SECURITIES (Details) - USD ($)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,122,249,000	$ 2,122,249,000	$ 2,183,880,000	$ 2,124,567,000
Gross Unrealized Gains	39,216,000	39,216,000	37,527,000	38,496,000
Gross Unrealized Losses	340,000	340,000	9,945,000	6,136,000
Estimated Fair Value	2,161,125,000	2,161,125,000	2,211,462,000	2,156,927,000
Gross gains recognized on available-for-sale securities		88,000	31,000
Gross losses recognized on available-for-sale securities			12,000
Available-for-sale securities, amortized cost, by contractual maturity [Abstract]
Maturing in one year or less	330,495,000	330,495,000
Maturing after one year through five years	1,037,343,000	1,037,343,000
Maturing after five years through ten years	51,766,000	51,766,000
Maturing after ten years	272,508,000	272,508,000
Mortgage-backed securities	430,137,000	430,137,000
Total	2,122,249,000	2,122,249,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Maturing in one year or less	331,904,000	331,904,000
Maturing after one year through five years	1,044,196,000	1,044,196,000
Maturing after five years through ten years	54,198,000	54,198,000
Maturing after ten years	294,027,000	294,027,000
Mortgage-backed securities	436,800,000	436,800,000
Estimated Fair Value	$ 2,161,125,000	$ 2,161,125,000	2,211,462,000	2,156,927,000
Available-for-sale securities, weighted average yield, by contractual maturity [Abstract]
Maturing in one year or less (in hundredths)	1.60%	1.60%
Maturing after one year through five years (in hundredths)	1.25%	1.25%
Maturing after five years through ten years (in hundredths)	5.13%	5.13%
Maturing after ten years (in hundredths)	5.87%	5.87%
Mortgage-backed securities (in hundredths)	2.04%	2.04%
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	$ 19,262,000	$ 19,262,000		259,030,000
Less Than 12 Months Unrealized Losses	139,000			557,000
12 Months or Longer Fair Value	36,649,000	36,649,000		522,344,000
12 Months or Longer Unrealized Losses	201,000			5,579,000
Total Fair Value Available-for-sale Securities	55,911,000	55,911,000		781,374,000
Total Unrealized Losses Available-for-sale Securities	340,000			6,136,000
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,249,043,000	1,249,043,000	1,236,827,000	1,213,310,000
Gross Unrealized Gains	6,674,000	6,674,000	5,129,000	4,093,000
Gross Unrealized Losses			3,868,000	2,349,000
Estimated Fair Value	1,255,717,000	1,255,717,000	1,238,088,000	1,215,054,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	1,255,717,000	1,255,717,000	1,238,088,000	1,215,054,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value				237,891,000
Less Than 12 Months Unrealized Losses				471,000
12 Months or Longer Fair Value				283,643,000
12 Months or Longer Unrealized Losses				1,878,000
Total Fair Value Available-for-sale Securities				521,534,000
Total Unrealized Losses Available-for-sale Securities				2,349,000
Government Agency Issued Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	203,852,000	203,852,000	214,804,000	204,918,000
Gross Unrealized Gains	3,301,000	3,301,000	4,675,000	4,751,000
Gross Unrealized Losses	275,000	275,000	731,000	439,000
Estimated Fair Value	206,878,000	206,878,000	218,748,000	209,230,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	206,878,000	206,878,000	218,748,000	209,230,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	13,493,000	13,493,000
Less Than 12 Months Unrealized Losses	114,000
12 Months or Longer Fair Value	21,535,000	21,535,000		24,565,000
12 Months or Longer Unrealized Losses	161,000			439,000
Total Fair Value Available-for-sale Securities	35,028,000	35,028,000		24,565,000
Total Unrealized Losses Available-for-sale Securities	275,000			439,000
Government Agency Issued Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	226,285,000	226,285,000	240,702,000	241,449,000
Gross Unrealized Gains	3,661,000	3,661,000	1,788,000	2,319,000
Gross Unrealized Losses	24,000	24,000	5,165,000	3,200,000
Estimated Fair Value	229,922,000	229,922,000	237,325,000	240,568,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	229,922,000	229,922,000	237,325,000	240,568,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value				3,822,000
Less Than 12 Months Unrealized Losses				24,000
12 Months or Longer Fair Value	10,748,000	10,748,000		203,520,000
12 Months or Longer Unrealized Losses	24,000			3,176,000
Total Fair Value Available-for-sale Securities	10,748,000	10,748,000		207,342,000
Total Unrealized Losses Available-for-sale Securities	24,000			3,200,000
Obligations Of States And Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	426,911,000	426,911,000	484,691,000	458,026,000
Gross Unrealized Gains	24,730,000	24,730,000	24,794,000	25,986,000
Gross Unrealized Losses	41,000	41,000	181,000	148,000
Estimated Fair Value	451,600,000	451,600,000	509,304,000	483,864,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	451,600,000	451,600,000	509,304,000	483,864,000
Available-for-sale securities with continuous unrealized loss positions [Abstract]
Less Than 12 Months Fair Value	5,769,000	5,769,000		17,317,000
Less Than 12 Months Unrealized Losses	25,000			62,000
12 Months or Longer Fair Value	4,366,000	4,366,000		10,616,000
12 Months or Longer Unrealized Losses	16,000			86,000
Total Fair Value Available-for-sale Securities	10,135,000	10,135,000		27,933,000
Total Unrealized Losses Available-for-sale Securities	41,000			148,000
Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,158,000	16,158,000	6,856,000	6,864,000
Gross Unrealized Gains	850,000	850,000	1,141,000	1,347,000
Estimated Fair Value	17,008,000	17,008,000	7,997,000	8,211,000
Available-for-sale securities, estimated fair value, by contractual maturity [Abstract]
Estimated Fair Value	$ 17,008,000	$ 17,008,000	$ 7,997,000	$ 8,211,000