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Net Loss per Share - Narrative (Details) shares in Millions					12 Months Ended
Net Loss per Share - Narrative (Details) shares in Millions	Aug. 19, 2019 $ / shares shares	Nov. 19, 2018 $ / shares shares	Sep. 15, 2017 $ / shares shares	Sep. 11, 2017 $ / shares shares	Dec. 31, 2019 shares	Dec. 31, 2018 shares	Dec. 31, 2017 shares	Aug. 14, 2019 $ / shares
Class of Stock [Line Items]
Allocation of undistributed earnings between stock classes, conversion ratio					1
2017 Notes \| Convertible Debt
Class of Stock [Line Items]
Conversion price (in usd per share) \| $ / shares			$ 104.06
2017 Notes \| Convertible Debt \| Class A common stock
Class of Stock [Line Items]
Conversion of stock, shares Issued (in shares) \| shares			4.1
Conversion price (in usd per share) \| $ / shares			$ 104.06
Cap price per share (in usd per share) \| $ / shares				$ 154.16
Capped Call Transaction, shares issued (in shares) \| shares				2.8
1.125% 2018 Senior Note Due 2024 \| Convertible Debt \| Class A common stock
Class of Stock [Line Items]
Conversion of stock, shares Issued (in shares) \| shares		4.9
Conversion price (in usd per share) \| $ / shares		$ 116.40
Cap price per share (in usd per share) \| $ / shares		$ 219.63
Capped Call Transaction, shares issued (in shares) \| shares		2.6
1.00% 2019 Convertible Senior Notes Due 2026 \| Convertible Debt \| Class A common stock
Class of Stock [Line Items]
Conversion of stock, shares Issued (in shares) \| shares	6.4
Conversion price (in usd per share) \| $ / shares	$ 148.48
Cap price per share (in usd per share) \| $ / shares	$ 280.15							$ 280.15
Capped Call Transaction, shares issued (in shares) \| shares	3.4
Common Stock Options, Restricted Stock Options And Restricted Stock Units
Class of Stock [Line Items]
Common stock outstanding that have been excluded from the computation of diluted net loss per share \| shares					8.2	8.1	7.0