Notes Payable - Borrowings (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Due within one year:
Current portion of medium-term notes	$ 10,887,523	$ 10,019,082
Weighted - average rate (as a percent)	0.76%	1.84%
Medium-term notes due in:
Two years	$ 3,764,817	$ 3,700,835
Three years	1,839,291	1,594,709
Four years	1,204,584	1,205,276
Five years	1,213,467	760,887
Thereafter	2,511,868	1,817,859
Total due after one year	10,534,027	9,079,566
Total	$ 21,421,550	$ 19,098,648
Weighted - Average Rate (as a percent)	1.61%	2.28%
Total Weighted - Average Rate (as a percent)	1.18%	2.05%
2021
Medium-term notes due in:
Weighted - Average Rate (as a percent)	1.08%	2.04%
2022
Medium-term notes due in:
Weighted - Average Rate (as a percent)	1.70%	2.15%
2023
Medium-term notes due in:
Weighted - Average Rate (as a percent)	1.78%	2.27%
2024
Medium-term notes due in:
Weighted - Average Rate (as a percent)	1.56%	2.25%
Thereafter
Medium-term notes due in:
Weighted - Average Rate (as a percent)	2.28%	2.89%
Notes Payable to Banks
Due within one year:
Current portion of medium-term notes	$ 2,507,363	$ 2,194,177
Weighted - average rate (as a percent)	0.44%	1.72%
Short-term debt, average outstanding during the quarter	$ 2,221,571	$ 1,977,214
Weighted - average rate, average outstanding during the quarter (as a percent)	1.05%	2.25%
Medium-term Notes
Due within one year:
Current portion of medium-term notes	$ 1,394,857	$ 1,152,770
Weighted - average rate (as a percent)	0.47%	1.98%
Short-term debt, average outstanding during the quarter	$ 1,120,213	$ 1,780,517
Weighted - average rate, average outstanding during the quarter (as a percent)	1.16%	2.33%
Long-term Debt
Due within one year:
Current portion of medium-term notes	$ 6,985,303	$ 6,672,135
Weighted - average rate (as a percent)	0.93%	1.85%