Notes Payable - Borrowings (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Due within one year:
Current portion of medium-term notes	$ 11,148,184	$ 10,019,082
Weighted - average rate (as a percent)	1.24%	1.84%
Medium-term notes due in:
Two years	$ 3,381,965	$ 3,700,835
Three years	1,588,751	1,594,709
Four years	1,311,362	1,205,276
Five years	1,027,532	760,887
Thereafter	2,207,226	1,817,859
Total due after one year	9,516,836	9,079,566
Total	$ 20,665,020	$ 19,098,648
Weighted - Average Rate (as a percent)	2.00%	2.28%
Total Weighted - Average Rate (as a percent)	1.59%	2.05%
2021
Medium-term notes due in:
Weighted - Average Rate (as a percent)	1.64%	2.04%
2022
Medium-term notes due in:
Weighted - Average Rate (as a percent)	1.90%	2.15%
2023
Medium-term notes due in:
Weighted - Average Rate (as a percent)	2.13%	2.27%
2024
Medium-term notes due in:
Weighted - Average Rate (as a percent)	1.95%	2.25%
Thereafter
Medium-term notes due in:
Weighted - Average Rate (as a percent)	2.57%	2.89%
Notes Payable to Banks
Due within one year:
Current portion of medium-term notes	$ 2,598,091	$ 2,194,177
Weighted - average rate (as a percent)	0.82%	1.72%
Short-term debt, average outstanding during the quarter	$ 2,043,407	$ 1,977,214
Weighted - average rate, average outstanding during the quarter (as a percent)	1.52%	2.25%
Medium-term Notes
Due within one year:
Current portion of medium-term notes	$ 1,203,723	$ 1,152,770
Weighted - average rate (as a percent)	1.18%	1.98%
Short-term debt, average outstanding during the quarter	$ 971,050	$ 1,780,517
Weighted - average rate, average outstanding during the quarter (as a percent)	1.79%	2.33%
Long-term Debt
Due within one year:
Current portion of medium-term notes	$ 7,346,370	$ 6,672,135
Weighted - average rate (as a percent)	1.39%	1.85%