Stock Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock compensation
Stock compensation expense	$ 133.1	$ 96.2	$ 69.9
Weighted-average fair value assumptions
Valuation method	Black-Scholes valuation model
Employee Stock
Stock compensation
Stock compensation expense	$ 3.2	$ 2.4	$ 2.4
Weighted-average fair value assumptions
Average risk-free interest rates (as a percent)	1.53%	0.90%	0.81%
Average expected life (in years)	6 months	6 months	6 months
Volatility (as a percent)	38.00%	48.00%	55.00%
Weighted-average fair value (in dollars per share)	$ 19.42	$ 18.82	$ 16.26
Stock Options
Weighted-average fair value assumptions
Average risk-free interest rates (as a percent)	1.80%	1.30%	1.35%
Average expected life (in years)	5 years 3 months	4 years 11 months 27 days	5 years 11 days
Volatility (as a percent)	49.00%	50.00%	49.00%
Weighted-average fair value (in dollars per share)	$ 53.41	$ 39.35	$ 34.55
Dividend yield (as a percent)	0.00%
Unrecognized compensation
Unrecognized compensation cost for nonvested option (in dollars)	$ 95.2
Vesting period of recognition of the unrecognized compensation cost of nonvested awards	1 year 7 months 6 days
Restricted Stock Units (RSUs)
Unrecognized compensation
Unrecognized compensation cost for nonvested option (in dollars)	$ 55.9
Vesting period of recognition of the unrecognized compensation cost of nonvested awards	1 year 4 months 24 days
Performance Stock Units (PSUs)
Stock compensation
Stock compensation expense		$ 0.0

The weighted-average grant-date fair value of awards granted during the reporting period.

No definition available.

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- Definition

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.

+ References

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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
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-Publisher FASB
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-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- References

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

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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- Definition

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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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.

+ References

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=6415400&loc=d3e5070-113901

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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