Investment Securities - Temporarily Impaired Securities (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Fair Value (Estimated) (Less than 12 months)	$ 254,875,000	$ 148,611,000	$ 287,408,000
Unrealized Losses (Less than 12 months)	(6,174,000)	(2,100,000)	(7,644,000)
Fair Value (Estimated) (12 months or more)	141,888,000	144,394,000	2,494,000
Unrealized Losses (12 months or more)	(6,686,000)	(3,843,000)	(150,000)
Fair Value (Estimated)	396,763,000	293,005,000	289,902,000
Unrealized Losses	(12,860,000)	(5,943,000)	(7,794,000)
U.S. Government-sponsored agencies
Schedule of Available-for-sale Securities [Line Items]
Fair Value (Estimated) (Less than 12 months)	6,946,000	7,161,000	10,824,000
Unrealized Losses (Less than 12 months)	(309,000)	(94,000)	(317,000)
Fair Value (Estimated) (12 months or more)	3,694,000	3,814,000	0
Unrealized Losses (12 months or more)	(206,000)	(86,000)	0
Fair Value (Estimated)	10,640,000	10,975,000	10,824,000
Unrealized Losses	(515,000)	(180,000)	(317,000)
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value (Estimated) (Less than 12 months)	183,335,000	132,025,000	198,068,000
Unrealized Losses (Less than 12 months)	(4,332,000)	(1,857,000)	(3,193,000)
Fair Value (Estimated) (12 months or more)	101,012,000	101,707,000	2,432,000
Unrealized Losses (12 months or more)	(3,755,000)	(2,693,000)	(146,000)
Fair Value (Estimated)	284,347,000	233,732,000	200,500,000
Unrealized Losses	(8,087,000)	(4,550,000)	(3,339,000)
State and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Fair Value (Estimated) (Less than 12 months)	64,594,000	9,425,000	78,516,000
Unrealized Losses (Less than 12 months)	(1,533,000)	(149,000)	(4,134,000)
Fair Value (Estimated) (12 months or more)	37,182,000	38,864,000	0
Unrealized Losses (12 months or more)	(2,725,000)	(1,061,000)	0
Fair Value (Estimated)	101,776,000	48,289,000	78,516,000
Unrealized Losses	$ (4,258,000)	(1,210,000)	(4,134,000)
Other equity securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value (Estimated) (Less than 12 months)		0	0
Unrealized Losses (Less than 12 months)		0	0
Fair Value (Estimated) (12 months or more)		9,000	62,000
Unrealized Losses (12 months or more)		(3,000)	(4,000)
Fair Value (Estimated)		9,000	62,000
Unrealized Losses		$ (3,000)	$ (4,000)