Investment Securities - Investment Securities (Parenthetical) (Detail)	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Contractual Maturities of Investment Securities [Abstract]
Federal statutory rate	21.00%	35.00%
Weighted-average maturity of available-for-sale investment securities	5 years 7 months 6 days		5 years 1 month 6 days
Weighted-average yield of available-for-sale investment securities	2.43%		2.25%
Weighted-average maturity of held-to-maturity investment securities	5 years 4 months 24 days		4 years 8 months 12 days
Weighted-average yield of held-to-maturity investment securities	2.36%		2.14%