Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities Available-for-Sale
Amortized Cost	$ 1,028,883	$ 1,097,645
Gross Unrealized/Unrecognized Holding Gains	453	4,645
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	25,915	10,409
Estimated Fair Value	1,003,421	1,091,881
Debt Securities Held-to-Maturity
Debt securities held-to-maturity, at amortized cost (fair value of $351,422 and $363,779 at December 31, 2018 and 2017, respectively)	351,808	357,197
Gross Unrealized/Unrecognized Holding Gains	3,289	7,333
Gross Unrealized/Unrecognized Holding Losses	3,675	751
Estimated Fair Value	351,422	363,779
Obligations of U.S. Treasury and other U.S. Government sponsored entities
Debt Securities Available-for-Sale
Amortized Cost		245,000
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		2,280
Estimated Fair Value		242,720
Obligations of States and Political Subdivisions
Debt Securities Held-to-Maturity
Debt securities held-to-maturity, at amortized cost (fair value of $351,422 and $363,779 at December 31, 2018 and 2017, respectively)	305,278	300,412
Gross Unrealized/Unrecognized Holding Gains	3,202	6,575
Gross Unrealized/Unrecognized Holding Losses	2,672	713
Estimated Fair Value	305,808	306,274
U.S. Government sponsored entity asset-backed securities
Debt Securities Available-for-Sale
Amortized Cost	1,028,883	852,645
Gross Unrealized/Unrecognized Holding Gains	453	4,645
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	25,915	8,129
Estimated Fair Value	1,003,421	849,161
Debt Securities Held-to-Maturity
Debt securities held-to-maturity, at amortized cost (fair value of $351,422 and $363,779 at December 31, 2018 and 2017, respectively)	46,530	56,785
Gross Unrealized/Unrecognized Holding Gains	87	758
Gross Unrealized/Unrecognized Holding Losses	1,003	38
Estimated Fair Value	$ 45,614	$ 57,505

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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