Investment Securities (Held to Maturity) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 447,979	$ 451,043
Gross unrealized gains	17,037	21,551
Gross unrealized losses	(1,023)	(65)
Allowance for credit losses	(77)	(78)	$ 0
Held to maturity debt securities	463,993	472,529
Held to maturity debt securities, net (fair value of $463,993 at March 31, 2021 (unaudited) and $472,529 at December 31, 2020)	447,902	450,965
Agency Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	8,998	7,600
Gross unrealized gains	1	6
Gross unrealized losses	(95)	(5)
Held to maturity debt securities	8,904	7,601
Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	50	62
Gross unrealized gains	1	2
Gross unrealized losses	0	0
Held to maturity debt securities	51	64
State And Municipal Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	428,631	433,655
Gross unrealized gains	16,970	21,442
Gross unrealized losses	(825)	(58)
Held to maturity debt securities	444,776	455,039
Corporate Obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	10,300	9,726
Gross unrealized gains	65	101
Gross unrealized losses	(103)	(2)
Allowance for credit losses			$ 0
Held to maturity debt securities	$ 10,262	$ 9,825