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Trading Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017
--- | --- | --- | ---
Schedule of Trading Securities [Line Items] |  |  |
Trading securities | $ 1,613,400 | $ 462,676 | $ 614,117
Trading Securities, Net Unrealized Gain (Loss) | (3,733) | (448) | 798
U.S. government agency residential mortgage-backed securities [Member] |  |  |
Schedule of Trading Securities [Line Items] |  |  |
Trading securities | 1,378,450 | 392,673 | 516,760
Trading Securities, Net Unrealized Gain (Loss) | (3,498) | (517) | 723
Municipal and other tax-exempt securities [Member] |  |  |
Schedule of Trading Securities [Line Items] |  |  |
Trading securities | 41,345 | 13,559 | 56,148
Trading Securities, Net Unrealized Gain (Loss) | (161) | 83 | 153
U.S. government agency debentures [Member] |  |  |
Schedule of Trading Securities [Line Items] |  |  |
Trading securities | 80,692 | 21,196 | 30,162
Trading Securities, Net Unrealized Gain (Loss) | 21 | 8 | (101)
Asset-backed securities [Member] |  |  |
Schedule of Trading Securities [Line Items] |  |  |
Trading securities | 72,309 | 23,885 | 0
Trading Securities, Net Unrealized Gain (Loss) | (100) | (26) | 0
Other debt securities [Member] |  |  |
Schedule of Trading Securities [Line Items] |  |  |
Trading securities | 40,604 | 11,363 | 11,047
Trading Securities, Net Unrealized Gain (Loss) | $ 5 | $ 4 | $ 23

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