Investment Securities (Narrative) (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) security position	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) security position	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($) security position	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Available for sale debt securities, at fair value	$ 948,614		$ 948,614				$ 976,919
Securities held to maturity, at fair value	$ 439,303		$ 439,303				$ 453,629
Total number of all held to maturity and available for sale securities in an unrealized loss position | security	16		16				85
Allowance for credit losses	$ (87)		$ (87)			$ 70	$ 0
Retained earnings	$ 685,509		$ 685,509				$ 695,273
Securities available for sale, number of securities in an unrealized loss position | position	11		11				50
Securities available for sale, amortized cost	$ 915,518		$ 915,518				$ 965,135
Unrealized loss	358		358				1,148
Held to maturity securities, proceeds from calls	12,600	$ 2,800	25,900	$ 12,100
Held to maturity securities, recognized gain on calls	44	29	55	0
Held to maturity securities, recognized loss on calls			0
Investment securities held to maturity, fair value	460,587		460,587				467,966
Allowance for credit losses	$ 86,259	$ 62,810	$ 86,259	$ 62,810	$ 75,143	55,525	$ 55,525	$ 55,353	$ 55,562
Number of securities in an unrealized loss position | security	5		5				35
AAA
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, at fair value	$ 53,744		$ 53,744				$ 56,033
Amount of total portfolio			12.00%
AA
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, at fair value	319,863		$ 319,863				333,450
Amount of total portfolio			73.00%
A
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, at fair value	$ 62,404		$ 62,404				62,652
Amount of total portfolio			14.00%
A rated or not rated
Schedule of Held-to-maturity Securities [Line Items]
Amount of total portfolio			1.00%
Accounting Standards Update 2016-13
Schedule of Held-to-maturity Securities [Line Items]
Allowance for credit losses						70
Allowance for credit losses						7,900
Accounting Standards Update 2016-13, Effect On Debt Securities
Schedule of Held-to-maturity Securities [Line Items]
Retained earnings						$ 52
Private Label Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Securities available for sale, number of securities in an unrealized loss position | position	1		1
Securities available for sale, amortized cost	$ 18,000		$ 18,000
Unrealized loss	2,000		2,000
Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Available for sale debt securities, at fair value	916,985		916,985				947,430
Allowance for credit losses	0		0				0
Securities available for sale, amortized cost	884,608		884,608				936,196
Unrealized loss	332		332				1,133
Held-to-maturity securities, amortized cost	89		89
Investment securities held to maturity, fair value	92		92				122
Allowance for credit losses	87		87
Mortgage-Backed Securities | AAA
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, at fair value	89		89				118
Mortgage-Backed Securities | AA
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, at fair value	0		0				0
Mortgage-Backed Securities | A
Schedule of Held-to-maturity Securities [Line Items]
Securities held to maturity, at fair value	$ 0		$ 0				$ 0