Derivatives (Details 4) lb in Thousands, gal in Thousands, bu in Thousands, T in Thousands	Dec. 31, 2017 bu	Dec. 31, 2017 lb	Dec. 31, 2017 gal	Dec. 31, 2017 T	Dec. 31, 2016 bu	Dec. 31, 2016 lb	Dec. 31, 2016 gal	Dec. 31, 2016 T
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	464,085	6,074	237,885	97	391,161	9,358	294,345	110
Non-exchange traded:
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	277,095	6,074	197,607	97	239,451	9,358	216,225	110
Non-exchange traded: | Corn
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	218,391			0	175,549	0	0	0
Non-exchange traded: | Soybeans
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	18,127	0	0	0	20,592	0	0	0
Non-exchange traded: | Wheat
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	14,577	0	0	0	7,177	0	0	0
Non-exchange traded: | Oats
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	25,953	0	0	0	36,025	0	0	0
Non-exchange traded: | Ethanol
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	197,607	0	0	0	215,081	0
Non-exchange traded: | Corn oil
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	6,074	0	0	0	9,358	0	0
Non-exchange traded: | Other
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	47	0	0	97	108	0	1,144	110
Exchange traded:
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	186,990	0	40,278	0	151,710	0	78,120	0
Exchange traded: | Corn
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	82,835	0	0	0	63,225	0	0	0
Exchange traded: | Soybeans
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	37,170	0	0	0	39,005	0	0	0
Exchange traded: | Wheat
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	65,640	0	0	0	45,360	0	0	0
Exchange traded: | Oats
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	1,345	0	0	0	4,120	0	0	0
Exchange traded: | Ethanol
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	39,438	0	0	0	78,120	0
Exchange traded: | Other
Amounts of quantities outstanding included in commodity derivative contracts
Nonmonetary notional amount	0	0	840	0