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Basis of Presentation and Summary of Significant Accounting Policies - Derivatives (Details) - Convertible Notes - USD ($)	Jun. 30, 2018	Jun. 19, 2018	May 22, 2018	Dec. 31, 2017	Aug. 31, 2014
2023 Convertible Senior Notes
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregate principal amount outstanding	$ 750,000,000		$ 750,000,000	$ 0
Interest rate (as a percent)			0.75%
Common stock price (USD per share)		$ 195.91	$ 187.09
Exercise price, conversion premium and bond hedge (USD per share)		248.48	248.48
Exercise price, warrant (USD per share)		$ 315.38	$ 315.38
Risk-free interest rate (as a percent)		2.80%	2.90%
Dividend yield (as a percent)		0.00%	0.00%
Annual coupon rate (as a percent)		0.75%	0.75%
Remaining contractual term (in years)		4 years 11 months 23 days	5 years
2023 Convertible Senior Notes \| Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Volatility (as a percent)		30.00%	30.00%
2023 Convertible Senior Notes \| Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Volatility (as a percent)		35.00%	35.00%
2019 Convertible Senior Notes
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Aggregate principal amount outstanding	$ 223,215,000			$ 245,000,000	$ 245,000,000.0
Interest rate (as a percent)					0.75%
Common stock price (USD per share)	$ 207.17		$ 187.09
Exercise price, conversion premium and bond hedge (USD per share)	$ 75.05		$ 75.05		$ 75.05
Risk-free interest rate (as a percent)	2.43%		2.47%
Dividend yield (as a percent)	0.00%		0.00%
Annual coupon rate (as a percent)	0.75%		0.75%
Remaining contractual term (in years)	1 year 1 month 21 days		1 year 3 months
2019 Convertible Senior Notes \| Minimum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Volatility (as a percent)	30.00%		30.00%
2019 Convertible Senior Notes \| Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Volatility (as a percent)	35.00%		35.00%