Fair Value of Assets and Liabilities - Additional Information about Servicing Assets and Liabilities Measured Using Different Market Servicing Rates and Different Prepayment (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Risks Inherent in Servicing Assets and Servicing Liabilities [Line Items]
Change in servicing rate	0.10%
Fair Value, Measurements, Recurring
Risks Inherent in Servicing Assets and Servicing Liabilities [Line Items]
Weighted-average market servicing rate assumptions	0.66%	0.63%
Servicing assets, rate increase	$ (7,749)	$ (5,673)
Servicing liabilities, rate increase	233	964
Servicing assets, rate decrease	7,760	5,812
Servicing liabilities, rate decrease	$ (222)	$ (825)