Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Held-to-Maturity
Amortized Cost	$ 40,927	$ 289,194
Gross Unrealized Gains	987	3,051
Gross Unrealized (Losses)	(59)	(1,415)
Estimated Fair Value	41,855	290,830
Available-for-Sale
Amortized Cost	3,428,146	2,187,623
Gross Unrealized Gains	31,740	6,037
Gross Unrealized (Losses)	(6,548)	(41,908)
Estimated Fair Value	3,453,338	2,151,752
U.S. Treasury
Available-for-Sale
Amortized Cost	449,729	0
Gross Unrealized Gains	112	0
Gross Unrealized (Losses)	(112)	0
Estimated Fair Value	449,729	0
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	194,207	157,523
Gross Unrealized Gains	1,313	518
Gross Unrealized (Losses)	(1,271)	(3,740)
Estimated Fair Value	194,249	154,301
Mortgage-backed securities
Held-to-Maturity
Amortized Cost	10,796	13,346
Gross Unrealized Gains	71	5
Gross Unrealized (Losses)	(59)	(412)
Estimated Fair Value	10,808	12,939
Available-for-Sale
Amortized Cost	1,738,584	1,552,487
Gross Unrealized Gains	8,510	3,097
Gross Unrealized (Losses)	(4,149)	(32,684)
Estimated Fair Value	1,742,945	1,522,900
State and political subdivisions
Held-to-Maturity
Amortized Cost	27,082	256,863
Gross Unrealized Gains	849	3,029
Gross Unrealized (Losses)	0	(954)
Estimated Fair Value	27,931	258,938
Available-for-Sale
Amortized Cost	860,539	320,142
Gross Unrealized Gains	20,983	171
Gross Unrealized (Losses)	(998)	(5,470)
Estimated Fair Value	880,524	314,843
Other securities
Held-to-Maturity
Amortized Cost	3,049	1,995
Gross Unrealized Gains	67	17
Gross Unrealized (Losses)	0	0
Estimated Fair Value	3,116	2,012
Available-for-Sale
Amortized Cost	185,087	157,471
Gross Unrealized Gains	822	2,251
Gross Unrealized (Losses)	(18)	(14)
Estimated Fair Value	$ 185,891	$ 159,708

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).

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