Fair Value Measurement - Debt (Details) - Senior Notes - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Carrying Amount \| Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible senior notes	$ 926,193	$ 900,303
Estimated Fair Value \| Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible senior notes	1,998,160	1,354,538
0.125% Convertible Senior Notes Due 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unamortized debt discount	167,016	184,698
Unamortized issuance costs	11,476	12,691
0.125% Convertible Senior Notes Due 2025 \| Carrying Amount \| Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible senior notes	621,508	602,611
0.125% Convertible Senior Notes Due 2025 \| Estimated Fair Value \| Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible senior notes	1,135,320	831,000
0.25% Convertible Senior Notes Due 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unamortized debt discount	36,029	42,280
Unamortized issuance costs	4,284	5,028
0.25% Convertible Senior Notes Due 2023 \| Carrying Amount \| Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible senior notes	304,685	297,692
0.25% Convertible Senior Notes Due 2023 \| Estimated Fair Value \| Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Convertible senior notes	$ 862,840	$ 523,538

Amount, after accumulated amortization, of debt discount (premium).

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