Notes Payable - Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Due within one year:
Short-term debt	$ 10,019,082	$ 7,757,050
Weighted - Average Rate (as a percent)	1.84%	2.10%
Medium-term notes due in:
2021	$ 3,700,835	$ 3,090,405
2022	1,594,709	2,220,651
2023	1,205,276	859,470
2024	760,887	881,738
Thereafter	1,817,859	1,434,383
Total due after one year	9,079,566	8,486,647
Total	$ 19,098,648	$ 16,243,697
Weighted - Average Rate (as a percent)	2.28%	2.48%
Total Weighted - Average Rate (as a percent)	2.05%	2.30%
2021
Medium-term notes due in:
Weighted - Average Rate (as a percent)	2.04%	2.11%
2022
Medium-term notes due in:
Weighted - Average Rate (as a percent)	2.15%	2.41%
2023
Medium-term notes due in:
Weighted - Average Rate (as a percent)	2.27%	2.19%
2024
Medium-term notes due in:
Weighted - Average Rate (as a percent)	2.25%	2.88%
Thereafter
Medium-term notes due in:
Weighted - Average Rate (as a percent)	2.89%	3.34%
Notes Payable to Banks
Due within one year:
Short-term debt	$ 2,194,177	$ 1,586,385
Weighted - Average Rate (as a percent)	1.72%	2.35%
Average Outstanding During the Year	$ 1,977,214	$ 1,432,470
Weighted - Average Rate (as a percent)	2.25%	1.83%
Medium-term Notes
Due within one year:
Short-term debt	$ 1,152,770	$ 1,826,380
Weighted - Average Rate (as a percent)	1.98%	2.29%
Average Outstanding During the Year	$ 1,780,517	$ 1,977,445
Weighted - Average Rate (as a percent)	2.33%	1.83%
Long-term Debt
Due within one year:
Current portion of long-term notes	$ 6,672,135	$ 4,344,285
Weighted - Average Rate (as a percent)	1.85%	1.93%