Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Held-to-Maturity
Amortized Cost	$ 289,194	$ 368,058
Gross Unrealized Gains	3,051	5,977
Gross Unrealized (Losses)	(1,415)	(737)
Estimated Fair Value	290,830	373,298
Available-for-Sale
Amortized Cost	2,187,623	1,612,324
Gross Unrealized Gains	6,037	2,100
Gross Unrealized (Losses)	(41,908)	(24,907)
Estimated Fair Value	2,151,752	1,589,517
U.S. Government agencies
Held-to-Maturity
Amortized Cost	16,990	46,945
Gross Unrealized Gains	0	7
Gross Unrealized (Losses)	(49)	(228)
Estimated Fair Value	16,941	46,724
Available-for-Sale
Amortized Cost	157,523	141,559
Gross Unrealized Gains	518	116
Gross Unrealized (Losses)	(3,740)	(1,951)
Estimated Fair Value	154,301	139,724
Mortgage-backed securities
Held-to-Maturity
Amortized Cost	13,346	16,132
Gross Unrealized Gains	5	8
Gross Unrealized (Losses)	(412)	(287)
Estimated Fair Value	12,939	15,853
Available-for-Sale
Amortized Cost	1,552,487	1,208,017
Gross Unrealized Gains	3,097	246
Gross Unrealized (Losses)	(32,684)	(20,946)
Estimated Fair Value	1,522,900	1,187,317
State and political subdivisions
Held-to-Maturity
Amortized Cost	256,863	301,491
Gross Unrealized Gains	3,029	5,962
Gross Unrealized (Losses)	(954)	(222)
Estimated Fair Value	258,938	307,231
Available-for-Sale
Amortized Cost	320,142	144,642
Gross Unrealized Gains	171	532
Gross Unrealized (Losses)	(5,470)	(2,009)
Estimated Fair Value	314,843	143,165
Other securities
Held-to-Maturity
Amortized Cost	1,995	3,490
Gross Unrealized Gains	17	0
Gross Unrealized (Losses)	0	0
Estimated Fair Value	2,012	3,490
Available-for-Sale
Amortized Cost	157,471	118,106
Gross Unrealized Gains	2,251	1,206
Gross Unrealized (Losses)	(14)	(1)
Estimated Fair Value	$ 159,708	$ 119,311

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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