Investment Securities - Amortized Cost and Estimated Fair Value of Held to Maturity Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 258,690	$ 256,567	$ 240,829
Unrealized Gains	1,966	4,499	2,959
Unrealized Losses	(4,805)	(1,411)	(4,410)
Fair Value (Estimated)	255,851	259,655	239,378
U.S. Government-sponsored agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	11,155	11,155	12,050
Unrealized Gains	0	0	0
Unrealized Losses	(515)	(180)	(317)
Fair Value (Estimated)	10,640	10,975	11,733
Mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	21,990	23,284	29,093
Unrealized Gains	469	568	880
Unrealized Losses	(289)	(128)	(135)
Fair Value (Estimated)	22,170	23,724	29,838
State and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	221,245	217,828	195,386
Unrealized Gains	1,497	3,931	2,079
Unrealized Losses	(4,001)	(1,103)	(3,958)
Fair Value (Estimated)	218,741	220,656	193,507
Corporate securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,300	4,300	4,300
Unrealized Gains	0	0	0
Unrealized Losses	0	0	0
Fair Value (Estimated)	$ 4,300	$ 4,300	$ 4,300