Investment Securities - Amortized Cost Basis, Unrealized Gains and Losses and Fair Value of AFS Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
AFS debt securities:
Amortized Cost	$ 3,651,711	$ 3,579,955
Gross Unrealized Gains	840	1,750
Gross Unrealized Losses	259,152	250,621
Fair Value	3,393,399	3,331,084
U.S. Treasuries
AFS debt securities:
Amortized Cost	411,640	398,021
Gross Unrealized Gains	5	39
Gross Unrealized Losses	12,865	10,711
Fair Value	398,780	387,349
U.S. Government agencies & GSEs
AFS debt securities:
Amortized Cost	312,408	281,708
Gross Unrealized Gains	259	269
Gross Unrealized Losses	15,241	14,477
Fair Value	297,426	267,500
State and political subdivisions
AFS debt securities:
Amortized Cost	181,829	182,546
Gross Unrealized Gains	0	5
Gross Unrealized Losses	18,566	18,502
Fair Value	163,263	164,049
Residential MBS, Agency & GSEs
AFS debt securities:
Amortized Cost	1,337,504	1,315,064
Gross Unrealized Gains	250	300
Gross Unrealized Losses	132,233	125,012
Fair Value	1,205,521	1,190,352
Residential MBS, Non-agency
AFS debt securities:
Amortized Cost	333,045	339,330
Gross Unrealized Gains	0	0
Gross Unrealized Losses	20,673	22,084
Fair Value	312,372	317,246
Commercial MBS, Agency & GSEs
AFS debt securities:
Amortized Cost	693,011	656,004
Gross Unrealized Gains	100	1,073
Gross Unrealized Losses	39,653	39,017
Fair Value	653,458	618,060
Commercial MBS, Non-agency
AFS debt securities:
Amortized Cost	20,165	24,269
Gross Unrealized Gains	0	0
Gross Unrealized Losses	542	675
Fair Value	19,623	23,594
Corporate bonds
AFS debt securities:
Amortized Cost	213,371	218,285
Gross Unrealized Gains	130	64
Gross Unrealized Losses	17,601	17,127
Fair Value	195,900	201,222
Asset-backed securities
AFS debt securities:
Amortized Cost	148,738	164,728
Gross Unrealized Gains	96	0
Gross Unrealized Losses	1,778	3,016
Fair Value	$ 147,056	$ 161,712

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://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

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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
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

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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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