Investment Securities (Narrative) (Detail) $ in Thousands		3 Months Ended
	Jan. 01, 2020 USD ($)	Mar. 31, 2020 USD ($) security position	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) security position	Dec. 31, 2018 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Available for sale debt securities, at fair value		$ 989,833		$ 976,919
Total Portfolio		$ 445,444		$ 453,629
Total number of all held to maturity and available for sale securities in an unrealized loss position | security		41		85
Allowance for credit losses	$ 0	$ (87)		$ 0
Retained earnings		686,397		695,273
Less allowance for credit losses	55,525	75,143	$ 55,353	55,525	$ 55,562
Securities available for sale, amortized cost		$ 955,485		$ 965,135
Securities available for sale, number of securities in an unrealized loss position | position		14		50
Unrealized loss		$ 551		$ 1,148
Held to maturity securities, proceeds from calls		13,300	9,300
Held to maturity securities, recognized gain on calls		11	0
Held to maturity securities, recognized loss on calls		0
Investment securities held to maturity, fair value		459,224		$ 467,966
Provision for credit losses		$ 14,717	$ 200
Number of securities in an unrealized loss position | security		27		35
AAA
Schedule of Held-to-maturity Securities [Line Items]
Total Portfolio		$ 54,389		$ 56,033
Amount of total portfolio		12.00%
AA
Schedule of Held-to-maturity Securities [Line Items]
Total Portfolio		$ 329,412		333,450
Amount of total portfolio		74.00%
A
Schedule of Held-to-maturity Securities [Line Items]
Total Portfolio		$ 59,837		62,652
Amount of total portfolio		13.00%
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
Retained earnings	8,300
Less allowance for credit losses	7,900
Provision for credit losses	1,000	$ 17
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, amortized cost		$ 18,000
Securities available for sale, number of securities in an unrealized loss position | position		1
Unrealized loss		$ 4,000
Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Available for sale debt securities, at fair value		960,177		947,430
Allowance for credit losses		0		0
Less allowance for credit losses		87
Securities available for sale, amortized cost		926,561		936,196
Unrealized loss		514		1,133
Held-to-maturity securities, amortized cost		104
Investment securities held to maturity, fair value		108		122
Mortgage-Backed Securities | AAA
Schedule of Held-to-maturity Securities [Line Items]
Total Portfolio		104		118
Mortgage-Backed Securities | AA
Schedule of Held-to-maturity Securities [Line Items]
Total Portfolio		0		0
Mortgage-Backed Securities | A
Schedule of Held-to-maturity Securities [Line Items]
Total Portfolio		$ 0		$ 0