Notes Payable and Long-Term Debt Phantom (Details) $ in Millions	Aug. 31, 2017 USD ($)
Unsecured debt | Private placement, payable in equal installments beginning in 2014 through 2018
Debt Instrument [Line Items]
Interest rate	6.18%
Debt Instrument, Face Amount	$ 400
Unsecured debt | Private placement, payable in installments through 2018
Debt Instrument [Line Items]
Interest rate	5.60%
Debt Instrument, Face Amount	$ 60
Unsecured debt | Private placement, payable in its entirety in 2019
Debt Instrument [Line Items]
Interest rate	4.08%
Debt Instrument, Face Amount	$ 130
Unsecured debt | Private placement, payable in its entirety in 2021
Debt Instrument [Line Items]
Interest rate	4.52%
Debt Instrument, Face Amount	$ 160
Unsecured debt | Private placement, payable in its entirety in 2023
Debt Instrument [Line Items]
Interest rate	4.67%
Debt Instrument, Face Amount	$ 130
Unsecured debt | Private placement, payable in 2023 152k [Member] [Member]
Debt Instrument [Line Items]
Interest rate	4.39%
Debt Instrument, Face Amount	$ 152
Unsecured debt | Private placement, payable in 2025 80k [Member]
Debt Instrument [Line Items]
Interest rate	3.85%
Debt Instrument, Face Amount	$ 80
Unsecured debt | Private placement, payable in 2025 100k [Member]
Debt Instrument [Line Items]
Interest rate	3.80%
Debt Instrument, Face Amount	$ 100
Unsecured debt | Private placement, payable in 2025 150k [Member] [Member]
Debt Instrument [Line Items]
Interest rate	4.58%
Debt Instrument, Face Amount	$ 150
Unsecured debt | Private placement, payable in its entirety in 2026
Debt Instrument [Line Items]
Interest rate	4.82%
Debt Instrument, Face Amount	$ 80
Unsecured debt | Private placement, payable in its entirety in 2027 [Member]
Debt Instrument [Line Items]
Interest rate	4.69%
Debt Instrument, Face Amount	$ 58
Unsecured debt | Private placement, payable in its entirety in 2028 [Member]
Debt Instrument [Line Items]
Interest rate	4.74%
Debt Instrument, Face Amount	$ 95
Unsecured debt | Private placement, payable in its entirety in 2031 [Member]
Debt Instrument [Line Items]
Interest rate	4.89%
Debt Instrument, Face Amount	$ 100
Unsecured debt | Private placement, payable in its entirety 2033 [Member]
Debt Instrument [Line Items]
Interest rate	4.71%
Debt Instrument, Face Amount	$ 100
Unsecured debt | Private placement, payable in its entirety in 2036 [Member]
Debt Instrument [Line Items]
Interest rate	5.40%
Debt Instrument, Face Amount	$ 125
Unsecured debt | Term loans, 430K payable in 2025 [Member]
Debt Instrument [Line Items]
Interest rate	2.25%
Notes Payable, Other Payables [Member] | Private placement, payable in equal installments beginning in 2014 through 2018
Debt Instrument [Line Items]
Interest rate	5.78%
Debt Instrument, Face Amount	$ 50
Notes Payable, Other Payables [Member] | Private placement, payable in equal installments beginning in 2017 through 2021
Debt Instrument [Line Items]
Interest rate	4.00%
Debt Instrument, Face Amount	$ 100
Revolving credit facility | Line of credit | Term loans from cooperative and other banks
Debt Instrument [Line Items]
Interest rate	5.59%
Minimum | Unsecured debt
Debt Instrument [Line Items]
Interest rate	1.30%
Maximum | Unsecured debt
Debt Instrument [Line Items]
Interest rate	15.25%