Convertible Senior Notes Due 2023 - Narrative (Details)	1 Months Ended	9 Months Ended
	Mar. 31, 2018 $ / shares shares	Sep. 30, 2019 USD ($)
Notes payable
Strike price of the warrant transactions (in dollars per share) | $ / shares	$ 80.9063
2023 Notes
Notes payable
Interest rate	0.625%
Effective interest rate	5.41%
Conversion rate for the notes (in shares per $1,000 principal amount)	0.0168545
Conversion price, per share of common stock (in dollars per share) | $ / shares	$ 59.33
Convertible note hedge options issued (in shares)	402,500
Common stock issued upon conversion of notes (in shares)	6,800,000
Warrants issued (in shares)	6,783,939
Number of shares per warrant entitled to holder (in shares)	1
Conversion of debt to equity | $		$ 0