Investment Securities - Age of Unrealized Losses and Fair Value of Related Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value
Less than Twelve Months	$ 60,712	$ 289,110
More than Twelve Months	30,400	306,513
Total	91,112	595,623
Unrealized Losses
Less than Twelve Months	(601)	(1,347)
More than Twelve Months	(318)	(4,235)
Total	(919)	(5,582)
U.S. Treasury securities
Fair Value
Less than Twelve Months		25,019
More than Twelve Months		0
Total		25,019
Unrealized Losses
Less than Twelve Months		(21)
More than Twelve Months		0
Total		(21)
U.S. government agency securities
Fair Value
Less than Twelve Months	0	0
More than Twelve Months	1,479	1,783
Total	1,479	1,783
Unrealized Losses
Less than Twelve Months	0	0
More than Twelve Months	(13)	(19)
Total	(13)	(19)
Obligations of states and state agencies
Fair Value
Less than Twelve Months	0	18,540
More than Twelve Months	1,010	8,755
Total	1,010	27,295
Unrealized Losses
Less than Twelve Months	0	(21)
More than Twelve Months	(32)	(62)
Total	(32)	(83)
Municipal bonds
Fair Value
Less than Twelve Months	6,777	0
More than Twelve Months	0	13,177
Total	6,777	13,177
Unrealized Losses
Less than Twelve Months	(69)	0
More than Twelve Months	0	(86)
Total	(69)	(86)
Obligations of states and political subdivisions
Fair Value
Less than Twelve Months	6,777	18,540
More than Twelve Months	1,010	21,932
Total	7,787	40,472
Unrealized Losses
Less than Twelve Months	(69)	(21)
More than Twelve Months	(32)	(148)
Total	(101)	(169)
Residential mortgage-backed securities
Fair Value
Less than Twelve Months	41,418	240,412
More than Twelve Months	27,911	282,798
Total	69,329	523,210
Unrealized Losses
Less than Twelve Months	(500)	(1,194)
More than Twelve Months	(273)	(4,068)
Total	(773)	(5,262)
Corporate and other debt securities
Fair Value
Less than Twelve Months	12,517	5,139
More than Twelve Months	0	0
Total	12,517	5,139
Unrealized Losses
Less than Twelve Months	(32)	(111)
More than Twelve Months	0	0
Total	$ (32)	$ (111)