Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Held-to-Maturity
Amortized Cost	$ 61,435	$ 289,194
Gross Unrealized Gains	797	3,051
Gross Unrealized (Losses)	(276)	(1,415)
Estimated Fair Value	61,956	290,830
Available-for-Sale
Amortized Cost	2,250,103	2,187,623
Gross Unrealized Gains	13,909	6,037
Gross Unrealized (Losses)	(23,901)	(41,908)
Estimated Fair Value	2,240,111	2,151,752
U.S. Government agencies
Held-to-Maturity
Amortized Cost	12,996	16,990
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(9)	(49)
Estimated Fair Value	12,987	16,941
Available-for-Sale
Amortized Cost	163,730	157,523
Gross Unrealized Gains	596	518
Gross Unrealized (Losses)	(2,749)	(3,740)
Estimated Fair Value	161,577	154,301
Mortgage-backed securities
Held-to-Maturity
Amortized Cost	12,847	13,346
Gross Unrealized Gains	10	5
Gross Unrealized (Losses)	(240)	(412)
Estimated Fair Value	12,617	12,939
Available-for-Sale
Amortized Cost	1,362,950	1,552,487
Gross Unrealized Gains	2,569	3,097
Gross Unrealized (Losses)	(19,842)	(32,684)
Estimated Fair Value	1,345,677	1,522,900
State and political subdivisions
Held-to-Maturity
Amortized Cost	33,597	256,863
Gross Unrealized Gains	751	3,029
Gross Unrealized (Losses)	(27)	(954)
Estimated Fair Value	34,321	258,938
Available-for-Sale
Amortized Cost	571,716	320,142
Gross Unrealized Gains	10,307	171
Gross Unrealized (Losses)	(1,233)	(5,470)
Estimated Fair Value	580,790	314,843
Other securities
Held-to-Maturity
Amortized Cost	1,995	1,995
Gross Unrealized Gains	36	17
Gross Unrealized (Losses)	0	0
Estimated Fair Value	2,031	2,012
Available-for-Sale
Amortized Cost	151,707	157,471
Gross Unrealized Gains	437	2,251
Gross Unrealized (Losses)	(77)	(14)
Estimated Fair Value	$ 152,067	$ 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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