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Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Held-to-maturity |  |
Amortized Cost | $ 335,946 | $ 40,927
Allowance for Credit Losses | (2,915) | 0
Net Carrying Amount | 333,031 | 40,927
Gross Unrealized Gains | 8,924 | 987
Gross Unrealized (Losses) | (30) | (59)
Estimated Fair Value | 341,925 | 41,855
Available-for-sale |  |
Available-for-sale, amortized cost | 3,397,043 | 3,263,151
Allowance for Credit Losses | (312) | 0
Gross Unrealized Gains | 79,663 | 31,740
Gross Unrealized (Losses) | (2,796) | (6,548)
Estimated Fair Value | 3,473,598 | 3,288,343
U.S. Treasury |  |
Available-for-sale |  |
Available-for-sale, amortized cost |  | 449,729
Allowance for Credit Losses |  | 0
Gross Unrealized Gains |  | 112
Gross Unrealized (Losses) |  | (112)
Estimated Fair Value |  | 449,729
U.S. Government agencies |  |
Available-for-sale |  |
Available-for-sale, amortized cost | 477,693 | 194,207
Allowance for Credit Losses | 0 | 0
Gross Unrealized Gains | 844 | 1,313
Gross Unrealized (Losses) | (1,300) | (1,271)
Estimated Fair Value | 477,237 | 194,249
Mortgage-backed securities |  |
Held-to-maturity |  |
Amortized Cost | 22,354 | 10,796
Allowance for Credit Losses | 0 | 0
Net Carrying Amount | 22,354 | 10,796
Gross Unrealized Gains | 683 | 71
Gross Unrealized (Losses) | 0 | (59)
Estimated Fair Value | 23,037 | 10,808
Available-for-sale |  |
Available-for-sale, amortized cost | 1,374,769 | 1,738,584
Allowance for Credit Losses | 0 | 0
Gross Unrealized Gains | 21,261 | 8,510
Gross Unrealized (Losses) | (1,094) | (4,149)
Estimated Fair Value | 1,394,936 | 1,742,945
State and political subdivisions |  |
Held-to-maturity |  |
Amortized Cost | 312,416 | 27,082
Allowance for Credit Losses | (2,307) | 0
Net Carrying Amount | 310,109 | 27,082
Gross Unrealized Gains | 8,148 | 849
Gross Unrealized (Losses) | (30) | 0
Estimated Fair Value | 318,227 | 27,931
Available-for-sale |  |
Available-for-sale, amortized cost | 1,416,136 | 860,539
Allowance for Credit Losses | (217) | 0
Gross Unrealized Gains | 55,111 | 20,983
Gross Unrealized (Losses) | (307) | (998)
Estimated Fair Value | 1,470,723 | 880,524
Other securities |  |
Held-to-maturity |  |
Amortized Cost | 1,176 | 3,049
Allowance for Credit Losses | (608) | 0
Net Carrying Amount | 568 | 3,049
Gross Unrealized Gains | 93 | 67
Gross Unrealized (Losses) | 0 | 0
Estimated Fair Value | 661 | 3,116
Available-for-sale |  |
Available-for-sale, amortized cost | 128,445 | 20,092
Allowance for Credit Losses | (95) | 0
Gross Unrealized Gains | 2,447 | 822
Gross Unrealized (Losses) | (95) | (18)
Estimated Fair Value | $ 130,702 | $ 20,896

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