Notes Payable - Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Due within one year:
Current portion of medium-term notes	$ 10,746,382	$ 10,019,082
Weighted - average rate (as a percent)	0.59%	1.84%
Medium-term notes due in:
Two years	$ 3,004,203	$ 3,696,699
Three years	2,809,551	1,592,315
Four years	927,119	1,202,817
Five years	1,342,250	762,003
Thereafter	2,966,172	1,818,299
Total due after one year	11,049,295	9,072,133
Total principal net of discounts	21,795,677	19,091,215
Hedging adjustments	53,240	7,433
Total	$ 21,848,917	$ 19,098,648
Weighted - average rate (as a percent)	1.37%	2.28%
Total weighted - average rate (as a percent)	0.98%	2.05%
Two years
Medium-term notes due in:
Weighted - average rate (as a percent)	1.00%	2.04%
Three years
Medium-term notes due in:
Weighted - average rate (as a percent)	1.24%	2.15%
Four years
Medium-term notes due in:
Weighted - average rate (as a percent)	1.67%	2.27%
Five years
Medium-term notes due in:
Weighted - average rate (as a percent)	1.03%	2.25%
Thereafter
Medium-term notes due in:
Weighted - average rate (as a percent)	1.92%	2.89%
Discount notes
Due within one year:
Current portion of medium-term notes	$ 1,797,175	$ 2,194,177
Weighted - average rate (as a percent)	0.11%	1.72%
Short-term debt, average outstanding during the quarter	$ 2,343,702	$ 1,977,214
Weighted - average rate, average outstanding during the quarter (as a percent)	0.63%	2.25%
Medium-term notes
Due within one year:
Current portion of medium-term notes	$ 2,645,146	$ 1,152,770
Weighted - average rate (as a percent)	0.19%	1.98%
Short-term debt, average outstanding during the quarter	$ 1,593,253	$ 1,780,517
Weighted - average rate, average outstanding during the quarter (as a percent)	0.60%	2.33%
Current portion of medium-term notes
Due within one year:
Current portion of medium-term notes	$ 6,304,061	$ 6,672,135
Weighted - average rate (as a percent)	0.90%	1.85%