Securities Available for Sale and Held to Maturity - Securities with Gross Unrealized Losses, Segregated by Length of Impairment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 1,223,365	$ 355,939
Estimated Fair Value, 12 Months or More	12,307	12,493
Estimated Fair Value, Total	1,235,672	368,432
Gross Unrealized (Losses), Less than 12 Months	(16,596)	(1,412)
Gross Unrealized (Losses), 12 Months or More	(479)	(493)
Gross Unrealized (Losses), Total	(17,075)	(1,905)
U.S. Government Agency Obligations [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	1,462	0
Estimated Fair Value, 12 Months or More	10,986	11,167
Estimated Fair Value, Total	12,448	11,167
Gross Unrealized (Losses), Less than 12 Months	(10)	0
Gross Unrealized (Losses), 12 Months or More	(467)	(481)
Gross Unrealized (Losses), Total	(477)	(481)
Obligations of States and Political Subdivisions [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	5,009	0
Estimated Fair Value, 12 Months or More	667	667
Estimated Fair Value, Total	5,676	667
Gross Unrealized (Losses), Less than 12 Months	(6)	0
Gross Unrealized (Losses), 12 Months or More	(3)	(3)
Gross Unrealized (Losses), Total	(9)	(3)
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	1,120	1,636
Estimated Fair Value, 12 Months or More	0	0
Estimated Fair Value, Total	1,120	1,636
Gross Unrealized (Losses), Less than 12 Months	(3)	(3)
Gross Unrealized (Losses), 12 Months or More	0	0
Gross Unrealized (Losses), Total	(3)	(3)
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	1,128,087	324,905
Estimated Fair Value, 12 Months or More	0	0
Estimated Fair Value, Total	1,128,087	324,905
Gross Unrealized (Losses), Less than 12 Months	(14,968)	(1,308)
Gross Unrealized (Losses), 12 Months or More	0	0
Gross Unrealized (Losses), Total	(14,968)	(1,308)
Other Residential Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	21,995	29,398
Estimated Fair Value, 12 Months or More	0	0
Estimated Fair Value, Total	21,995	29,398
Gross Unrealized (Losses), Less than 12 Months	(76)	(101)
Gross Unrealized (Losses), 12 Months or More	0	0
Gross Unrealized (Losses), Total	(76)	(101)
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	65,692	0
Estimated Fair Value, 12 Months or More	654	659
Estimated Fair Value, Total	66,346	659
Gross Unrealized (Losses), Less than 12 Months	(1,533)	0
Gross Unrealized (Losses), 12 Months or More	(9)	(9)
Gross Unrealized (Losses), Total	$ (1,542)	$ (9)