Share-based compensation (Details 4) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2011 years	Sep. 30, 2010	Sep. 30, 2011 years	Sep. 30, 2010 years
Long-term incentive plan (LTIP)
Share-based compensation
Payout low end of range (as a percent)			0.00%
Payout high end of range (as a percent)			200.00%
LTIP linked to TRS
Share-based compensation
Period for calculating total return to shareholders (in years)			3
LTIP linked to TRS and other performance conditions (in shares)
Outstanding, beginning of period (in shares)	199,563	132,588	126,782	36,198
Granted (in shares)			75,015	97,191
Forfeited (in shares)	(1,063)	(5,806)	(3,297)	(6,607)
Outstanding, end of period (in shares)	198,500	126,782	198,500	126,782
Total weighted-average grant-date fair value			$ 2.7	$ 2.2
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 25.99	$ 20.42	$ 20.33	$ 14.85
Granted (in dollars per share)			$ 35.46	$ 22.45
Forfeited (in dollars per share)	$ 30.67	$ 22.45	$ 25.10	$ 21.53
Outstanding, end of period (in dollars per share)	$ 25.97	$ 20.33	$ 25.97	$ 20.33
Total weighted-average grant-date fair value			2.7	2.2
Risk-free interest rate (as a percent)			1.25%	1.30%
Expected life (in years)			3	3
Expected volatility (as a percent)			27.80%	27.90%
Range of expected volatility for Peer Group, minimum (as a percent)			21.20%	22.30%
Range of expected volatility for Peer Group, maximum (as a percent)			82.60%	52.30%
Grant date fair value (in dollars per share)	$ 35.46	$ 22.45	$ 35.46	$ 22.45
Unrecognized compensation cost	2.7		2.7
Weighted-average period over which unrecognized compensation cost expected to be recognized (in years)	1.5		1.5
LTIP linked to other performance conditions
LTIP linked to TRS and other performance conditions (in shares)
Outstanding, beginning of period (in shares)	185,767	184,535	161,310	24,131
Granted (in shares)			113,831	160,939
Cancelled (in shares)			(81,908)
Forfeited (in shares)	(1,596)	(23,225)	(9,062)	(23,760)
Outstanding, end of period (in shares)	184,171	161,310	184,171	161,310
Total weighted-average grant-date fair value			2.8	3.0
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 22.63	$ 18.69	$ 18.66	$ 16.99
Granted (in dollars per share)			$ 24.96	$ 18.95
Cancelled (in dollars per share)			$ 18.38
Forfeited (in dollars per share)	$ 22.74	$ 18.95	$ 19.61	$ 18.90
Outstanding, end of period (in dollars per share)	$ 22.63	$ 18.66	$ 22.63	$ 18.66
Total weighted-average grant-date fair value			2.8	3.0
Unrecognized compensation cost	$ 2.8		$ 2.8
Weighted-average period over which unrecognized compensation cost expected to be recognized (in years)	1.9		1.9
2011-2013 LTIP
Share-based compensation
Period for calculating performance goals based on average consolidated net income (in years)			3
Period for calculating performance goals based on average net income (in years)			3
2009-2011 LTIP
Share-based compensation
Average period used to calculate performance goals (in years)			2
2010-2012 LTIP
Share-based compensation
Average period used to calculate performance goals (in years)			2

Represents the average period used to calculate performance goals.

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Represents the period for calculation of performance goals based on consolidated net income.

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Represents the period for calculation of performance goals based on subsidiaries net income.

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Represents the weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect during the reporting period as a result of the occurrence of a terminating event specified in the contractual agreement of the plan or when performance conditions met.

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Represents the number of years used in the calculation of total return to shareholders.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were cancelled during the reporting period.

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The weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect during the reporting period as a result of the cancelled in during the period.

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Represents the total weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Represents the estimated fair value at grant date for nonvested equity-based awards on other than stock (or unit) option plans.

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Represents the high end of range of payout depending on achievement of the goals.

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Represents the low end of range of payout depending on achievement of the goals.

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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 number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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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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The estimated measure of the maximum 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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The estimated measure of the minimum 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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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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