Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Held-to-Maturity
Amortized Cost	$ 47,455	$ 289,194
Gross Unrealized Gains	1,256	3,051
Gross Unrealized (Losses)	(71)	(1,415)
Estimated Fair Value	48,640	290,830
Available-for-Sale
Amortized Cost	2,324,169	2,187,623
Gross Unrealized Gains	27,011	6,037
Gross Unrealized (Losses)	(8,793)	(41,908)
Estimated Fair Value	2,342,387	2,151,752
U.S. Government agencies
Held-to-Maturity
Amortized Cost	999	16,990
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	0	(49)
Estimated Fair Value	999	16,941
Available-for-Sale
Amortized Cost	197,382	157,523
Gross Unrealized Gains	1,654	518
Gross Unrealized (Losses)	(1,380)	(3,740)
Estimated Fair Value	197,656	154,301
Mortgage-backed securities
Held-to-Maturity
Amortized Cost	12,225	13,346
Gross Unrealized Gains	103	5
Gross Unrealized (Losses)	(59)	(412)
Estimated Fair Value	12,269	12,939
Available-for-Sale
Amortized Cost	1,346,839	1,552,487
Gross Unrealized Gains	5,971	3,097
Gross Unrealized (Losses)	(7,050)	(32,684)
Estimated Fair Value	1,345,760	1,522,900
State and political subdivisions
Held-to-Maturity
Amortized Cost	32,236	256,863
Gross Unrealized Gains	1,090	3,029
Gross Unrealized (Losses)	(12)	(954)
Estimated Fair Value	33,314	258,938
Available-for-Sale
Amortized Cost	617,987	320,142
Gross Unrealized Gains	18,841	171
Gross Unrealized (Losses)	(270)	(5,470)
Estimated Fair Value	636,558	314,843
Other securities
Held-to-Maturity
Amortized Cost	1,995	1,995
Gross Unrealized Gains	63	17
Gross Unrealized (Losses)	0	0
Estimated Fair Value	2,058	2,012
Available-for-Sale
Amortized Cost	161,961	157,471
Gross Unrealized Gains	545	2,251
Gross Unrealized (Losses)	(93)	(14)
Estimated Fair Value	$ 162,413	$ 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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