Derivative Financial Instruments - Offsetting of Derivative Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Zero Cost Collars [Member]
Derivative [Line Items]
Liability Derivatives, Gross amounts of recognized liabilities	$ 1,257	$ 453
Liability Derivatives, Gross amounts offset in the balance sheets	0	0
Liability Derivatives, Net amounts of liabilities presented in the balance sheets	1,257	453
Liability Derivatives, Gross amounts not offset in the balance sheets, Financial instruments	0	0
Liability Derivatives, Gross amounts not offset in the balance sheets, Cash collateral pledged	(700)	(650)
Liability Derivatives, Net amount after master netting	557	$ (197)
Forward [Member]
Derivative [Line Items]
Liability Derivatives, Gross amounts of recognized liabilities	267
Liability Derivatives, Gross amounts offset in the balance sheets	0
Liability Derivatives, Net amounts of liabilities presented in the balance sheets	267
Liability Derivatives, Gross amounts not offset in the balance sheets, Financial instruments	0
Liability Derivatives, Gross amounts not offset in the balance sheets, Cash collateral pledged	(120)
Liability Derivatives, Net amount after master netting	$ 147