v2.4.0.6

Summary of significant accounting policies (Details 2)	12 Months Ended
	Dec. 31, 2012 Rate
Derivative financial instruments
Number of interest rate swaps designated as cash flow hedges	2
Long-term incentive plan
Long-term incentive plan
Cliff vesting period	3 years
Portion of vested notional units expected to be redeemed in cash	0.33
Portion of vested notional units expected to be redeemed in shares	0.67
Long-term incentive plan | Officers
Long-term incentive plan
Cliff vesting period	3 years
Long-term incentive plan | Non-officers
Long-term incentive plan
Vesting period on ratable basis	3 years
Long-term incentive plan | Senior officers
Long-term incentive plan
Vesting period on ratable basis	3 years
Vesting percentage of performance units	33.00%
Long-term incentive plan | Maximum
Long-term incentive plan
Vesting percentage of notional units	150.00%
Aggregate number of shares which may be issued from treasury	1,300,000

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Represents the number of interest rate swaps designated as cash flow hedges for accounting purposes.

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Represents the cliff vesting period of equity-based compensation awards.

+ References

No definition available.

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- Definition

Represents the portion of vested notional units that are expected to be redeemed in cash under equity-based compensation plan.

+ References

No definition available.

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- Definition

Represents the portion of vested notional units that are expected to be redeemed in shares of common stock under equity-based compensation plan.

+ References

No definition available.

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- Definition

Represents the vesting percentage of notional units depending on the level of achievement of target levels under equity-based compensation plan.

+ References

No definition available.

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- Definition

Represents the vesting percentage of performance units depending on the level of achievement of target levels under the equity-based compensation plan.

+ References

No definition available.

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- Definition

Period 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, 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 (a)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

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).

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph a

-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.



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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