Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Debt Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,045,737	$ 1,293,506
Gross Unrealized Gains	31,048	46,886
Gross Unrealized Losses	(1,247)	(919)
Fair Value	1,075,538	1,339,473
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,023	50,031
Gross Unrealized Gains	987	1,362
Gross Unrealized Losses	0	0
Fair Value	51,010	51,393
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	22,695	25,067
Gross Unrealized Gains	1,218	1,103
Gross Unrealized Losses	(6)	(13)
Fair Value	23,907	26,157
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,098	40,861
Gross Unrealized Gains	614	970
Gross Unrealized Losses	0	(32)
Fair Value	32,712	41,799
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,440	37,489
Gross Unrealized Gains	541	731
Gross Unrealized Losses	(8)	(69)
Fair Value	32,973	38,151
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	64,538	78,350
Gross Unrealized Gains	1,155	1,701
Gross Unrealized Losses	(8)	(101)
Fair Value	65,685	79,950
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	823,040	1,050,369
Gross Unrealized Gains	24,490	40,426
Gross Unrealized Losses	(1,214)	(773)
Fair Value	846,316	1,090,022
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	85,441	89,689
Gross Unrealized Gains	3,198	2,294
Gross Unrealized Losses	(19)	(32)
Fair Value	$ 88,620	$ 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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