Offsetting of Assets and Liabilities (Details) - USD ($) $ in Thousands |
Jun. 30, 2018 |
Dec. 31, 2017 |
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| Offsetting of Assets and Liabilities [Line Items] | |||||||||||||||||||
| Financial derivatives–assets, at fair value- | [1] | $ 30,669 | $ 28,165 | ||||||||||||||||
| Fair value repurchase agreements | [1] | 214,411 | 155,949 | ||||||||||||||||
| Derivative Liabilities | [1] | (25,675) | (36,273) | ||||||||||||||||
| Reverse repurchase agreements | [1] | (1,421,506) | (1,209,315) | ||||||||||||||||
| Derivative Liability, Not Offset, Policy Election Deduction | (18,031) | (18,708) | |||||||||||||||||
| Securities Sold under Agreements to Repurchase, Not Offset, Policy Election Deduction | (214,411) | (155,949) | |||||||||||||||||
| Derivative Asset, Not Offset, Policy Election Deduction | 18,031 | 18,708 | |||||||||||||||||
| Securities Purchased under Agreements to Resell, Not Offset, Policy Election Deduction | 214,411 | 155,949 | |||||||||||||||||
| Derivative, Collateral, Obligation to Return Cash | [2],[3] | (1,775) | (1,720) | ||||||||||||||||
| Securities Purchased under Agreements to Resell, Collateral, Obligation to Return Cash | [2],[3] | 0 | 0 | ||||||||||||||||
| Derivative, Collateral, Right to Reclaim Cash | [2],[3] | 7,644 | 17,565 | ||||||||||||||||
| Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Cash | [2],[3] | 17,923 | 18,558 | ||||||||||||||||
| Derivative Asset, Fair Value, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election | 10,863 | 7,737 | |||||||||||||||||
| Securities Purchased under Agreements to Resell, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election | 0 | 0 | |||||||||||||||||
| Derivative Liability, Fair Value, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election | 0 | 0 | |||||||||||||||||
| Securities Sold under Agreements to Repurchase, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election | 0 | 0 | |||||||||||||||||
| Securities Collateral relating to Reverse Repurchase Agreements | 1,690,000 | 1,410,000 | |||||||||||||||||
| Financial Derivatives - Assets [Member] | |||||||||||||||||||
| Offsetting of Assets and Liabilities [Line Items] | |||||||||||||||||||
| Financial derivatives–assets, at fair value- | 30,669 | [4] | 28,165 | [5] | |||||||||||||||
| Financial Instruments, Owned and Pledged as Collateral, at Fair Value | [2],[3] | 0 | 0 | ||||||||||||||||
| Excess cash collateral | 3,100 | 6,400 | |||||||||||||||||
| Repurchase Agreements [Member] | |||||||||||||||||||
| Offsetting of Assets and Liabilities [Line Items] | |||||||||||||||||||
| Fair value repurchase agreements | 214,411 | [4],[6] | 155,949 | [5],[7] | |||||||||||||||
| Financial Instruments, Owned and Pledged as Collateral, at Fair Value | [2],[3] | 0 | 0 | ||||||||||||||||
| Financial Derivatives - Liabilities [Member] | |||||||||||||||||||
| Offsetting of Assets and Liabilities [Line Items] | |||||||||||||||||||
| Derivative Liabilities | (25,675) | [4] | (36,273) | [5] | |||||||||||||||
| Financial Instruments, Owned and Pledged as Collateral, at Fair Value | [2],[3] | 0 | 0 | ||||||||||||||||
| Excess cash collateral | 9,800 | 16,600 | |||||||||||||||||
| Reverse Repurchase Agreements [Member] | |||||||||||||||||||
| Offsetting of Assets and Liabilities [Line Items] | |||||||||||||||||||
| Financial Instruments, Owned and Pledged as Collateral, at Fair Value | [2],[3] | $ 1,189,172 | $ 1,034,808 | ||||||||||||||||
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