Notes Payable - Borrowings (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Due within one year:
Current portion of medium-term notes	$ 9,593,477	$ 10,746,382
Weighted - average rate (as a percent)	0.49%	0.59%
Medium-term notes due in:
Two years	$ 2,912,497	$ 3,004,203
Three years	2,509,857	2,809,551
Four years	1,308,945	927,119
Five years	1,853,777	1,342,250
Thereafter	3,358,018	2,966,172
Total due after one year	11,943,094	11,049,295
Total principal net of discounts	21,536,571	21,795,677
Hedging adjustments	23,739	53,240
Total	$ 21,560,310	$ 21,848,917
Weighted - average rate (as a percent)	1.25%	1.37%
Total weighted - average rate (as a percent)	0.91%	0.98%
Two years
Medium-term notes due in:
Weighted - average rate (as a percent)	0.96%	1.00%
Three years
Medium-term notes due in:
Weighted - average rate (as a percent)	1.16%	1.24%
Four years
Medium-term notes due in:
Weighted - average rate (as a percent)	1.25%	1.67%
Five years
Medium-term notes due in:
Weighted - average rate (as a percent)	0.84%	1.03%
Thereafter
Medium-term notes due in:
Weighted - average rate (as a percent)	1.81%	1.92%
Discount notes
Due within one year:
Current portion of medium-term notes	$ 1,675,995	$ 1,797,175
Weighted - average rate (as a percent)	0.06%	0.11%
Short-term debt, average outstanding during the quarter	$ 1,625,416	$ 2,343,702
Weighted - average rate, average outstanding during the quarter (as a percent)	0.11%	0.63%
Medium-term notes
Due within one year:
Current portion of medium-term notes	$ 2,514,691	$ 2,645,146
Weighted - average rate (as a percent)	0.14%	0.19%
Short-term debt, average outstanding during the quarter	$ 2,725,057	$ 1,593,253
Weighted - average rate, average outstanding during the quarter (as a percent)	0.16%	0.60%
Current portion of medium-term notes
Due within one year:
Current portion of medium-term notes	$ 5,402,791	$ 6,304,061
Weighted - average rate (as a percent)	0.78%	0.90%