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Offsetting of Assets and Liabilities (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Offsetting Assets and Liabilities [Line Items] |  |  |  |  |
Financial derivatives–assets, at fair value | [1] | $ 23,049 |  | $ 8,792 |
Reverse repurchase agreements | [1] | 26,769 |  | 81,461 |
Financial derivatives–liabilities, at fair value | [1] | (333) |  | (1,863) |
Repurchase agreements | [1] | (1,500,632) |  | (1,597,206) |
Derivative Liability, Fair Value, Amount Offset Against Collateral |  | (310) |  | (1,046) |
Securities sold under agreements to repurchase available for offset |  | (26,769) |  | (81,461) |
Derivative Asset, Fair Value, Amount Offset Against Collateral |  | 310 |  | 1,046 |
Securities purchased under agreements to resell available for offset |  | 26,769 |  | 81,461 |
Derivative, Collateral, Obligation to Return Cash | [3] | (8,223) | [2] | 0 | [4]
Cash collateral received | [3] | 0 | [2] | 0 | [4]
Derivative, Collateral, Right to Reclaim Cash | [3] | 22 | [2] | 697 | [4]
Cash Collateral Pledged | [3] | 25,339 | [2] | 18,707 | [4]
Derivative Asset, Fair Value, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election |  | 14,516 |  | 7,746 |
Reverse Repurchase Agreements and Related Collateral Net |  | 0 |  | 0 |
Derivatives and related collateral, net |  | (1) |  | (120) |
Repurchase agreements and related collateral, net |  | 0 |  | 0 |
Securities Sold under Agreements to Repurchase, Fair Value of Collateral |  | 1,550,000 |  | 1,660,000 |
Derivative Financial Instruments, Assets [Member] |  |  |  |  |
Offsetting Assets and Liabilities [Line Items] |  |  |  |  |
Excess cash collateral |  | 600 |  | 5,400 |
Derivative Financial Instruments, Liabilities [Member] |  |  |  |  |
Offsetting Assets and Liabilities [Line Items] |  |  |  |  |
Excess cash collateral |  | 900 |  | 1,400 |
Repurchase Agreements [Member] |  |  |  |  |
Offsetting Assets and Liabilities [Line Items] |  |  |  |  |
Financial Instruments Pledged as Collateral | [3] | 1,448,524 | [2] | 1,497,038 | [4]
Reverse Repurchase Agreements [Member] |  |  |  |  |
Offsetting Assets and Liabilities [Line Items] |  |  |  |  |
Financial Instruments Pledged as Collateral | [3] | 0 | [2] | 0 | [4]
Derivative Financial Instruments, Assets [Member] |  |  |  |  |
Offsetting Assets and Liabilities [Line Items] |  |  |  |  |
Financial Instruments Pledged as Collateral | [3] | 0 | [2] | 0 | [4]
Derivative Financial Instruments, Liabilities [Member] |  |  |  |  |
Offsetting Assets and Liabilities [Line Items] |  |  |  |  |
Financial Instruments Pledged as Collateral | [3] | $ 0 | [2] | $ 0 | [4]
[1] | In the Company's Consolidated Balance Sheet, all balances associated with the repurchase agreements and financial derivatives are presented on a gross basis.
[2] | For the purpose of this presentation, for each row the total amount of financial instruments transferred or pledged and cash collateral (received) or pledged may not exceed the applicable gross amount of assets or (liabilities) as presented here. Therefore, the Company has reduced the amount of financial instruments transferred or pledged as collateral related to the Company's repurchase agreements and cash collateral pledged on the Company's financial derivative assets and liabilities. Total financial instruments transferred or pledged as collateral on the Company's repurchase agreements as of September 30, 2018 were $1.55 billion. As of September 30, 2018 total cash collateral on financial derivative assets and liabilities excludes $0.6 million and $0.9 million respectively of net excess cash collateral.
[3] | When collateral is pledged to or pledged by a counterparty, it is often pledged or posted with respect to all positions with such counterparty, and in such cases such collateral cannot be specifically identified as relating to a specific asset or liability. As a result, in preparing the above table, the Company has made assumptions in allocating pledged or posted collateral among the various rows.
[4] | For the purpose of this presentation, for each row the total amount of financial instruments transferred or pledged and cash collateral (received) or pledged may not exceed the applicable gross amount of assets or (liabilities) as presented here. Therefore the Company has reduced the amount of financial instruments transferred or pledged as collateral related to the Company's repurchase agreements and cash collateral pledged on the Company's financial derivative assets and liabilities. Total financial instruments transferred or pledged as collateral on the Company's repurchase agreements as of December 31, 2017 were $1.66 billion. As of December 31, 2017 total cash collateral on financial derivative assets and liabilities excludes $5.4 million and $1.4 million, respectively of net excess cash collateral.

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