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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

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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 -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 + Details
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- Definition 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 Reference 1: http://www.xbrl.org/2003/role/disclosureRef -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 + Details
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