Derivative Financial Instruments - Offsetting of Derivative Liabilities and Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Zero Cost Collars [Member]
Derivative [Line Items]
Liability Derivatives, Gross amounts of recognized liabilities	$ 684
Liability Derivatives, Gross amounts offset in the balance sheets	0
Liability Derivatives, Net amounts of liabilities presented in the balance sheets	684
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	0
Liability Derivatives, Net amount after master netting	684
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]
Liability Derivatives, Gross amounts of recognized liabilities	1,911
Liability Derivatives, Gross amounts offset in the balance sheets	0
Liability Derivatives, Net amounts of liabilities presented in the balance sheets	1,911
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	0
Liability Derivatives, Net amount after master netting	$ 1,911
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