Investment Securities - Investment Securities (Parenthetical) (Detail)	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Contractual Maturities of Investment Securities [Abstract]
Federal statutory rate	21.00%	21.00%	21.00%
Weighted-average maturity of available-for-sale investment securities	4 years 1 month 6 days		5 years 4 months 24 days
Weighted-average yield of available-for-sale investment securities	2.65%		2.57%
Weighted-average maturity of held-to-maturity investment securities	4 years		5 years 2 months 12 days
Weighted-average yield of held-to-maturity investment securities	2.50%		2.46%