| Financial Instruments |
Financial Instruments Fair Value Measurements
We classify our cash equivalents and marketable debt securities within Level 1 or Level 2 because we use quoted market prices or alternative pricing sources and models utilizing market observable inputs to determine their fair value. Certain other assets are classified within Level 3 because factors used to develop the estimated fair value are unobservable inputs that are not supported by market activity.
The following tables summarize our assets measured at fair value on a recurring basis and the classification by level of input within the fair value hierarchy (in millions): | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Fair Value Measurement at Reporting Date Using | | Description | | March 31, 2024 | | Quoted Prices in Active Markets for Identical Assets
(Level 1) | | Significant Other Observable Inputs
(Level 2) | | Significant Unobservable Inputs
(Level 3) | | Cash | | $ | 6,495 | | | | | | | | | Cash equivalents: | | | | | | | | | | Money market funds | | 24,465 | | | $ | 24,465 | | | $ | — | | | $ | — | | | U.S. government and agency securities | | 808 | | | 808 | | | — | | | — | | | | | | | | | | | | Time deposits | | 350 | | | — | | | 350 | | | — | | | Corporate debt securities | | 189 | | | — | | | 189 | | | — | | | Total cash and cash equivalents | | 32,307 | | | 25,273 | | | 539 | | | — | | | Marketable securities: | | | | | | | | | | U.S. government securities | | 10,345 | | | 10,345 | | | — | | | — | | | U.S. government agency securities | | 3,468 | | | 3,468 | | | — | | | — | | | Corporate debt securities | | 12,000 | | | — | | | 12,000 | | | — | | | | | | | | | | | | Total marketable securities | | 25,813 | | | 13,813 | | | 12,000 | | | — | | | Restricted cash equivalents | | 858 | | | 858 | | | — | | | — | | | Other assets | | 101 | | | — | | | — | | | 101 | | | Total | | $ | 59,079 | | | $ | 39,944 | | | $ | 12,539 | | | $ | 101 | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Fair Value Measurement at Reporting Date Using | | Description | | December 31, 2023 | | Quoted Prices in Active Markets for Identical Assets
(Level 1) | | Significant Other Observable Inputs
(Level 2) | | Significant Unobservable Inputs
(Level 3) | | Cash | | $ | 6,265 | | | | | | | | | Cash equivalents: | | | | | | | | | | Money market funds | | 32,910 | | | $ | 32,910 | | | $ | — | | | $ | — | | | U.S. government and agency securities | | 2,206 | | | 2,206 | | | — | | | — | | | | | | | | | | | | Time deposits | | 261 | | | — | | | 261 | | | — | | | Corporate debt securities | | 220 | | | — | | | 220 | | | — | | | Total cash and cash equivalents | | 41,862 | | | 35,116 | | | 481 | | | — | | | Marketable securities: | | | | | | | | | | U.S. government securities | | 8,439 | | | 8,439 | | | — | | | — | | | U.S. government agency securities | | 3,498 | | | 3,498 | | | — | | | — | | | Corporate debt securities | | 11,604 | | | — | | | 11,604 | | | — | | | | | | | | | | | | Total marketable securities | | 23,541 | | | 11,937 | | | 11,604 | | | — | | | Restricted cash equivalents | | 857 | | | 857 | | | — | | | — | | | Other assets | | 101 | | | — | | | — | | | 101 | | | Total | | $ | 66,361 | | | $ | 47,910 | | | $ | 12,085 | | | $ | 101 | |
Unrealized Losses
The following tables summarize our available-for-sale marketable debt securities and cash equivalents with unrealized losses as of March 31, 2024 and December 31, 2023, aggregated by major security type and the length of time that individual securities have been in a continuous loss position (in millions): | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | March 31, 2024 | | Less than 12 months | | 12 months or greater | | Total | | Fair Value | | Unrealized Losses | | Fair Value | | Unrealized Losses | | Fair Value | | Unrealized Losses | | U.S. government securities | $ | 3,098 | | | $ | (11) | | | $ | 5,825 | | | $ | (278) | | | $ | 8,923 | | | $ | (289) | | | U.S. government agency securities | 447 | | | (1) | | | 3,108 | | | (146) | | | 3,555 | | | (147) | | | Corporate debt securities | 882 | | | (4) | | | 9,302 | | | (450) | | | 10,184 | | | (454) | | | Total | $ | 4,427 | | | $ | (16) | | | $ | 18,235 | | | $ | (874) | | | $ | 22,662 | | | $ | (890) | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | December 31, 2023 | | Less than 12 months | | 12 months or greater | | Total | | Fair Value | | Unrealized Losses | | Fair Value | | Unrealized Losses | | Fair Value | | Unrealized Losses | | U.S. government securities | $ | 336 | | | $ | (1) | | | $ | 7,041 | | | $ | (275) | | | $ | 7,377 | | | $ | (276) | | | U.S. government agency securities | 71 | | | — | | | 3,225 | | | (164) | | | 3,296 | | | (164) | | | Corporate debt securities | 647 | | | (3) | | | 10,125 | | | (491) | | | 10,772 | | | (494) | | | Total | $ | 1,054 | | | $ | (4) | | | $ | 20,391 | | | $ | (930) | | | $ | 21,445 | | | $ | (934) | |
The allowance for credit losses and the gross unrealized gains on our marketable debt securities were not material as of March 31, 2024 and December 31, 2023. Contractual Maturities
The following table classifies our marketable debt securities by contractual maturities (in millions): | | | | | | | March 31, 2024 | | Due within one year | $ | 7,965 | | | Due after one year to five years | 17,848 | | | Total | $ | 25,813 | |
Instruments Measured at Fair Value on Non-recurring Basis
Our non-marketable equity securities accounted for using the measurement alternative are measured at fair value on a non-recurring basis and are classified within Level 3 of the fair value hierarchy because we use significant unobservable inputs to estimate their fair value. Assets remeasured at fair value on a non-recurring basis within Level 3 during the three months ended March 31, 2024 and 2023 were $154 million and $119 million, respectively.
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