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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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Apr. 30, 2019	Mar. 31, 2019	Apr. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents (in shares)	12,989	10,058	8,828	11,379
0.25% Convertible Senior Notes Due 2023 \| Senior Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Face value	$ 173,018,000		$ 173,018,000		$ 345,000,000			$ 345,000,000
Conversion price	$ 26.95		$ 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		$ 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		$ 800,000,000		$ 800,000,000	$ 100,000,000	$ 700,000,000
Conversion price	$ 51.56		$ 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		$ 79.32
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents (in shares)	865	2,122	977	2,715
RSUs and PSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents (in shares)	3,394	3,831	3,425	4,952
Senior Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents (in shares)	8,721	4,098	4,422	3,709
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total common stock equivalents (in shares)	9	7	4	3