Derivatives (Volume of Contracts Outstanding) (Details) lb in Thousands, gal in Thousands, bu in Thousands, T in Thousands	Jun. 30, 2018 lb	Jun. 30, 2018 bu	Jun. 30, 2018 T	Jun. 30, 2018 gal	Dec. 31, 2017 lb	Dec. 31, 2017 bu	Dec. 31, 2017 T	Dec. 31, 2017 gal	Jun. 30, 2017 lb	Jun. 30, 2017 bu	Jun. 30, 2017 T	Jun. 30, 2017 gal
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	6,158	598,844	77	474,625	6,074	464,085	97	237,885	4,658	433,265	160	261,848
Non-exchange Traded
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	6,158	370,044	77	334,261	6,074	277,095	97	197,607	4,658	264,685	100	257,018
Non-exchange Traded | Corn
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	272,979	0	0		218,391	0		0	184,197	0	0
Non-exchange Traded | Soybeans
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	49,208	0	0	0	18,127	0	0	0	31,532	0	0
Non-exchange Traded | Wheat
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	11,163	0	0	0	14,577	0	0	0	7,340	0	0
Non-exchange Traded | Oats
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	36,612	0	0	0	25,953	0	0	0	41,526	0	0
Non-exchange Traded | Ethanol
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount		0	0	332,761	0	0	0	197,607	0	0	0	256,518
Non-exchange Traded | Corn Oil
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	6,158	0	0	0	6,074	0	0	0	4,658	0	0	0
Non-exchange Traded | Other
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	82	77	1,500	0	47	97	0	0	90	100	500
Exchange Traded
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	228,800	0	140,364	0	186,990	0	40,278	0	168,580	60	4,830
Exchange Traded | Corn
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	133,730	0	0	0	82,835	0	0	0	94,895	0	0
Exchange Traded | Soybeans
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	45,775	0	0	0	37,170	0	0	0	27,470	0	0
Exchange Traded | Wheat
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	48,105	0	0	0	65,640	0	0	0	43,925	0	0
Exchange Traded | Oats
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	1,190	0	0	0	1,345	0	0	0	2,290	0	0
Exchange Traded | Ethanol
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	0	140,364	0	0	0	39,438	0	0	0	3,990
Exchange Traded | Other
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount					0	0	0	840	0	0	60	840