Investment Securities, Part II (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($) securities	Dec. 31, 2017 USD ($) securities	Dec. 31, 2016 USD ($) securities	Dec. 31, 2015 USD ($)
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
One year or less	$ 87,434,000	$ 87,434,000
After 1 through 5 years	282,827,000	282,827,000
After 5 through 10 years	203,980,000	203,980,000
After ten years	4,338,000	4,338,000
Fair Value	578,579,000	$ 578,579,000	$ 619,848,000
Available-for-sale, Weighted Average Yield, Maturities Year One (in hundredths)		2.28%
Available-for-sale , Weighted Average Yield, Maturities After 1 through 5 Years (in hundredths)		3.30%
Available-for-sale, Weighted Average Yield, Maturities After 5 through 10 Years (in hundredths)		4.11%
Available-for-sale , Weighted Average Yield, Maturities After 10 Years (in hundredths)		2.74%
Available-for-sale , Weighted Average Yield, Maturities (in hundredths)		3.42%
Available-for-sale, Full Tax-equivalent Yield, Maturities Year One (in hundredths)		2.67%
Available-for-sale, Full Tax-equivalent Yield, Maturities After 1 through 5 Years (in hundredths)		3.82%
Available-for-sale, Full Tax-equivalent Yield, Maturities After 5 through 10 years (in hundredths)		4.88%
Available-for-sale, Full Tax-equivalent Yield, Maturities After Ten Years (in hundredths)		3.55%
Available-for-sale, Full Tax-equivalent Yield, Maturities (in hundredths)		4.01%
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	39,993,000	$ 39,993,000
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	29,339,000	29,339,000
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	0	0
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	0	0
Held-to-maturity, at amortized cost (estimated fair value of $68,457 and $73,096 at December 31, 2017 and 2016, respectively)	69,332,000	$ 69,332,000	74,231,000
Held To Maturity Weighted Average Yield Maturities Year One		1.76%
Held To Maturity Weighted Average Yield Maturities After 1 Through 5 Years		2.08%
Held To Maturity Weighted Average Yield Maturities After 5 Through 10 Years		0.00%
Held To Maturity Weighted Average Yield Maturities After 10 Years		0.00%
Held To Maturity Weighted Average Yield Maturities		1.90%
Held To Maturity Tax Equivalent Yield Maturities Year One		1.76%
Held To Maturity Tax Equivalent Yield Maturities After 1 Through 5 Years		2.08%
Held To Maturity Tax Equivalent Yield Maturities After 5 Through 10 Years		0.00%
Held To Maturity Tax Equivalent Yield Maturities After 10 Years		0.00%
Held To Maturity Tax Equivalent Yield Maturities		1.90%
Tax rate used to calculate tax-equivalent yields (in hundredths)		35.00%
Percentage investment book value exceeds total stockholders' equity (in hundredths)		10.00%
Investment Securities Pledged as Collateral	479,398,000	$ 479,398,000	509,296,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	289,151,000	289,151,000	484,981,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(3,251,000)	(3,251,000)	(9,411,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	82,396,000	82,396,000	9,525,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2,613,000)	(2,613,000)	(1,673,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	371,547,000	371,547,000	494,506,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (5,864,000)	(5,864,000)	(11,084,000)
Details of Trust Preferred Securities [Abstract]
Coupon Rate of Trust Preferred Security	floating rate coupon of 3-month LIBOR plus 90 basis points
Number of Ratios Utilized in OTTI Credit Model	29
OTTI Cash Flow Initial Default Rate	2.00%
Period For Initial Default Rate Used In OTTI Analysis	two years
Default Rate Applied For Remainder Of OTTI Analysis	36 basis points
Recovery Rate Used In OTTI Cash Flow Analysis	10.00%
Credit losses on trust preferred securities held [Abstract]
Beginning of period		1,111,000	1,111,000	$ 1,111,000
Additions related to OTTI losses not previously recognized		0	0	0
Reductions due to sales / (recoveries)		0	0	0
Reductions due to change in intent or likelihood of sale		0	0	0
Additions related to increases in previously recognized OTTI losses		0	0	0
Reductions due to increases in expected cash flows		0	0	0
End of period	$ 1,111,000	1,111,000	1,111,000	$ 1,111,000
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	39,105,000	39,105,000
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	29,352,000	29,352,000
Held-to-maturity Securities, Debt Maturities, Year Six Through Ten, Fair Value	0	0
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	0	0
Held-to-maturity Securities, Fair Value	68,457,000	68,457,000	73,096,000
PreTSL XXVIII
Details of Trust Preferred Securities [Abstract]
Book Value	2,893,000	2,893,000
Market Value	2,548,000	2,548,000
Unrealized Gains (Losses)	(345,000)	(345,000)
Other-than- temporary Impairment Recorded To-date		(1,111,000)
U.S. Treasury securities and obligations of U.S. government corporations & agencies
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
One year or less	70,462,000	70,462,000
