Loans - Schedule of Financing Receivables Credit Quality Indicators (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Total
Financing Receivable, before Allowance for Credit Loss, Total	$ 7,341,912		$ 7,261,933
90-Days Delinquencies	19,144		6,286
Farm & Ranch
2021
Total	558,717		2,021,189
Current period charge-offs	0		0
Current period recoveries	0		0
Current period Farm & Ranch net charge-offs	0		0
2020
Total	2,029,701		859,880
Current period charge-offs	0		0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
2019
Total	805,952		524,541
Current period charge-offs	0		0
Financing Receivable, Originated Two Years before Latest Fiscal Year, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
2018
Total	489,046		562,007
Current period charge-offs	0		0
Financing Receivable, Originated Three Years before Latest Fiscal Year, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
2017
Total	460,291		515,561
Current period charge-offs	0		0
Financing Receivable, Originated Four Years before Latest Fiscal Year, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
Prior
Total	1,452,339		1,140,603
Current period charge-offs	0		0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
Revolving Loans - Amortized Cost Basis
Total	506,921		552,657
Current period charge-offs	0		0
Financing Receivable, Revolving, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
Total
Financing Receivable, before Allowance for Credit Loss, Total	6,302,967		6,176,438
Current period charge-offs	0		0
Financing Receivable, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0	$ 0	0
90-Days Delinquencies	19,144		6,286
Rural Utilities
2021
Total	10,979		667,489
Current period charge-offs	0		0
Current period recoveries	0		0
Current period Farm & Ranch net charge-offs	0		0
2020
Total	663,330		809,921
Current period charge-offs	0		0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
2019
Total	801,799		8,260
Current period charge-offs	0		0
Financing Receivable, Originated Two Years before Latest Fiscal Year, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
2018
Total	8,181		89,842
Current period charge-offs	0		0
Financing Receivable, Originated Three Years before Latest Fiscal Year, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
2017
Total	89,621		31,275
Current period charge-offs	0		0
Financing Receivable, Originated Four Years before Latest Fiscal Year, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
Prior
Total	650,486		641,145
Current period charge-offs	0		0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
Revolving Loans - Amortized Cost Basis
Total	22,708		12,480
Current period charge-offs	0		0
Financing Receivable, Revolving, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0		0
Total
Financing Receivable, before Allowance for Credit Loss, Total	2,247,104		2,260,412
Current period charge-offs	0		0
Financing Receivable, Allowance for Credit Loss, Recovery	0		0
Current period Farm & Ranch net charge-offs	0	$ 0	0
90-Days Delinquencies	0		0
Acceptable | Farm & Ranch
2021
Total	542,638		1,947,618
2020
Total	1,923,390		774,315
2019
Total	724,202		484,345
2018
Total	443,929		500,768
2017
Total	399,931		465,277
Prior
Total	1,334,771		1,068,693
Revolving Loans - Amortized Cost Basis
Total	482,993		535,742
Total
Financing Receivable, before Allowance for Credit Loss, Total	5,851,854		5,776,758
Acceptable | Rural Utilities
2021
Total	10,979		667,489
2020
Total	639,730		809,921
2019
Total	801,799		8,260
2018
Total	8,181		89,842
2017
Total	89,621		31,275
Prior
Total	650,486		641,145
Revolving Loans - Amortized Cost Basis
Total	22,708		12,480
Total
Financing Receivable, before Allowance for Credit Loss, Total	2,223,504		2,260,412
Special mention | Farm & Ranch
2021
Total	16,079		70,171
2020
Total	102,656		79,744
2019
Total	56,723		18,317
2018
Total	16,292		8,530
2017
Total	2,569		13,111
Prior
Total	23,003		21,328
Revolving Loans - Amortized Cost Basis
Total	11,804		7,656
Total
Financing Receivable, before Allowance for Credit Loss, Total	229,126		218,857
Special mention | Rural Utilities
2021
Total	0		0
2020
Total	23,600		0
2019
Total	0		0
2018
Total	0		0
2017
Total	0		0
Prior
Total	0		0
Revolving Loans - Amortized Cost Basis
Total	0		0
Total
Financing Receivable, before Allowance for Credit Loss, Total	23,600		0
Substandard | Farm & Ranch
2021
Total	0		3,400
2020
Total	3,655		5,821
2019
Total	25,027		21,879
2018
Total	28,825		52,709
2017
Total	57,791		37,173
Prior
Total	94,565		50,582
Revolving Loans - Amortized Cost Basis
Total	12,124		9,259
Total
Financing Receivable, before Allowance for Credit Loss, Total	221,987		180,823
Substandard | Rural Utilities
2021
Total	0		0
2020
Total	0		0
2019
Total	0		0
2018
Total	0		0
2017
Total	0		0
Prior
Total	0		0
Revolving Loans - Amortized Cost Basis
Total	0		0
Total
Financing Receivable, before Allowance for Credit Loss, Total	$ 0		$ 0