Stock compensation (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Stock compensation
Stock compensation expense	$ 7.4	$ 4.1	$ 21.5	$ 10.7
Weighted-average fair value assumptions
Valuation method			Black-Scholes valuation model
Employee Stock Options
Weighted-average fair value assumptions
Average risk-free interest rates (as a percent)	0.54%	0.75%	0.99%	1.12%
Average expected life (in years)	3.69	3.08	3.29	2.96
Volatility (as a percent)	70.00%	75.00%	71.00%	74.00%
Weighted-average fair value (in dollars per share)	$ 8.15	$ 6.72	$ 7.35	$ 5.09
Dividend yield (as a percent)			0.00%
Assumed annualized forfeiture rate (as a percent)			5.00%
Unrecognized compensation
Unrecognized compensation cost for nonvested option	$ 23.0		$ 23.0
Weighted-average period of recognition of unrecognized compensation cost of nonvested awards (in years)			3.09
Employee Stock Purchase Plan
Weighted-average fair value assumptions
Average risk-free interest rates (as a percent)	0.57%	0.72%	0.57%	0.72%
Average expected life (in years)	0.50	0.50	0.50	0.50
Volatility (as a percent)	35.00%	49.00%	35.00%	49.00%
Weighted-average fair value (in dollars per share)	$ 1.13	$ 0.65	$ 1.13	$ 0.65

Represents the expected annualized forfeiture rate for options granted.

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

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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.

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As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

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The weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, using a decimal to express in number of years.

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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 period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior.

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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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