Investment Securities - Carrying Values of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Abstract]
Investment in subsidiary bank	$ 355,303	$ 318,926
Securities available for sale, Percent	100.00%	100.00%
US Treasury [Member]
Debt Securities, Available-for-sale [Abstract]
Investment in subsidiary bank	$ 128,585	$ 83,155
Securities available for sale, Percent	36.19%	26.07%
Other securities (FHLB, FHLMC and FNMA) [Member]
Debt Securities, Available-for-sale [Abstract]
Investment in subsidiary bank	$ 15,229	$ 34,871
Securities available for sale, Percent	4.29%	10.93%
State and political subdivisions [Member]
Debt Securities, Available-for-sale [Abstract]
Investment in subsidiary bank	$ 211,489	$ 200,900
Securities available for sale, Percent	59.52%	63.00%