Convertible Senior Notes Due 2023 - Narrative (Details)			1 Months Ended	12 Months Ended
	Mar. 15, 2018 USD ($) $ / shares shares	Mar. 14, 2018 USD ($) trading_day $ / shares	Mar. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Notes payable
Percentage of last reported sales price threshold per $1,000 principal amount		98.00%
Total cost of the convertible note hedge transactions	$ 92,900,000			$ 0	$ 92,897,000	$ 0
Strike price of the warrant transactions (in dollars per share) | $ / shares			$ 80.9063
Proceeds from sale of warrants			$ 65,700,000	0	65,688,000	0
Debt financing costs			$ 10,400,000	0	10,435,000	$ 0
2023 Notes
Notes payable
Principal amount of the 2023 Notes	$ 402,500,000			402,500,000	$ 402,500,000
Threshold trading days (whether or not consecutive) | trading_day		20
Consecutive trading day period (in days) | trading_day		30
Minimum percentage of the conversion price on trading day		130.00%
Conversion option stock price trigger | $ / shares		$ 77.13
Threshold trading days period prior to a conversion date during which principal amount of notes for such trading day was less than 98% of the product of the last reported sale price of common stock | trading_day		5
Consecutive trading day period prior to a conversion date during which principal amount of notes for such trading day was less than 98% of the product of the last reported sale price of common stock | trading_day		10
Conversion rate for the notes (in shares per $1,000 principal amount)			0.0168545
Conversion price, per share of common stock | $ / shares	$ 59.33	$ 59.33
Amount of principal value to be repaid		$ 402,500,000
Shares of common stock issued upon conversion of Notes | shares	6,800,000
Warrants issued | shares			6,783,939
Number of shares per warrant | shares			1
Debt discount			$ 76,400,000
Effective interest rate (as percent)			5.41%
Conversion option reported as debt discount and APIC			$ 2,000,000.0
Deferred financing costs			$ 8,400,000
Debt converted				$ 0
Convertible Senior Notes due 2023 granted to initial purchasers
Notes payable
Principal amount of the 2023 Notes		$ 350,000,000
Interest rate (as a percent)		0.625%
Convertible Senior Notes due 2023 over-allotment option
Notes payable
Principal amount of the 2023 Notes	$ 52,500,000
Over-allotment period for additional principal amount	30 days
Overallotment option | 2023 Notes
Notes payable
Convertible note hedge options issued (in shares) | shares	402,500