Securities (Summary of Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available for Sale and Held to Maturty Securities [Line Items]
Available for sale securities amortized cost basis	$ 846,099	$ 790,801
Available for sale securities unrealized gains	1,154	3,688
Available for sale securities unrealized losses	(34,606)	(13,535)
Available for sale, at fair value	812,647	780,954
Held to maturity securities	10,863	12,541
Held to maturity securities unrealized gains	0	180
Held to maturity securities unrealized losses	(206)	0
Held to maturity securities fair value	10,657	12,721
Total available for sale and held to maturity securities amortized cost	856,962	803,342
Total available for sale and held to maturity securities unrealized gains	1,154	3,868
Total available for sale and held to maturity securities unrealized losses	(34,812)	(13,535)
Total available for sale and held to maturity securities fair value	823,304	793,675
Obligations of U.S. government-sponsored enterprises [Member]
Schedule of Available for Sale and Held to Maturty Securities [Line Items]
Available for sale securities amortized cost basis	201,398	161,479
Available for sale securities unrealized gains	9	0
Available for sale securities unrealized losses	(7,934)	(3,875)
Available for sale, at fair value	193,473	157,604
Mortgage-backed securities issued by U.S. government-sponsored agencies and U.S. government-sponsored enterprises [Member]
Schedule of Available for Sale and Held to Maturty Securities [Line Items]
Available for sale securities amortized cost basis	616,552	594,944
Available for sale securities unrealized gains	1,143	3,671
Available for sale securities unrealized losses	(24,833)	(7,733)
Available for sale, at fair value	592,862	590,882
Held to maturity securities	10,863	12,541
Held to maturity securities unrealized gains	0	180
Held to maturity securities unrealized losses	(206)	0
Held to maturity securities fair value	10,657	12,721
Obligations of states and political subdivisions [Member]
Schedule of Available for Sale and Held to Maturty Securities [Line Items]
Available for sale securities amortized cost basis	935	2,355
Available for sale securities unrealized gains	2	4
Available for sale securities unrealized losses	0	0
Available for sale, at fair value	937	2,359
Individual name issuer trust preferred debt securities
Schedule of Available for Sale and Held to Maturty Securities [Line Items]
Available for sale securities amortized cost basis	13,303	18,106
Available for sale securities unrealized gains	0	0
Available for sale securities unrealized losses	(839)	(1,122)
Available for sale, at fair value	12,464	16,984
Corporate bonds [Member]
Schedule of Available for Sale and Held to Maturty Securities [Line Items]
Available for sale securities amortized cost basis	13,911	13,917
Available for sale securities unrealized gains	0	13
Available for sale securities unrealized losses	(1,000)	(805)
Available for sale, at fair value	$ 12,911	$ 13,125

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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