Investment Securities - Schedule of Proceeds from Sales or Calls and Realized Gain and Losses of Available-for-sale Investment Securities and Other Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales of investment securities available-for-sale	$ 39,637	$ 27,983	$ 296,075	$ 295,615
Proceeds from calls of available-for-sale debt securities	82,410	316,361	388,145	378,476
Total	122,047	344,344	684,220	674,091
Realized gains on sales of available-for-sale debt securities	4,942	340	11,082	3,443
Realized gains on calls of available-for-sale debt securities	10	86	50	89
Realized losses on sales of available-for-sale debt securities	(30)	(2)	(500)	(2,017)
Realized losses on calls of available-for-sale debt securities	(1)	0	(26)	(29)
Debt securities gains (losses), net	$ 4,921	$ 424	$ 10,606	$ 1,486