Derivatives (Volume of Contracts Outstanding) (Details) lb in Thousands, gal in Thousands, bu in Thousands, T in Thousands	Jun. 30, 2019 lb	Jun. 30, 2019 bu	Jun. 30, 2019 T	Jun. 30, 2019 gal	Dec. 31, 2018 lb	Dec. 31, 2018 bu	Dec. 31, 2018 T	Dec. 31, 2018 gal	Jun. 30, 2018 lb	Jun. 30, 2018 bu	Jun. 30, 2018 T	Jun. 30, 2018 gal
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	8,809	1,292,468	3,359	310,137	2,920	506,417	66	323,975	6,158	598,844	77	474,625
Non-exchange Traded
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	8,809	866,106	3,179	213,884	2,920	313,612	66	246,863	6,158	370,044	77	334,261
Non-exchange Traded | Corn
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	648,434	0	0	0	250,408	0	0	0	272,979	0	0
Non-exchange Traded | Soybeans
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	59,594	0	0	0	22,463	0	0	0	49,208	0	0
Non-exchange Traded | Wheat
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	93,621	0	0	0	14,017	0	0	0	11,163	0	0
Non-exchange Traded | Oats
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	40,582	0	0	0	26,230	0	0	0	36,612	0	0
Non-exchange Traded | Ethanol
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	0	211,352	0	0	0	244,863		0	0	332,761
Non-exchange Traded | Corn oil
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	8,809	0	0	0	2,920	0	0	0	6,158	0	0	0
Non-exchange Traded | Other
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	23,875	3,179	2,532	0	494	66	2,000		82	77	1,500
Exchange Traded
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	426,362	180	96,253	0	192,805	0	77,112	0	228,800	0	140,364
Exchange Traded | Corn
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	317,405	0	0	0	130,585	0	0	0	133,730	0	0
Exchange Traded | Soybeans
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	52,762	0	0	0	26,985	0	0	0	45,775	0	0
Exchange Traded | Wheat
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	55,150		0	0	33,760	0	0	0	48,105	0	0
Exchange Traded | Oats
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	1,045	0	0	0	1,475	0	0	0	1,190	0	0
Exchange Traded | Ethanol
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	0	82,988	0	0	0	77,112	0	0	0	140,364
Exchange Traded | Propane
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	0	13,230
Exchange Traded | Other
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	180	35