Investment Securities - Schedule of Investment Securities' Gross Unrealized Losses and Fair Value by Investment Category (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	$ 394,889	$ 73,219
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	3,707	302
Available-for-sale debt securities, 12 months or longer, Fair Value	23,099	718,120
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	275	22,748
Available-for-sale debt securities, Total, Fair Value	417,988	791,339
Available-for-sale debt securities, Total, Unrealized Loss	3,982	23,050
State and political subdivisions
Held-to-maturity debt securities:
Held-to-maturity, Less than 12 months, Fair Value	523	91,228
Held-to-maturity, Less than 12 months, Unrealized Loss	0	155
Held-to-maturity, 12 months or longer, Fair Value	0	58,203
Held-to-maturity, 12 months or longer, Unrealized Loss	0	639
Held-to-maturity, Total, Fair Value	523	149,431
Held-to-maturity, Total, Unrealized Loss	0	794
CMO
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	47,376	17,143
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	481	40
Available-for-sale debt securities, 12 months or longer, Fair Value	7,999	212,208
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	61	6,798
Available-for-sale debt securities, Total, Fair Value	55,375	229,351
Available-for-sale debt securities, Total, Unrealized Loss	542	6,838
FNMA MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	310,312	34,214
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	2,681	162
Available-for-sale debt securities, 12 months or longer, Fair Value	6,522	407,638
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	105	12,776
Available-for-sale debt securities, Total, Fair Value	316,834	441,852
Available-for-sale debt securities, Total, Unrealized Loss	2,786	12,938
FHLMC MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	35,354	16,025
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	541	21
Available-for-sale debt securities, 12 months or longer, Fair Value	2,836	76,469
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	50	2,373
Available-for-sale debt securities, Total, Fair Value	38,190	92,494
Available-for-sale debt securities, Total, Unrealized Loss	591	2,394
GNMA MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	1,847	5,837
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	4	79
Available-for-sale debt securities, 12 months or longer, Fair Value	5,742	21,805
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	59	801
Available-for-sale debt securities, Total, Fair Value	7,589	27,642
Available-for-sale debt securities, Total, Unrealized Loss	$ 63	$ 880

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