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Farmer Mac Guaranteed Securities and USDA Guaranteed Securities - Farmer Mac Guaranteed Securities and USDA Securities (Details) - USD ($) $ in Thousands |  |  | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2017 | Dec. 31, 2016 | Dec. 31, 2016 | Oct. 01, 2016
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Transfer of USDA Guaranteed Securities- Available-for-sale to Held-to-maturity; Unrealized holding gains |  |  |  | $ 73,100
Trading Securities [Abstract] |  |  |  |
Transfer of Farmer Mac Guaranteed Securities - Available-for-sale to Held-to-maturity; Unrealized holding gains |  |  |  | 700
Transfer of Farmer Mac Guaranteed Securities and USDA Guaranteed Securities - Available-for-sale to Held-to-maturity; Unrealized holding gains |  |  |  | $ 73,800
Farmer Mac Guaranteed Securities and USDA Securities [Member] |  |  |  |
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Unamortized Premium/(Discount) | $ 64,172 |  |  |
Held-to-Maturity Securities [Abstract] |  |  |  |
Held-to-Maturity Unpaid Principal Balance | 4,186,923 | $ 3,086,504 | $ 3,086,504 |
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment | 4,251,095 | 3,158,456 | 3,158,456 |
Held to Maturity; Unrealized Holding Gains | 5,177 | 7,192 | 7,192 |
Held to Maturity - Unrealized Holding Loss | (71,098) | (99,004) | (99,004) |
Held-to-maturity Securities, Fair Value | 4,185,174 | 3,066,644 | 3,066,644 |
Available-for-sale Securities [Abstract] |  |  |  |
Available-for-sale Securities, Amortized Cost Basis | 5,459,514 |  |  |
Available-for-sale, at fair value | 5,443,746 |  |  |
Farmer Mac Guaranteed Securities [Member] |  |  |  |
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Unamortized Premium/(Discount) | (852) | (1,833) | (1,833) |
Held-to-Maturity Securities [Abstract] |  |  |  |
Held-to-Maturity Unpaid Principal Balance | 2,159,662 | 1,151,064 | 1,151,064 |
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment | 2,158,810 | 1,149,231 | 1,149,231 |
Held to Maturity; Unrealized Holding Gains | 5,177 | 7,192 | 7,192 |
Held to Maturity - Unrealized Holding Loss | (6,357) | (3,414) | (3,414) |
Held-to-maturity Securities, Fair Value | 2,157,630 | 1,153,009 | 1,153,009 |
USDA Securities [Member] |  |  |  |
Trading Securities [Abstract] |  |  |  |
Trading, at fair value | 14,864 | 20,388 | 20,388 |
Institutional Credit [Member] | Farmer Mac Guaranteed Securities and USDA Securities [Member] |  |  |  |
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Unamortized Premium/(Discount) |  | 71,952 | 71,952 |
Institutional Credit [Member] | Farmer Mac Guaranteed Securities [Member] |  |  |  |
Held-to-Maturity Securities [Abstract] |  |  |  |
Held-to-Maturity Unpaid Principal Balance | 2,128,974 | 1,115,465 | 1,115,465 |
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment | 2,127,808 | 1,113,189 | 1,113,189 |
Held to Maturity; Unrealized Holding Gains | 5,091 | 7,187 | 7,187 |
Held to Maturity - Unrealized Holding Loss | (6,073) | (3,175) | (3,175) |
Held-to-maturity Securities, Fair Value | 2,126,826 | 1,117,201 | 1,117,201 |
Available-for-sale Securities [Abstract] |  |  |  |
Available-for-Sale Securities Unpaid Principal Balance | 5,459,655 | 4,889,007 | 4,889,007 |
Available-for-sale Securities, Amortized Cost Basis | 5,459,514 | 4,888,904 | 4,888,904 |
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax | 30,811 | 28,715 | 28,715 |
Available-for-sale Securities, Unrealized Losses | (46,579) | (63,934) | (63,934) |
Available-for-sale, at fair value | 5,443,746 | 4,853,685 | 4,853,685 |
Institutional Credit [Member] | Held-to-maturity Securities [Member] | Farmer Mac Guaranteed Securities [Member] |  |  |  |
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Unamortized Premium/(Discount) | (1,166) | (2,276) | (2,276) |
Institutional Credit [Member] | Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member] |  |  |  |
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Unamortized Premium/(Discount) | (141) | (103) | (103) |
USDA Guarantees [Member] |  |  |  |
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Unamortized Premium/(Discount) | 300 | 400 | 400 |
USDA Guarantees [Member] | Farmer Mac Guaranteed Securities [Member] |  |  |  |
Held-to-Maturity Securities [Abstract] |  |  |  |
Held-to-Maturity Unpaid Principal Balance | 30,688 | 35,599 | 35,599 |
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment | 31,002 | 36,042 | 36,042 |
Held to Maturity; Unrealized Holding Gains | 86 | 5 | 5 |
Held to Maturity - Unrealized Holding Loss | (284) | (239) | (239) |
Held-to-maturity Securities, Fair Value | 30,804 | 35,808 | 35,808 |
USDA Guarantees [Member] | USDA Securities [Member] |  |  |  |
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Unamortized Premium/(Discount) | 65,024 | 73,785 | 73,785 |
Held-to-Maturity Securities [Abstract] |  |  |  |
Held-to-Maturity Unpaid Principal Balance | 2,027,261 | 1,935,440 | 1,935,440 |
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment | 2,092,285 | 2,009,225 | 2,009,225 |
Held to Maturity; Unrealized Holding Gains | 0 | 0 | 0 |
Held to Maturity - Unrealized Holding Loss | (64,741) | (95,590) | (95,590) |
Held-to-maturity Securities, Fair Value | 2,027,544 | 1,913,635 | 1,913,635 |
Trading Securities [Abstract] |  |  |  |
Trading Securities Unpaid Principal Balance | 14,204 | 19,360 | 19,360 |
Trading Securities, Debt, Amortized Cost | 15,296 | 20,737 | 20,737 |
Trading Securities, Unrealized Gains | 31 | 41 |  |
Trading Securities, Unrealized Holding Loss | (463) | (390) |  |
Trading, at fair value | 14,864 | 20,388 | 20,388 |
USDA Guarantees [Member] | Available-for-sale Securities [Member] | Farmer Mac Guaranteed Securities [Member] |  |  |  |
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Unamortized Premium/(Discount) | 314 | 443 | 443 |
USDA Guarantees [Member] | Trading Securities [Member] | USDA Securities [Member] |  |  |  |
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Unamortized Premium/(Discount) | $ 1,092 | $ 1,377 | 1,377 |
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Farmer Mac Guaranteed Securities [Member] | USDA Guarantee [Member] | Available-for-sale Securities [Member] |  |  |  |
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 |  |  | 2,000,000 |
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Farmer Mac Guaranteed Securities [Member] | Farmer Mac Guaranteed Securities [Member] | Available-for-sale Securities [Member] |  |  |  |
Schedule of Held-to-Maturity, Available-for-sale, and Trading Securities [Line Items] |  |  |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 |  |  | $ 32,800 |

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