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Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Held-to-Maturity |  |
Amortized Cost | $ 42,237 | $ 289,194
Gross Unrealized Gains | 1,107 | 3,051
Gross Unrealized (Losses) | (42) | (1,415)
Estimated Fair Value | 43,302 | 290,830
Available-for-Sale |  |
Amortized Cost | 2,327,106 | 2,187,623
Gross Unrealized Gains | 34,043 | 6,037
Gross Unrealized (Losses) | (5,015) | (41,908)
Estimated Fair Value | 2,356,134 | 2,151,752
U.S. Government agencies |  |
Held-to-Maturity |  |
Amortized Cost | 0 | 16,990
Gross Unrealized Gains | 0 | 0
Gross Unrealized (Losses) | 0 | (49)
Estimated Fair Value | 0 | 16,941
Available-for-Sale |  |
Amortized Cost | 177,712 | 157,523
Gross Unrealized Gains | 1,629 | 518
Gross Unrealized (Losses) | (1,202) | (3,740)
Estimated Fair Value | 178,139 | 154,301
Mortgage-backed securities |  |
Held-to-Maturity |  |
Amortized Cost | 11,549 | 13,346
Gross Unrealized Gains | 93 | 5
Gross Unrealized (Losses) | (42) | (412)
Estimated Fair Value | 11,600 | 12,939
Available-for-Sale |  |
Amortized Cost | 1,332,130 | 1,552,487
Gross Unrealized Gains | 9,405 | 3,097
Gross Unrealized (Losses) | (3,741) | (32,684)
Estimated Fair Value | 1,337,794 | 1,522,900
State and political subdivisions |  |
Held-to-Maturity |  |
Amortized Cost | 28,692 | 256,863
Gross Unrealized Gains | 931 | 3,029
Gross Unrealized (Losses) | 0 | (954)
Estimated Fair Value | 29,623 | 258,938
Available-for-Sale |  |
Amortized Cost | 658,984 | 320,142
Gross Unrealized Gains | 22,290 | 171
Gross Unrealized (Losses) | (72) | (5,470)
Estimated Fair Value | 681,202 | 314,843
Other securities |  |
Held-to-Maturity |  |
Amortized Cost | 1,996 | 1,995
Gross Unrealized Gains | 83 | 17
Gross Unrealized (Losses) | 0 | 0
Estimated Fair Value | 2,079 | 2,012
Available-for-Sale |  |
Amortized Cost | 158,280 | 157,471
Gross Unrealized Gains | 719 | 2,251
Gross Unrealized (Losses) | 0 | (14)
Estimated Fair Value | $ 158,999 | $ 159,708

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