Derivatives (Volume of Contracts Outstanding) (Details) lb in Thousands, gal in Thousands, bu in Thousands, T in Thousands	Mar. 31, 2019 lb	Mar. 31, 2019 bu	Mar. 31, 2019 T	Mar. 31, 2019 gal	Dec. 31, 2018 lb	Dec. 31, 2018 bu	Dec. 31, 2018 T	Dec. 31, 2018 gal	Mar. 31, 2018 lb	Mar. 31, 2018 bu	Mar. 31, 2018 T	Mar. 31, 2018 gal
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	6,739	1,168,112	2,713	373,930	2,920	506,417	66	323,975	5,048	717,171	90	392,851
Non-exchange Traded
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	6,739	818,315	2,508	235,452	2,920	313,612	66	246,863	5,048	441,111	90	284,743
Non-exchange Traded | Corn
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	624,612	0	0		250,408	0		0	335,887	0	0
Non-exchange Traded | Soybeans
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	42,859	0	0	0	22,463	0	0	0	48,003	0	0
Non-exchange Traded | Wheat
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	118,909	0	0	0	14,017	0	0	0	16,639	0	0
Non-exchange Traded | Oats
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	26,361	0	0	0	26,230	0	0	0	40,555	0	0
Non-exchange Traded | Ethanol
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount		0	0	233,420	0	0	0	244,863		0	0	280,243
Non-exchange Traded | Corn oil
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	6,733	0	0	0	2,920	0	0	0	5,048	0	0	0
Non-exchange Traded | Other
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	6	5,574	2,508	2,032	0	494	66	2,000		27	90	4,500
Exchange Traded
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	349,797	205	138,478	0	192,805	0	77,112	0	276,060	0	108,108
Exchange Traded | Corn
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	197,210	0	0	0	130,585	0	0	0	146,505	0	0
Exchange Traded | Soybeans
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	47,860	0	0	0	26,985	0	0	0	52,460	0	0
Exchange Traded | Wheat
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	103,955		0	0	33,760	0	0	0	74,805	0	0
Exchange Traded | Oats
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	770	0	0	0	1,475	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	110,758	0	0	0	77,112	0	0	0	108,108
Exchange Traded | Gasoline
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	0	12,936
Exchange Traded | Propane
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	0	14,784
Exchange Traded | Other
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	2	205	0