Debt - LT Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term debt	$ 2,097,940	$ 2,640,129
Accounts receivable, net	9,205,343	8,482,687
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months	158,633
Long-term Debt, Maturities, Repayments of Principal in Year Two	350,491
Long-term Debt, Maturities, Repayments of Principal in Year Three	299,049
Long-term Debt, Maturities, Repayments of Principal in Year Four	496,220
Long-term Debt, Maturities, Repayments of Principal in Year Five	347,018
Long-term Debt, Maturities, Repayments of Principal after Year Five	605,162
Investment Income, Interest and Dividend	22,568	54,815	$ 47,860
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	138,303	209,512	$ 213,913
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum Borrowing Capacity	2,000,000
Long-term debt	$ 0	10,000
Debt Instrument, Interest Rate, Effective Percentage	1.26%
Facility Fee	0.20%
Debt Instrument, Basis Spread on Variable Rate	1.18%
North American Asset Securitization Program [Member]
Debt Instrument [Line Items]
Maximum Borrowing Capacity	$ 1,200,000
Long-term debt	$ 0	400,000
Debt Instrument, Interest Rate, Effective Percentage	0.56%
Facility Fee	0.40%
Debt Instrument, Basis Spread on Variable Rate	0.40%
Accounts receivable, net	$ 2,207,700	2,217,800
5.125% Notes, Due 2021 [Member]
Debt Instrument [Line Items]
Long-term debt	0	130,691
3.50% Notes, Due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt	348,918	348,088
Debt Instrument, Fair Value	$ 360,500	358,500
Debt Instrument, Interest Rate, Stated Percentage	3.50%
4.50% Notes, Due 2023 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 298,701	298,148
Debt Instrument, Fair Value	$ 321,500	316,000
Debt Instrument, Interest Rate, Stated Percentage	4.50%
3.25% Notes, Due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 496,034	495,045
Debt Instrument, Fair Value	$ 540,500	515,500
Debt Instrument, Interest Rate, Stated Percentage	3.25%
4.00% Notes, Due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 346,999	346,368
Debt Instrument, Fair Value	$ 383,000	367,000
Debt Instrument, Interest Rate, Stated Percentage	4.00%
7.50% Senior Debentures, Due 2027 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 109,939	109,857
Debt Instrument, Fair Value	$ 140,000	135,000
Debt Instrument, Interest Rate, Stated Percentage	7.50%
3.875% Notes, Due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt	$ 495,223	494,648
Debt Instrument, Fair Value	$ 564,000	516,500
Debt Instrument, Interest Rate, Stated Percentage	3.875%
Other obligations with various interest rates and due dates [Member]
Debt Instrument [Line Items]
Other Notes Payable, Noncurrent	$ 2,126	$ 7,284