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Net Loss Per Share - Shares Excluded From Computation Of Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Apr. 30, 2019	Apr. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents (in shares)	8,344	13,363
0.25% Convertible Senior Notes Due 2023 \| Senior Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Face value	$ 345,000,000		$ 345,000,000		$ 345,000,000
Conversion price					$ 26.95
0.25% Convertible Senior Notes Due 2023 \| Senior Notes \| Capped Call
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Conversion price					$ 40.68
0.125% Convertible Senior Notes Due 2025 \| Senior Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Face value	$ 800,000,000	$ 700,000,000	$ 800,000,000	$ 100,000,000
Conversion price				$ 51.56
0.125% Convertible Senior Notes Due 2025 \| Senior Notes \| Capped Call
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Conversion price				$ 79.32
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents (in shares)	1,050	3,258
RSUs and PSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents (in shares)	3,316	6,732
Senior Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents (in shares)	3,968	3,332
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents (in shares)	10	41