SECURITIES (Amortized Cost, Gross Unrealized Gains and Losses, and Estimated Fair Values of Investment Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 960,846	$ 947,597
Gross Unrealized Gains	12,942	10,480
Gross Unrealized (Losses)	(5,427)	(11,313)
Estimated Fair Value	968,361	946,764
Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	285,921	274,007
Gross Unrealized Gains	7,582	4,962
Gross Unrealized (Losses)	(1,304)	(3,079)
Estimated Fair Value	292,199	275,890
Corporate and Other Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	114,997	123,674
Gross Unrealized Gains	1,241	892
Gross Unrealized (Losses)	(816)	(2,786)
Estimated Fair Value	115,422	121,780
Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	546,038	536,031
Gross Unrealized Gains	4,119	4,626
Gross Unrealized (Losses)	(3,253)	(5,371)
Estimated Fair Value	546,904	535,286
Other Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,890	13,885
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(54)	(77)
Estimated Fair Value	$ 13,836	$ 13,808