Stock-based compensation Share-based Compensation Disclosures (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2011 Years	Dec. 31, 2011 Years	Dec. 31, 2010	Dec. 31, 2009
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value		$ 3.7	$ 2.5	$ 2.4
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	5.5	5.5	4.4	6.1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	4.1	4.1	3.4	4.6
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	6.3	6.3
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term		4.2
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	9.1	9.1
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	4.4	4.4
Share-based Compensation Arrangement by Share-based Payment Award, Terms of Award		10
Allocated Share-based Compensation Expense	7.3	24.3	14.3	12.9
Performance Based Grant, Number of Options Granted	1.2
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	27.1	27.1
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition		2.8
Performance Based Grant, Number of Tranches	3
Total Shareholder Return, Minimum	0	0
Performance Based Grant, Fair Value Amount	5.2
Performance Based Grant, Weighted-Average Value Per Share	$ 4.55
Performance Based Grant, Expected Annual Amortization Expense	1.3
Performance Based Grant, Amortization Expense Recognized	0.1
Employee Service Share-based Compensation, Cash Received from Exercise of Stock Options		$ 7.3	$ 2.2	$ 2.1
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period, Maximum		4

Performance Based Grant, Amortization Expense Recognized

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Total Shareholder Return, Minimum - Minimum Total Shareholder Return (as defined in the share-based compensation award agreements) to meet secondary vesting condition.

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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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Aggregate proceeds received by the entity during the annual period from exercises of stock or unit options and conversion of similar instruments granted under equity-based payment arrangements.

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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 maximum vesting term over which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition.

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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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The total dollar difference between fair values of the underlying shares reserved for issuance and exercise prices of vested portions of options outstanding and currently exercisable under the option plan as of the balance sheet date.

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The weighted average period between the balance sheet date and expiration for all vested portions of options outstanding and currently exercisable (or convertible) under the plan, which may be expressed in a decimal value for number of years.

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The total accumulated difference between fair values of underlying shares on dates of exercise and exercise price on options which were exercised (or share units converted) into shares during the reporting period under the plan.

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The total dollar difference between fair values of the underlying shares reserved for issuance and exercise prices pertaining to options outstanding under the plan as of the balance sheet date.

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Description of pertinent provisions of equity-based compensation awards that have actual or potential impact upon the company's financial statements.

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