Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Held-to-Maturity
Amortized Cost	$ 323,306	$ 368,058
Gross Unrealized Gains	2,369	5,977
Gross Unrealized (Losses)	(2,837)	(737)
Estimated Fair Value	322,838	373,298
Available-for-Sale
Amortized Cost	2,060,798	1,612,324
Gross Unrealized Gains	2,857	2,100
Gross Unrealized (Losses)	(65,841)	(24,907)
Estimated Fair Value	1,997,814	1,589,517
U.S. Government agencies
Held-to-Maturity
Amortized Cost	34,983	46,945
Gross Unrealized Gains	0	7
Gross Unrealized (Losses)	(113)	(228)
Estimated Fair Value	34,870	46,724
Available-for-Sale
Amortized Cost	145,808	141,559
Gross Unrealized Gains	0	116
Gross Unrealized (Losses)	(4,348)	(1,951)
Estimated Fair Value	141,460	139,724
Mortgage-backed securities
Held-to-Maturity
Amortized Cost	13,933	16,132
Gross Unrealized Gains	0	8
Gross Unrealized (Losses)	(698)	(287)
Estimated Fair Value	13,235	15,853
Available-for-Sale
Amortized Cost	1,471,687	1,208,017
Gross Unrealized Gains	127	246
Gross Unrealized (Losses)	(52,188)	(20,946)
Estimated Fair Value	1,419,626	1,187,317
State and political subdivisions
Held-to-Maturity
Amortized Cost	272,396	301,491
Gross Unrealized Gains	2,369	5,962
Gross Unrealized (Losses)	(2,020)	(222)
Estimated Fair Value	272,745	307,231
Available-for-Sale
Amortized Cost	291,731	144,642
Gross Unrealized Gains	12	532
Gross Unrealized (Losses)	(9,304)	(2,009)
Estimated Fair Value	282,439	143,165
Other securities
Held-to-Maturity
Amortized Cost	1,994	3,490
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(6)	0
Estimated Fair Value	1,988	3,490
Available-for-Sale
Amortized Cost	151,572	118,106
Gross Unrealized Gains	2,718	1,206
Gross Unrealized (Losses)	(1)	(1)
Estimated Fair Value	$ 154,289	$ 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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