Investment Securities (Narrative) (Detail)	3 Months Ended
	Mar. 31, 2024 USD ($) security position	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) security position
Schedule of Held-to-maturity Securities [Line Items]
Available for sale debt securities, at fair value	$ 1,666,306,000		$ 1,690,112,000
Held to maturity debt securities	$ 354,671,000		$ 363,080,000
Total number of all held to maturity and available for sale securities in an unrealized loss position | security	954		808
Debt securities, available-for-sale, accrued interest, after allowance for credit loss	$ 5,200,000		$ 4,900,000
Investments which pay principal on periodic basis, amortized cost	1,500,000,000
Investments which pay principal on periodic basis, fair value	1,310,000,000
Proceeds from sales of available for sale debt securities		$ 0
Proceeds from calls on securities	$ 0
Securities available for sale, number of securities in an unrealized loss position | position	434		436
Proceeds from sale of held-to-maturity securities	$ 0	0
Held to maturity securities, proceeds from calls	1,200,000	3,100,000
Held to maturity securities, recognized gain on calls	0	0
Held to maturity securities, recognized loss on calls	1,200	$ 5,000
Held-to-maturity, debt securities, allowance	$ 17,000		$ 31,000
Number of securities in an unrealized loss position | security	520		372
AAA
Schedule of Held-to-maturity Securities [Line Items]
Amount of total portfolio	18.00%
AA
Schedule of Held-to-maturity Securities [Line Items]
Amount of total portfolio	72.00%
A
Schedule of Held-to-maturity Securities [Line Items]
Amount of total portfolio	10.00%
A rated or not rated
Schedule of Held-to-maturity Securities [Line Items]
Amount of total portfolio	1.00%
Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Available for sale debt securities, at fair value	$ 1,259,881,000		$ 1,285,609,000