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, 2021	Dec. 31, 2020
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 2,018,072	$ 355,939
Estimated Fair Value, 12 Months or More	66,755	12,493
Estimated Fair Value, Total	2,084,827	368,432
Gross Unrealized (Losses), Less than 12 Months	(15,919)	(1,412)
Gross Unrealized (Losses), 12 Months or More	(830)	(493)
Gross Unrealized (Losses), Total	(16,749)	(1,905)
US Treasury Securities [Member]
Schedule of Available For Sale and Held to Maturity Securities [Line Items]
Estimated Fair Value, Less than 12 Months	278,615
Estimated Fair Value, 12 Months or More	0
Estimated Fair Value, Total	278,615
Gross Unrealized (Losses), Less than 12 Months	(2,143)
Gross Unrealized (Losses), 12 Months or More	0
Gross Unrealized (Losses), Total	(2,143)
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	0
Estimated Fair Value, 12 Months or More	9,355	11,167
Estimated Fair Value, Total	9,355	11,167
Gross Unrealized (Losses), Less than 12 Months	0	0
Gross Unrealized (Losses), 12 Months or More	(362)	(481)
Gross Unrealized (Losses), Total	(362)	(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	5,506	0
Estimated Fair Value, 12 Months or More	667	667
Estimated Fair Value, Total	6,173	667
Gross Unrealized (Losses), Less than 12 Months	(2)	0
Gross Unrealized (Losses), 12 Months or More	(2)	(3)
Gross Unrealized (Losses), Total	(4)	(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	1,131	1,636
Estimated Fair Value, 12 Months or More	0	0
Estimated Fair Value, Total	1,131	1,636
Gross Unrealized (Losses), Less than 12 Months	(12)	(3)
Gross Unrealized (Losses), 12 Months or More	0	0
Gross Unrealized (Losses), Total	(12)	(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	1,553,063	324,905
Estimated Fair Value, 12 Months or More	56,089	0
Estimated Fair Value, Total	1,609,152	324,905
Gross Unrealized (Losses), Less than 12 Months	(10,637)	(1,308)
Gross Unrealized (Losses), 12 Months or More	(457)	0
Gross Unrealized (Losses), Total	(11,094)	(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	18,583	29,398
Estimated Fair Value, 12 Months or More	0	0
Estimated Fair Value, Total	18,583	29,398
Gross Unrealized (Losses), Less than 12 Months	(95)	(101)
Gross Unrealized (Losses), 12 Months or More	0	0
Gross Unrealized (Losses), Total	(95)	(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	161,174	0
Estimated Fair Value, 12 Months or More	644	659
Estimated Fair Value, Total	161,818	659
Gross Unrealized (Losses), Less than 12 Months	(3,030)	0
Gross Unrealized (Losses), 12 Months or More	(9)	(9)
Gross Unrealized (Losses), Total	$ (3,039)	$ (9)