Summary of Significant Accounting Policies - Convertible Debt Securities (Details)	Sep. 30, 2019	May 14, 2018	Jul. 31, 2017
2024 Convertible Notes
Debt Conversion [Line Items]
Stated interest rate			2.875%
Senior Notes | 2025 Convertible Notes
Debt Conversion [Line Items]
Stated interest rate		2.375%
Measurement Input, Discount Rate | Liability Component of Convertible Debt | 2024 Convertible Notes
Debt Conversion [Line Items]
Fair value assumptions, discount rate	0.08