Share-based compensation plans (Details) (USD $)	3 Months Ended
Sep. 30, 2011 DecimalType	Sep. 30, 2010 DecimalType	Dec. 31, 2010
Share-based compensation plans [Abstract]
Number of shares underlying options	290,000	367,000
Weighted-average exercise price per share	$ 31.47	$ 19.05
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected dividend yield - weighted average	1.30%	2.10%
Expected dividend yield - range	1.3	2.1
Expected volatility - weighted average	36.00%	36.00%
Expected volatility range - minimum	36.00%	35.00%
Expected volatility range - maximum	37.00%	39.00%
Risk-free interest rate - weighted average	1.20%	1.40%
Risk-free interest rate range - minimum	0.50%	0.60%
Risk-free interest rate range - maximum	1.90%	1.90%
Expected term in years - weighted average	3.8	3.8
Expected term in years range - minimum	1.9	1.9
Expected term in years range - maximum	5.3	5.3
Weighted-average fair value estimates at grant date	$ 2,400,000	$ 4.65
Nonvested share activity [Line Items]
Beginning balance - shares			328,600
Stock units granted - shares	159,500
Cancelled awards - shares	(14,900)
Vested - shares	(149,700)
Ending balance - shares	323,500		328,600
2005 Equity Incentive Plan
Nonvested share activity [Line Items]
Beginning balance - shares			299,500
Stock units granted - shares	143,700
Cancelled awards - shares	(14,900)
Vested - shares	(120,600)
Ending balance - shares	307,700		299,500
Weighted-average grant-date fair value [Abstract]
Beginning Balance - grant-date fair value			$ 22.84
Stock units granted - grant-date fair value	$ 30.43
Cancelled awards - grant-date fair value	$ 23.40
Vested - grant-date fair value	$ 24.07
Ending Balance - grant-date fair value	$ 25.99		$ 22.84
Non-Employee Directors' Equity Plan
Nonvested share activity [Line Items]
Beginning balance - shares			29,100
Stock units granted - shares	15,800
Cancelled awards - shares	0
Vested - shares	(29,100)
Ending balance - shares	15,800		29,100
Weighted-average grant-date fair value [Abstract]
Beginning Balance - grant-date fair value			$ 22.84
Stock units granted - grant-date fair value	$ 30.43
Cancelled awards - grant-date fair value	$ 23.40
Vested - grant-date fair value	$ 24.07
Ending Balance - grant-date fair value	$ 25.99		$ 22.84

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- Definition

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

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- Definition

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.

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- Definition

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).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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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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- Definition

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

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

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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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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- Definition

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

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The maximum 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 minimum 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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- Definition

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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If different dividend rates are used during the contractual term, then disclose the range of expected dividends used.

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

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

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The range of expected volatilities used and the weighted-average expected volatility for an entity using a valuation technique with different volatilities during the contractual term.

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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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Gross number of share options (or share units) granted during the period.

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- Definition

The weighted average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on stock options awarded under the plan during the reporting period.

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- Definition

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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