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Derivative Instruments - Notional and Fair Value Amounts of Derivative Instruments (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Derivative [Line Items] |  |
Notional Amount | $ 278,200 | $ 246,800
Fair Value | 2,647 | 15,550
Bank of America, N.A. |  |
Derivative [Line Items] |  |
Notional Amount | 144,500 | 85,100
Fair Value | 870 | 6,320
Barclays Bank PLC |  |
Derivative [Line Items] |  |
Notional Amount | 28,500 | 48,900
Fair Value | 247 | 1,828
Citigroup Inc. |  |
Derivative [Line Items] |  |
Notional Amount | 0 | 0
Fair Value | 0 | 0
Credit Suisse International |  |
Derivative [Line Items] |  |
Notional Amount | 16,100 | 21,100
Fair Value | 55 | 1,444
Societe Generale |  |
Derivative [Line Items] |  |
Notional Amount | 89,100 | 91,700
Fair Value | $ 1,475 | $ 5,958

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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