Investment Securities (Amortized Cost of held to Maturity Debt Securities by Year of Originations and Credit Rating) (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 439,303	$ 453,629
Not Rated	439,390	453,629
Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Not Rated	6,840	6,599
Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Not Rated	89	118
State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Not Rated	424,166	437,074
Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Not Rated	8,295	9,838
AAA
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	53,744	56,033
AAA | Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	6,840	6,599
AAA | Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	89	118
AAA | State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	46,815	49,316
AAA | Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	0	0
AA
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	319,863	333,450
AA | Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	0	0
AA | Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	0	0
AA | State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	317,252	330,322
AA | Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	2,611	3,128
A
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	62,404	62,652
A | Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	0	0
A | Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	0	0
A | State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	57,145	56,317
A | Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	5,259	6,335
BBB
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	1,516	1,469
BBB | Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	0	0
BBB | Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	0	0
BBB | State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	1,116	1,119
BBB | Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	400	350
Not Rated
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	1,863	25
Not Rated | Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	0	0
Not Rated | Mortgage-Backed Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	0	0
Not Rated | State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	1,838	0
Not Rated | Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 25	$ 25