Mortgage Banking Activities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Mortgage Loan Activity [Line Items]
Mortgage loans serviced for others		$ 1,400,000	$ 1,300,000
Fair value of mortgage servicing rights		11,700	10,900
Private Investor [Member]
Mortgage Loan Activity [Line Items]
Mortgage loans serviced for others		16,404	23,404
Private Investor [Member] | Adjustable Rate Residential Mortgages [Member] | One-to Four-Family [Member]
Mortgage Loan Activity [Line Items]
Mortgage loans serviced for others	$ 27,900
Percentage of mortgage service release premium as part of gain recognized on sale	0.45%
FHLMC, FNMA and Private Investor [Member]
Mortgage Loan Activity [Line Items]
Customer escrow balances		$ 9,600	$ 15,300