Interest rate swap agreements (Details) (USD $)	9 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Derivative Instruments and Hedging Activities Disclosure [Abstract]
The percentage of effectiveness of interest rate swap agreements	100.00%	100.00%
Interest rate swap hedge ineffectiveness recognized in earnings	$ 0	$ 0
Cash flow hedge loss to be reclassified within twelve month	8,500,000
Cash flow hedge liability at fair value	9,319,000		20,700,000
Collateral obligation requirements	$ 0

The percentage of effective notional amount of interest rate hedge agreements.

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- Definition

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 30

-Section 50

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-Number 133

-Paragraph 45

-Subparagraph b(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 amount of the obligation to return cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.

+ References

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-Publisher FASB

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- Definition

The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 205G

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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-Paragraph 44C

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-SubTopic 25

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- Definition

Fair value as of the balance sheet date of all interest rate derivative liabilities designated as cash flow hedging instruments.

+ References

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