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Debt 0.75 Senior convertible notes Narrative (Details)		1 Months Ended		12 Months Ended
	May 13, 2017	Nov. 30, 2018 USD ($) d $ / shares	May 31, 2017 USD ($) $ / shares	Dec. 31, 2018 USD ($) d	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2017 USD ($)	May 12, 2017 USD ($)
Debt Instrument [Line Items]
Proceeds from issuance of convertible debt, net of issuance costs				$ 836,600,000	$ 389,000,000	$ 0
Accretion of debt discount				17,900,000	9,400,000	100,000
Senior Notes | Convertible Notes due 2023
Debt Instrument [Line Items]
Interest rate on convertible notes		0.75%
Principal amount		$ 750,000,000		850,000,000	0
Proceeds from issuance of convertible debt, net of issuance costs		$ 836,600,000
Conversion Ratio	0.0100918	0.0060869
Conversion price of convertible notes | $ / shares		$ 164.29
Contractual coupon interest				500,000
Accretion of debt discount				$ 2,600,000
Amortization of debt discount						200,000
Effective interest rate on notes				5.63%
Holder's repurchase price percentage in event of fundamental change				100.00%
Debt Instrument, Convertible, Exercised Option To Purchase Additional Aggregate Principal		$ 100,000,000
Conversion amount				$ 1,000
Number of trading days | d		60
Redemption price percentage				100.00%
Senior Notes | Convertible Notes due 2023 | Minimum
Debt Instrument [Line Items]
Number of trading days | d				20
Redemption price percentage				130.00%
Senior Notes | Convertible Notes due 2023 | Maximum
Debt Instrument [Line Items]
Number of trading days | d				30
Senior Notes | Convertible Notes due 2022
Debt Instrument [Line Items]
Interest rate on convertible notes			0.75%					0.75%
Principal amount				$ 400,000,000.0	400,000,000.0		$ 50,000,000.0	$ 350,000,000.0
Proceeds from issuance of convertible debt, net of issuance costs			$ 389,000,000
Conversion price of convertible notes | $ / shares			$ 99.09
Debt Instrument, Percent of Conversion Price, Last Reported Sale Price of Common Stock				140.00%
Contractual coupon interest				$ 3,000,000	1,900,000
Accretion of debt discount				$ 13,400,000	8,200,000
Amortization of debt discount					1,000,000	$ 1,600,000
Effective interest rate on notes				5.10%
Holder's repurchase price percentage in event of fundamental change				100.00%
Conversion amount				$ 1,000
Redemption price percentage				100.00%
Additional paid-in capital | Senior Notes | Convertible Notes due 2023
Debt Instrument [Line Items]
Recognized additional paid-in capital from cash conversion option				$ 174,400,000
Additional paid-in capital | Senior Notes | Convertible Notes due 2022
Debt Instrument [Line Items]
Recognized additional paid-in capital from cash conversion option					$ 72,600,000
Debt Instrument Conversion Term One | Senior Notes | Convertible Notes due 2023 | Minimum
Debt Instrument [Line Items]
Percentage of applicable conversion price				130.00%
Number of trading days | d				20
Debt Instrument Conversion Term One | Senior Notes | Convertible Notes due 2023 | Maximum
Debt Instrument [Line Items]
Number of trading days | d				30
Debt Instrument Conversion Term One | Senior Notes | Convertible Notes due 2022 | Minimum
Debt Instrument [Line Items]
Percentage of applicable conversion price				130.00%
Number of trading days | d				20
Debt Instrument Conversion Term One | Senior Notes | Convertible Notes due 2022 | Maximum
Debt Instrument [Line Items]
Number of trading days | d				30
Debt Instrument Conversion Term Two | Senior Notes | Convertible Notes due 2023
Debt Instrument [Line Items]
Conversion amount				$ 1,000
Number of trading days | d				5
Debt Instrument Conversion Term Two | Senior Notes | Convertible Notes due 2023 | Maximum
Debt Instrument [Line Items]
Percentage of applicable conversion price				98.00%
Number of trading days | d				5
Debt Instrument Conversion Term Two | Senior Notes | Convertible Notes due 2022
Debt Instrument [Line Items]
Number of trading days | d				5
Debt Instrument Conversion Term Two | Senior Notes | Convertible Notes due 2022 | Maximum
Debt Instrument [Line Items]
Percentage of applicable conversion price				98.00%
Number of trading days | d				5

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