Mortgage Loan Losses Allowance - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mortgage Repurchase Reserve [Abstract]
Pre-tax charges for loan losses related to mortgage loans sold	$ 2,900	$ 2,000	$ 2,700
Mortgage repurchase reserve	$ 14,000	$ 12,700