Securities - Amortized Cost and Fair Value of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 850,672	$ 737,005
Gross Unrealized Gains	21,392	33,478
Gross Unrealized Losses	(3,070)	(90)
Fair Value	868,994	770,393
Interest receivable	3,200	3,100
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	85,222	9,980
Gross Unrealized Gains	326	302
Gross Unrealized Losses	(248)	0
Fair Value	85,300	10,282
Obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	68,635	78,755
Gross Unrealized Gains	2,570	4,149
Gross Unrealized Losses	0	0
Fair Value	71,205	82,904
Collateralized mortgage obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	206,325	202,975
Gross Unrealized Gains	3,858	6,410
Gross Unrealized Losses	(1,198)	(89)
Fair Value	208,985	209,296
Residential mortgage-backed securities of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	60,855	66,960
Gross Unrealized Gains	553	818
Gross Unrealized Losses	(966)	0
Fair Value	60,442	67,778
Commercial mortgage-backed securities of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	347,825	258,875
Gross Unrealized Gains	9,339	14,806
Gross Unrealized Losses	(658)	0
Fair Value	356,506	273,681
Corporate obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	500	2,021
Gross Unrealized Gains	0	5
Gross Unrealized Losses	0	(1)
Fair Value	500	2,025
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	81,310	117,439
Gross Unrealized Gains	4,746	6,988
Gross Unrealized Losses		0
Fair Value	$ 86,056	$ 124,427