Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Debt Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,184,790	$ 1,293,506
Gross Unrealized Gains	26,743	46,886
Gross Unrealized Losses	(3,256)	(919)
Fair Value	1,208,277	1,339,473
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		50,031
Gross Unrealized Gains		1,362
Gross Unrealized Losses		0
Fair Value		51,393
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,408	25,067
Gross Unrealized Gains	1,041	1,103
Gross Unrealized Losses	(7)	(13)
Fair Value	21,442	26,157
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	29,752	40,861
Gross Unrealized Gains	239	970
Gross Unrealized Losses	(39)	(32)
Fair Value	29,952	41,799
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	53,186	37,489
Gross Unrealized Gains	440	731
Gross Unrealized Losses	(429)	(69)
Fair Value	53,197	38,151
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	82,938	78,350
Gross Unrealized Gains	679	1,701
Gross Unrealized Losses	(468)	(101)
Fair Value	83,149	79,950
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	973,543	1,050,369
Gross Unrealized Gains	21,761	40,426
Gross Unrealized Losses	(2,476)	(773)
Fair Value	992,828	1,090,022
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	107,901	89,689
Gross Unrealized Gains	3,262	2,294
Gross Unrealized Losses	(305)	(32)
Fair Value	$ 110,858	$ 91,951

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

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Amortized cost excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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