Freestanding Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Total net settlement amount - asset positions	$ 4,900	$ 5,500
Total net settlement amount - liability positions	1,500	9,000
Cash Margin, Received	2,387	30,941
Excess cash deposited	865	$ 0
Counterparty A
Derivative [Line Items]
Net derivative liability position with counterparty	(500)
Cash Margin, Received	1,900
Net derivative liability position	(800)
Over-collateralized positions	1,600
Counterparty B
Derivative [Line Items]
Net derivative asset position	800
Over-collateralized positions	14,600
Excess cash deposited	$ 23,500