Derivatives (Volume of Contracts Outstanding) (Details) lb in Thousands, gal in Thousands, bu in Thousands, T in Thousands	Sep. 30, 2018 lb	Sep. 30, 2018 bu	Sep. 30, 2018 T	Sep. 30, 2018 gal	Dec. 31, 2017 lb	Dec. 31, 2017 bu	Dec. 31, 2017 T	Dec. 31, 2017 gal	Sep. 30, 2017 lb	Sep. 30, 2017 bu	Sep. 30, 2017 T	Sep. 30, 2017 gal
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	2,560	562,872	115	324,087	6,074	464,085	97	237,885	5,782	533,750	110	225,251
Non-exchange Traded
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	2,560	362,632	115	231,813	6,074	277,095	97	197,607	5,782	311,850	110	201,521
Non-exchange Traded | Corn
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	277,774	0	0		218,391	0		0	222,287	0	0
Non-exchange Traded | Soybeans
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	45,755	0	0	0	18,127	0	0	0	44,463	0	0
Non-exchange Traded | Wheat
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	7,948	0	0	0	14,577	0	0	0	8,598	0	0
Non-exchange Traded | Oats
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	31,155	0	0	0	25,953	0	0	0	36,451	0	0
Non-exchange Traded | Ethanol
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount		0	0	230,813	0	0	0	197,607		0	0	201,521
Non-exchange Traded | Corn Oil
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	2,560	0	0	0	6,074	0	0	0	5,782	0	0	0
Non-exchange Traded | Other
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	115	1,000	0	47	97	0		51	110	0
Exchange Traded
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	200,240	0	92,274	0	186,990	0	40,278	0	221,900	0	23,730
Exchange Traded | Corn
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	123,250	0	0	0	82,835	0	0	0	113,990	0	0
Exchange Traded | Soybeans
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	31,855	0	0	0	37,170	0	0	0	45,220	0	0
Exchange Traded | Wheat
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	44,130	0	0	0	65,640	0	0	0	61,795	0	0
Exchange Traded | Oats
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	1,005	0	0	0	1,345	0	0	0	895	0	0
Exchange Traded | Ethanol
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	0	92,274	0	0	0	39,438	0	0	0	22,890
Exchange Traded | Other
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount					0	0	0	840	0	0	0	840