Derivative Instruments - Schedule of gain or loss recognized on interest rate derivatives designated as cash flow hedges (Details) (Cash flow hedging, USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Cash flow hedging
Interest Rate Cash Flow Hedges [Roll Forward]
Balance at beginning of period	$ (10,079)	$ (12,791)	$ (11,706)	$ (16,624)
Amount of gain (loss) recognized in cumulative other comprehensive (loss) income	537	(1,087)	(324)	274
Amount of loss reclassified from cumulative other comprehensive (loss) income into interest expense	1,268	1,265	3,756	3,737
Unrealized gain on derivative instruments	1,805	178	3,432	4,011
Balance at end of period	$ (8,274)	$ (12,613)	$ (8,274)	$ (12,613)

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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4D

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624177-113959

+ Details

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- Definition

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

+ References

+ Details

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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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624163-113959

+ Details

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- Definition

Amount of unrealized gain (loss) related to derivatives designated as cash flow hedging instruments, recorded in results of operations to the extent that the cash flow hedge is determined to be ineffective.

+ References

+ Details

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