Investment Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Available-for-sale Securities

The following is a summary of the amortized cost and fair value of investment securities available-for-sale and investment securities held-to-maturity at December 31, 2017 and 2016:

	Investment Securities Available-for-Sale
(Dollars in thousands)	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
December 31, 2017
Government sponsored agencies	$	203,099	$	765	$	948	$	202,916
State and political subdivisions	350,088		310		4,428		345,970
Residential mortgage-backed securities	151,752		5		1,626		150,131
Collateralized mortgage obligations	1,042,240		89		8,484		1,033,845
Corporate bonds	193,230		1,156		1,592		192,794
Preferred stock and trust preferred securities	36,237		1,715		62		37,890
Total	$	1,976,646	$	4,040	$	17,140	$	1,963,546
December 31, 2016
U.S. Treasury securities	$	5,788	$	5	$	—	$	5,793
Government sponsored agencies	216,890		189		2,068		215,011
State and political subdivisions	311,704		163		11,779		300,088
Residential mortgage-backed securities	276,162		112		3,992		272,282
Collateralized mortgage obligations	323,965		63		4,003		320,025
Corporate bonds	90,859		16		1,401		89,474
Preferred stock and trust preferred securities	31,353		1,018		80		32,291
Total	$	1,256,721	$	1,566	$	23,323	$	1,234,964

Held-to-maturity Securities

	Investment Securities Held-to-Maturity
(Dollars in thousands)	Amortized Cost		Unrecognized Gains		Unrecognized Losses		Fair Value
December 31, 2017
State and political subdivisions	$	676,593	$	3,856	$	17,933	$	662,516
Trust preferred securities	500		—		110		390
Total	$	677,093	$	3,856	$	18,043	$	662,906
December 31, 2016
State and political subdivisions	$	622,927	$	2,648	$	17,354	$	608,221
Trust preferred securities	500		—		190		310
Total	$	623,427	$	2,648	$	17,544	$	608,531

Proceeds from sales of securities and the associated gains and losses recorded in earnings

Proceeds from sales of securities and the associated gains and losses recorded in earnings are listed below:

	For the years ended December 31,
(Dollars in thousands)	2017			2016			2015
Proceeds	$	409,220		$	41,446		$	40,301
Gross gains	178			325			631
Gross losses	(7,566		)	(196		)	(1		)

Amortized cost and fair value of debt securities by contractual maturity

The following is a summary of the amortized cost and fair value of investment securities at December 31, 2017, by maturity, for both available-for-sale and held-to-maturity investment securities. The maturities of residential mortgage-backed securities and collateralized mortgage obligations are based on scheduled principal payments. The maturities of all other debt securities are based on final contractual maturity.

	December 31, 2017
(Dollars in thousands)	Amortized Cost		Fair Value
Investment Securities Available-for-Sale:
Due in one year or less	$	300,364	$	298,644
Due after one year through five years	751,998		746,082
Due after five years through ten years	616,027		611,822
Due after ten years	306,868		305,174
Preferred stock	1,389		1,824
Total	$	1,976,646	$	1,963,546
Investment Securities Held-to-Maturity:
Due in one year or less	$	89,359	$	89,149
Due after one year through five years	237,113		233,022
Due after five years through ten years	152,299		148,185
Due after ten years	198,322		192,550
Total	$	677,093	$	662,906

Summary of continuous unrealized loss position of securities

The following schedule summarizes information for both available-for-sale and held-to-maturity investment securities with gross unrealized losses at December 31, 2017 and 2016, aggregated by category and length of time that individual securities have been in a continuous unrealized loss position. As of December 31, 2017, the Corporation's securities portfolio consisted of 2,329 securities, 1,518 of which were in an unrealized loss position.

	Less Than 12 Months				12 Months or More				Total
(Dollars in thousands)	Fair Value		Gross Unrealized Losses		Fair Value		Gross Unrealized Losses		Fair Value		Gross Unrealized Losses
December 31, 2017
Government sponsored agencies	$	63,818	$	510	$	24,621	$	438	$	88,439	$	948
State and political subdivisions	437,407		12,268		349,242		10,093		786,649		22,361
Residential mortgage-backed securities	93,508		383		56,576		1,243		150,084		1,626
Collateralized mortgage obligations	713,525		7,235		73,707		1,249		787,232		8,484
Corporate bonds	71,447		1,138		47,878		454		119,325		1,592
Preferred stock and trust preferred securities	—		—		11,164		172		11,164		172
Total	$	1,379,705	$	21,534	$	563,188	$	13,649	$	1,942,893	$	35,183
December 31, 2016
Government sponsored agencies	$	105,702	$	1,707	$	15,023	$	361	$	120,725	$	2,068
State and political subdivisions	758,063		28,158		26,810		975		784,873		29,133
Residential mortgage-backed securities	244,239		3,992		—		—		244,239		3,992
Collateralized mortgage obligations	279,001		3,778		14,754		225		293,755		4,003
Corporate bonds	80,536		1,401		—		—		80,536		1,401
Preferred stock and trust preferred securities	10,699		80		310		190		11,009		270
Total	$	1,478,240	$	39,116	$	56,897	$	1,751	$	1,535,137	$	40,867