Loans and Allowance for Losses and Concentrations of Credit Risk - Loan (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 4,004,968	$ 3,873,755
Allowance for Loan and Lease Losses, Real Estate	7,017	6,796
Loans and Leases Receivable, Net Amount	5,515,052	5,266,786
Financing Receivable, Allowance for Credit Losses	9,184	8,866	$ 7,435	$ 6,563
Total provision for losses	335	1,758	1,002
Financing Receivable, Allowance for Credit Losses, Write-downs	17	327	130
Real Estate Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Lease Receivable Unpaid Principal Balance	5,527,166	5,275,024
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums	(5,097)	(1,442)
Loans and Leases Receivable, Gross	5,522,069	5,273,582
Allowance for Loan and Lease Losses, Real Estate	7,017	6,796
Loans and Leases Receivable, Net Amount	5,515,052	5,266,786
Real Estate Loan [Member] \| Farm & Ranch [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Lease Receivable Unpaid Principal Balance	4,588,323	4,198,733
Real Estate Loan [Member] \| Rural Utilities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Lease Receivable Unpaid Principal Balance	938,843	1,076,291
Unsecuritized [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Lease Receivable Unpaid Principal Balance	4,010,065	3,875,197
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums	(5,097)	(1,442)
Loans and Leases Receivable, Gross	4,004,968	3,873,755
Allowance for Loan and Lease Losses, Real Estate	5,565	5,493
Loans and Leases Receivable, Net Amount	3,999,403	3,868,262
Unsecuritized [Member] \| Farm & Ranch [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Lease Receivable Unpaid Principal Balance	3,071,222	2,798,906
Unsecuritized [Member] \| Rural Utilities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Lease Receivable Unpaid Principal Balance	938,843	1,076,291
In Consolidated Trusts [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Lease Receivable Unpaid Principal Balance	1,517,101	1,399,827
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums	0	0
Loans and Leases Receivable, Gross	1,517,101	1,399,827
Allowance for Loan and Lease Losses, Real Estate	1,452	1,303
Loans and Leases Receivable, Net Amount	1,515,649	1,398,524
In Consolidated Trusts [Member] \| Farm & Ranch [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Lease Receivable Unpaid Principal Balance	1,517,101	1,399,827
In Consolidated Trusts [Member] \| Rural Utilities [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Lease Receivable Unpaid Principal Balance	0	0
Crops [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses	4,394	4,081	3,365	2,791
Total provision for losses	313	944	574
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(228)	0
Permanent Plantings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses	2,126	2,469	1,723	931
Total provision for losses	(343)	816	792
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(70)	0
Livestock [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses	1,460	1,211	1,375	1,781
Total provision for losses	249	(151)	(406)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	(13)	0
Part-Time Farm [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses	474	481	405	408
Total provision for losses	10	92	127
Financing Receivable, Allowance for Credit Losses, Write-downs	(17)	(16)	130
Agriculture Storage and Processing [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses	720	606	533	649
Total provision for losses	114	73	(116)
Financing Receivable, Allowance for Credit Losses, Write-downs	0	0	0
Other Commodity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Allowance for Credit Losses	10	18	34	$ 3
Total provision for losses	(8)	(16)	31
Financing Receivable, Allowance for Credit Losses, Write-downs	$ 0	$ 0	$ 0

Unpaid principal balance of loans before unamortized premiums, discounts and other cost basis adjustments

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The reserve to cover probable credit losses related to specifically identified fixed maturity real estate loans as well as probable credit losses inherent in the remainder of that loan portfolio.

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-Publisher FASB
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A valuation allowance for financing receivables that are expected to be uncollectible.

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Amount of direct write-downs of financing receivables charged against the allowance.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

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Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

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Amount of deferred fees paid by borrowers and unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield. Excludes amounts for loans and leases covered under loss sharing agreements.

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