Investment Securities (Narrative) (Detail)	3 Months Ended
	Mar. 31, 2021 USD ($) security position	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($) security position	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Available for sale debt securities, at fair value	$ 1,216,936,000		$ 1,105,489,000
Securities held to maturity	$ 447,902,000		$ 450,965,000
Total number of all held to maturity and available for sale securities in an unrealized loss position | security	122		49
Allowance for credit losses	$ 77,000		$ 78,000		$ 0
Retained earnings	748,574,000		718,090,000
Securities available for sale, amortized cost	1,197,401,000		$ 1,073,570,000
Proceeds from sales of securities	9,442,000	$ 0
Loss on sale of securities	$ 0
Securities available for sale, number of securities in an unrealized loss position | position	81		42
Unrealized loss	$ 7,424,000		$ 971,000
Amortized cost, net of allowance for credit losses	447,979,000		451,043,000
Held to maturity securities, proceeds from calls	6,800,000	13,300,000
Held to maturity securities, recognized gain on calls	0	11,000
Held to maturity securities, recognized loss on calls	33,000	0
Investment securities held to maturity, fair value	463,993,000		472,529,000
Allowance for credit losses	$ 85,591,000	$ 75,143,000	$ 101,466,000	$ 55,525,000	$ 55,525,000
Number of securities in an unrealized loss position | security	41		7
AAA
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	$ 66,255,000		$ 65,492,000
Amount of total portfolio	15.00%
AA
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	$ 311,659,000		314,410,000
Amount of total portfolio	70.00%
A
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	$ 58,866,000		59,748,000
Amount of total portfolio	13.00%
A rated or not rated
Schedule of Held-to-maturity Securities [Line Items]
Amount of total portfolio	2.00%
Private Label Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Securities available for sale, amortized cost	$ 17,445
Securities available for sale, number of securities in an unrealized loss position | position	1
Unrealized loss	$ 690
Available-for-sale Securities
Schedule of Held-to-maturity Securities [Line Items]
Proceeds from sales of securities	9,400,000
Gain on sale of securities	230,000
Accounting Standards Update 2016-13
Schedule of Held-to-maturity Securities [Line Items]
Allowance for credit losses				70,000
Allowance for credit losses				7,900,000
Accounting Standards Update 2016-13, Effect On Debt Securities
Schedule of Held-to-maturity Securities [Line Items]
Retained earnings				$ 52,000
Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Available for sale debt securities, at fair value	1,060,152,000		938,413,000
Securities available for sale, amortized cost	1,042,480,000		910,393,000
Unrealized loss	6,243,000		852,000
Amortized cost, net of allowance for credit losses	50,000		62,000
Held-to-maturity securities, amortized cost	50,000
Investment securities held to maturity, fair value	51,000		64,000
Allowance for credit losses	77,000
Mortgage-backed securities | AAA
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	50,000		62,000
Mortgage-backed securities | AA
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0		0
Mortgage-backed securities | A
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity	0		$ 0
Investments
Schedule of Held-to-maturity Securities [Line Items]
Available for sale debt securities, at fair value	1,110,000,000
Securities available for sale, amortized cost	$ 1,090,000,000.00