Balance Sheet Offsetting (Details) - Interest and commodity agreements - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Offsetting Derivative Assets [Abstract]
Gross amounts recognized	$ 23,224	$ 18,031
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	23,224	18,031
Financial instruments	(21,783)	(18,031)
Collateral	(1,441)	0
Net amount	0	0
Offsetting Derivative Liabilities [Abstract]
Gross amounts recognized	21,783	31,075
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	21,783	31,075
Financial instruments	(21,783)	(18,031)
Collateral	0	(11,148)
Net amount	$ 0	$ 1,896