Net Loss Per Share Attributable to Common Stockholders - Anti-Dilutive Securities (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Anti-dilutive securities
Antidilutive securities (in shares)	20,350,477	16,928,742	17,251,486
Conversion price (in dollars per share)	$ 70.90
Stock options issued and outstanding
Anti-dilutive securities
Antidilutive securities (in shares)	7,705,848	7,978,369	10,710,427
Nonvested restricted stock units issued and outstanding
Anti-dilutive securities
Antidilutive securities (in shares)	8,490,517	8,262,902	5,665,459
Class A common stock reserved for Twilio.org
Anti-dilutive securities
Antidilutive securities (in shares)	795,673	572,676	635,014
Class A common stock committed under 2016 ESPP
Anti-dilutive securities
Antidilutive securities (in shares)	207,792	113,312	235,372
Conversion spread
Anti-dilutive securities
Antidilutive securities (in shares)	3,150,647	233	0
Unvested shares subject to repurchase
Anti-dilutive securities
Antidilutive securities (in shares)	0	1,250	5,214