Investment Securities - Securities With Unrealized Losses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Held-to-Maturity
Held-to-maturity debt securities, less than 12 months, estimated fair value	$ 3,489	$ 40,141
Held-to-maturity debt securities, less than 12 months, gross unrealized losses	(30)	(210)
Held-to-maturity debt securities, 12 months or more, estimated fair value	2,300	79,014
Held-to-maturity debt securities,12 months or more, gross unrealized losses	(29)	(1,205)
Held-to-maturity debt securities, estimated fair value	5,789	119,155
Held-to-maturity debt securities, gross unrealized losses	(59)	(1,415)
Available-for-Sale
Available-for-sale debt securities, less than 12 months, estimated fair value	834,731	319,262
Available-for-sale debt securities, less than 12 months, gross unrealized losses	(2,561)	(2,786)
Available-for-sale debt securities, 12 months or more, estimated fair value	438,262	1,278,434
Available-for-sale debt securities, 12 months or more, gross unrealized losses	(3,987)	(39,122)
Available-for-sale debt securities, estimated fair value	1,272,993	1,597,696
Available-for-sale debt securities, gross unrealized losses	(6,548)	(41,908)
U.S. Treasury
Available-for-Sale
Available-for-sale debt securities, less than 12 months, estimated fair value	299,667
Available-for-sale debt securities, less than 12 months, gross unrealized losses	(112)
Available-for-sale debt securities, 12 months or more, estimated fair value	0
Available-for-sale debt securities, 12 months or more, gross unrealized losses	0
Available-for-sale debt securities, estimated fair value	299,667
Available-for-sale debt securities, gross unrealized losses	(112)
U.S. Government agencies
Held-to-Maturity
Held-to-maturity debt securities, less than 12 months, estimated fair value		992
Held-to-maturity debt securities, less than 12 months, gross unrealized losses		(2)
Held-to-maturity debt securities, 12 months or more, estimated fair value		15,948
Held-to-maturity debt securities,12 months or more, gross unrealized losses		(47)
Held-to-maturity debt securities, estimated fair value		16,940
Held-to-maturity debt securities, gross unrealized losses		(49)
Available-for-Sale
Available-for-sale debt securities, less than 12 months, estimated fair value	14,454	26,562
Available-for-sale debt securities, less than 12 months, gross unrealized losses	(130)	(221)
Available-for-sale debt securities, 12 months or more, estimated fair value	86,239	108,636
Available-for-sale debt securities, 12 months or more, gross unrealized losses	(1,141)	(3,519)
Available-for-sale debt securities, estimated fair value	100,693	135,198
Available-for-sale debt securities, gross unrealized losses	(1,271)	(3,740)
Mortgage-backed securities
Held-to-Maturity
Held-to-maturity debt securities, less than 12 months, estimated fair value	3,489	0
Held-to-maturity debt securities, less than 12 months, gross unrealized losses	(30)	0
Held-to-maturity debt securities, 12 months or more, estimated fair value	2,300	12,177
Held-to-maturity debt securities,12 months or more, gross unrealized losses	(29)	(412)
Held-to-maturity debt securities, estimated fair value	5,789	12,177
Held-to-maturity debt securities, gross unrealized losses	(59)	(412)
Available-for-Sale
Available-for-sale debt securities, less than 12 months, estimated fair value	405,540	163,560
Available-for-sale debt securities, less than 12 months, gross unrealized losses	(1,326)	(1,146)
Available-for-sale debt securities, 12 months or more, estimated fair value	349,293	1,037,679
Available-for-sale debt securities, 12 months or more, gross unrealized losses	(2,823)	(31,538)
Available-for-sale debt securities, estimated fair value	754,833	1,201,239
Available-for-sale debt securities, gross unrealized losses	(4,149)	(32,684)
State and political subdivisions
Held-to-Maturity
Held-to-maturity debt securities, less than 12 months, estimated fair value		39,149
Held-to-maturity debt securities, less than 12 months, gross unrealized losses		(208)
Held-to-maturity debt securities, 12 months or more, estimated fair value		50,889
Held-to-maturity debt securities,12 months or more, gross unrealized losses		(746)
Held-to-maturity debt securities, estimated fair value		90,038
Held-to-maturity debt securities, gross unrealized losses		(954)
Available-for-Sale
Available-for-sale debt securities, less than 12 months, estimated fair value	111,088	129,075
Available-for-sale debt securities, less than 12 months, gross unrealized losses	(975)	(1,406)
Available-for-sale debt securities, 12 months or more, estimated fair value	2,730	132,020
Available-for-sale debt securities, 12 months or more, gross unrealized losses	(23)	(4,064)
Available-for-sale debt securities, estimated fair value	113,818	261,095
Available-for-sale debt securities, gross unrealized losses	(998)	(5,470)
Other securities
Available-for-Sale
Available-for-sale debt securities, less than 12 months, estimated fair value	3,982	65
Available-for-sale debt securities, less than 12 months, gross unrealized losses	(18)	(13)
Available-for-sale debt securities, 12 months or more, estimated fair value	0	99
Available-for-sale debt securities, 12 months or more, gross unrealized losses	0	(1)
Available-for-sale debt securities, estimated fair value	3,982	164
Available-for-sale debt securities, gross unrealized losses	$ (18)	$ (14)

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