Stock compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Stock compensation
Stock compensation expense	$ 36.6	$ 33.8	$ 72.8	$ 64.4
Weighted-average fair value assumptions
Valuation method			Black-Scholes valuation model
Dividend yield (as a percent)			0.00%
Research and Development Expense [Member]
Stock compensation
Stock compensation expense	24.8	22.9	$ 49.0	44.4
Selling, General and Administrative Expenses [Member]
Stock compensation
Stock compensation expense	$ 11.8	$ 10.9	$ 23.8	$ 20.0
Stock Options
Weighted-average fair value assumptions
Average risk-free interest rates (as a percent)	2.71%	1.56%	2.47%	1.79%
Average expected life (in years)	5 years 4 months 13 days	5 years 4 months 28 days	5 years 6 months	5 years 4 months 10 days
Volatility (as a percent)	46.00%	50.00%	45.00%	49.00%
Weighted-average fair value (in dollars per share)	$ 28.65	$ 60.44	$ 39.47	$ 52.87
Restricted Stock Units (RSUs)
Weighted-average fair value assumptions
Average risk-free interest rates (as a percent)	2.52%	1.38%	2.50%	1.31%
Average expected life (in years)	6 months	6 months	6 months	6 months
Volatility (as a percent)	30.00%	42.00%	51.00%	42.00%
Weighted-average fair value (in dollars per share)	$ 12.11	$ 17.55	$ 16.25	$ 18.80

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 2
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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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For each plan, identification of the award pricing model or other valuation method used in calculating the weighted average fair values disclosed. The model is also used to calculate the compensation expense that is shown within the balance sheet, income statement, and cash flow. Examples of valuation techniques are lattice models (binomial model), closed-form models (Black-Scholes-Merton formula), and a Monte Carlo simulation technique. Fair value is the amount at which an asset or liability could be bought or incurred or sold or settled in a current transaction between willing parties, that is, other than in a forced or liquidation sale. May include disclosures about the assumptions underlying application of the method selected.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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