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Debt Instruments - Convertible Senior Notes (Details) $ / shares in Units, $ in Thousands		9 Months Ended
	Mar. 05, 2021 USD ($)	Sep. 30, 2021 USD ($) day $ / shares	Sep. 30, 2020 USD ($)
Debt Instrument [Line Items]
Issuance of convertible note, net of issuance costs | $		$ 930,775	$ 0
2026 Convertible Notes
Debt Instrument [Line Items]
Issuance of convertible note, net of issuance costs | $	$ 930,000
2026 Convertible Notes | Common Class A
Debt Instrument [Line Items]
Debt instrument, convertible, conversion ratio		0.0097272
Debt instrument, convertible, conversion price (in dollars per share) | $ / shares		$ 102.80
2026 Convertible Notes | Fastly Conversion Option
Debt Instrument [Line Items]
Debt instrument, convertible, threshold percentage of stock price trigger		130.00%
Debt instrument, convertible, threshold trading days		20
Debt instrument, convertible, threshold consecutive trading days		30
2026 Convertible Notes | Holder Conversion Option One | Common Class A
Debt Instrument [Line Items]
Debt instrument, convertible, threshold percentage of stock price trigger		130.00%
Debt instrument, convertible, threshold trading days		20
Debt instrument, convertible, threshold consecutive trading days		30
2026 Convertible Notes | Holder Conversion Option Two | Common Class A
Debt Instrument [Line Items]
Debt instrument, convertible, threshold percentage of stock price trigger		98.00%
Debt instrument, convertible, threshold trading days		5
Debt instrument, convertible, threshold consecutive trading days		10
Convertible Debt | 2026 Convertible Notes
Debt Instrument [Line Items]
Debt Instrument, face amount | $	$ 948,800
Interest rate, stated percentage	0.00%
Debt instrument, face amount, additional principal issuable | $	$ 123,800
Discount and transaction costs | $	$ 18,600
Convertible Debt | 2026 Convertible Notes | Fastly Conversion Option
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage		100.00%
Convertible Debt | 2026 Convertible Notes | Fundamental Change
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage		100.00%

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The price per share of the conversion feature embedded in the debt instrument.

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