After 1 through 5 years	38,359,000	38,359,000
After 5 through 10 years	4,949,000	4,949,000
After ten years	0	0
Fair Value	113,770,000	113,770,000	136,324,000
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount	39,993,000	39,993,000
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount	29,339,000	29,339,000
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	0	0
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	0	0
Held-to-maturity, at amortized cost (estimated fair value of $68,457 and $73,096 at December 31, 2017 and 2016, respectively)	69,332,000	69,332,000	74,231,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	58,584,000	58,584,000	125,257,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(540,000)	(540,000)	(2,508,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	47,972,000	47,972,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1,494,000)	(1,494,000)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	106,556,000	106,556,000	125,257,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (2,034,000)	$ (2,034,000)	(2,508,000)
Number of AFS securities in Unrealized Loss Positions | securities	11	11
Held-to-maturity Securities, Continuous Unrealized Loss Position [Abstract]
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 34,101,000	$ 34,101,000	53,295,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(525,000)	(525,000)	(1,338,000)
Held-to-maturity Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	14,540,000	14,540,000	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(453,000)	(453,000)	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value	48,641,000	48,641,000	53,295,000
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (978,000)	$ (978,000)	(1,338,000)
Number of HTM securities in Unrealized Loss Positions | securities	7	7
Credit losses on trust preferred securities held [Abstract]
Held-to-maturity Securities, Fair Value	$ 68,457,000	$ 68,457,000	73,096,000
Obligations of states and political subdivisions
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
One year or less	16,897,000	16,897,000
After 1 through 5 years	74,454,000	74,454,000
After 5 through 10 years	73,297,000	73,297,000
After ten years	1,618,000	1,618,000
Fair Value	166,266,000	166,266,000	162,705,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	42,618,000	42,618,000	93,405,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(769,000)	(769,000)	(2,804,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	9,267,000	9,267,000	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(256,000)	(256,000)	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	51,885,000	51,885,000	93,405,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (1,025,000)	$ (1,025,000)	(2,804,000)
Number of AFS securities in Unrealized Loss Positions | securities	39	39
Mortgage-backed securities
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
One year or less	$ 75,000	$ 75,000
After 1 through 5 years	168,002,000	168,002,000
After 5 through 10 years	125,734,000	125,734,000
After ten years	0	0
Fair Value	293,811,000	293,811,000	314,991,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	187,949,000	187,949,000	266,319,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,942,000)	(1,942,000)	(4,099,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	22,609,000	22,609,000	5,878,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(518,000)	(518,000)	(270,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	210,558,000	210,558,000	272,197,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (2,460,000)	$ (2,460,000)	$ (4,369,000)
Number of AFS securities in Unrealized Loss Positions | securities	26	26	2
Trust preferred securities
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
One year or less	$ 0	$ 0
After 1 through 5 years	0	0
After 5 through 10 years	0	0
After ten years	2,548,000	2,548,000
Fair Value	2,548,000	2,548,000	$ 1,652,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	0	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	2,548,000	2,548,000	1,652,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(345,000)	(345,000)	(1,398,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,548,000	2,548,000	1,652,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (345,000)	$ (345,000)	$ (1,398,000)
Number of AFS securities in Unrealized Loss Positions | securities	1	1	1
Other securities
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
One year or less	$ 0	$ 0
After 1 through 5 years	2,012,000	2,012,000
After 5 through 10 years	0	0
After ten years	172,000	172,000
Fair Value	2,184,000	2,184,000	$ 4,176,000
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Fair Value	0	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	0	0	1,995,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0	(5,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0	0	1,995,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 0	$ 0	$ (5,000)
Number of AFS securities in Unrealized Loss Positions | securities			1