Balance Sheet Offsetting (Details) - Interest and commodity agreements - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Offsetting Derivative Assets [Abstract]
Gross amounts recognized	$ 47,528	$ 29,503
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	47,528	29,503
Financial instruments	(43,105)	(29,503)
Collateral	(1,337)	0
Net amount	3,086	0
Offsetting Derivative Liabilities [Abstract]
Gross amounts recognized	43,105	37,164
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	43,105	37,164
Financial instruments	(43,105)	(29,503)
Collateral	0	(7,661)
Net amount	$ 0	$ 0