Freestanding Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Net settlement amount, asset positions	$ 5.5	$ 0.3
Net settlement amount, liability positions	9.0	$ 8.6
Counterparty A [Member]
Derivative [Line Items]
Net derivative liability position	0.5
Cash margin received	5.4
Net derivative asset position	4.0
Over-collateralized positions	0.9
Counterparty B [Member]
Derivative [Line Items]
Net derivative asset position	1.7
Over-collateralized positions	6.1
Excess cash deposited	$ 23.6