Investment Securities - Amortized Cost and Estimated Fair Value of Available for Sale Securities (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 6,686,000	$ 3,843,000	$ 150,000
Amortized Cost	311,630,000	303,845,000	314,060,000
Unrealized Gains	194,000	859,000	1,948,000
Unrealized Losses	(8,055,000)	(4,532,000)	(3,384,000)
Fair Value (Estimated)	303,769,000	300,172,000	312,624,000
U.S. Government-sponsored agencies
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	206,000	86,000	0
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,755,000	2,693,000	146,000
Amortized Cost	302,935,000	293,689,000	295,424,000
Unrealized Gains	191,000	722,000	1,407,000
Unrealized Losses	(7,798,000)	(4,422,000)	(3,204,000)
Fair Value (Estimated)	295,328,000	289,989,000	293,627,000
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,725,000	1,061,000	0
Amortized Cost	5,765,000	6,860,000	15,407,000
Unrealized Gains	3,000	16,000	474,000
Unrealized Losses	(257,000)	(107,000)	(176,000)
Fair Value (Estimated)	5,511,000	6,769,000	15,705,000
Other equity securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		3,000	4,000
Amortized Cost	2,930,000	3,296,000	3,229,000
Unrealized Gains	0	121,000	67,000
Unrealized Losses	0	(3,000)	(4,000)
Fair Value (Estimated)	$ 2,930,000	$ 3,414,000	$ 3,292,000