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Derivative Instruments - Cash flow hedges (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Effects of derivative instruments |  |
Amount of Gain Recognized in OCI on Derivatives (Effective Portion) | $ 1,616 | $ 434
Cash Flow Hedging |  |
Effects of derivative instruments |  |
Amount of Gain Recognized in OCI on Derivatives (Effective Portion) | 1,031 | 335
Amount of Loss (Gain) Reclassified from Accumulated OCI into Income (Effective Portion) | 24 | 226
Cash Flow Hedging | Interest rate contracts | Interest expense Member |  |
Effects of derivative instruments |  |
Amount of Gain Recognized in OCI on Derivatives (Effective Portion) | 1,031 | 335
Amount of Loss (Gain) Reclassified from Accumulated OCI into Income (Effective Portion) | $ 24 | $ 226

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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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959 + Details
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