Investment Securities - Amortized Cost and Fair Value of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 6,964,566	$ 5,784,754
Unrealized Gains	143,187	197,714
Unrealized Losses	(73,677)	(11,385)
Basis Adjustments	(6,126)	(968)
Fair Value	7,027,950	5,970,115
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	237,345	9,909
Unrealized Gains	0	299
Unrealized Losses	(898)	0
Basis Adjustments	4,130	0
Fair Value	240,577	10,208
U.S. government-sponsored entities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,582,406	841,133
Unrealized Gains	16,714	5,744
Unrealized Losses	(26,168)	(3,921)
Basis Adjustments	(10,256)	(968)
Fair Value	1,562,696	841,988
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,349,118	3,249,002
Unrealized Gains	44,243	91,086
Unrealized Losses	(38,660)	(990)
Basis Adjustments	0	0
Fair Value	3,354,701	3,339,098
States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,540,053	1,405,868
Unrealized Gains	72,248	86,325
Unrealized Losses	(3,018)	(31)
Basis Adjustments	0	0
Fair Value	1,609,283	1,492,162
Pooled trust preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,751	13,763
Unrealized Gains	0	0
Unrealized Losses	(4,384)	(5,850)
Basis Adjustments	0	0
Fair Value	9,367	7,913
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	241,893	265,079
Unrealized Gains	9,982	14,260
Unrealized Losses	(549)	(593)
Basis Adjustments	0	0
Fair Value	$ 251,326	$ 278,746

Debt Securities, Available-For-Sale, Accumulated Basis Adjustments, Before Tax

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- Definition

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).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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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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Amortized cost excluding accrued interest, before 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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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