Notes Payable - Borrowings (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Due within one year:
Current portion of medium-term notes	$ 10,935,083	$ 10,019,082
Weighted - average rate (as a percent)	0.63%	1.84%
Medium-term notes due in:
Two years	$ 3,277,494	$ 3,700,835
Three years	2,326,812	1,594,709
Four years	1,056,062	1,205,276
Five years	1,242,150	760,887
Thereafter	2,751,684	1,817,859
Total due after one year	10,654,202	9,079,566
Total	$ 21,589,285	$ 19,098,648
Weighted - average rate (as a percent)	1.47%	2.28%
Total weighted - average rate (as a percent)	1.05%	2.05%
Two years
Medium-term notes due in:
Weighted - average rate (as a percent)	0.96%	2.04%
Three years
Medium-term notes due in:
Weighted - average rate (as a percent)	1.44%	2.15%
Four years
Medium-term notes due in:
Weighted - average rate (as a percent)	1.73%	2.27%
Five years
Medium-term notes due in:
Weighted - average rate (as a percent)	1.35%	2.25%
Thereafter
Medium-term notes due in:
Weighted - average rate (as a percent)	2.07%	2.89%
Discount notes
Due within one year:
Current portion of medium-term notes	$ 2,358,943	$ 2,194,177
Weighted - average rate (as a percent)	0.21%	1.72%
Short-term debt, average outstanding during the quarter	$ 2,313,477	$ 1,977,214
Weighted - average rate, average outstanding during the quarter (as a percent)	0.79%	2.25%
Medium-term notes
Due within one year:
Current portion of medium-term notes	$ 2,065,148	$ 1,152,770
Weighted - average rate (as a percent)	0.27%	1.98%
Short-term debt, average outstanding during the quarter	$ 1,312,909	$ 1,780,517
Weighted - average rate, average outstanding during the quarter (as a percent)	0.81%	2.33%
Current portion of medium-term notes
Due within one year:
Current portion of medium-term notes	$ 6,510,992	$ 6,672,135
Weighted - average rate (as a percent)	0.90%	1.85%