Investment Securities (Available-for-sale Securities) (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities
Securities of a single issuer, other than US Treasury, government agencies and US government sponsored agencies exceeding 10% of Stockholders' Equity	$ 0
Amortized Cost	808,689,000	$ 797,732,000
Gross Unrealized Gains	3,363,000	11,040,000
Gross Unrealized Losses	(21,142,000)	(13,585,000)
Available-for-sale securities	790,910,000	795,187,000
States and political subdivisions
Schedule of Available-for-sale Securities
Amortized Cost	96,730,000	100,039,000
Gross Unrealized Gains	1,052,000	1,786,000
Gross Unrealized Losses	(577,000)	(256,000)
Available-for-sale securities	97,205,000	101,569,000
Residential mortgage-backed securities
Schedule of Available-for-sale Securities
Amortized Cost	699,788,000	684,100,000
Gross Unrealized Gains	2,195,000	2,582,000
Gross Unrealized Losses	(20,237,000)	(13,018,000)
Available-for-sale securities	681,746,000	673,664,000
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities
Amortized Cost	6,974,000	7,004,000
Gross Unrealized Gains	0	11,000
Gross Unrealized Losses	(110,000)	(39,000)
Available-for-sale securities	6,864,000	6,976,000
Bank-issued trust preferred securities
Schedule of Available-for-sale Securities
Amortized Cost	5,197,000	5,195,000
Gross Unrealized Gains	116,000	141,000
Gross Unrealized Losses	(218,000)	(207,000)
Available-for-sale securities	$ 5,095,000	5,129,000
Equity securities (a)
Schedule of Available-for-sale Securities
Amortized Cost		1,394,000
Gross Unrealized Gains		6,520,000
Gross Unrealized Losses		(65,000)
Available-for-sale securities		$ 7,849,000