Securities Available for Sale and Held to Maturity - Securities with Gross Unrealized Losses, Segregated by Length of Impairment (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 426,031	$ 352,661
Estimated Fair Value, 12 Months or More	18,231	481,177
Estimated Fair Value, Total	444,262	833,838
Gross Unrealized (Losses), Less than 12 Months	(986)	(1,347)
Gross Unrealized (Losses), 12 Months or More	(517)	(5,984)
Gross Unrealized (Losses), Total	(1,503)	(7,331)
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	0	6,585
Estimated Fair Value, 12 Months or More	11,347	12,886
Estimated Fair Value, Total	11,347	19,471
Gross Unrealized (Losses), Less than 12 Months	0	(105)
Gross Unrealized (Losses), 12 Months or More	(496)	(602)
Gross Unrealized (Losses), Total	(496)	(707)
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	0	0
Estimated Fair Value, 12 Months or More	6,221	6,216
Estimated Fair Value, Total	6,221	6,216
Gross Unrealized (Losses), Less than 12 Months	0	0
Gross Unrealized (Losses), 12 Months or More	(12)	(53)
Gross Unrealized (Losses), Total	(12)	(53)
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	353	23,544
Estimated Fair Value, 12 Months or More	0	18,529
Estimated Fair Value, Total	353	42,073
Gross Unrealized (Losses), Less than 12 Months	(1)	(107)
Gross Unrealized (Losses), 12 Months or More	0	(272)
Gross Unrealized (Losses), Total	(1)	(379)
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	399,494	112,879
Estimated Fair Value, 12 Months or More	0	278,120
Estimated Fair Value, Total	399,494	390,999
Gross Unrealized (Losses), Less than 12 Months	(953)	(230)
Gross Unrealized (Losses), 12 Months or More	0	(3,305)
Gross Unrealized (Losses), Total	(953)	(3,535)
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	11,155	158,341
Estimated Fair Value, 12 Months or More	0	151,271
Estimated Fair Value, Total	11,155	309,612
Gross Unrealized (Losses), Less than 12 Months	(28)	(738)
Gross Unrealized (Losses), 12 Months or More	0	(1,631)
Gross Unrealized (Losses), Total	(28)	(2,369)
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	15,029	51,312
Estimated Fair Value, 12 Months or More	663	14,155
Estimated Fair Value, Total	15,692	65,467
Gross Unrealized (Losses), Less than 12 Months	(4)	(167)
Gross Unrealized (Losses), 12 Months or More	(9)	(121)
Gross Unrealized (Losses), Total	$ (13)	$ (288)