|
|
| Cash equivalents |
|
|
| Amortized Cost |
|
2,998
|
| Estimated Fair Value |
|
2,998
|
| U.S. government agency securities | Level 2 |
|
|
| Cash equivalents |
|
|
| Amortized Cost |
|
6,491
|
| Estimated Fair Value |
|
$ 6,491
|