Derivative Financial Instruments - Offsetting of Derivative Assets (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Zero Cost Collars [Member]
Derivative [Line Items]
Asset Derivatives, Gross amounts of recognized assets	$ 2,827
Asset Derivatives, Gross amounts offset in the balance sheets	0
Asset Derivatives, Net amounts of assets presented in the balance sheets	2,827
Asset Derivatives, Gross amounts not offset in the balance sheets, Financial instruments	0
Asset Derivatives, Gross amounts not offset in the balance sheets, Cash collateral pledged	0
Asset Derivatives, Net amount	2,827
Forward [Member]
Derivative [Line Items]
Asset Derivatives, Gross amounts of recognized assets	2,352
Asset Derivatives, Gross amounts offset in the balance sheets	0
Asset Derivatives, Net amounts of assets presented in the balance sheets	2,352
Asset Derivatives, Gross amounts not offset in the balance sheets, Financial instruments	0
Asset Derivatives, Gross amounts not offset in the balance sheets, Cash collateral pledged	0
Asset Derivatives, Net amount	$ 2,352